Master's thesis of Engineering: Studies on film forming corrosion inhibitors on aluminium alloy, AA2024-T3

STUDIES ON FILM FORMING CORROSION INHIBITORS ON ALUMINIUM ALLOY, AA2024-T3

A thesis submitted in fulfilment of the requirements for the degree

of Master of Engineering

Chathumini R. Samarawickrama

Bachelor of Engineering (Honours) - Monash University, Australia

School of Engineering

College of Science, Technology, Engineering and Maths

RMIT University

September 2020

DECLARATION

I certify that except where due acknowledgement has been made, the work is that of the author

alone; the work has not been submitted previously, in whole or in part, to qualify for any other

academic award; the content of the thesis is the result of work which has been carried out

since the official commencement date of the approved research program; any editorial work,

paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and

guidelines have been followed.

Chathumini R. Samarawickrama

30 September 2020

ACKNOWLEDGEMENTS

Without the unconditional support that I have received from many individuals, I would not have

been able to successfully complete this thesis in due time and manner.

First of all, I would like to extend my eternal gratitude to my parents and my sister supported

me emotionally as well financially throughout the entire period. If not for their encouragement

and the continuous faith that they had in me, I would not have had the strength to overcome

the challenges that I came across during the completion of this work.

Secondly, but most importantly, I would like to thank all of my supervisors, Dr. Liam Ward,

Prof. Ivan Cole and Dr. Patrick Keil for the immense support, guidance, expertise as well as

the numerous opportunities provided over these two years. The confidence that my

supervisors showed in me surely played a significant role in pursuing my career in the research

industry. I could not have asked for better supervisors such immense patience, empathy as

well as kindness.

I would also like to thank the staff members in RMIT Microscopy and Microanalysis Facility

(RMMF), especially Dr. Edwin Mayes and Dr. Matthew Field for the assistance extended with

the Focused ion beam techniques, Transmission electron microscopy technique and other

surface analytical techniques.

I would also like to extend my thanks to my friends, especially my closest friends for constantly

being present and look after my well-being when times were difficult. I would also like to thank

the Rapid Discovery and Fabrication (RDF) team at RMIT University for being supportive and

for making me feel belonged.

Thank you.

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DECLARATION ......................................................................................................................................... ii

ACKNOWLEDGEMENTS .......................................................................................................................... iii

TABLE OF CONTENTS .............................................................................................................................. iv

LIST OF FIGURES .................................................................................................................................... vii

LIST OF TABLES ........................................................................................................................................ x

EXECUTIVE SUMMARY ............................................................................................................................ 1

1. CHAPTER 1: INTRODUCTION ........................................................................................................... 4

Background ............................................................................................................................. 4 1.1

Scope, significance and justification ....................................................................................... 7 1.2

Structure of the thesis ............................................................................................................ 9 1.3

2. CHAPTER 2: LITERATURE REVIEW ................................................................................................. 11

Properties and applications of AA2024-T3 ........................................................................... 11 2.1

Corrosion of aluminium and its alloys .................................................................................. 12 2.2

2.3 Microstructure and corrosion of AA2024-T3 ........................................................................ 16

S-phase and Ɵ-phase (Al2CuMg and Al2Cu) .................................................................. 16 2.3.1

AlCuFeMn(Si) ................................................................................................................. 19 2.3.2

2.4 Corrosion protection of aluminium and its alloys................................................................. 20

Inorganic corrosion inhibitors ....................................................................................... 23 2.4.1

Film-forming corrosion inhibitors ................................................................................. 24 2.4.2

Electrochemical and surface analytical techniques .............................................................. 31 2.5

Justification for the study ..................................................................................................... 34 2.6

3. CHAPTER 3: EXPERIMENTAL PROCEDURE ..................................................................................... 36

3.1 Specimen preparation ........................................................................................................... 36

Surface preparation ...................................................................................................... 36 3.1.1

Solution preparation ..................................................................................................... 37 3.1.2

3.2 Introduction to the experimental techniques ...................................................................... 38

Electrochemical techniques .......................................................................................... 38 3.2.1

Surface analytical techniques ....................................................................................... 41 3.2.2

3.3 Experimental methods .......................................................................................................... 43

Electrochemical techniques .......................................................................................... 43 3.3.1

Surface analytical techniques ....................................................................................... 46 3.3.2

Film Stability Study ........................................................................................................ 49 3.3.3

4. CHAPTER 4: RESULTS: INHIBITOR-INDUCED FILM FORMATION ................................................... 51

4.1 Evaluation of inhibitor performance ..................................................................................... 52

4.1.1 Free Corroding Potential ............................................................................................... 52

4.1.2 Effect of concentration ................................................................................................. 53

4.1.3 Effect of pH on inhibited systems ................................................................................. 57

4.2 Evaluation of the mechanism of film-formation ................................................................... 59

4.2.1 EIS Studies for the uninhibited system ......................................................................... 59

4.2.2 EIS Studies for inhibited system .................................................................................... 60

4.2.3 Modelling of equivalent circuits for inhibited and uninhibited systems ...................... 62

4.2.4 Linear polarization resistance studies ........................................................................... 65

4.3 Surface analysis of inhibited systems ................................................................................... 66

4.3.1 Energy dispersive x-ray spectroscopy (EDS) of 2-MBT system ..................................... 66

4.3.2 Energy dispersive x-ray spectroscopy (EDS) of Na-MPA system................................... 69

4.4 Analysis of pure aluminium and copper ............................................................................... 70

4.4.1 Effect of inhibitors on pure aluminium ......................................................................... 70

4.4.2 Effect of inhibitors on pure copper ............................................................................... 73

5. CHAPTER 5: RESULTS: LONG-TERM STABILITY OF THE INHIBITOR-INDUCED FILM ...................... 76

5.1 Studies for uninhibited and inhibited systems over 168 hours (7 Days) immersion ............ 77

5.1.1 Linear polarization resistance (LPR) studies ................................................................. 77

5.1.2 Potentiodynamic scan (PDS) studies ............................................................................. 79

5.1.3 Electrochemical impedance spectroscopy (EIS) studies ............................................... 81

5.2 LPR studies of inhibited systems after 336 hours (14 Days) immersion ............................... 87

5.3 Surface analysis ..................................................................................................................... 88

5.3.1 Surface analysis of the uninhibited system .................................................................. 88

5.3.2 Surface analysis using SEM of the 2-MBT inhibited alloy surface................................. 89

5.3.3 Surface analysis using SEM of the Na-MPA inhibited alloy surface .............................. 90

Surface analysis of 2-MBT inhibited systems over longer exposure periods of 1440 hours 5.4 (60 days) ............................................................................................................................................ 94

6. CHAPTER 6: RESULTS: FILM DEGRADATION STUDIES ................................................................... 99

6.1 Linear polarization resistance (LPR) studies ....................................................................... 100

6.1.1 Film breakdown studies after a 7-day pre-treatment period ..................................... 100

6.1.2 Film breakdown studies after a 30-day pre-treatment period ................................... 101

6.1.3 Film breakdown studies after a 60-day pre-treatment period ................................... 102

6.1.4 Effect of the pre-treatment period on the film stability ............................................. 104

6.2 Electrochemical impedance spectroscopy (EIS) Studies ..................................................... 106

6.3 Potentiodynamic scanning (PDS) ........................................................................................ 109

7. CHAPTER 7: DISCUSSION ............................................................................................................. 113

7.1 Effectiveness of inhibitor-induced film formation and associated mechanisms ................ 113

7.1.1 Preliminary evaluation of performance of inhibitors ................................................. 113

7.1.2 Effect of presence of intermetallic particles on inhibitor-induced film-formation .... 118

7.2 Inhibitor-induced film stability and growth ........................................................................ 120

7.2.1 Electrochemical analysis of inhibitor-induced film formation and its stability .......... 120

7.2.2 Surface analysis of film growth and stability .............................................................. 126

7.3 Inhibitor-induced film degradation studies ........................................................................ 129

8. CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ............................ 134

8.1 Inhibitor-induced film formation ........................................................................................ 134

8.2 Long-term stability of the inhibitor-induced film ............................................................... 135

8.3 Film degradation studies ..................................................................................................... 136

8.4 Future work and recommendations ................................................................................... 137

REFERENCES ........................................................................................................................................ 139

LIST OF FIGURES

Figure 2-1: Pitting corrosion mechanism for S-phase particle/aluminium matrix ............................... 13

Figure 2-2: Natural oxide layer formed on the aluminium surface ...................................................... 14

Figure 2-3: Pourbaix diagram for the system Al/H2O at 25oC ............................................................... 14

Figure 2-4 : A polished surface of A2024-T3 - Legend for the phase colours appears in Table 4.1 ..... 16

Figure 2 -2-5: Schematic diagram displaying different modes of possible Cu-redistribution from the Cu-

rich intermetallic particles in AA2024-T3.............................................................................................. 18

Figure 2-6: Hierarchy of localised corrosion attack on AA2024-T3 ...................................................... 19

Figure 2-7:Hypothesised schematic diagram of rare-earth metal-based inhibitors deposited on the s-

phase intermetallic particle within 30 minutes of immersion and 24 hours of immersion. ................ 25

Figure 2-8: Schematic diagram on mechanism of corrosion inhibitor on AA2024-T3 .......................... 28

Figure 3-1: Electrochemical Setup ........................................................................................................ 44

Figure 4-1: Comparison of OCP of inhibited and uninhibited systems over 1 hour ............................. 52

Figure 4-2: Comparison of Potentiodynamic scans at different 2-mercaptobenzothiazole (2-MBT)

inhibitor concentrations ....................................................................................................................... 53

Figure 4-3: Comparison of potentiodynamic scans at different 3-mercaptopropionic acid (3-MPA)

inhibitor concentrations ....................................................................................................................... 55

Figure 4-4: Comparison of potentiodynamic scans at different 3-mercaptopropionic acid (3-MPA)

inhibitor concentrations ....................................................................................................................... 56

Figure 4-5: Comparison between 2-mercaptobenzothiazole (2-MBT) at a pH of 4.5 and 7 ................ 57

Figure 4-6: Comparison between 3-mercaptobenzothiazole (3-MPA) and Na-mercaptopropionate (Na-

MPA) ..................................................................................................................................................... 58

Figure 4-7: (a) Nyquist plot of the uninhibited system; (b) Bode plot of the uninhibited system ....... 59

Figure 4-8: (a) Nyquist plot of the 2-MBT system; (b) Bode plot of the 2-MBT system ....................... 60

Figure 4-9: (a) Nyquist plot of the Na-MPA system; (b) Bode plot of the Na-MPA system .................. 61

Figure 4-10: Comparison of EIS data between the inhibited and uninhibited system ......................... 62

Figure 4-11: Simplified equivalent circuit for the inhibited and uninhibited system ........................... 62

Figure 4-12: Comparison of polarization resistance data for uninhibited and inhibited systems over 72

hours ..................................................................................................................................................... 65

Figure 4-13: Elemental composition through EDS after 7 days of immersion in 2-MBT solution. ...... 67

Figure 4-14: Elemental composition through EDS after 7 days of immersion in 2-MBT solution,

displaying the effects of trenching ....................................................................................................... 68

vii

Figure 4-15: Elemental composition through EDS after 7 days of immersion in Na-MPA solution ..... 69

Figure 4-16: Potentiodynamic scans for inhibited and uninhibited pure aluminium systems ............. 70

Figure 4-17: Polarization resistance for uninhibited and inhibited systems with pure aluminium over

72 hours ................................................................................................................................................ 72

Figure 4-18: Potentiodynamic scans for inhibited and uninhibited pure copper systems ................... 74

Figure 4-19: Comparison of linear polarization resistance of inhibited and uninhibited systems with

pure copper ........................................................................................................................................... 75

Figure 5-1:Comparison of polarization resistance with and without inhibitors on aluminium alloy,

AA2024-T3 over 168 hours (7 days). ..................................................................................................... 77

Figure 5-2: Potentiodynamic scans of inhibited systems after 30 min and 168 hours (7 days) ........... 79

Figure 5-3: (a) Nyquist plots in the 2-MBT inhibited system over 168 hours (7 days) ......................... 81

Figure 5-4: Suggested equivalent circuit for the 2-MBT system over 168 hours (7 days) .................... 82

Figure 5-5:(a) Nyquist plots for the Na-MPA inhibited system over 168 hours (7 days) ...................... 84

Figure 5-6: Linear polarization resistance, Rp of inhibited systems during 7 – 14 days exposure period.

.............................................................................................................................................................. 87

Figure 5-7:(a) Sample as received (polished); (b) after 1 hr of immersion in an uninhibited chloride

electrolyte; (c) after 24 hr of immersion in an uninhibited solution .................................................... 88

Figure 5-8: SEM images of AA2024-T3 after exposure to a 2-MBT induced 0.1 M NaCl solution for

different time periods. .......................................................................................................................... 89

Figure 5-9: Topography of AA2024-T3 after 168 hours (7 days) in 2-MBT induced NaCl solution....... 90

Figure 5-10 : SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1 M NaCl solution for

different time periods. .......................................................................................................................... 91

Figure 5-11:Overall topography SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1 M

NaCl solution for 7 days ........................................................................................................................ 92

Figure 5-12: Overall topography SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1

M NaCl solution for 7 days showing the corroded areas with the pits formed .................................... 92

Figure 5-13: EDS results of AA2024-T3 after 7 days in Na-MPA induced NaCl solution ....................... 93

Figure 5-14: FIB image of AA2024-T3 sample that has been exposed to 2-MBT inhibited NaCl after 60

days ....................................................................................................................................................... 94

Figure 5-15: Image of a TEM lamella of AA2024-T3 that has been exposed to 2-MBT inhibited NaCl

after 60 days ......................................................................................................................................... 95

Figure 5-16: Labelled image of the analysed lamella sample ............................................................... 96

Figure 5-17:Transition between the aluminium matrix and the intermetallic particle ........................ 96

viii

Figure 5-18: (a) TEM image of the intermetallic particle region; (b) TEM image of the transition region;

(c) TEM image of the aluminium matrix region .................................................................................... 97

Figure 6-1: Polarization resistance after a 7-day immersion period in 2-MBT inhibited system is

exposed to an uninhibited solution for 7 days ................................................................................... 100

Figure 6-2: Polarization resistance after a 30-day immersion period in 2-MBT inhibited system is

exposed to an uninhibited solution for 24 hours ............................................................................... 101

Figure 6-3: Polarization resistance after a 60-day immersion period in 2-MBT inhibited system is

exposed to an uninhibited solution for 24 hours ............................................................................... 103

Figure 6-4: Effect of immersion time period in an inhibitor induced NaCl solution on film degradation

............................................................................................................................................................ 105

Figure 6-5: Nyquist plot after being exposed to an uninhibited system at different time intervals (60-

day treatment period in 2-MBT) ......................................................................................................... 106

Figure 6-6: Bode plot after being exposed to an uninhibited system at different time intervals (60-day

treatment period in 2-MBT) ................................................................................................................ 107

Figure 6-7: Potentiodynamic scanning after being exposed to an uninhibited system at different time

intervals (60-day treatment period in 2-MBT) .................................................................................... 110

Figure 6-8: Image of the sample which was treated for 60 days and then was exposed to a NaCl only

solution for 7 days............................................................................................................................... 112

Figure 7-1: Schematic diagram of the equivalent circuit for the inhibitor system ............................. 124

Figure 7-2: Schematic diagram of the equivalent circuit for the degradation of the passivating layer in

the absence of inhibitor ...................................................................................................................... 131

LIST OF TABLES

Table 2-1: Atomic percentage of elements in nominated phases ........................................................ 16

Table 4-1: Ecorr and icorr values at different inhibitor concentrations ............................................... 56

Table 4-2: Fitted equivalent circuit parameters of uninhibited, 2-MBT and Na-MPA systems ............ 63

Table 4-3: The fitted parameters for the uninhibited system (a) model in parallel; (b) model in series

.............................................................................................................................................................. 64

Table 4-4: Ecorr and icorr parameters for the uninhibited and inhibited systems with pure aluminium

.............................................................................................................................................................. 71

Table 4-5: Ecorr and icorr with and without inhibitors for pure copper .............................................. 74

Table 5-1: Ecorr and icorr values of inhibited systems after 0 hours and 168 hours (7 days) ............. 80

Table 5-2: The fitted parameters from the equivalent circuit (figure 5-5) for the 2-MBT system ....... 82

Table 5-3: The fitted parameters of the corresponding equivalent circuits for the Na-MPA system over

168 hours (7 days) ................................................................................................................................. 85

Table 6-1: The fitted parameters from the equivalent circuit (figure 6-7) for the film degradation

process ................................................................................................................................................ 108

Table 6-2: icorr and Ecorr values obtained from tafel extrapolation (60-day treatment period in 2-MBT)

............................................................................................................................................................ 110

EXECUTIVE SUMMARY Aluminium alloy, AA2024-T3 is used in many structural applications in the aerospace industry,

to include fuselage, wings, spars and other critical load bearing structures, due to its high

strength to weight ratio and high damage tolerance. However, the susceptibility of this alloy to

various forms of corrosion has limited certain applications and led to this alloy being widely

studied by researchers to explore ways to mitigate corrosion. Currently, the most commonly

utilized methods are through the use of protective coatings and corrosion inhibitors. The

limitations of chromate-based inhibitors associated with the toxic and carcinogenic effects has

stimulated research into the use of alternative low toxicity inhibitors, to include film-forming

corrosion inhibitors. While many studies have been conducted in the past to determine the

performance and inhibition efficiency of a wide range of such inhibitors, limited amount of work

have been done in understanding the underlying mechanisms on the film-forming process,

interactions with the oxide layer and overall long-term performance. In particular, little

information is available regarding the stability of the film over extended time periods in the

presence and absence of the inhibitor.

A series of experiments utilizing both electrochemical and surface analytical techniques were

developed to evaluate the inhibitor-induced film growth, stability and subsequent breakdown,

on AA2024-T3 when exposed to NaCl solution. For this study, two inhibitors were utilized,

namely 2-mercaptobenzothiazole (2-MBT) and Na-mercaptopropionate (Na-MPA). The first

part of the study focussed on understanding the film forming behaviour and optimizing the

inhibitor systems for longer term durability studies. Potentiodynamic scanning (PDS) studies

concluded optimum conditions for longer term studies to be 0.1 mM concentration for both 2-

MBT and Na-MPA, at pH values of 4.5 and 7 respectively. Both the inhibitors resulted in a

current density one order of magnitude lower than that of the uninhibited solution. Electro-

impedance spectroscopy (EIS), linear polarisation resistance (LPR) studies and surface

analysis revealed important information about the inhibitor films formed, interaction with the

surface and the influence of the intermetallic particles (IMP’s) present in the alloy. Dissolution

of the oxide layer promoted trenching to occur in the vicinity of the intermetallic particles

(IMP’s) and subsequent re-deposition of Cu remnants. For 2-MBT, the thiol groups triggered

film formation due to the strong affinity for Cu to S. for Na-MPA, the carboxylate groups in

addition to the thiol groups interacted with the surface and contributed to film formation.

Studies on pure Al and Cu confirmed the selectivity of the inhibitor groups in promoting film

formation on the intermetallic particles and/or the Al matrix. Generally, initial trenching

promoted the film-formation process for both inhibitors.

For long term inhibitor performance, LPR data showed continued protection by the inhibitor 2-

MBT over a 60-day period, whereas the Na-MPA inhibitor only showed protection for 8-9 days.

Limited protection over longer exposure periods was attributed to insufficient inhibitor

concentration being present, which in turn, was thought to reduce the protectivity offered by

the film. The equivalent circuits fitted through EIS showed that the dominating factor over a

168-hour (7 days) period was the charge transfer resistance. The porosity of the inhibitor/oxide

layer controlled the amount of active species migrating to the alloy surface. Over longer

exposure periods, 2-MBT showed lowered charged transfer resistance due to inhibitor

blocking the pores and thereby forming active free surface. In the case of Na-MPA, the charge

transfer resistance reduced over time, indicating increased porosity development over time,

allowing more of the corrosive medium to reach the active surface. This study also

demonstrated an inhibitor/oxide layer, approximately 20 nm thick, formed over the alloy

surface after 60 days of exposure to 2-MBT.

The film breakdown studies revealed 7 days and 30 days pre-conditioning in the 2-MBT

inhibited solution was insufficient time to develop and sustain a protective layer, when exposed

to the uninhibited solution for 68 hours (7 days). After 60 days pre-conditioning however, the

film that had developed showed continued protection over a 168-hour period (7 day) while

demonstrating self-repairing abilities. Through EIS equivalent fitting, the charge transfer

resistance appeared to be the dominating factor where the highly soluble corrosion products

were thought to block the pores, thus creating a barrier between the active species and the

alloy surface.

In summary, this study has provided a deeper understanding of the mechanisms of film

formation of two inhibitor systems used for the protection of AA2024-T3 alloys. The study has

highlighted important findings surrounding interactions of the inhibitor species with the alloy

surface, the naturally occurring oxide layer and the intermetallic precipitates that are present

at the surface, with respect to the overall longer-term durability and breakdown of these

systems.

1. CHAPTER 1: INTRODUCTION

1.1 Background Aluminium being the most abundant metal on earth has led to the development of a wide

range of aluminium alloys tailored for a wide range of industrial sectors. The selection of an

appropriate alloy for many applications requires taking properties such as ductility, tensile

strength, weldability, density, formability and most importantly corrosion resistivity into

consideration. One of the major benefits in terms of structural applications is the high strength

to weight ratios that can be achieved as a result of the low density of aluminium. To tailor the

properties, aluminium alloys are produced by the addition of other elements such as copper,

iron, magnesium, zinc and silicon at different percentages which may add up to 15% of the

alloy by weight. The ability to tailor the metal properties required for a desired application via

the addition of various combinations of alloying elements makes these aluminium alloys a

highly attractive choice in industry. Consequently, a range of commercially graded aluminium

alloys with various mechanical properties are currently available for common applications such

as in the aerospace, automotive and marine industry. Favourable mechanical properties are

produced through various strengthening mechanisms to include the incorporation of a range

of distinctive intermetallic particles as well as by the methods of production.

One of the major benefits of pure aluminium is its high corrosion resistance in many aggressive

environments. The corrosion resistance of pure aluminium can be largely attributed to the

formation of a naturally occurring oxide layer over the metal surface. Aluminium has a high

affinity for oxygen and therefore, when the metal surface is exposed to air, it very rapidly

develops a compactly adhering, self-healing and protective oxide layer, shielding the surface

from further corrosion [1]. However, if the oxide layer becomes damaged, this can trigger

corrosion processes to initiate. Once localised corrosion has been initiated, self-healing

cannot occur and thus the oxide layer becomes non protective. Hence, a uniform oxide layer

is crucial to resist corrosion [2]. Although pure aluminium can provide adequate corrosion

resistance in many environments, the poor mechanical properties limit its usage in many

structural applications. Such limitations can be overcome through alloying with certain

elements to achieve the required structural properties. However, the high corrosion resistance

of pure aluminium is generally compromised with alloying additions. Here, the presence of

intermetallic particles may promote the formation of a non-uniform oxide layer over the metal

surface.

From the broad range of aluminium alloys from the 1xxx series through to the 7xxx series,

aluminium alloy, AA2024 is used in many structural applications in the aerospace industry,

such as fuselage, wings, spars and other critical load bearing structures, due to its light weight,

high strength and high damage tolerance. However, the most significant drawback of this alloy

is the relatively low corrosion resistance in comparison to pure aluminium and even other

aluminium alloys such as those from the 6xxx series. The main alloying additions in AA2024-

T3 are Cu and Mg with typical compositions of 4.3-4.5% and 1.3-1.4% respectively.

Consequently the formation of intermetallic precipitates through the interaction of these

alloying additions with the Al matrix, such as the S-phase Al2CuMg and other deleterious

phases create a heterogeneous metal surface, thus increasing the susceptibility to localised

corrosion [2-4]. Localized corrosion is essentially manifested by the formations of pits more

often around the intermetallic particles. Similarly, to pitting corrosion, aluminium alloys could

also undergo crevice corrosion if a crevice is present and the substrate is exposed to a

corrosive medium. Aluminium alloys are also susceptible to intergranular corrosion, where

selective dissolution takes place as a results of the formation of electrochemical micro-

coupling along the grain boundary [5], as well as other forms of corrosion such as filiform

corrosion, erosion corrosion and stress corrosion cracking [6]. The presence of these

intermetallic particles can hinder the uniform development of an oxide layer over the metal

surface, subsequently reducing the corrosion protection of the alloy. Therefore, over time, the

corrosion engineering sector has developed methods to mitigate against such forms of

corrosion. This commonly includes the use of corrosion inhibiting compounds, protective

coatings or the combination of both to be employed to minimize corrosion.

A corrosion inhibitor can be considered to be a chemical substance which, when added in

small concentrations to the environment, will decrease the corrosion rate of the system. From

the wide range of inhibitors available for mitigating corrosion of Al alloy systems, chromate-

based inhibitors still remain the most common choice for AA 2024-T3 alloy, due to its excellent

inhibiting performance and low cost. However, the main drawback with chromate-based

inhibitors is its highly toxic and carcinogenic nature, thus precluding their usage in many

applications. Consequently, significant efforts have focussed on the development of more

“environmentally friendly” and less toxic inhibitors to replace carcinogenic conventional

chromate-based inhibitors. Consequently, numerous studies have been carried out on

assessing the performance of a range of inhibitors and the corrosion mechanism.

Studies have focussed on a range of different types of inhibitors, including organic, inorganic

and rare-earth based corrosion inhibitors. In particular, organic inhibitors are known to perform

well with aluminium alloy, AA2024-T3 and an extensive amount of work has been done in this

respect. Different inhibitors exhibit different mechanisms according to the nature of the

inhibitor compound and their composition. Inhibitors could be classified as anodic, cathodic or

mixed type where the inhibitors can shift the metal surface into the passivation range, block

the cathodic sites or even form a barrier over the metal surface. From previous studies, it has

been indicated that a certain percentage of organic inhibitors which are typically considered

to be mixed inhibitors provide protection by forming a film over the metal surface [7-9]. These

film-forming inhibitors have been studied intensively and researchers have suggested

potential corrosion mechanisms as to how these inhibitors could be providing protection to the

AA2024 surface [10]. Despite the work that has been done on assessing the performance and

the inhibiting mechanisms, limited studies have been conducted on understanding and

evaluating the mechanisms of inhibitor-induced film formation of organic inhibitors. In addition,

as it is crucial to achieve a solution that is effective in the long-term, it is essential to understand

the long-term effects and durability of such film-forming inhibitor compounds on a particular

alloy surface when subject to specific environmental conditions.

Corrosion studies are mainly built upon electrochemical techniques and surface analytical

techniques to obtain quantitative and qualitative data respectively. Authors have utilized a

range of electrochemical techniques such as potentiodynamic scans, electrochemical

impedance spectroscopy, linear polarization resistance and many more along with various

surface analytical techniques to achieve visual evidence to confirm the results obtained via

the electrochemical techniques. Scanning electron microscopy, atomic force microscopy,

focused ion beam and X-ray photoelectron microscopy are amongst the most equipped

methods. Previous studies have demonstrated the use these methods to evaluate the

performance of inhibitors, the multi-layer interactions, film thickness and subsequently the

mechanism of inhibitors.

1.2 Scope, significance and justification As per the literature, there are hundreds of corrosion inhibitors in the market and many more

currently being tested. It is already well-established that some inhibitors form films over the

surface of the alloy. Whilst pure aluminium naturally forms an oxide layer to retard the

corrosion process, addition of some inhibitors to aluminium alloys is known to follow a similar

path by displaying specific film-forming characteristics. Many studies have been done on

determining the efficiency of different inhibitors and interpretation of the mechanisms of

corrosion inhibitors.

However, there is a lack of knowledge on the behaviour of film forming inhibitors that are both

known to be very efficient and environmentally friendly. In particular, little work has been done

on the time resolved stability of these films and their interactions with naturally occurring /

corrosion induced oxide / hydroxide layers. Therefore, it would be important to study the

mechanism of film formation in depth to identify key characteristics which trigger film formation

as these could be very effective replacements to chromate-based inhibitors. It would also be

significantly useful to understand if the inhibitor-induced film formed would persist in the

presence of inhibitor over time and if it is removed and exposed to an inhibitor-free solution.

The main focus of this study was to determine how the mechanism of inhibitor film formation

influences the corrosive activity of aluminium alloy, AA2024-T3. In particular the main

objectives of this study were to determine:

• The optimum conditions for inhibitors to induce protective films on AA2024-T3

• The long-term stability of the inhibitor-induced film with the continuous presence of an

inhibitor

• The degradation process of the formed film once exposed to a neutral chlorine solution

• The correlations between data obtained from electrochemical techniques and surface

analysis techniques

• The impact of molecular structure on the film formation behaviour

• Interaction between the naturally-occurring oxide layer and the inhibitor-induced film

Therefore, the key focus of this study was based on understanding the mechanisms

associated with the formation and development of inhibitor induced films and assessing how

they were related to the attachment of the inhibitor molecule to the surface, build-up of the film

layers and restriction of anodic/cathodic activity. In addition, this study also concentrated on

the film development with time as a function of the properties of AA2024-T3 alloys and to

apprehend the long-term film stability.

Significantly limited work has been conducted on determining the stability of the inhibitor-

induced film in the absence of a continuous supply of inhibitor and the effectiveness of it as a

barrier against corrosion of the selected substrate. Consequently, this has become one of the

main objectives of this study.

This study has utilised a range of electrochemical techniques in conjunction with surface

characterisation methods to provide quantitative results with visual aid, thus allowing for a

deeper understanding of film formation, long term stability and film breakdown associated with

a selection of organic inhibitors, using both quantitative and qualitative data.

1.3 Structure of the thesis This thesis presents work on determining and understanding the mechanism behind the film-

formation process of organic inhibitors on aluminium alloy, AA2024-T3. This work also

demonstrates the stability of the inhibitor-induced film in the presence and absence of a

continuous supply of inhibitor.

Following on from Chapter 1 which outlines an introduction to the study including background,

scope, aims and objective, chapter 2 includes the background knowledge that is required to

understand the problem in need of attention and a summary of the previous studies that has

been conducted in relation to the selected inhibitors in evaluating the performance. This

section also lays out the basic information that has been used as a reference point for this

study and a critical review of the literature.

Chapter 3 lays out an introduction to the experimental techniques used in this study along with

the parameters and steps taken to conduct a series of experiments to obtain the required

results.

Chapter 4 presents the results on analysing the effects of concentration and pH on the

selected inhibitors and the potential mechanism of the film-formation via potentiodynamic

scans, electrochemical impedance spectroscopy and energy dispersive spectroscopy. This

section explores the interaction between the inhibitor molecule and the metal surface as well

as the work done on investigating the interaction between the oxide layer and the inhibitor film.

In addition, analysis of pure aluminium and copper was conducted to understand the

performance of the inhibitor with these elements independently, to assist with understanding

the mechanism of the film-formation and the effect of intermetallic precipitates present on film

forming behaviour.

Chapter 5 includes a series of electrochemical test results in conjunction with surface

analytical data to demonstrate each inhibitor film growth and stability over longer immersion

times. This provides valuable insights to the long-term performance of the selected inhibitors

for aluminium alloy, AA2024-T3.

Chapter 6 contains the linear polarization resistance results achieved to examine the tenacity

of the inhibitor-induced film in the absence of inhibitor once the film has been developed. This

chapter also contains results exploring the minimum immersion time required for the film to be

sustainable in a corrosive medium.

Chapter 7 discusses the results in Chapters 4, 5 and 6 to summarize the significance of the

analysed data and how these results could be employed to solve the research gaps discussed

in section 1.2.

Chapter 8 provides the main conclusions drawn from this study and recommendations for

future work to build up on the results achieved through this study.

2. CHAPTER 2: LITERATURE REVIEW

Corrosion is a process which is significantly influenced by the metallurgy of the chosen metal.

In particular, the microstructure and heterogeneous nature of aluminium alloy, AA2024-T3 can

significantly influence the corrosion behaviour, the stability and nature of the oxide layer

formed and the interaction of the surface with corrosion inhibitors, particularly film forming

inhibitors. This chapter explores in detail the structure of the alloy, AA2024-T3 to understand

the microstructure and the role of intermetallic particles influencing the process of film

formation. In addition, the selected inhibitors are also introduced and the associated inhibition

mechanisms via electrochemical and surface analytical techniques. For this purpose, these

techniques such as open circuit potential, potentiodynamic scans, electrochemical impedance

spectroscopy, energy dispersive spectroscopy, scanning electron microscopy and optical

microscopy techniques have been discussed.

2.1 Properties and applications of AA2024-T3 Aluminium, the world’s third most common element and one of the lightest engineering

materials, is the most widely used metal in alloy form after steel. Pure aluminium, known for

its corrosion resistance and high electrical conductivity is also soft and ductile, hence

aluminium alloys were produced for structural engineering applications. The presence of other

elements has resulted in improved mechanical properties such as tensile strength, fatigue

resistance, ductility, good machinability and weldability. A range of aluminium alloys are

available, depending on the alloying additions, fabrication process (cast or wrought) and

strengthening mechanisms (heat treatable / non heat treatable). Common alloys used in many

structural engineering applications include, 1xxxx, 2xxx, 4xxxx, 5xxxx, 6xxxx and 7xxxx series

which have varying properties and thereby tailored for a wide range of specific applications.

AA2024-T3, an alloy from the 2xxx series, has widespread applications and is extensively

used in the aircraft construction industry due to a range of properties, to include high fracture

tough performance, super-plasticity, lightweight, high damage tolerance as well as high

durability. This material is being employed in the fuselage structure and aircraft wings in the

form of sheets in T3 conditions. Additional strengthening mechanisms such as cold working

due to forming processes may also be involved. This alloy was originally developed to allow

for moderate amounts of cold working through deformation to be conducted, followed by

extended periods of ageing, accounting for the increased strength properties.

According to the work done by Huda and co-workers [1], this alloy has a grain size of about

19.6 µm which is considered to be relatively smaller than other engineered metals and

therefore can be classified as a fine-grained material. As per the Hall-Petch relationship,

tensile strength is inversely proportional to the grain size and hence, strengthening through

grain refinement could be an additional strengthening mechanism [11].

Due to the alloying additions which includes, copper (Cu), magnesium (Mg), manganese

(Mn), iron (Fe),and silicon (Si) the microstructure of this alloy is highly complex, consisting of

an aluminium matrix and a range of intermetallic (IM) particles. Boag, et al. [12] studied the

complexity of the microstructure of AA2024-T3. Due to the small sample sizes, classifications

and technique resolutions, there were many variations in the exact composition of AA2024-T3

alloy. The incorporation of Cu, as the main alloying element, resulted in the development of

this alloy as part of the foundation of the aerospace construction industry due to the desirable

mechanical properties achieved through the excellent solubility and the ability to strengthen

through age hardening [1, 13].

2.2 Corrosion of aluminium and its alloys Pure aluminium is known to have high resistance to corrosion. Despite it being a very reactive

metal, aluminium remains resistant to corrosion due to the formation of a passive layer or an

oxide layer on the surface of the metal. These layers act as a barrier between the metal and

the environment. Aluminium has a very high affinity with oxygen that when a new aluminium

surface come into contact with air, it very rapidly acquires a thin, compact, tightly adhering,

protective and most importantly self-healing film of aluminium oxide. The extent of resistance

to corrosion solely depends on the inertness of this film. However, if the surface film is

defective through dissolution, mechanical damage or other factors, corrosion processes

initiate and in the case of localised corrosion processes, self-healing cannot occur. Therefore,

the relationship between aluminium and oxygen availability is key to the mechanisms of

corrosion protection and the conditions that can result in breakdown of protective barriers.

The corrosion mechanisms of aluminium are discussed in general as background. Exposing

to a saline medium causes the aluminium to dissolve into its respective ions and / or induce

oxide layer breakdown, forming aluminium ions as shown below [8, 14]:

At the anode (oxidation):

Al → Al3+ + 3 e- ------- Equation (2.1)

Al3+ + 3 H2O→ Al (OH) 3 + 3 H+ ------- Equation (2.2)

At the cathode (reduction):

2 H+ + 2 e- → H2 ------- Equation (2.3)

O2 + 2 H2O + 4 e- → 4 OH- ------- Equation (2.4)

Pitting is a form of localized corrosion which is triggered by the presence of chloride ions. Pits

Figure 2-1: Pitting corrosion mechanism for S-phase particle/aluminium matrix

are propagated as illustrated in figure 2.1 below [15].

The negatively charged Cl- ions start to migrate into the pits as reactions (2.1) and (2.2)

produce a positive charge. The alkaline environment destabilize the aluminium oxide layer

and initiate aluminium dissolution inside the pit [15].

As mentioned before, the high resistivity of pure aluminium is a result of a thick oxide layer

over the metal surface. The alumina layer is composed of a highly compact amorphous

protective layer and over longer periods of time, a thicker porous layer is formed over this

inner layer. The oxide layer has been shown to be about 5-10 nm in thickness at room

temperature as shown in figure 2.2 [8, 16].

Figure 2-2: Natural oxide layer formed on the aluminium surface

[16]

The naturally-occurring oxide layer over aluminium is stable between pH values of 5 and 8.5

as shown in the Pourbaix diagram below [8, 17].The passivity of a metal can be disrupted by

several external factors such as pH, temperature, presence of impurities which causes

formation of aluminium-soluble complexes.

Figure 2-3: Pourbaix diagram for the system Al/H2O at 25oC

[8]

From figure 2.3, it can be observed that when exposed to pH levels between 4 and 9, the

aluminium surface is covered with a thicker oxide layer, implying that the oxide layer is

unstable in very acidic (low pH) or highly alkaline (high pH) environments.

Aluminium alloys are generally tailored to meet the requirements of many engineering

applications, particularly those which are structural, however this is usually at the expense of

corrosion resistance. Additions of various alloying components affects the corrosion

resistance to varying degrees, thus making it difficult to categorise the overall performance of

aluminium alloys in general. This is associated with the heterogenic nature of these alloys, the

nature and composition of alloying additions and the resultant precipitates that form and are

dispersed throughout the matrix. The effects of intermetallic precipitates on corrosion

performance is discussed in more detail in the next section.

2.3 Microstructure and corrosion of AA2024-T3 According to the study by Boag et al. [12], 9 different intermetallic phases have been shown

to be present in the 2024 alloy as listed in table 2.1. Figure 2.4 provides a visual representation

Table 2-1: Atomic percentage of elements in nominated phases

Figure 2-4 : A polished surface of A2024-T3 - Legend for the phase colours appears in Table 4.1

of the different phases mentioned in the table 2.1 [12].

As evident, the intermetallic particles in AA2024-T3 mostly consist of copper with a percentage

ranging between 3.80 and 4.40 out of the total intermetallic particles which contributes to a

2.3.1 S-phase and Ɵ-phase (Al2CuMg and Al2Cu)

percentage of 6.6 to 7.1.

The more common precipitates found in the 2XXX alloy series are the S-phase and Ɵ-phase.

The s-phase and Ɵ-phase particles can be distinguished by their composition. Ɵ-phase is a

binary alloy consisting of Al and Cu, typically with an Al:Cu ratio of 70:30 and formula Al2Cu,

whereas S-phase particles contain additions of Mg as well as Al and Cu, with an Al:Cu:Mg

ratio of 60:20:20 and typical formula Al2CuMg. Whilst many studies have relied on shape-

based classification to determine anodic S-phase particles and cathodic particles, Boag et al.

established that S-phase and Ɵ-phase have similar shapes [12].

The Ɵ-phase can be identified by its spherical shape which ranges from 0.73 um to 6.18 um

and represents about 40% of all intermetallic particles in the alloy. It has been found that after

mechanically polishing the alloy surface to 1 um using active oxide polishing suspension

(OPS), the percentage of theta phase at the alloy surface increased to 48% of the area of the

intermetallic particles along with a subsequent decrease in the amount of S-phase particles

present. This could be a result of selective dissolution of magnesium due to chemical effect of

OPS polishing. Kikuchi pattern analysis revealed that while both the S-phase and Ɵ-phase

particles occur in clusters, backscattered electron microscopy was able to distinguish the Ɵ-

phase from the S-phase due to its high atomic density [2]. From these two intermetallic

species, the S-phase (Al2CuMg) plays an important role as S-phase precipitation formation is

a key aspect for strengthening of AA2024-T3 during age-hardening, and generally plays a key

role in multi-phase strengthening [1]. However, the de-alloyed S-phase particles with Cu

remnants are detrimental to maintaining corrosion resistivity as they lead to the formation of

preferential anodic sites due to the dissolution of the aluminium matrix [18].

The S-phase particle, Al2CuMg, is a spherical intermetallic particle formed during the

solidification and aging processed during the manufacture of the alloy. The S-phase

intermetallic has been found to be more active than the matrix and is more prone to dissolution.

Another explanation as shown in figure 2.5 which is a schematic diagram depicting the stages

of redistribution of copper on AA2024-T3 exposed to a corrosive medium as per the study

carried out by Williams et al. [19].

Figure 2 -2-5: Schematic diagram displaying different modes of possible Cu-redistribution from the Cu-rich intermetallic particles in AA2024-T3

The authors have explained that dealloying of the s-phase particle (stage 1 in figure 2.5) is a

result of magnesium dissolving as Mg2+ which inhibits re-passivation and subsequently leading

to the dissolution of aluminium. This process then leaves a porous copper sponge with a high

surface area. As shown in stage II, over time, either due to fluid convection of the solution or

mechanical disruption by corrosion products, could cause the Cu clusters to be detached from

the surface. Detached Cu clusters could undergo anodic dissolution to form Cu2+ which then

redeposits over the metal surface as shown in stage III [19].

Luo [1] has provided an alternative explanation for this type of corrosion which initiates with

the dealloying of the s-phase particles located at the alloy surface and cathodic reactions

occurring at the periphery region. This process led to the formation of an elevated alkaline

environment with a higher pH around the alloy surface which disrupts the naturally occurring

oxide layer. As the alloy attempts to repair the native oxide layer, this causes hydrogen

evolution which leads to trenching around the s-phase intermetallic particles [1].

2.3.2 AlCuFeMn(Si)

In addition to the Ɵ-phase and S-phase intermetallic particles that form, more complex

intermetallic formation based upon the presence of Al, Cu, Fe, Mn and Si are known to have

pronounced effects on the alloy properties. Intermetallic particles, AlCuFeMn(Si), based upon

alloying additions are irregular in shape and are much larger in size than other intermetallic

particles in AA2024-T3 [1, 12]. These inclusions in the aluminium matrix are classified as

being cathodic with respect to the S-phase IM particles in the matrix. In order to trigger pit

initiation, the particle size and the location is crucial [1]. As the AlCuFeMn(Si) inclusions are

less active than the S-phase particles, their cathodic nature is observed around the dealloying

S-phase particles. At high chloride concentrations and higher exposure times, these particles

have also been observed to undergo trenching. AlCuFeMn(Si) particles promote oxygen

reduction to form OH- ions which results in an increase in pH. As the electrochemical response

is dependent on the pH of the environment, Fe-rich intermetallic particles tend to show

increased cathodic efficiency with increasing pH.

Boag et al. [4] has clarified these two explanations in relation to the time exposed to the

corrosive medium. Figure 2.6 below represents a timeline of different stages of the corrosion

Figure 2-6: Hierarchy of localised corrosion attack on AA2024-T3

process of AA2024-T3 when exposed to a 0.1M NaCl solution [4].

During the first 5 minutes, the S-phase particles act as the anode and undergo corrosion at

an accelerated rate by dealloying, which subsequently leads to trenching and formation of Cu-

rich remnants. During the next 5-15 minutes, the process explained by Luo [1] takes place,

whereby the Cu-rich remnant hosts the cathodic reactions and the increased pH which results,

disrupts the native oxide layer. After 15 minutes, the activity of these sites becomes reduced

upon isolation from the matrix as an oxide is developed in the trench beneath the intermetallic

particle. After about 15 minutes of observing trenching around the S-phase particles, the

AlCuFeMn intermetallic particles undergo trenching. Towards the end of this process,

trenching was exhibited by AlCuFeMnSi inclusion which was completed after about 120 min

exposure [4]. These observations are consistent with those of Ilevabare et al. [20] and

Schneider et al. [21]

Corrosion of aluminium alloys are mainly triggered by the presence of intermetallic particles

or impurities. Therefore, many studies have been undertaken to explore ways towards

protection against corrosion.

2.4 Corrosion protection of aluminium and its alloys

In the past decade, many studies have been carried out to evaluate methods to protect

aluminium and its alloys. Currently, there are several methods to inhibit corrosion such as

anodising processes, conversion coatings, paints, organic coatings and the utilization of

corrosion inhibitors [8].

Anodising process is an electrochemical process to favour the oxide layer formation where

the aluminium metal is connected as an anode in an electrolytic cell. Increasing the thickness

of the oxide layer significantly more than the naturally-occurring oxide layer is the base

mechanism of inhibiting corrosion via this process [22-24].

Conversion coating production involves the strong bonding of low solubility phosphates and

chromium oxides to the metal surface through chemical oxidation-reduction processes [25].

Due to the negative impacts on the environment, phosphates and chromium based coatings

have been replaced with coatings containing cerium and other metallic ions or combinations

[26, 27].

Organic coatings hinder corrosion by forming a barrier between the metal surface and the

corrosive medium [15, 28, 29]. Recent studies have been conducted to expand the options

available by exploring organic-inorganic combinations [15, 30, 31]. A variety of studies have

been done to discover the efficiency of different inhibitors with concentration, pH as well as

temperature as variables [32-35]. Majority of the papers briefly note what the mechanism of

the inhibitors could be. However, there depth of knowledge provided for the proposed

mechanisms of the corrosion inhibitors used is limited.

In order to maintain a continuous inhibiting surface film, a minimum concentration of the

inhibitor should be present as corrosion inhibition is known to be reversible [15]. In addition,

adequate circulation is also required to avoid any stagnant areas to achieve good inhibition

efficiency [15].

The utilization of corrosion inhibitors is one of the most commonly used methods to prevent

corrosion. Corrosion inhibitors are chemical compounds which are essentially mixed into a

chemical stream to prevent or retard the rate of corrosion. According to ISO 8044, corrosion

inhibitors are defined as chemical substances that can retard the corrosion rate even with a

small concentration, whilst the concentration of the corroding agent remains unchanged [15].

This is one of the more cost-effective method to control corrosion, increasing the service

lifetime of cheap, less corrosion resistant alloys when exposed to corrosive environments. In

particular, the incorporation of chemical inhibitors into protective coatings for enhanced

durability is of significant commercial interest to a range of industries and markets.

Originally, the most effective corrosion inhibitors were based on chromate, phosphates and

other arsenic compounds. However, due to their toxic nature and thereby the negative impact

on the environment, this led to the development of alternative, environmentally friendly and

more cost-effective options to overtake the market. In the past, a considerable amount of

research has been done to manufacture hundreds of improved inhibitors.

Corrosion inhibitors can be categorized as either inorganic or organic compounds. Inorganic

inhibitors retard corrosion by reducing the anodic or cathodic reactions whereas organic

inhibitors could be anodic inhibitors, cathodic inhibitors or mixed-type inhibitors [1, 3, 36]. In

addition, inhibition through film formation is also associated with both types of inhibitor

compounds. These different types of inhibitors are discussed in more detail.

Anodic inhibitors

Anodic inhibitors, also known as passivating inhibitors, induce a significant anodic shift in the

corrosion potential and drive the metallic surface into the passivation range. These types of

inhibitors can be inorganic or organic in nature. Chromate-based inhibitors were and still are

in some countries the most widely used in a wide variety of applications due to the low cost

and high effectiveness. It is crucial to have a sufficient inhibitor concentration to ensure that

the entire surface is covered, otherwise this leads to accelerated localised corrosion at the

exposed areas [36].

Cathodic Inhibitors

Cathodic inhibitors, which can be inorganic or organic in nature, serve their purpose by either

reducing the rate of the cathodic reactions or by selectively precipitating over cathodic areas

which in turn increase the surface impedance and reduce the diffusion of reducible special to

these areas. In other words, these inhibitors act as cathodic poisons, cathodic precipitates and

oxygen scavengers.

Mixed-type Inhibitors

Inhibitors which display both anodic and cathodic effects are names mixed-type inhibitors.

These inhibitors, generally organic in nature, act upon the full surface of a metal and form a

hydrophobic film over the metal surface. The inhibition efficiency is achieved by either one of

the routes: (i) adsorption on anodic and cathodic sites and subsequently blocking the reactions

and/or (ii) formation of a protective film which acts as a barrier on the surface. The growing

demand for environmentally friendly chemicals has brought about a reduction in the use

chromate-based inhibitors whilst providing opportunities for developing organic inhibitors.

Compared to inorganic inhibitors, organic inhibitors have shown to be more efficient under

2.4.1 Inorganic corrosion inhibitors

certain circumstances [36].

A range of inorganic anions have been previously utilised to inhibit corrosion of aluminium

such as sulphates, nitrates, and phosphate ions. Sulphates act as corrosion inhibitors by

competitive adsorption with chloride ions to circumvent the initiation of pit formation [37]. In

the case of nitrates, these ions are incorporated into the alumina layer [38], while phosphates

block off the active sites on the oxide film or the metal surface [39]. However, another study

demonstrated that sulphate ions are capable of preventing the initiation as well as the

propagation of pits by interacting with the dissolved metal cations in the electrolyte [40]. In

addition, other inorganic inhibitors based on chromates, tungstate and molybdates have also

been utilized to mitigate corrosion of aluminium in HCl and the kinetics behind this has been

studied by Abd El Aal and co-authors [34]. They discovered that the rate of oxide film

breakdown decreases with increasing inhibitor concentration which follows a direct logarithmic

law and the first step of the process includes the adsorption of oxy-anions [32].

Nevertheless, due to the carcinogenic and toxic nature of some of the above-mentioned

inorganic inhibitors, rare earth corrosion inhibitors, particularly cerium-based inhibitors, have

been explored extensively. Here, the proposed mechanism in inhibiting corrosion on

aluminium alloy AA2024-T3 is through the formation of Ce-oxide layers over the Cu-rich

intermetallic particles, to block the cathodic sites and prevent any further activities on these

sites [26, 41, 42]. This phenomenon was shown to be independent of the copper content. It

was further suggested that the inorganic rare-earth metal based inhibitors such as CeCl3 can

only provide cathodic protection at the intermetallic sites and not the matrix, and are unable

to protect against activities that occur below the surface, as demonstrated by trails of

2.4.2 Film-forming corrosion inhibitors

corrosion products being observed [41].

Inorganic film-forming inhibitors

In recent years, researchers have explored the possibility of multi-functional inorganic

inhibitors where the inhibitors are capable of restricting both anodic and cathodic activities on

the metal surface [43]. The properties of these mixed inhibitors have been achieved by

combining two or more existing inhibitors to achieve synergistic inhibition through film

formation [44]. Garcia et al. [41] has studied an organic inhibitor, Cerium dibutylphosphate

with AA2024-T3 where the Ce ions triggered a cerium oxide formation over the intermetallic,

whilst the dibutylphosphate component formed a film over the entire metal surface. This

ensures that there is a uniform barrier over the overall surface acting as a barrier for the

chloride ions to reach the metal surface. In addition, Ho et al. [45] suggested that this inhibitor

Ce(dbp)3 acts as a more effective inhibitor at higher chloride concentrations as pit initiation to

some extent is favourable to the deposition of the inhibitor-induced film [45]. Similarly, Tianhui

et al. [26] have studied Cerium tartrate as a corrosion inhibitor for aluminium alloy, AA2024-

T3 to understand the mechanisms of a rare-earth metal based organic corrosion inhibitor.

They concluded that this environmentally friendly compound forms a protective layer over the

metal surface immediately upon immersion and both anodic and cathodic activities are

hindered. Whilst Ce(III) ions are converted to cerium oxide/hydroxide blocking the cathodic

sites, the carboxylic groups in the tartrate component, bonds with the aluminium matrix

influencing the formation of a layer over the matrix as well. Catubig et al. [46] studied the rare

earth (RE) based organic inhibitors Ce(MAcet)3 and Pr(MAcet)3 with aluminium alloy. Here, it

was established that the cathodic inhibition was led by the increased concentration and the

anodic inhibition was influenced by longer immersion periods [44]. Figure 2.7 demonstrates a

hypothesis of how the inhibitor compounds deposits over the s-phase particles within the first

30 minute of immersion, which continues to occur whilst this also triggers the passive film

Figure 2-7:Hypothesised schematic diagram of rare-earth metal-based inhibitors deposited on the S-phase intermetallic particle within 30 minutes of immersion and 24 hours of immersion.

formation over the aluminium matrix to thicken over extended periods up to 24 hours.

A study has been conducted by combining chitosan, a linear polysaccharide, due to its film-

forming properties along with a corrosion inhibitor based upon cerium (III) ions. The chitosan

pre-layer infused with cerium ions acts as a reservoir for the Ce based corrosion inhibitor and

assists in prolonged delivery due to its capability to immobilize the corrosion inhibitor [47].

Organic film-forming inhibitors

In terms of organic film-forming inhibitors, there are different families of inhibitors, such as

triazoles, thiazoles, thiols, quinoline and mercapto-based compounds [7, 9, 48]. The

derivatives of these have varying results due to their functional groups as well as their

structure. Harvey et al. [49] has investigated 28 inhibitors for AA2024 and AA7075 to

understand the effects of thiol, carboxylate, hydroxyl and amino groups; substitution of

nitrogen for carbon and sulphur for nitrogen, as well as positioning of the groups on an

aromatic ring [49]. Ward et al. [50] and White et al. [51] conducted a high-throughput testing

method to investigate the performance of over one hundred inhibitors and the author

summarized the characteristics of the stronger inhibitors as shown below [50]:

1. The presence of 2 or more N atoms connected in close proximity

2. The presence of 5-6 membered hetero-aromatic rings

3. The presence of one or more S atoms connected to the aromatic ring

In general, for higher efficiency, a strong co-ordination bond on the ligand attached to the

carbon chain or ring is necessary and therefore, inhibition is boosted in the sequence O < N

< S [52]. Many studies have mainly focused on the sulphur atom in the inhibitor molecules

due to its significant importance and this hypothesis is consistent with the work carried out by

Harvey et al. [49].

Organic inhibitors such as Benzotriazole (BT), 2-mercaptobenzothiazole (2-MBT), 2-

mercaptobenzimidazole and Na-mercaptopropionic acid (Na-MPA) are just a few of the most

widely used inhibitors which are well-known to be highly effective. According to the study done

by Harvey et al. [49], the principle mechanism of corrosion inhibition is by the formation of a

barrier film or a complex compound on the surface which would inhibit reactions. Benzotriazole

(BT), a corrosion inhibitor which has been studied in detail, is known to form a film which is

approximately about 10 – 20 molecules thick [49]. BT undergoes a reversible physical

adsorption to inhibit corrosion by possible formation of Cu/BT complexes on Cu-rich

intermetallic areas. As this mechanism is reversible, this process is time-dependent and could

initiate degradation of the film formed after a certain immersion period [53]. Due to the

formation of Cu-BT complexes, the cathodic reaction is restricted, which subsequently

prevents corrosion of the metal [53].

It has also been concluded that BT is an effective corrosion inhibitor for Cu as it forms a film

over the surface due to the affinity between the triazole ring and the copper oxides [7]. Swift

[54] has carried out an investigation with XPS and ToFSIMS analysis which displayed

evidence for the presence of C6-BTA and Cu6C-BTA molecular ions, subsequently acting as

a testimony for inhibitor-copper bonding within an intermolecular network, which led to the

formation of an inhibitor-induced film [54].

Another study carried out on imidiazoline, using EIS, investigated the formation and

destruction of the inhibitor film [55]. This investigation also varying the inhibitor concentration

to understand the film stability. The intention of this experiment was to stimulate a scenario

where the supply of the inhibitor was hindered due to a blockage. Through EIS, it was found

that the electron transfer resistance as well as the inhibitor layer resistance immediately

decreased which then recovered over the next few hours and then decreased again. The

author concluded that the inhibitor film was likely to be composed of a multi-layered structure

where the inner most layer was the metal-inhibitor structure and the other layers were inhibitor

layers with different molecular cross-linking. It was further shown that when exposed to a lower

concentration solution, the film displayed excellent self-repairing properties [55].

Whelan et al. [56] have demonstrated in their work that nitrogen-rich compounds are popular

choices as corrosion inhibitors as a result of their ability to bind with chlorides and circumvent

them reaching the metal surface [56]. In addition, similar to sulphur-containing compounds,

they have also been classified as good corrosion inhibitors due to their strong affinity towards

copper. These are generally known to work with pure copper and Cu-rich aluminium alloys

where the cathodic reactions are initiated at the Cu-rich intermetallic particles [49, 57].

Whilst the majority of the inhibitors are known to be triggered by the presence of the Cu-rich

intermetallic particles, 8-Hydroxyquinoline has chelating properties which cause it to react with

the aluminium matrix, as this is known to form an aluminium-chelate complex upon exposure

to 8-HQ [58]. Previous studies conducted on pure aluminium, revealed that the passive

alumina formed layer was strengthened in comparison to the untreated sample. The

adsorption of chloride ions on to the metal surface as well as the breakdown of the oxide layer

Figure 2-8: Schematic diagram on mechanism of corrosion inhibitor on AA2024-T3

in acidic environments were averted by the adsorption of 8-HQ [59].

Catubig et al. [44] has suggested that upon immersion of AA2024 in 0.1 mM rare-earth

mercaptoacetate inhibitor, the inhibitor undergoes preferential deposition over the S-phase

intermetallic particles which hinders the de-alloying process. However, previous studies have

shown that thiol-containing systems require some initiation of corrosion prior to the inhibitor

forming complexes with the intermetallic. Figure 2.8 shows a schematic that represents the

suggested mechanism [46].

The information presented above has provided information in general on film forming inhibitors

and associated mechanisms. While a detailed review of the mechanism for all film forming

inhibitors is beyond the scope of this thesis, more detailed information on two specific film

forming inhibitors, namely 2-mercaptobenzothiazole (2-MBT) and 3-mercaptopropionic acid

(3-MPA), is presented below, which form the basis for this research program

2-mercaptobenzothiazole (2-MBT)

Balaskas et al. [7] demonstrated that both cathodic and anodic current densities were

diminished by the presence of 2-mercaptobenzothiazole. Whilst the presence of a film over

cathodically active IM particles was thought to be responsible for the reduction in cathodic

current, the reduction of the anodic current density was associated with film formation over

the aluminium matrix. In addition, due to the presence of polar functional groups, inhibitor 2-

mercaptobenzothiazole has the ability to react with copper present in the intermetallic particles

to form Cu-MBT complexes which can result in film build-up over the metal surface, restricting

any further cathodic activities and thereby hindering corrosion [7, 9, 53, 60, 61]. The same

mechanism was suggested by Zheludkevich et al. [9], whereby 2-MBT forms a film

immediately after immersion. However, Zheludkevich, et al. [9] reported further information

regarding the mechanisms of 2-MBT induced film formation on AA2024-T3 . EIS results have

shown that the inhibitors influenced the formation of a thin film over the aluminium oxide layer

which caused a reduction in corrosion activity. The results of this study have also

demonstrated that 2-MBT created precipitates over Cu-rich intermetallic particles, possibly

due to the Cu-S covalent bond. Eventually, the inhibitor film was shown to retard the rate of

both the anodic and cathodic reactions to prevent further corrosion of the alloy.

Studies conducted by Ryan et al. [48] have shown that whilst 2-MBT forms a transparent film,

Na-MBT forms a thin brown film over the aluminium alloy surface and when the film was

removed, only a few corroded areas were observed which were shallow pits associated with

Cu-rich intermetallic particles. The author also suggested that one reason for excellent

inhibition could be due to the possibility of sulphur atoms being geometrically located with

sufficient separation distance between sulphur atoms to create stable bonds with either

aluminium or oxygen atoms on the substrate [48].

3-Mercaptopropionic acid (3-MPA)

The compound 3-mercaptopropionic acid is another inhibitor containing a thiol group and a

carboxylic group as its main functional groups.

According to a study done by Harvey et al. mercaptopropionic acid reported an inhibition

efficiency of approaching 100% with AA2024-T3 [49]. Similar to other organic inhibitors

containing a thiol group, Harvey suggested a mechanism, which is also applicable to the

sodium salt of 3-mercaptopropionic acid, whereby the sulphur binds with the Cu-rich

intermetallic particles. A study has been conducted by Sunarya [62] to evaluate the

performance of 3-mercaptopropionic acid on carbon steel. The author has discovered that 3-

MPA behaves as a cathodic-type inhibitor with about inhibition efficiency percentage between

85% and 90% [62]. However, no significant studies have been done on this inhibitor with

aluminium alloy, AA2024-T3.

In addition, 3-mercaptopropionic acid is a compound subject to possible dimerization. It has

been determined that the oxidation of 3-MPA occurred leading to formation of the

corresponding dithiol dimer, dithiodipropionic acid (dTdPA) [63]. A study by Ihs and Liedberg

[64] conducted with copper and 3-MPA has suggested that a solution of 3-MPA at a pH of 3.5

forms a multilayered structure over the copper surface and the first layer the thiol groups are

linked to the metal surface whereas the subsequent layers are consistent of dimers of 3-MPA

[64].

All corrosion-based studies in the literature have employed many electrochemical and surface

analytical techniques to evaluate the corrosion resistivity as well as the inhibitor performance.

Many techniques have been explored and constantly developed to determine many aspects

associated with corrosion and inhibitor film formation. Some of the more important techniques

are described in more detail in the following section

2.5 Electrochemical and surface analytical techniques A range of electrochemical and surface analytical techniques have been adopted as standard

practice for assessing corrosion behaviour and in particular for detailed studies on the

mechanisms associated with the use of corrosion inhibitors. The applications of these

techniques to studying inhibitor systems, the information they reveal, and their advantages

and limitations are discussed in this section. Researchers have investigated the use of a range

of techniques on its own as well as in combination with another technique to cover multiple

aspects and data.

One of the most basic but informative techniques from electrochemical corrosion studies is

open circuit potential. This provides information on the variation of potential over time which

can be applied to understand any cathodic or anodic shifts in the potential as well as the

stability of the system. In addition, with respective to inhibitor systems, the variation of the

working electrode potential recorded could be used to interpret the effect of inhibitor and its

stability.

Another more commonly used technique to evaluate the performance of a corrosion inhibitor

is potentiodynamic scanning, which generate polarization curves or Tafel plots. Previous

studies have conducted full scans with respective to the open circuit potential as well as

cathodic and anodic scans separately to understand the behaviour of the inhibitor in addition

to the corrosion rate [9, 10, 19, 26, 32-34, 53, 58, 60, 61, 65-68]. As this technique can

accelerate corrosion, this allows researchers to consider this as tool for predicting, to some

degree, rates of corrosion over extended periods of time. The data collected via

potentiodynamic scanning could provide information on kinetics of electron-transfer reactions,

corrosion mechanism as well as an indication of the corrosion rate.

Electrochemical impedance spectroscopy is also a widely used technique to determine

inhibition mechanisms, inhibitor performance as well as film-forming behaviour [7, 9, 69-71].

In regard to the corrosion characteristics of inhibitor-induced film formation, Tan et al. [55] has

demonstrated the use of EIS to understand the formation and destruction of inhibitor layers by

analysing the electrode surface electrochemical kinetics parameters. This allows the reader

to conceptualise the potential equivalent circuit which could closely represent the inhibition

system [55]. In general, many studies have been done to understand the different stages of

the corrosion inhibition process by conducting EIS tests at different time intervals of exposure

to the inhibitor solution [41, 72-74] as well as varying inhibitor concentration [66, 75].

Another electrochemical technique used in corrosion studies is linear polarization resistance.

This non-destructive technique provides the ability to monitor the polarization resistance over

a given period of time. Polarization resistance is a parameter that can be used to indicate the

resistivity against corrosion and is important in monitoring film forming behaviour of inhibitor

film (and oxide layer) formation which could then be analysed further to calculate the corrosion

rate of a system. Previous studies have been conducted with inhibitor systems to determine

the resistance over extended time periods [9, 76].

Other techniques utilized by researchers include electrochemical noise [7] and cyclic

voltammetry [74, 77]. Electrochemical noise (ECN) is a technique which monitors the

fluctuations of current and potential as a result of events such as film breakdown, pit

propagation and other reactions corresponding to metal dissolution or hydrogen discharge

[78].

Cyclic voltammetry measures the current response of a cyclic potential sweep carried out

between two or more fixed values. This provides information on the thermodynamics of redox

processes as well as the kinetics of electron transfer processes [79].

Electrochemical techniques provide important quantitative data, in terms of the

electrochemistry and electrochemical effects associated with a corrosion system and

particularly an inhibited corrosion system. However, they are limited in terms of providing

information on the structural / morphological aspects associated with corrosion and inhibition

processes and any elemental and compositional changes incurred, particularly at the system

interfaces. Here, a number of surface analytical techniques are utilised to compliment the

information provided by the electrochemical analysis.

Optical and scanning electron microscopes are two widely used instruments to observe the

morphological effects of corrosion activity, corrosion processes and effect of inhibitors.

Scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy

(EDS) can gather information on the composition of the surface of the samples, whether it be

from single point analysis, line scans or surface area maps. In addition to morphological

aspects of corrosion and associated scale and / or inhibitor formation being observed, the

presence of intermetallic particles and their association with corrosion, scale formation and

inhibitor film formation can be monitored. Scanning electron microscopy has been mostly used

to visualise the corrosion activity around the intermetallic particles with and without inhibitor

exposure. [34, 58, 69, 80, 81] [75]. In addition to the standard images obtained via secondary

electrons, SEM also allows for capture of backscattered images which provide data on

different chemical compositions due to varying atomic density.

Another surface analytical technique which has been widely used is x-ray photoelectron

spectroscopy (XPS). Many authors have utilised this to determine the valence states of

elements within the substrate itself, inhibitor layers formed over the metals surface as well as

the corrosion products formed [82] [54] [83] [26] [84] [85] [86] [75]. Consequently, this technique

provides a certain level of information on the molecular bonding associated with scale and /

or inhibitor film formation, that has taken place at the metal surface. Specific to organic inhibitor

studies, Swift has conducted a research by using XPS and time of flight secondary ion mass

spectrometry (ToFSIMS) to characterize thin films formed by corrosion inhibitors on different

metal surfaces. This study provided evidence for the presence of C6-BTA and Cu6C-BTA

molecular ions and subsequently acting as a testimony for the inhibitor-copper bonding with

intermolecular network, which leads to the formation of an inhibitor-induced film [54]. Recloux

et al. [53] has performed ToFSIMS to map out the surface composition to determine if the

inhibitor, benzotriazole is absorbed on to specific site such as the Cu-rich intermetallic particles

or if it is evenly distributed. This is one of the methods used to determine if the film formation

is uniform over the metal surface or localised around the intermetallic particles.

Atomic force microscopy (AFM) is a techniques that has been used to determine the thickness

of an inhibitor layer over the metal surface via height maps [26] [86] [9] [75]. One reason for

utilizing this technique is due to its ability to achieve non-destructive imaging at high resolution.

In addition, AFM can also be used to capture high resolution 3D-imaging of the topography of

a sample surface [87]. This can provide information on any film or layer over the metal surface

as well as any pits formed.

Fourier transform infra-red spectroscopy (FTIR) is another technique which has been used in

previous studies to determine the chemical composition as well as the type of bonding

associated with inhibitor systems. FTIR spectroscopy results could determine if an inhibitor

film is adsorbed on the aluminium alloy surface in terms of bonding characteristics identified

from the FTIR spectra [84] [47] [26] [72] [75]. This can be utilized to recognize the functional

groups involved in bonding and any linking between the metal surface as well as the inhibitor

layer. The interaction between the oxide layer and the inhibitor layer could also be evaluated

by identifying any complexes formed during the film formation.

2.6 Justification for the study

The compounds 2-mercaptobenzothiazole and 3-mercaptopropionic acid have been chosen

for this study based upon the previous study published by Harvey et al. [49], as these inhibitors

have shown promising corrosion efficiencies and distinctive inhibiting mechanisms due to their

structures. The literature has shown that 2-mercaptobenzothiazole has been widely studied

using a variety of techniques. The studies have suggested different possible mechanisms for

corrosion inhibition and also its performance [7, 9, 49, 73]. Although it is known to form a

transparent film over the metal surface, there have been limited studies focusing on the film-

formation process and its stability. Similarly, limited studies have been conducted on 3-MPA

as a corrosion inhibitor for AA2024-T3, particularly on understanding the film-forming process

and its long-term tenacity.

Many studies have been published in regard to the potential mechanism around the affinity of

copper towards sulphur and the Cu-inhibitor complex formation as discussed previously, but

limited work has been done in understanding the mechanisms of film formation over extended

time periods. Aluminium alloy, AA2024-T3 has a naturally occurring oxide layer over the metal

surface and the majority of the studies have not considered this aspect when discussing the

formation of an inhibitor layer over the metal surface. The interaction between the passive

oxide layer and the inhibitor layer could be a key aspect n understanding the mechanism of

film formation over the aluminium alloy, AA2024-T3. There is an abundance of information

available from the literature which investigate a wide and diverse range of different type of

inhibitors with different metal substrates, in terms of analysing the performance, inhibition

efficiency and possible mechanisms. However, there is a lack of information in regard to

investigating the long-term inhibitor-induced film stability. As it is crucial to ensure that the

inhibitor film continues to maintain the film stability to ensure long lasting protection of the

metal, long-term monitoring of the film growth and the persistency should be investigated in

detail. Furthermore, very few studies have been done on evaluating the persistency of the

inhibitor-induced film once it has been developed. Although sufficient work has been

conducted with a continuous presence of inhibitor, there is a lack of studies concerning the

response of the inhibitor when subjected to a discontinued supply of inhibitor. Investigating

the film-breakdown process over time could provide useful information to understand the

boundaries of film stability.

3. CHAPTER 3: EXPERIMENTAL PROCEDURE

The literature review demonstrated a range of electrochemical techniques and surface

analytical techniques which are being commonly used in corrosion studies. It is important to

tailor these techniques to suit the study being undertaken and the objectives that needs to be

accomplished. This chapter outlines the experimental methodology utilized in this study

including the specifications of the materials used as well as the techniques. An introductory

section for each method utilized is included to emphasize on the purpose of each technique

while the following sections discusses in detail the specific parameters and steps taken.

3.1 Specimen preparation AA2024-T3 aluminium alloy extruded rods with a diameter of 25 mm from Calm Aluminium

Pty Ltd were utilized in this study. Composition of the aluminium alloy as provided by Calm

Aluminium P/L is shown below.

Table 3.1: Composition of the AA2024-T3 rods

Al

Cu

Mg

Fe

Mn

Si

Zn

Cr

Ti

Unspecified

Element

Percentage

3.80-

1.20-

0.30-

Remainder

0.50

0.25

0.10

0.15

4.90

1.80

0.90

(% weight)

3.1.1 Surface preparation

Prior to conducting all the experiments, the samples were cut into 5-10 mm thick discs. The

samples were then ground using 240, 400, 800 and 1200 grit silicon carbide paper using an

automated grinder for consistency. The samples were then diamond polished with 9 µm, 3 µm

and 1 µm diamond paste respectively to achieve the appropriate surface finish for the

electrochemical studies and surface analysis. The samples were cleaned with ethanol in an

ultrasonic bath between each diamond polish for 90 seconds. After the final round, the

samples were cleaned in the ultrasonic bath for 5 minutes to ensure that any particles from

the grinding or the polishing process weren’t left behind on the surface. The samples were

then dried with N2 gas and stored in a desiccator until further use. To ensure consistency

between results from the experimental techniques and the surface analytical techniques, all

3.1.2 Solution preparation

the samples were prepared as mentioned above.

Two inhibitors were selected for this study, namely 2-mercaptobenzothiazole (2-MBT) and Na-

mercaptopropionate (Na-MPA). In this study, the concentrations of both the base corrosion

medium (NaCl solution) and selected inhibitors were kept constant at 0.1 M and 10-3 M

respectively. 2-mercaptobenzothiazole (98%) was obtained from Sigma Aldrich. The inhibitor

solutions were made to the standard (10-3 M) concentration in the chloride base using de-

ionized water. Due to the insoluble nature of 2-mercaptobenzothiazole, it was first dissolved

in 10 ml of ethanol which was then mixed in to 0.1 M NaCl solution. The solutions were stirred

for 1 hour using a magnetic stirrer at room temperature to ensure complete mixing.

Similarly, the inhibitor, Na-mercaptopropionate was synthesized using 3-Mercaptopropionic

acid, obtained from Sigma Aldrich, and 1 M NaOH from. A solution of 10-3 M concentration in

0.1 M chloride base was made using de-ionized water. As 3-mercaptopropionic acid has a

pungent odor, this inhibitor solution was diluted under a fume hood and was allowed to stir for

1 hour using a magnetic stirrer at room temperature.

The inhibitor solutions were utilized within a maximum of 7 days to ensure that the optimum

effectiveness was achieved.

3.2 Introduction to the experimental techniques 3.2.1 Electrochemical techniques

Electrochemistry is a very important tool in studying corrosion. This is one of the most widely

used methods in quantifying corrosion behavior. Here, a range of electrochemical techniques

were adopted to quantify the corrosion behavior. All corrosion monitoring electrochemical

techniques were performed using a Biologic VMP-300 Potentiostat with a conventional three-

electrode electrochemical cell, namely the Al alloy samples as the working electrode (WE),

AgCl/KCl electrode as the reference electrode (RE) and platinum mesh as the secondary

electrode (SE). All electrochemical tests and analysis of the data was subsequently carried

out using EC-Lab software.

Open circuit potential

Open circuit potential (OCP) is defined as the free corroding potential or the different in

potential between the working electrode and the reference electrode when no external force

or current is applied. This technique was conducted as the first step of each experiment to

ensure that the cell system had stabilized before the other techniques were carried out. A

stable system is crucial to obtaining accurate data, given that even small fluctuations could

affect electrochemical data significantly causing incorrect conclusions and implications to be

drawn.

However, in this study, this basic technique has also been used to monitor any cathodic and

anodic shifts that have taken place when treated with an inhibitor in comparison to an

untreated sample being exposed to a sodium chloride solution. OCP was also used to monitor

cathodic shifts occurring during potentiodynamic scanning.

Potentiodynamic scans

Potentiodynamic polarization scan provide significant insights into the corrosion rate, pitting

susceptibility, passivity, cathodic and anodic behavior of the electrochemical system.

Potentiodynamic scans are considered to be a destructive technique as they accelerate the

corrosion process when anodically polarized and thus induce irreversible processes. As

conventional corrosion studies could take up to several days or weeks to determine the

corrosion activity of a sample, this technique could be used to collect and analyze data within

a few hours. Anodic and cathodic scans can be conducted independently of each other to

determine the corrosion behavior of each region separately or as a combined scan (cathodic

to anodic) to determine the influence of the cathodic scan on the anodic polarization.

Linear polarisation resistance

This a non-destructive electrochemical method to measure the corrosion rate. This technique

can run for an extended time period without the scan itself disturbing the corrosion process in

any significant capacity. The system’s polarization resistance Rp is measured in Ohms using

the potential (E) and current density (i) data generated over a small voltage range either side

of the OCP. This method can also be used as a means to represent and quantify the corrosion

rate of the system. As monitoring of LPR is a rapid method, the Rp value and hence the rate

of corrosion can be calculated within minutes which allow for instant feedback.

Equation can be derived from Ohm’s law [88].

𝑉 𝐼

𝑉 = 𝐼𝑅 → 𝑅 = ------ Eq. (3.1)

Therefore, the slope of an E vs i graph could be used to quantify Rp by calculating the gradient

of the linear region around the OCP. A linear polarization scan is carried out over a potential

range of 30 mV across the OCP and the gradient is calculated over a smaller potential window

which is typically less than 10 mV. The polarization resistance, Rp is inversely proportional to

the corrosion current density which could subsequently be used to calculate the corrosion rate

[89]. As the corrosion current density is directly proportional to the corrosion rate, an increased

polarization resistance is an indication of a reduced corrosion rate. Thereby, monitoring the

Rp over extended time periods allows to monitor the change in the rate of corrosion over a

given period without destructing the sample.

This electrochemical method can provide powerful information on the long-term effects of an

inhibitor. In regards to film formation specifically, Rp data can be directly associated with the

film thickness. It can be assumed that as the film builds up, inhibitor performance is improved.

Therefore, increasing polarization resistance over time could imply that the inhibitor film is

building up over the surface. The technique, corrosimetry on EC-Lab V11-31 allows the Rp

data to be recorded at specified time intervals over extended time periods. The data is usually

plotted in a single graph presenting observed trends in changing polarization resistance with

time.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a widely used technique in

electrochemistry. This technique uses an AC current with a small amplitude to monitor the

impedance characteristics of an electrochemical cell. An impedance spectrum is generated

by scanning the AC signal over a wide range of frequencies. EIS is classified as a non-

destructive technique as it only uses a small excitation signal to measure impedance, Z. This

technique can be utilized to obtain parameters such as the existence of surface films,

interfacial corrosion and mass transfer. However, EIS is considered to be a complex technique

due to the difficulty associated with data interpretation and equivalent circuit fitting.

The resulting EIS data in this study is presented as standard Nyquist and Bode plots. These

plots are built based on the responses over a given range of frequency. Nyquist plot contains

two components, real and imaginary components which represents impedance as a vector of

magnitude |Z| at an angle of f (arg Z) commonly known as the phase angle. Each point on the

Nyquist plot provides the impedance at a particular frequency. Bode plots include the

impedance |Z| corresponding to the logarithmic frequency, log (freq) and phase-shift. Bode

plots can be utilized to gather information about the system behavior in regard to the

equivalent circuit. The time constants can be identified via the Bode plots which indicate

dielectric features within the inhibitor or oxide layers. In addition, this technique has also been

used to determine the polarization resistance which can then be compared with linear

polarization resistance results for consistency,

The first step taken to analyzing EIS data is to determine the suitable equivalent circuit model.

An equivalent circuit model is a representation of the system on the basis of its

electrochemistry. In order to undertake the most effective approach to fitting the correct model,

an understanding of the system in terms of the physical electrochemistry is first developed.

The next step is to ensure that the model that describes the system the best is consistent with

the model that fits to the data obtained via EIS [90].

Potentio electrochemical impedance spectroscopy (PEIS) technique performs impedance

measurements in potentiostatic mode by applying a sinusoidal wave around a potential that

3.2.2 Surface analytical techniques

can be set to a fixed value or relative to the cell equilibrium potential.

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is a widely used surface analytical technique for

gathering information on the surface topography and composition of a given sample by

providing magnified images. The use of a focused beam of electrons allows the SEM to

capture magnified images from 10 times up to 300,000 times. This instrument is preferred over

optical microscopy due to its ability to deliver high resolution images at high magnification,

typically with a resolution of 5 nm, and enhanced depth of field [91]. An electron gun produces

a beam of high energy electrons which then is focused on the surface of the specimen directed

by the assistance of magnetic lenses. The SEM has two detectors, namely secondary

electrons (SE) and backscatter electrons (BSE). Secondary electrons are used to capture the

surface of the specimen and are low energy electrons with energy less than 50 eV and these

are easily collected by the detector. On the other hand, backscattered electrons are high

energy electrons with energy greater than 50 eV and the BSE detector collects the electrons

which are scattered back from the sample. BSE imaging is completely dependent on the

atomic number of the sample [92]. During this study a JEOL JSM7200F High resolution

scanning Electron microscopy was utilized.

Scanning electron microscopy was used as the primary technique to observe the extent of

corrosion taken place with and without the presence of each inhibitor used in this study. As

the SEM offers imaging of a sample with high resolution at significantly higher magnitudes,

this allows to closely observe if any, corrosion around specific intermetallic particles as well

as any small pits formed which cannot be observed via an optical microscope or the naked

eye. Therefore, this technique was widely used to examine the topography of the treated and

untreated sample upon exposure to sodium chloride. This was particularly important for the

studies involving long-term exposure to sodium chloride in the presence of each inhibitor and

the film breakdown studies, where images could confirm the various stages of film formation

and film breakdown as indicated by the results from the electrochemical techniques at different

time intervals.

Energy dispersive x-ray spectroscopy (EDS)

Energy dispersive x-ray spectroscopy (EDS) which is also known as EDX is a very user

friendly micro-analytical technique to determine the chemical composition of the surface of the

specimen in a quantitative and qualitative manner. The energy of the x-rays emitted off of the

sample are used as a representation of the elements present on the surface of the sample.

EDS technique includes a wide range of means of output. The results can be obtained as a

spectrum, a map or even quantitative analysis with concentrations.

In this study, as it is important to determine the presence of sulphur containing inhibitor over

the metal surface, this technique could provide confirmation on the presence of inhibitor by

obtaining a compositional map of the surface of a treated metal surface. In addition, this could

also be utilized to gather information on the effect of the presence of Cu-rich intermetallic

particles on the inhibitor film formation.

Focused ion beam (FIB) / transmission electron microscopy (TEM)

Focused ion beam (FIB) is technique that involves the use of an ion beam with an extremely

fine probe size usually smaller than 10 nm. The advantage of the FIB technique is that this ion

beam can be utilized to undertake milling of the sample for further surface analysis or even

shave off the top layer to analyse a cross sectional area of the specimen at depth typically 10

nm or more below the surface. In order to prepare the samples, the samples should be sputter

coated with Iridium initially, to ensure that the inhibitor layers or the oxide layers remain intact

and unharmed from the milling process. A layer of platinum can also be deposited over the

surface to protect the surface of the sample when observing the cross-sectional areas. FIB is

a very useful tool when combined with a SEM as the electron and ion beams are made to

intersect at an angle of 52o at a point close to the surface and this allows for achieve

immediate, high resolution SEM images of the cross-sectional interfaces FIB-milled surface.

In addition to utilizing the FEI Scios FIB SEM for imaging, the ion beam is also used for

preparing transmission electron microscopy (TEM) lamella specimens. TEM requires very thin

samples for analysis. FIB is one method for preparing nano-scale thin TEM lamella samples.

The high-resolution imaging can assist with identification and accurate milling of the area that

needs to be extracted. Here, a JEOL 2100F transmission electron microscope was utilized.

3.3 Experimental methods 3.3.1 Electrochemical techniques

A Biologic VMP-300 Potentiostat in conjunction with EC-Lab software was used for

undertaking all electrochemical tests and electrochemical analysis. The cell setup used for

this study incorporated a standard 3-electrode system. A saturated AgCl/KCl electrode was

used as the reference electrode, platinum electrode as the counter electrode and the AA2024-

T3 sample as the working electrode with an exposed working area of 1 cm2. The cell

apparatus is shown in figure 3.1.

Reference electrode

Counter electrode

Figure 3-1: Electrochemical Setup

Working electrode

In order to minimize effects due to crevice corrosion associated with stagnant electrolyte

between the o-ring of the cell and the metal sample, the samples were sufficiently tightened.

Subsequently, after each experiment, the sample surface was observed to ensure

consistency, in regard to the absence of uniform-ring induced crevice corrosion. In addition,

the cell was carefully monitored to ensure the absence of any bubbles over the surface of the

sample, inside the capillary tube and at the reference electrode.

All electrochemical techniques have been replicated three times and the standard deviations

were calculated in order to present the uncertainties of the data. Based on the measurements

system analysis, these methodologies have been evaluated to results in a relative error of

10% or better and therefore, the experiments were only replicated three times.

Open circuit potential

The time period for monitoring the OCP was adjusted according to the technique being used.

This was based upon a series of trial and error tests to determine how long it would take for

the system to stabilize. The corresponding time periods were chosen to be 30 minutes when

determining the short-term performance via potentiodynamic scans and electrochemical

impedance spectroscopy. For long term exposure tests such as linear polarization resistance

experiments, the OCP was conducted for 2 hours to allow sufficient stabilization between

readings. For linear polarization, as the scanning range is only about 30 mV, any minor

fluctuations could affect the readings significantly whereas for potentiodynamic scan and

electrochemical impedance spectroscopy, minor fluctuations will not be reflected in the results.

However, for pure aluminium and copper, the OCP duration was kept constant at 15 min as

the system becomes stable over a relatively short period due to their homogeneous nature.

Potentiodynamic scanning

To determine the influence of the inhibitors on the cathodic and anodic reactions,

potentiodynamic scans were carried out at a scan rate of 1 mV/s over the range -600 mV vs

OCP to 600 mV vs OCP in the direction of cathodic to anodic. Prior to each full cathodic-

anodic scan, open circuit potential measurements were conducted for 30 minutes to ensure

that the cell system achieved steady state conditions. All data was plotted in the conventional

E vs log i format followed by a Tafel fitting to the polarization curves to calculate the Ecorr and

icorr values. The icorr values calculated provide an indication of the corrosion rate of the system.

All Tafel fittings were conducted through the “Tafel fit” feature in EC-Lab software. The Tafel

equation was fitted across +/- 50 mV from the point of OCP for each potentiodynamic scan.

Conducting a cathodic and anodic sweep in a single potentiodynamic scan could potentially

affect the results of the anodic scan as the sample has already being polarized prior to

undergoing anodic polarization and this has been taken into consideration when evaluating

the data.

Linear polarisation resistance

LPR scans were conducted using the corrosimetry (CM) technique in EC Lab. Initially a range

of ± 15 mV vs OCP was chosen for the LPR experiments in order to calculate Rp. However,

due to the heterogeneous nature of AA2024-T3, the sample tended to be significantly unstable

over this range. Subsequently, the potential range was changed to -10 mV to 20 mV vs OCP

for the LPR scan with a range of ± 2.5 mV for calculating the gradient. Prior to conducting the

scan, the sample was allowed to stabilize for 2 hours. This minimized the fluctuations and

noise in scans as the OCP was more constant. All tests were conducted with using a scan

rate of 1 mV/s.

Electrochemical impedance spectroscopy

The Potentio-electrochemical impedance spectroscopy (PEIS) technique in EC Lab was

employed to obtain the impedance results. Initially, the system was allowed to stabilize for 2

hours at the free corroding potential using the OCP technique. The measurements were

collected over a frequency range of 100 kHz to 10 mHz with Na = 6 points per decade and a

signal amplitude, Nd of 15 mV. EIS processing and fitting the model was conducted using the

EC-Lab V 11.31 software. For long term monitoring, a loop was created to run an EIS scan

after different time intervals as per the requirement via technique builder option in EC-Lab

V11.31. The cell setup was grounded using a faraday cage.

The Z fit, which is the software impedance fitting tool was used to fit an electrical circuit for a

selected system. The initial parameters were entered into the software as a starting point

based on the Nyquist plot and the minimization tool, Randomize + Simplex was used as the

3.3.2 Surface analytical techniques

iteration method to yield the closest parameters to the experimental data.

For post-analysis of samples subjected to inhibitor exposure and / or electrochemical testing,

two methods of sample preparation were employed. For samples used to identify the extent

of corrosion taken place, the samples were rinsed with de-ionized water followed by cleaning

in ethanol in an ultrasonic bath. However, for samples where the focus of the post-analysis

was on the film-forming behavior, the samples were rinsed only with de-ionized water. The

step associated with the used of ethanol in an ultrasonic bath was omitted to ensure that the

film remained intact. All samples were dried with a N2 gas gun at a low pressure and stored in

a desiccator until further use.

Optical microscopy

An upright materials microscope, Leica DM2700 M was used with the Leica Application Suite

X (LAS X) as the software platform for basic image analysis at magnifications ranging from 5x

to 100x.

Scanning electron microscopy (SEM)

For scanning electron microscopy (SEM) analysis, JEOL JSM7200F high resolution scanning

electron microscope was used. The samples were cleaned with compressed air to ensure that

the sample does not contain any loose particles on the surface, which could contaminate the

SEM chamber. The sample was fitted on to a suitable holder prior to placing it inside the

microscope. The sample was analysed at an aperture size scale of 3 which is equivalent to

an aperture diameter of 50 microns respectively. The larger the aperture size, the clearer the

signal, whereas the smaller the aperture size, the better the resolution. Therefore, the aperture

size was adapted accordingly. A working distance of 10 mm, an accelerating voltage of 10 kV

or 20 kV and a beam current of 1-2 nA were used as the parameters. At a higher beam current,

the film-formed over the metal surface could be damaged and therefore to avoid this, the beam

current this was maintained at a low level. As part of the set-up procedure, the sample was

optimised via objective lens alignment for beam focus and astigmatism.

Energy dispersive X-ray spectroscopy (EDS)

Here, the JEOL JSM7200F high resolution scanning electron microscope was used. This

equipment has the built-in option for EDS, complete with AztecEnergy software from Oxford

instruments for acquisition and analysis. For obtaining images, the accelerating voltage of the

SEM was maintained at 20 kV and it was reduced to 10 kV for EDS analysis as lower

accelerating voltage ensures more accurate detection of a wide range of elements. It should

also be noted that non-appearance of any elements in the compositional maps is only an

indication of presence of lower levels relative to the other more prominent elements. As

required, a map, line or a point scan was obtained to determine the composition of the metal

surface. This technique was utilized instead of other elemental analysis instruments such as

XPS, as this provides the results with a visual aid which would assist in understanding the

difference in process occurring over the aluminium matrix and the intermetallic particles.

However, it is appreciated that in terms of interaction volume, whilst XPS provides insights to

the near surface region chemical composition within a few nanometres in depth, EDS is able

to generate compositional maps by analysing a few micrometres in depth. The elements of

interest such as sulphur was manually added to the list of elements to monitor its presence.

However, under vacuum, sulphur tends to decompose, making it a difficult task to analyse the

presence of sulphur over the metal surface, thus limiting the presence / absence as a means

for direct interpretation for the existence of an inhibitor layer. The data was analysed via

mapping to obtain a visual representation and as qualitative data and also element peak

profiles were collected to identify the percentage of each element present as quantitative data.

Focused ion beam (FIB) / transmission electron microscopy (TEM)

Focused ion beam (FIB) technique is utilized in the preparation of a TEM lamella as the sample

is required to have approximately a thickness of about 100 nm for it to be considered as am

electron-transparent sample. As the milling process require the use of high beam currents,

this could also damage and mill away the metal surface including the film formed. Therefore,

prior to placing the samples under the microscope, the samples were sputter coated with

iridium up to 10 nm thickness. For the purpose of milling out a TEM sample, the FEI FIB-SEM

dual beam system was utilized. Upon insertion of the sample into the microscope, a 50 nm

thick platinum coating was deposited, followed by another thicker platinum coating of 100 nm

thickness. To ensure a smooth and defect free FIB sample, the ion beam voltage was

progressively reduced on each cut. The process of milling a sample was carried out with the

assistance of 12x5 “Ex-situ” lift-out method under AutoTEM feature. The sample was then

extracted using a micromanipulator and mounted it on to the lamella grid. The thinning process

was then conducted by interchanging the tilt angle and milling it at an ion beam current of 48

3.3.3 Film Stability Study

pA. The sample was then stored under vacuum until further analysis in the TEM.

The purpose of this study was to understand the stability of a film once it had developed. The

process involved conducting constant immersion (free corroding) studies, by immersing the

AA2024-T3 samples in an inhibited environment for pre-determined periods of time, follow into

on the degradation process of the film formed.

The samples were prepared as described in section 1.1, then subsequently fully immersed in

an inhibitor solution ensuring that the sample is placed upright or perpendicular to the base of

the beaker, i.e. with the polished surface in the vertical plane. This avoids any corrosion

products directly being deposited over the metal surface, as the presence of these corrosion

products over the surface could accelerate certain forms of localised corrosion and this will

not be considered to be free corrosion. The beakers were loosely covered with a lid and the

solution was re-filled with de-ionised water as the level of the solution reduced over time. The

samples were removed from the solution after 7 days, 30 days and 60 days. Different time

periods were adopted in order to gain a deeper understanding on the extent of film formation

and stability as a function of exposure time.

Once the samples were removed from the solution, they were rinsed with distilled water

followed by lightly drying with N2 gas. Samples were then subject to LPR measurements (as

described previously) in uninhibited 0.1 M NaCl. Readings were taken at 2-hour intervals the

exception being the first reading taken after 30 minutes, for immersion periods of 7-14 days,

depending on the stability of the film, or until it showed signs of degradation.

Surface morphology studies were carried out using optical microscopy to compliment the LPR

data. For morphology studies, samples were subject to inhibited conditions for up to 60 days

(as described above). Subsequently, samples were subject to the degradation process

described above, i.e. uninhibited conditions, however the samples were removed from the

electrochemical cell after 1 day, 3 days, 7 days and 14 days (if applicable) and then examined

under the optical microscope. Micrographs taken from samples exposed to the same

conditions, but initial immersion in the uninhibited solution served as controls. This allowed for

visual comparison of samples exposed to an inhibited environment initially compared with the

uninhibited exposure conditions.

4. CHAPTER 4: RESULTS: INHIBITOR-INDUCED FILM FORMATION

The literature has shown that while performance of conventional organic (and inorganic)

inhibitors for AA2024 series has been studied extensively and optimised with respect to many

conditions such as inhibitor and environment concentration, solution pH, temperature, there is

still a lack of knowledge surrounding film forming inhibitor durability and long term

performance. The main focus of this research study was to develop a deeper understanding

of the film-forming behaviour of established inhibitors over time, rather than optimising inhibitor

performance. Specifically, the focus was on mechanisms associated with film formation, film

durability and film breakdown.

Prior to undertaking longer term film durability studies of the selected inhibitor system, the

conditions for studying film forming behaviour need to be optimised as well as the type of films

formed. This chapter is divided into three sections. In the first section, the effects of inhibitor

concentration and solution pH on film forming behaviour of the aluminium alloy, at constant

(room) temperature and NaCl concentration, were studied using potentiodynamic scanning

(PDS) in order to optimise the conditions for the film durability and stability studies (Chapter

5). The second section is focussed on electrochemical impedance spectroscopy (EIS),

polarisation resistance (Rp) and energy dispersive spectroscopy (EDS) studies of the surface

exposed to the inhibitors to identify the type and nature of the films formed. The third section

is focussed on electrochemical studies on pure aluminium and copper to serve as a base

comparison and establish the role that intermetallic particles may play in film formation and

overall effect on the longer-term stability.

4.1 Evaluation of inhibitor performance 4.1.1 Free Corroding Potential Figure 4.1 represents the free corroding potential over a period of 1 hour for both inhibited and

Time /hr

-0.4

0.2

0.4

0.6

0.8

-0.5

-0.6

V / e w E

-0.7

-0.8

Uninhibited

2-MBT

Na-MPA

Figure 4-1: Comparison of OCP of inhibited and uninhibited systems over 1 hour

uninhibited systems.

Generally, a shift to more positive potentials was observed initially for all three systems, before

levelling out, indicating a “settling in” period during the first 15 minutes or so of exposure. This

was more pronounced for the 2-MBT system compared with the Na-MPA and more-so the

uninhibited system. Further, the 2-MBT inhibitor system displayed a significant cathodic shift

for the full duration, when compared with the Na-MPA and uninhibited systems, as indicated

by the more negative potentials. It can be further observed that both inhibitor systems show a

distinct cathodic shift initially, before climbing to more anodic potentials. The starting Ewe of

2-MBT was observed to be -0.64 V before rising to more positive values. Similarly, in the case

of Na-MPA, the starting corrosion potential was observed to be at -0.58 V before rising. All

traces (both inhibitors and the uninhibited solution) showed an initial move to the negative

immediately after exposure but due to the instability in the variations at this point the

significance of this initial drop is unclear. For both inhibitor systems, there seems to be a

significant anodic shift of about approximately 150 mV over the first 15 min. After about 15

min, the OCP seems to be stabilizing around -0.57 V and -0.53 V for 2-MBT and Na-MPA

4.1.2 Effect of concentration

respectively.

2-mercaptobenzothiazole (2-MBT) inhibitor system

Potentiodynamic scans for uninhibited and 2-MBT inhibited systems, as a function of inhibitor

concentration (0.1 mM, 0.5 mM and 1.0 mM), are shown in figure 4.2. All tests were carried

out in 0.1 M NaCl at room temperature and at “unbuffered” pH value of 4.5. However, with

varying concentration the pH fluctuated by approximately ± 0.25. Ecorr and icorr values

obtained from Tafel Extrapolation are presented in figure 4.4 and table 4.1

Prior to conducting the potentiodynamic scans, the samples were allowed to stabilise at the

Figure 4-2: Comparison of Potentiodynamic scans at different 2-mercaptobenzothiazole (2-MBT) inhibitor Figure 4.1: Comparison of Potentiodynamic scans at different 2-mercaptobenzothiazole (2-MBT) concentrations inhibitor concentrations

open circuit potential for 30 min.

Analysis of the curves reveal that for all inhibitor concentrations, the curves displayed cathodic

shifts to the left, resulting in more negative Ecorr values, when compared to the uninhibited

system. In addition, all curves were shifted down, resulting in lower icorr values being observed

for inhibited systems compared to the uninhibited system. The lowest concentration (0.1 mM)

induced the most positive value for the inhibited polarisation curves, resulting in an Ecorr value

of -0.54 mV being observed. At higher concentrations, the Ecorr values were shifted more

cathodically resulting in Ecorr values of -0.66 mV and -0.58 mV being observed for 0.5 mM

and 1.0 mM concentrations respectively. A reduction in the icorr value was observed as the

inhibitor concentration was increased showing a value of approximately 0.99 µA/cm2 at 0.1

mM compared with 0.48 µA/cm2 and 0.37 µA/cm2 at 0.5 mM and 1.0 mM respectively.

Analysis of the cathodic portions of the curves reveal the shallow nature (low gradient)

associated with these curves tend to suggest the presence of diffusion limited processes.

Analysis of the anodic portions of the curves reveal between the Ecorr and a potential of

approx. -0.5 V, the lower rates of increase in current with potential may be attributed to some

form of protection / passivation associated with the inhibitor / alloy system over this potential

range. This region is more pronounced for the midrange and higher inhibitor concentrations

as these systems have more negative Ecorr values and hence the range is greater. However,

at potentials more positive than -0.5 V, the change in the shape of the curves, showing

pronounced inflexion and a sudden increase in current with potential, suggests breakdown of

any protective / passivating layer formed. The identical nature of these curves beyond this

critical breakdown potential suggests further anodic processes occurring are similar in nature.

3-mercaptopropioninc acid (3-MPA) inhibitor system

Potentiodynamic scans for uninhibited and 3-MPA inhibited systems, as a function of inhibitor

concentration (0.1 mM, 0.5 mM and 1.0 mM), are shown in figure 4.3. All tests were carried

out in 0.1 M NaCl at room temperature and at “unbuffered” pH value of 3.5. However, with

varying concentration the pH fluctuated by approximately ± 0.15. The system was allowed to

stabilize for 30 minutes before conducting the potentiodynamic scan. The Ecorr and icorr values

Figure 4-3: Comparison of potentiodynamic scans at different 3-mercaptopropionic acid (3-MPA) inhibitor Figure 4.2: Comparison of potentiodynamic scans at different 3-mercaptopropionic acid (3-MPA) concentrations inhibitor concentrations

obtained from Tafel Extrapolation are presented in figure 4.4 and table 4.1.

The results reveal that as for the 2-MBT system, a shift to the left and lowering of the inhibited

polarisation curves for all three concentrations was observed, compared to the uninhibited

system, indicating a cathodic shift in Ecorr values and a lowering in the icorr values. Further

analysis reveals that at the lower inhibitor concentration of 0.1 mM, there is a more negative

shift in the polarization curve in comparison to the inhibitor concentrations of 0.5 mM and 1.0

mM. Similar to 2-MBT, it could also be seen that the cathodic arm of the curve for all

concentrations plateaus out, implying a possible diffusion-limited process. However, it should

be highlighted that the icorr value for 1.0 mM concentration is significantly higher at 5.14 µA/cm2

which is also relatively greater than the uninhibited system with an icorr of 4.58 µA/cm2. This

suggests that at high concentrations, the inhibitor could potentially accelerate corrosion. In

addition, the concentrations 0.1 mM and 0.5 mM have resulted in low icorr values of 1.69

µA/cm2 and 0.77 µA/cm2 respectively, compared with the uninhibited system value of 4.58

µA/cm2. Unlike the 2-MBT system, it can be observed that the anodic region approx. 50 mV

more positive than the Ecorr, did not show the same depressed current values, instead

exhibiting an increase in current with potential, which implies either the absence of, or the

breakdown of any passivity or protective layer form, that may be present in the 2-MBT system.

This increase in current with voltage is more pronounced at higher concentrations, whereas

the curve at 0.1 mM concentration has a lower gradient. It can be seen that this inhibitor

performs relatively better at lower concentrations than at higher concentrations with AA2024-

T3.

Tafel extrapolation of 2-MBT and 3-MPA inhibited system

Figure 4.4 below shows the variation in icorr values at different inhibitor concentrations for both

inhibitors, 2-MBT and 3-MPA.

6.0

) 2

5.0

4.0

3.0 2-MBT

m c / A µ ( r r o c i

2.0 3-MPA

1.0 Uninhibited

Inhibitor concentration

Figure 4-4: Comparison of potentiodynamic scans at different 3-mercaptopropionic acid (3-MPA) inhibitor concentrations

Table 4-1: Ecorr and icorr values at different inhibitor concentrations

0.0 1 mM 0.5 mM 0.1 mM 0 mM

Conc. (mM)

icorr (µA/cm2)

Ecorr (mV)

icorr (µA/cm2) Ecorr(mV)

1 mM

0.372 ± 0.064

-581.2 ± 23.5

5.137 ± 0.117

-500.9 ± 12.9

0.5 mM

0.475 ± 0.024

-662.4 ± 34.2

1.691 ± 0.045

-466.7 ± 20.7

0.1 mM

0.998 ± 0.091

-538.0 ± 29.4

0.768 ± 0.014

-540.7 ± 16.2

Uninhibited 4.583 ± 0.348

-480.8 ± 5.3

4.583 ± 0.348

-480.8 ± 5.3

Table 4.1 provides a summary of the Ecorr and icorr values for both systems in comparison with

the uninhibited system. Figure 4.4 allows to visualise the trends and understand the overall

performance of each inhibitor with varying concentration. The icorr values can be used as a

direct indication of the corrosion rate of the system. For 2-MBT, as the concentration

increases, the icorr value, or the corrosion rate can be observed to decrease from 0.99 µA/cm2

at 0.1 mM to 0.37 µA/cm2 at 1.0 mM. In contrast, for 3-MPA, the icorr value increases from

4.1.3 Effect of pH on inhibited systems

0.77 µA/cm2 at 0.1 mM to 5.14 µA/cm2 at 1.0 mM.

2-mercaptobenzothiazole (2-MBT) inhibited system

To understand the effects of pH on the 2-MBT system, a buffered solution of 2-MBT at a

concentration of 1 mM and pH of 7 was used with the potentiodynamic scan experiments

Figure 4-5: Comparison between 2-mercaptobenzothiazole (2-MBT) at a pH of 4.5 and 7

repeated.

Figure 4.5 presents the results obtained for the unbuffered pH of 4.5 and the buffered pH of

7. It is quite evident that both cathodic and anodic current densities have been suppressed at

the unbuffered pH. However, the gradient of the cathodic arm appears to have reduced

indicating a more pronounced diffusion limited process at neutral pH. Through the Tafel

extrapolation analysis, the current density was evaluated to be 0.57 µA /cm2 at neutral pH and

0.37 µA /cm2 at the as-received pH. Therefore, a minute reduction in the corrosion inhibition

was observed for the buffered solution at the same concentration. Furthermore, with the

unbuffered solution, the Tafel plot shows re-passivation which is not observed with the

buffered solution which could imply better capability of film repair.

3 – mercaptopropionic acid (3-MPA) inhibited system

Although the inhibitor 3-MPA performs well at a concentration of 0.1 mM, in order to maintain

consistency of inhibitor concentration through the study, the sodium salt of 3-MPA at a

concentration of 1 mM was synthesised and tested. This was achieved when 1.0 M of NaOH

was added, which also had the effect of adjusting the pH up to 7. The inhibitor, Na-MPA was

synthesized in-situ by incorporating required molar compositions of 1.0 M NaOH and 1.0 mM

3-MPA as per the below dissociation equation.

𝐻𝑆𝐶𝐻2𝐶𝐻2𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝐻𝑆𝐶𝐻2𝐶𝐻2𝐶𝑂𝑂−𝑁𝑎+ + 𝐻2𝑂 ---------- Equation (4.1)

Potentiodynamic scans were conducted for Na-MPA and 3-MPA at 1 mM concentration to

verify if the modified inhibitor performs better than the base compound, as shown in in figure

Figure 4-6: Comparison between 3-mercaptobenzothiazole (3-MPA) and Na-mercaptopropionate (Na-MPA)

4.6.

Analysis of figure 4.6 shows a significant reduction in the corrosion rate with the synthesized

Na-MPA compound; 3-MPA exhibited an icorr of 5.14 µA/cm2, while Na-MPA yielded an icorr

value of 0.47 µA/cm2, which implies that the icorr has reduced by an order of magnitude. It

could also be observed that during cathodic polarization, the current response from Na-MPA

system is significantly lower than that of 3-MPA. Therefore, it is suggested that the use of Na-

MPA at a pH of 7 shows substantially higher inhibition in comparison to that of 3-MPA at a pH

of 3.5.

4.2 Evaluation of the mechanism of film-formation 4.2.1 EIS Studies for the uninhibited system

Electrochemical impedance spectroscopy was utilized to determine the inhibitor performance

with respect to impedance data. This technique allowed for determining the interfacial

processes as well the build-up of the layers to formulate a hypothesis, particularly via an

equivalent circuit.

Figure 4-7: (a) Nyquist plot of the uninhibited system; (b) Bode plot of the uninhibited system

(a) (b)

As a reference point, electrochemical impedance spectroscopy scan was conducted for the

uninhibited system with 0.1M NaCl solution as the electrolyte and the corresponding Nyquist

and Bode plots are shown in figure 4.7.

Analysis of the Nyquist plot, shown in figure 4.7 (a) yields the polarization resistance is about

18 kOhms and falls in the 104 Ohm range. It should also be highlighted that these plots present

two time constants with one around the 10 kHz and the other around 1 Hz. However, from the

magnitude response shown in the Bode plot, it can also be seen that at 1 mHz, log (Z) reaches

4.2.2 EIS Studies for inhibited system

about 4.4, which results in an impedance magnitude of approximately 18 kOhm.

Data obtained from the EIS studies conducted on the 2-MBT inhibited system, in the form of

Nyquist and Bode plots are shown in Figure 4.8 (a) and 4.8 (b) respectively. For this series of

experiments, the electrolytes mediums utilized were 2-MBT and Na-MPA inhibitor solutions at

1 mM concentration and a pH of 4.5 and 7 respectively.

Figure 4-8: (a) Nyquist plot of the 2-MBT system; (b) Bode plot of the 2-MBT system

(a) (b)

Analysis of the Nyquist impedance plot from figure 4.8 (a) reveals the polarization resistance

value of 120 kOhm. Hence the polarization resistance falls in the 105 kOhm range which is

one order of magnitude greater than that of the uninhibited system. Analysis of the Bode

impedance plot in figure 4.7(b) reveals two time constants. The time constant observed at the

high frequency is likely due to a protective layer over the metal surface whereas the peak at

the low frequency range could be associated with interfacial activities at the metal surface.

The implications of these results with time evolution are further discussed in Chapter 6.

Similarly, the Nyquist and Bode impedance graphs shown in figure 4.9, present the data for

the Na-MPA system. Here there are similarities to the 2-MBT system.

Figure 4-9: (a) Nyquist plot of the Na-MPA system; (b) Bode plot of the Na-MPA system

(a) (b)

The Nyquist impedance plot, shown in figure 4.9 (a) shows similar polarization resistance to

that observed for the 2-MBT system, and therefore is significantly higher than that observed

for the uninhibited system. However, whilst the Bode plot shown in figure 4.9 (b) also shows

two time constants, the frequencies at which the peaks are observed are different. The peak

observed at the low frequency could still be a reflection of the interfacial activities at the metal

surface, similar to the 2-MBT system. However, the peak at the high frequency could be due

to the film growth or an oxide layer modification. The semicircle displayed in the Nyquist plot

for 2-MBT is steeper than that observed for Na-MPa, which suggests that the capacitance in

the 2-MBT system is lower than that of Na-MPA. The implications of these results with time

evolution are further discussed in Chapter 6.

4.2.3 Modelling of equivalent circuits for inhibited and uninhibited systems The uninhibited system and both inhibited systems were fitted into one equivalent circuit

including two R-C circuits in series with an additional resistor to account for the electrolyte

resistance as shown below (figure 4.10). This equivalent circuit was chosen as this proved to

be the best fit model and resulted in a deviation of less than 4% for all plots However, in the

literature, many studies have demonstrated equivalent circuits with two R-C cycles or R-CPE

in parallel [7, 9, 53]. However, the difference in the deviations between the two models is even

less than ± 0.5 %.

Figure 4-10: Comparison of EIS data between the inhibited and uninhibited system

Figure 4-11: Simplified equivalent circuit for the inhibited and uninhibited system

(a) (b)

The parameters for R1, R2, R3, C2 and C3 for each system are shown below.

Table 4-2: Fitted equivalent circuit parameters of uninhibited, 2-MBT and Na-MPA systems

Parameter NaCl 2-MBT Na-MPA

R1 95.4 Ohm 94.74 Ohm 87.56 Ohm

R2 2,944 Ohm 4,887 Ohm 11,047 Ohm

C2 43.6E-06 F 11.6E-06 F 19.53E-06 F

R3 16,923 Ohm 104,385 Ohm 111,609 Ohm

C3 0.14E-03 F 7.48E-06 F 46.65E-06 F

Each system has displayed two time constants with the one at high frequency being

associated with the oxide layer in the case of the uninhibited system and inhibitor/oxide layer

in the case of the inhibited system. The time constant at low frequency may be associated

with the interfacial corrosion activity. From the fitted data, it is quite evident that the R2 for the

uninhibited system which is associated with the passivating layer is lower than that recorded

for the inhibited system which implies a lower corrosion resistivity whilst the increased

capacitance, C2 is an indication of reduced film thickness. However, it could be easily

observed that more dominant factors are R3 and C3 which correspond to the charge transfer

resistance and double layer capacitance. The 2-MBT and Na-MPA inhibitor systems produce

a decrease in double layer capacitance with respect to that of the uninhibited system implying

the presence of a thicker passivating layer in the inhibited systems. Therefore, the EIS

measurements have displayed the presence of a protective film inhibiting corrosion at the alloy

surface.

As per the literature, some studies have utilized the equivalent circuit in table 4.3 (a) below [7,

9, 86]. In order to determine the best fit model, the fitted parameters from a model with 2 R-C

cycles in parallel and in series were evaluated.

The selected equivalent circuit has been equipped with a capacitor, C instead of a constant

phase element, CPE. In reality, as recorded in previous literature [7, 86], for corrosion

systems, frequency dispersion should be taken into consideration. For a pure capacitor,

characteristic parameter, n would be 1 indicating no capacitance dispersion. However, during

the measurements, it was noted that there was no significant variation in n-value amongst the

measurements. Therefore, as the n-value was close to 1.0, the process as assumed to have

a linear effect and that the CPE resembles a capacitor; thereby the corresponding pseudo

capacitance was calculated.

Table 4-3: The fitted parameters for the uninhibited system (a) parallel circuit model; (b) series circuit model

(a) (b)

Model Model

R1 (Ohm)

95.4

R1 (Ohm)

95.5

R2 (Ohm)

2,944

R2 (Ohm)

4,196

C2 (F)

43.6E-06

C2 (F)

44.4E-06

R3 (Ohm)

16,923

R3 (Ohm)

15,663

C3 (F)

0.14E-03

C3 (F)

0.10E-03

Dev.

3.59%

Dev.

4.8 %

When the two models are compared, the values of each component appear to be consistent

with minor variations. It should be highlighted that in both cases, R2 and C2 are correspondent

to the inhibitor/oxide layer where as R3 and C3 are associated with the charge transfer

resistance and double layer capacitance. Although, the difference in values is very minor, the

series circuit model displays a better fit model with a marginally lower deviation. Therefore,

the series circuit model was chosen for the subsequent inhibited systems as they displayed

4.3 Surface analysis of inhibited systems Energy dispersive spectroscopy (EDS) was employed to determine the surface composition

of the AA2024 after free immersion in each inhibitor for 7 days. This technique wasn’t utilized

for the uninhibited system as the alloy surface appears to be too coarse after 7 days of

immersion due to severe corrosion and EDS cannot be accurately conducted over such rough

surfaces. After immersion in inhibitor solutions, 2-MBT and Na-MPA, samples were observed

4.3.1 Energy dispersive x-ray spectroscopy (EDS) of 2-MBT system

under the JEOL JSM 7200F scanning electron microscope.

Figure 4.13 displays the elemental composition in the form of surface area elemental maps,

as observed on a 2-MBT inhibitor treated AA2024-T3 sample.

The elemental mapping interprets the presence of different intermetallic particles. The larger

two intermetallic particles to the left of the electron image in figure 4.13, containing both Mg

and Cu are likely to be associated with S-phase Al2CuMg particle, whereas the smaller

particles on the right hand side of the electron micrograph, can be seen to contain Fe and Mn

and therefore likely to be associated with AlCuFeMn particles. Further analysis reveals that

some trenching has occurred around the S-phase particles which has been discussed in

Figure 4-13: Elemental composition through EDS after 7 days of immersion in 2-MBT solution.

previous studies [4].

Secondly, the presence of oxygen is associated not only with the S-phase particles but is also

very prominent around the periphery area of the S-phase particles where trenching has

occurred. The presence of sulphur and carbon within the S-particles suggests the presence

of inhibitor molecules and direct association with these intermetallic particles. It is also

important to highlight that the presence of sulphur is more concentrated at the outline of

intermetallic particles. However, the results are also indicative of the lack of inhibitor over the

AlCuFeMn IM particles which is considered to be Cu-deficient, therefore suggesting that the

association of inhibitor with Cu-rich particles are more pronounced.

Interestingly, further analysis was conducted in understanding the necessity of the initial step

of minor trenching around the S-phase IM particles via SEM and EDS on a sample which was

exposed to a solution consisted of 1 mM 2-MBT and 0.1 M NaCl. Figure 4.14 below shows

Figure 4-14: Elemental composition through EDS after 7 days of immersion in 2-MBT solution, displaying the effects of trenching

the influence of trenching on the film-formation process.

The two intermetallic particles shown in figure 4.14 are likely to be associated with the S-phase

particles as seen from the elemental maps of Mg and Cu. However, it could also be seen that

in the SEM Image, the top particle shows some trenching around the intermetallic, whereas 2

the bottom particle shows no visible trenching, which be confirmed via the elemental map of

O. Here, O is present in the trenched (upper) particle while being minimal in the un-trenched

(lower) particle for detection. It should be highlighted that the elemental maps of S, C and N

shows that these elements are significantly more prominent over the trenched intermetallic

particle in comparison to the un-trenched particles. The presence of S, C and N within the

trenched particle and lack thereof in the un-trenched particle suggests not only the presence

of the inhibitor with these IMP’s, but that they are confined to the trenched particle suggests

that initial trenching could be aiding the inhibitor-induced film formation process. However, the

non-appearance of these elements in the maps is an indication of comparatively lower levels

4.3.2 Energy dispersive x-ray spectroscopy (EDS) of Na-MPA system

of presence and not complete absence of it.

Figure 4.15 shows the elemental composition in the form of surface area elemental maps, as

Figure 4-15: Elemental composition through EDS after 7 days of immersion in Na-MPA solution

observed on the Na-MPA inhibitor treated AA2024-T3 sample.

Similar observations can be made when compared to that of 2-MBT. The electron image in

figure 4.15 shows a combination of intermetallic particles with some trenching around these

particles evident. The presence of Cu, O and S and lack of Al around the intermetallic particles

support the association of an inhibitor layer with particles that have undergone trenching, as

observed previously for the 2-MBT sample. These observations are further discussed in

Chapter 7.

4.4 Analysis of pure aluminium and copper

In order to provide a base reference point for the film forming inhibitor studies and the role of

IM particles on film durability, a series of experiments were planned to understand the

behaviour and the performance of each selected inhibitor with pure aluminium and pure

copper as these two elements are the most abundant in the alloy AA2024-T3. This would act

as supporting material to understand if the inhibitor’s performance is mainly driven by the

aluminium matrix or the Cu-rich intermetallic particles itself.

4.4.1 Effect of inhibitors on pure aluminium Potentiodynamic scans

Figure 4.16 shows potentiodynamic scans for uninhibited, 2-MBT and Na-MPA inhibited pure

aluminium in in 0.1 M NaCl. The 2-MBT and Na-MPA system were utilized at an inhibitor

concentration of 1 mM and a pH of 4.5 and 7 respectively. The system was allowed to stabilize

for 15 min at the open circuit potential. The Ecorr and icorr values determined from Tafel

Figure 4-16: Potentiodynamic scans for inhibited and uninhibited pure aluminium systems

extrapolation of the curves are shown in table 4.3 below.

Table 4-0-1: Ecorr and icorr parameters for the uninhibited and inhibited systems with pure aluminium

Solution

Ecorr (mV)

icorr (µA/cm2)

0.1M NaCl only

-551.8 ± 4.9

0.035 ± 0.002

1 mM Na-MPA

-679.6 ± 44.2

0.029 ± 0.007

1 mM 2-MBT

-713.2 ± 31.4

0.060 ± 0.013

Analysis of the curves reveal that pure aluminium exhibited very low icorr values for all

conditions. The icorr value of 0.035 µA/cm2 for uninhibited conditions is approximately 2 orders

of magnitude lower than that of AA2024-T3. However, the lowest icorr value recorded was

0.029 µA/cm2 upon treatment with Na-MPa whilst 2-MBT resulted in an icorr value of 0.06

µA/cm2 which is still one order of magnitude lower than that obtained for AA2024-T3 under

similar conditions.

In addition, it could also be observed that Na-MPA and 2-MBT inhibitors have caused a

noticeable cathodic shift in the Ecorr values, typically by about 127.8 mV and 161.4 mV for NA-

MPA and 2-MBT respectively.

Further analysis of the curves show that the rate of current density increases with voltage

monitored during anodic polarization is significantly lower than that observed for 2-MBT

indicating an increased durability for protective / passivating layer formation. Another point to

be highlighted is the fact that in the presence of inhibitors, the anodic scan has resulted in a

significant fluctuation in current in comparison to the uninhibited system. The noise observed

in the anodic arm may be a cause of localised or metastable activation and passivation

processes caused by the presence of the inhibitor. In terms of the cathodic polarization, Na-

MPA shows a reduced current density in comparison to the uninhibited system, whilst 2-MBT

has exhibited an increased current density.

Linear polarization resistance (LPR)

tests were conducted with pure aluminium over 3 days to understand the performance of the

inhibitors with this metal in the presence of a thick homogeneous oxide layer. The results of

Figure 4-17: Polarization resistance for uninhibited and inhibited systems with pure aluminium over 72 hours

this study are shown in figure 4.17.

It should be noted that when pure aluminium is exposed to the uninhibited solution,

polarisation resistance values are significantly higher than those obtained for AA2024-T3. This

provides support for pure aluminium providing high corrosion resistance whereby the

presence of a thick oxide layer protects the metal surface from the corrosive medium and

thereby resulting in high Rp values. It should be noted that immediately upon immersion, the

polarization resistance is comparatively low which then increases rapidly after about 6 hours

of exposure to NaCl. This is followed by a gradual drop after about 12 hours which then

continues to gradually rise after 24 hours, before appearing to level out between 60 to 72

hours exposure.

For the inhibited systems, Na-MPA shows much higher polarization resistance, typically

around 1.6 MOhm over 72 hours (3 days), when compared to the 2-MBT system with a Rp of

0.18 MOhms. Upon immersion in Na-MPA, there is a slight drop in the Rp after about 4 hours

of immersion which then plateaus out at a Rp of 1.1 MOhms for a further 24 hours. The Rp of

this system then increases which then again plateaus out at about 2 MOhm for the remainder

of the test duration period.

When comparing the Na-MPA and uninhibited systems, higher resistance values are observed

during the initial 4 hours for the inhibited systems which then displays the vice versa for the

next 20 hours. In contrast to the uninhibited system which show a gradual increase in

polarisation resistance over the next 30 hours before attaining its maximum resistance value

of 2 MOhm, the inhibited system rises sharply to this value and remains stable for the

remaining 30 hours. These observations indicate that the formation of an inhibitor film

significantly influence the overall passive nature of the aluminium alloy and subsequently the

natural oxide growth. These findings are discussed in more detail in Chapter 7.

For the 2-MBT system, the polarization resistance, although much lower than for the

uninhibited and Na-MPA inhibited systems, still falls in the 105 Ohm range which implies that

the metal surface continues to be protected. As these results fall within the same range as

those observed AA2024-T3, any role that a protective oxide layer associated with pure

aluminium plays is likely to be quite different to that for the Na-MPA inhibited system and is

considered in more detail in Chapter 7.

4.4.2 Effect of inhibitors on pure copper Potentiodynamic scans (PDS)

Potentiodynamic scans were conducted for pure Cu under both inhibited and uninhibited

conditions after allowing the system to stabilize for 15 minutes at the OCP. The 2-MBT and

Na-MPA solutions were utilized at an inhibitor concentration of 1 mM and a pH of 4.5 and 7

respectively. The results obtained are shown in figure 4.18. Ecorr and icorr results are presented

in table 4.5.

Figure 4-18: Potentiodynamic scans for inhibited and uninhibited pure copper systems

Table 0-2: Ecorr and icorr with and without inhibitors for pure copper

Solution

Ecorr (mV)

icorr (µA/cm2)

0.1M NaCl only

-122.8 ±

2.811 ± 0.13

1 mM Na-MPA

-265.9 ± 2.42

2.496 ± 0.058

1 mM 2-MBT

-153.5 ± 10.4

0.148 ± 0. 23

Analysis of figure 4.18 reveals the presence of the inhibitor produces a cathodic shift with the

Ecorr value shifting from -122.8 mV for uninhibited to Cu to -153.5 mV (approximately 30 mV)

and -265.9 mV (approximately 140 mV) for 2-MBT and Na-MPA systems respectively.

Analysis of the icorr results reveal the value for 2-MBT is notably lower (approx. one order of

magnitude) than that of both Na-MPA and the uninhibited system (0.15 µA/cm2 compared with

2.50 µA/cm2 and 2.81 µA/cm2 respectively). The potentiodynamic scan for 2-MBT shows a

plateau at the beginning of the anodic arm (-0.15 V) up until a potential of 0.05 V indicating

that during this period it is a diffusion-controlled process. This observation implies that the

protective layer was maintained during anodic polarization until it reached a potential of about

0.1 V which then showed increased rate of current density suggesting the breakdown of any

passivation / protective layer.

Linear polarization resistance (LPR)

LPR tests conducted over 3 days are shown in figure 4.19. The results are presented as

variation in polarization resistance, Rp, with time when exposed to 2-MBT, Na-MPA and

Figure 4-19: Comparison of linear polarization resistance of inhibited and uninhibited systems with pure copper

uninhibited NaCl.

Analysis of figure 4.19 reveals 2-MBT shows much higher polarization resistance when

exposed to pure copper in comparison with not only uninhibited Cu and Na-MPA inhibited Cu,

but also with AA2024-T3 and with pure aluminium. The findings suggest that 2-MBT could be

an excellent corrosion inhibitor for copper. Polarization resistance values for 2-MBT initiates

in the range of 106 Ohm which constantly increase over time, reaching the 107 Ohm range

after 24 hours, whereas the uninhibited system and the Na-MPA system exhibited Rp values

in the 104 Ohm range. With this LPR trend, it seems to be the resistance continues to increase,

which signifies any film build up over the metal surface, is acting as a barrier between the

corrosive medium and the metal surface increasing corrosion inhibition. In contrast, as the Na-

MPA system has resulted in a similar Rp range to the uninhibited system this could be an

indication that the metal surface is unprotected.

5. CHAPTER 5: RESULTS: LONG-TERM STABILITY OF THE INHIBITOR-INDUCED FILM

Durability and stability, particularly for inhibitor systems that rely on film formation is of the

utmost importance when considering the longer-term performance of corrosion inhibitors. The

literature survey has shown that one of the main limitations of current film forming inhibitor

systems is knowledge of their performance and behaviour over extended periods of time. Very

limited work has been done on understanding the film build-up over an extended time period.

It is crucial therefore to monitor not only the time period a system takes to form a uniform

stable film over the metal surface to ensure effective corrosion inhibition, but also the stability

of this film over extended periods of exposure.

This chapter focuses mainly on the long-term behaviour and stability of the two inhibitors

selected for the study, namely 2-MBT and Na-MPA and sustainability of the films formed over

extended time periods. As shown from the pH studies conducted in Chapter 4, all tests were

conducted in the “unbuffered” conditions. Both linear polarization resistance and

electrochemical impedance spectroscopy (EIS) techniques were used to monitor the corrosion

rate over 7-14 days. Potentiodynamic scans (PDS) were conducted after 30 min and 7 day

exposure, while being exposed to an inhibited solution. Similarly, electrochemical impedance

spectroscopy (EIS) has also been conducted simultaneously over a 7-day period to monitor

the corrosion rate via impedance results at different time intervals. As LPR and EIS are non-

destructive techniques, it is possible to obtain results from the same sample at different time

intervals without interfering with its natural corrosion process. For the potentiodynamic scans,

as these are destructive and corrosion accelerating techniques, a scan was taken immediately

upon immersion and after 7 days of immersion on a different sample, to confirm if the

performance of the inhibitor film has been sustained. As a visual aid for the electrochemical

techniques, SEM images were collected at different time intervals to determine the extent of

corrosion, film forming behaviour and influence of IMP’s on the system’s ability to provide

continuing protection through film formation. Therefore, this chapter outlines the techniques

used and the results obtained to establish the long-term stability of the inhibitor-induced film.

5.1 Studies for uninhibited and inhibited systems over 168 hours (7 Days) immersion

5.1.1 Linear polarization resistance (LPR) studies

Here, LPR studies were conducted on both uninhibited and inhibited systems for 168 hours (7

days). The systems were allowed to stabilise for a 2-hour period prior to commencing the

measurements

Figure 5.1 shows the variation of Rp with time over 168 hours for AA2024-T3 in both

uninhibited (0.1M NaCl only) solution and inhibited solutions, with 2-MBT and Na-MPA at

Figure 5-1:Comparison of polarization resistance with and without inhibitors on aluminium alloy, AA2024-T3 over 168 hours (7 days).

inhibitor concentrations of 1mM and an initial pH of 4.5 and 7 respectively.

For the uninhibited system, the observed fluctuations during the first 5 hours could be

associated with instability in the naturally occurring oxide layer formation during the initial

stages, resulting in discontinuities in the oxide layer build up and/or breakdown of the oxide

layer as a result of the heterogeneous nature of the alloy surface and the presence of

intermetallic particles. After 5 hours immersion, a spike in the Rp can be observed which then

drops again. Between 1 and 36 hours exposure, there is a gradual increment in the

polarization resistance followed by a gradual decrement between 36 hours and 60 hours. After

60 hours, a slight increment in the Rp could be observed over time. However, after around

120 hours, the Rp seems to be quite stable around 2 x 104 Ohms. It can be observed that the

uninhibited system follows a sinusoidal pattern behaviour over time. In addition, the solution

resistivity and the significantly large amounts of corrosion products observed, could be the

cause of any further fluctuations. It should be noted that the Rp in an uninhibited solution falls

within the 104 Ohm range and this has been utilized as a base reference to determine the

inhibitor performance.

Figure 5.1 also shows the variation in Rp with time for the 2-MBT inhibited system after 168

hours exposure. It should be noted that during the first 24 hours, an overall gradual increment

in the polarization resistance could be observed. This has been followed by a slight drop in

the polarization resistance over the next 6 hours. In addition, some fluctuation could be seen

up to 72 hours, which has resulted in an irregular sinusoidal behaviour. The Rp values appear

to have stabilized after about 96 hours, although some fluctuations are evident. Nevertheless,

there is an overall gradual increase in the polarisation resistance from 1 x 105 Ohms to 3 x 105

Ohms over the given time period The Rp values fall in the 105 Ohm range, an order of

magnitude higher than the uninhibited system over the given time period.

For the Na-MPA system, as shown in figure 5.1, significantly larger fluctuations in polarization

resistance can be observed when compared with the 2-MBT results. During the first 12 hours,

a slight increment in the resistance is observed, which then drops, reaching a similar Rp value

as for the 2-MBT system after about 24-hour exposure. After 24 hours, whilst the 2-MBT

system shows a rising trend in the Rp, the Na-MPA system showed a decline. This reduction

in the polarization resistance continued up to 48 hours of exposure, which was then followed

by a minor increment. After 72 hours exposure, the system appears to have stabilised although

there is increased number, and severity, of spikes observed here compared to the 2-MBT

system. Overall, the Rp values are slightly lower for the 2-MBT system compared to the Na-

MPA system. However, Rp values have managed to remain within the 105 Ohm range over

the 168 hours (7 days), similar to that observed for 2-MBT and thereby an order of magnitude

5.1.2 Potentiodynamic scan (PDS) studies

higher than that of the uninhibited system.

Potentiodynamic scans conducted for inhibited systems after 0 and 168 hours (7 days)

exposure are shown in figure 5.2. Ecorr and icorr values, determined from Tafel extrapolation

Figure 5-2: Potentiodynamic scans of inhibited systems after 30 min and 168 hours (7 days)

are summarised in table 5.1

Table 5-1: Ecorr and icorr values of inhibited systems after 0 hours and 168 hours (7 days)

2-MBT

Na-MPA

(30 min)

(168 hrs)

(30 min)

(168 hrs)

icorr / µAcm-2

0.372 ± 0.064

0.012 ± 0.003

0.465 ± 0.124

0.231 ± 0.084

Ecorr / mV

-581.2 ± 23.5

-655.2 ± 75.3

-586.5 ± 66.9

-411.5 ± 46.3

Over a 168-hour period, significant changes in the anodic and cathodic behaviour of both 2-

MBT and Na-MPA systems are evident after 168 hours (7 days) exposure. For the 2-MBT

system, the full scan was observed to shift to the left and downwards after, indicating a

cathodic shift in Ecorr values and a lowering of the icorr values. A cathodic shift of about -75

mV can be observed. Analysis of the cathodic regions reveal not only a significant lowering of

the cathodic arm after 168 hours (7 days) exposure, but also a change in the gradient, an

increase in gradient i.e. increased rate of change of current with voltage being observed after

one week exposure Here, for any given cathodic potential over the full cathodic potential

range, the cathodic current value is lower after 168 hours exposure .

Analysis of the anodic curves show regions of plateauing, becoming more frequent and more

pronounced after 168 hours exposure. These regions are likely to be associated with

passivation and passivation breakdown. Passivation breakdown and re-passivation regions

are evident at two pints after one-week exposure, with one around -0.6 V and another at -0.5

V. As shown in table 5.1, the icorr was reduced from 0.372 µA/cm2 to 0.012 µA/cm2 after 168

hours (7 days) exposure. Moreover, after 168 hours of exposure the anodic arm has exhibited

light noise which may have originated from localised and metastable activation and

passivation processes.

In contrast, For the Na-MPA system, after 168 hours of exposure, a shift to the right can be

observed after 168 hours exposure indicating an anodic shift in potential. A potential shift in

the Ecorr value of about +175 mV can be observed. Further, a slight reduction in the icorr value

(0.465 µA/cm2 to 0.231 µA/cm2) can be observed after one-week exposure. While there are no

significant changes to the slopes of the cathodic arm over the exposure period, a significant

sharp rise in the initial region of the anodic curve can be observed after one week exposure,

suggesting breakdown of any protective layer that has formed, almost immediately, followed

by a sharp rise in anodic current, the rate of increase in current becoming less with further

increase in anodic potential. For any given anodic potential over the full anodic potential

range, the anodic current value is lower after 168 hours exposure

Furthermore, the potentiodynamic scan for the uninhibited system hasn’t been included as the

sample surface is extensively corroded after 168 hours (7 days) of exposure that the readings

5.1.3 Electrochemical impedance spectroscopy (EIS) studies

obtained from such rough corroded surface cannot be considered to be accurate.

2-mercaptobenzothiazole (2-MBT) inhibited system

EIS results taken for the 2-MBT inhibited system after 1 hour, 1 day, 3 days and 7 days, and

140,000

120,000

100,000

)

80,000

1 hour

m h O / Z (

60,000

24 hours (1 day)

I -

72 hours (3 days)

40,000

168 hours (7 days)

20,000

50,000

100,000

150,000

200,000

250,000

300,000

represented as both Nyquist and Bode plots, are shown in figure 5.3 below.

Figure 5-3: (a) Nyquist plots in the 2-MBT inhibited system over 168 hours (7 days)

Re (Z/Ohm)

Figure 5-3: (b) Bode plots in the 2-MBT inhibited system over 168 hours (7 days)

Figure 5-4: Suggested equivalent circuit for the 2-MBT system over 168 hours (7 days)

Table 5-2: The fitted parameters from the equivalent circuit (figure 5.4) for the 2-MBT system

1 hour 24 hours 72 hours 168 hours

(1 day) (3 days) (7 days)

R1 (Ohm) 93.11 77.27 86.59 82.88

R2 (Ohm) 4,887 11,209 7,283 6,599

C2 (F) 7.47E-06 9.17E-06 10.02E-06 11.46E-06

R3 (Ohm) 104,385 174,789 140,000 319,141

C3 (F) 11.55E-06 16.64 E-06 17.8E-06 20.46E-06

A simple equivalent circuit has been fitted to fit the plots at different time intervals where R1 is

the electrolyte resistance, R2 and R3, the polarization resistance and C2 and C3, the

capacitance of the potential layers. This equivalent circuit was chosen as this was the best fit

model to accommodate two time constants with a deviation below 2 %. However, in previous

studies, inhibited systems have been fitted with three time constants [7, 9, 53, 70]. By

analysing the plot in figure 5.3(b), it could be observed that the time constants at high

frequencies aren’t distinctive enough to fit them into two different R-C cycles. Therefore, to

avoid overfitting the data, an equivalent circuit with two R-C cycles was chosen to

accommodate for the inhibitor/oxide layer and the interfacial activities.

The inductive loops were ignored during the process of fitting the model. The time constants

observed at the high frequency of 10 Hz reaching a phase angle of up to 80o, may be

associated with the inhibitor layer and the native oxide layer. These time constants display

capacitive behaviour with dielectric properties. On the other hand, the time-dependent process

at low frequency of 1 Hz could be associated with the interfacial activities at the alloy surface.

As time evolves, has exhibited the high to mid-range frequency time-dependent process from

the phase angle plot becomes more pronounced. Some fluctuations can be observed after 1

hour of exposure at very low frequencies around 10 mHz and this could be due to noise or

OCP shift in between data points during the scan. It should be highlighted that after 24 hours

(1 day), 72 hours (3 days), 168 hours (7 days), the time constant at 10 Hz remains constant

with a similar capacitance and phase angle but the time constant at 1 Hz demonstrates

significant variance. The maximum phase angle value for the 24 hours (1 day) and 168 hours

(7 days) plot reaches a value of about 65o, whilst the peak from the 72 hours (3 days) plot

attains a maximum value around 60o. The same trend could be observed in the log (|Z|) vs

frequency graph at very low frequencies of about 10 mHz. However, the log |z| graph after 1

hour shows a higher capacitance overall than that observed for longer exposure periods where

the capacitance shows no significant change until the frequency reaches about 100 mHz.

Such trends and observations are reflected in the data presented in table 5-2.

The Nyquist plot shows that as time evolves, the polarization resistance, R1+R2+R3 has

significantly increased from 108 kΩ to 320 kΩ which is approximately a three-fold increase.

This is also evident from the Bode plot, where the log (|Z|) at the frequency of 0.01Hz shows

increment over the period of 7 days, indicating a rise in polarization resistance. However, it

should be noted that the polarization resistance after 3 days has shown a decrement in

comparison to that after 1 day which has then increased again after 7 days. Such observations

imply change in chemistry during the film formation process after 3 days and possible

breakdown of the protecting layer which then self-repairs. More importantly, upon analysis, it

should be highlighted that the charge transfer resistance is the more dominant factor

contributing to the variation in polarization resistance.

Na-mercaptopropionate (Na-MPA) inhibitor system

EIS results taken for the Na-MPA inhibited system after 1 hour, 1 day, 3 days and 7 days, and

Figure 5-5:(a) Nyquist plots for the Na-MPA inhibited system over 168 hours (7 days)

Figure 5-5: (b) Bode plots for the Na-MPA system over 168 hours (7 days)

represented as both Nyquist and Bode plots, are shown in figure 5.5 below.

Table 5-3: The fitted parameters of the corresponding equivalent circuits for the Na-MPA system over 168 hours (7 days)

Figure 5-5: (b) Bode plots for the Na-MPA system over 168 hours (7 days)

Similar to the 2-MBT system, the suggested equivalent circuits have been chosen based on

the best fit model with a deviation less than 3.6% for all plots and also based on the 2-MBT

inhibitor system and the corresponding equivalent circuit for each time interval is presented in

table 5.3. It should be noted that unlike the 2-MBT system, where one equivalent circuit model

was adopted for all exposure conditions, for the Na-MPA system, different circuits have been

modelled for the various exposure times. Similar to the 2-MBT system, the maximum value in

the phase angle plot at the high frequency is associated with the presence of the inhibitor/oxide

layer, whereas the one at the low frequency corresponds to interfacial corrosion activities. It

should also be highlighted that the polarization resistance reaches about 122.7 kOhms after

1 hour of exposure. However, after 24 hours (1 day), one intermediate time constant and one

low frequency time constant could be observed in addition to the high frequency peak. The

fluctuations observed in both the Nyquist and Bode plots at very low frequencies were ignored

as this could be due to potential noise or drifting of the OCP during the EIS measurements.

As the Nyquist plot shows a Warburg component where the line is approximately at a 45o

angle, this has been incorporated into the second R-C cycle in the equivalent circuit as shown

in table 5.3 (b)

For longer immersion periods of 72 hours (3 days) and 168 hours (7 days), around the

frequency value of 10 mHz, a e time constant exists which reaches a low phase angle of about

35o. However, the peak at the mid-range frequency reaches a phase angle of about 80o which

is about the same angle obtained in the presence of an inhibitor protective layer. For 1 hour,

72 hour and 168 hour exposure times, an equivalent circuit with two R-C cycles have been

fitted as the Nyquist plots show two semicircles each although at different frequency ranges

as displayed in figure 5.5 (a. However, it is worth noticing that unlike the 2-MBT system, the

capacitance of the Na-MPA system starts to show variance after reaching a frequency of 1

kHz. The log |Z| vs frequency plot shows a decreasing capacitance trend as the exposure time

increases. The maximum log |Z| at 10mHz is obtained after 1 hour of exposure, while the

lowest value was observed after 168 hours (7 days) of exposure.

In relation to the polarization resistance, from the data recorded in the table 5.3, there seems

to be a drop-in polarization resistance after 24 hours (1 day) exposure from 1.21 x 105 Ohms

to 9.61 x 104 Ohms which has then subsequently increased over the next 48 hours (2 days)

to about 1.04 x 105 Ohms. However, another drop in the polarization resistance can be seen

after 168 hours (7 days) where the double layer resistance adds up to about 7.45 x 104 Ohms.

As for the 2-MBT system, it should be highlighted that the main dominating factor for the

variation in the polarization resistance is R3 which is associated with the charge transfer

resistance, although it is not as dominant here as for the 2-MBT system.

5.2 LPR studies of inhibited systems after 336 hours (14 Days) immersion

The polarisation resistance was monitored for longer exposure periods up to 336 hours (14

days) to understand the longer-term performance of the inhibitors, 2-MBT and Na-MPA. The

Figure 5-6: Linear polarization resistance, Rp of inhibited systems during 7 – 14 days exposure period.

results from this study are shown in figure 5.6.

Up to 168 hours (7 days) exposure (refer figure 5.1), the Na-MPA system was showing

slightly lower Rp values than the 2-MBT system, although both systems fall within the 105

Ohm range. However, beyond 168 hours (7 days) of exposure the two inhibitors displayed

significantly different performances. As shown in figure 5.6, the Rp for the 2-MBT system

remained fairly constant for the complete period of 168 – 336 hours (7-14 days) exposure,

sitting around the 5 x 105 Ohm range. Initially a slight drop during the first few hours of this

exposure timeframe can be observed followed by a slight rise, however beyond 216 hours

the Rp values levelled out. Overall, the 2MBT system Rp range was an order of magnitude

higher than the uninhibited system.

In contrast, the Na-MPA system shows a reduction in the Rp value at around 180 hours

exposure. The rate at which the resistance values drop is initially high (up to around 216

hours) although this appears to gradually lessen with continuing exposure, levelling out at

around 264 hours exposure. Here it can be observed that the Rp values are significantly less

than for the 2-MBT system, falling within the 104 Ohm range (typically around 3 X 104 Ohm).

These values are similar to those observed for the uninhibited system, suggesting that the

performance of the 2-MBT system is similar to that of the uninhibited system, in terms of

polarisation resistance. In addition, greater degree of fluctuations in the Rp plots can be

observed for the Na-MPA system compared to the 2-MBT system, indicating possible

reduced stability of any protective films that have formed during exposure.

For the uninhibited system, LPR results weren’t obtained over the 168 – 336 hours (7 – 14

days) due to the significant corrosion observed over this time period precluding the generation

of any meaningful data.

5.3 Surface analysis 5.3.1 Surface analysis of the uninhibited system

Optical microscopy was used to visually determine the extent of corrosion taken place on the

metal surface in the uninhibited system. Here AA2024-T3 samples were exposed to an

uninhibited 0.1 M NaCl solution and images were captured before exposure, after 1 hour and

24 hours of exposure, as displayed in figure 5.7 respectively.

(a) (b) (c)

Figure 5-7:(a) Sample as received (polished); (b) after 1 hr of immersion in an uninhibited chloride electrolyte; (c) after 24 hr of immersion in an uninhibited solution

The image 5.7 (a) shows the presence of a range of intermetallic particles with a few pits which

are likely to be associated with the surface preparation process. However, after 1 hour of

exposure to NaCl solution, a significant amount of pitting can be observed. There is some

discoloration over the surface with deep pits represented by black circles. Brown discoloration

around the edges of the pits is likely to be associated with corrosion and formation of corrosion

products, as shown in figure 5.7 (b). After 24 hours the metal sample has a completely

corroded topography, showing corrosion over the whole of the metal surface, interspersed

with a greater number of larger pits. These images were used as a reference point to

5.3.2 Surface analysis using SEM of the 2-MBT inhibited alloy surface

understand the effectiveness of the inhibitors on AA2024-T3.

Scanning electron microscopy images captured using a secondary electron (SE) signal mode

of the 2-MBT inhibited alloy surface after 24 hours (1 day), 72 hours (3 days) and 168 hours

(7 days) exposure are shown in figure 5.8.

IM Particles

Al Matrix

Figure 5-8: SEM images of AA2024-T3 after exposure to a 2-MBT induced 0.1 M NaCl solution for different time periods.

After 24 hours After 72 hours After 168 hours

The images presented in figure 5.8 were taken from different samples for the three exposure

times, as damage may occur to the surface film formed once removed from solution. The

images show the presence of intermetallic particles (light regions) within the Al alloy matrix

(dark regions). Generally, the alloy surface does not show any significant signs of corrosion,

either localised (pitting) or uniform, after 24 and 72 hours (1 and 3 days) exposure, and even

up to 168 hours (7 days) of exposure. All these SEM images taken at lower magnifications

show similar features for the shorter exposure times, specifically in terms of the absence of

any corrosion products.

However, After 168 hours (7 days) of exposure, at higher magnifications (x1,000) , SEM

images reveal different morphologies. Here, dark regions (indicated with the red arrow),

possible associated with strands of inhibitors covering the metal surface can be observed,

covering selected areas of the surfae as shown in figure 5.9. Here, both sparsely populated

and densely populated regions of coverage can be observed in Figures 5.9 (a) and 5.9 (b)

(a) (b)

respectively.

Figure 5-9: Topography of AA2024-T3 after 168 hours (7 days) in 2-MBT induced NaCl solution

5.3.3 Surface analysis using SEM of the Na-MPA inhibited alloy surface

Inhibitor strands

Similarly to the 2-MBT system, SEM images were obtained of the alloy surface after 24 hours

(1 day), 72 hours (3 days) and 168 hours (7 days) exposure to the Na-MPA inhibited system.

Figure 5-10 : SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1 M NaCl solution for different time periods.

After 24 hours After 36 hours After 168 hours

The SEM images, as shown in figure 5.10, show that the metal is well protected with almost

no visible signs of localised (pits) or uniform corrosion after 24 hours. After 72 hours of

exposure however, there is evidence of pit formation. After 168 hours of exposure, as shown

in figure 5.10 (c), further pitting is evident. However, this SEM image was taken at a region on

the surface which showed no signs of film formation and in general, very little pitting. A

significant proportion of the exposed area showed film formation and larger pit formation,

visible to the naked eye. These observations are shown in figures 5.11 to 5.13 and described

in further detail below.

Figure 5.11 shows low powered SEM image of the exposed sample area. The presence of

large pits can be observed along with large regions of film formation over the surface. In

addition, there are regions where there is the absence of any film, likely to be associated with

spallation of this layer. In particular, film formation appears to be associated with these pits,

as shown by the localised areas showing both pit and film formation. It is likely that a film

associated with the inhibitor, is incorporated within the corrosion products that have formed

around these pits.

Figure 5-11:Overall topography SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1 M NaCl solution for 7 days

Figure 5.12 shows a more detailed image of the layer that has formed around the pit. However,

it should also be noted that the area covered by this layer has not been affected by the NaCl

and remains protected. Inspection of figure 5.12 reveals there may potentially have been a

protective layer formed over the metal surface, but due to localised corrosion or pitting, the

peripheral area of the pit shows incomplete coverage or spallation of the film layer. This has

led to formation of rings of corrosion products around the pit.

Figure 5-12: Overall topography SEM images of AA2024-T3 after exposure to a Na-MPA induced 0.1 M NaCl solution for 7 days showing the corroded areas with the pits formed

Pits formed

The crystalized structure observed could be an effect of incorporation of salt crystals within

the oxide layer.

In order to confirm the composition of this layer, an EDS scan was conducted, and the results

Figure 5-13: EDS results of AA2024-T3 after 7 days in Na-MPA induced NaCl solution

are shown in figure 5.13.

Analysis of the left side region in the micrograph, show the presence of Al and selected regions

of Cu, indicative of the aluminium matrix and Cu based IMP’s. Lower detection of oxygen,

sulphur and carbon in comparison to other elements suggest the absence of any prominent

oxide and / or inhibitor layer, supporting that any inhibitor film / oxide layer may have formed

and subsequently spalled away. Analysis of the right hand region, showing the strong

presence of oxygen, sulphur and carbon, and reduced presence of aluminium, supports the

formation and presence of an oxide layer with an inhibitor film layer or the incorporation of

inhibitor species (sulphur and carbon) within the oxide layer, thus forming a mixed inhibitor /

oxide layer. This will be further discussed in detail in Chapter 7.

5.4 Surface analysis of 2-MBT inhibited systems over longer exposure periods of 1440 hours (60 days)

The results presented for studies conducted over 168 hours (7 days) have indicated that 2-

MBT inhibitor outperforms the Na-MPA in terms of film formation, protection and overall

durability. Here longer-term immersion studies were conducted on the 2-MBT inhibitor for 1440

hours (60 days) and analysed using the focussed ion beam (FIB) technique. FIB was used to

Figure 5-14: FIB image of AA2024-T3 sample that has been exposed to 2-MBT inhibited NaCl after 60 days

capture the formation of the inhibitor layer over the metal surface.

Figure 5.14 shows the alloy surface after 1440 hours (60 days) of free immersion in 2-MBT

solution with the presence of inhibitor over the surface.

To investigate this further, particularly to determine the existence of a combined and or mixed

inhibitor / oxide layer, a TEM lamella cross section of the exposed sample was milled out using

the FIB technique. The Conventional TEM (CTEM) imaging mode in the transmission electron

microscopy was utilized [93]. Both the aluminium matrix as well as the intermetallic particle

can be observed in the TEM lamella image, as shown in figure 5.15.

Intermetallic particle

Treated surface

Figure 5-15: Image of a TEM lamella of AA2024-T3 that has been exposed to 2-MBT inhibited NaCl after 60 days

Aluminium matrix

As indicated, the right side of the cross-sectional specimen is the top surface exposed to the

inhibitor solution for 60 days. The top right-hand corner of the specimen shows the presence

of an intermetallic particle, while the bottom right hand region indicates the aluminium matrix.

The observed cracks are due to stress induced in the sample during the thinning process as

well as the analysis. A magnified region of the cross section of the aluminium matrix is shown

in figure 5.16. Here the presence of the platinum (Pt) layer as well as the iridium layer over

the metal surface, as part of the preparation process can be observed. One method to

hypothesis the composition of the layers is by the analysis of the molecular weight. The layer

below the iridium layer is likely to associated with an oxide layer and the relatively lower

molecular weight (MW: 101.96 g/mol) resulting in a lighter layer, relative to the others.

However, the dark layer below the oxide layer, currently unidentified, could potentially be the

inhibitor layer given the relatively high molecular mass (MW: 167.3 g/mol) of the inhibitor itself

being significantly higher than that of the oxide layer, yet lower than Iridium (MW: 192.2 g/mol).

However, as this layer is less than 20 nm in width, EDS scans could not provide accurate

Figure 5-16: Labelled image of the analysed lamella sample

results to provide further clarification.

Figure 5.17 displays the transition point from the intermetallic particle to the aluminium matrix,

indicated by the red line to distinguish between these two areas.

Visibly thicker inhibitor

layer over the IM particle

than the Al matrix

IMP

Figure 5-17:Transition between the aluminium matrix and the intermetallic particle

Al matrix

However, this transition point can also be easily observed via the differences in surface

topographies being observed in the different regions of the sample. Another important aspect

to be noted is that the dark unidentified layer is visibly thicker and more prominent over the

intermetallic particle compared with that over the aluminium matrix.

Further analysis of figure 5.17 revealed that the contrast of the oxide layer changed over the

two regions, becoming brighter over the intermetallic particle region compared with the

aluminium matrix region. The thickness of the dark layer also showed a significant increase in

thickness over the intermetallic particle region.

Further detailed analysis of the layers and interfacial regions was conducted through higher

magnification images of the three regions (Al matrix; transition; IMP regions) as shown in figure

Figure 5-18: (a) TEM image of the intermetallic particle region; (b) TEM image of the transition region; (c) TEM image of the aluminium matrix region

5.18.

Figure 5.18 (a) presents the cross-section of the topography over an intermetallic particle.

Whilst there is a very prominent darker layer over the metal surface, the thickness of the oxide

layer seems to be less than that over the aluminium matrix as seen in figures 5.18 (b) and (c).

The total thickness of the combined layer seems to be around the 10 nm range over the

intermetallic particle. Figure 5.18 (b) shows the transition area between the aluminium matrix

and the intermetallic particle. This image shows the darker unidentified layer appearing to be

relatively lighter, and less distinct from the oxide layer. It appears that this layer has started to

blend with the oxide layer. Overall, the thickness of the double layer seems to have increased

up to about 20 nm over the aluminium matrix. For the layers covering the aluminium matrix,

as shown in figure 5.18 (c), the existence of two distinct layers is no longer evident, showing

the presence of a single film with a thickness of about 20 nm.

6. CHAPTER 6: RESULTS: FILM DEGRADATION STUDIES

The performance of the two selected inhibitors (2-MBT and Na-MPA) for extended time

periods in terms of exposing the AA2024 to a continuous supply of inhibitor to maintain the

film, has been explored in Chapter 5. While the major role of an inhibitor is to maintain its

performance when exposed to corrosive mediums, it is also important to understand the

behaviour of this inhibitor-induced film once developed and the response to the absence of a

continuous supply of inhibitor or an irregular supply of inhibitor. For instance, if a defect is

initiated in a film with a wide crack, a film of moisture could be present connecting the crack

to the corrosive medium. In the presence of the inhibitor, the inhibitors could diffuse into the

defect through the moisture film itself. However, if a moisture droplet in a large defect does

not reach the paint film, the alloy surface will not have any supply of inhibitor. Therefore, it is

important to understand the ability to self-repair and maintain itself in the absence of inhibitor.

In this chapter, the degradation process of the 2-MBT film was assessed by exposing a treated

sample to an uninhibited solution of neutral 0.1 M NaCl solution after being exposed to an

inhibited solution for a pre-determined period of time. The aim was to provide an insight as to

how long it would take for a stable film to be formed over the surface as well as how long it

takes for the formed film to degrade. A range of electrochemical techniques were used at

various time intervals in conjunction with the constant immersion studies to monitor and record

changes associated with the film stability and film breakdown behaviour after exposure to the

uninhibited solution.

6.1 Linear polarization resistance (LPR) studies 6.1.1 Film breakdown studies after a 7-day pre-treatment period

LPR studies were conducted with the parameters and adopted procedure as detailed in

Chapter 3. Figure 6.1 shows change in Rp with time which has been exposed to an uninhibited

0.1 M NaCl solution for 7 days after having been immersed in a 1 mM 2-MBT inhibited 0.1M

Figure 6-1: Polarization resistance after a 7-day immersion period in 2-MBT inhibited system is exposed to an uninhibited solution for 7 days

NaCl solution for 7 days.

LPR results obtained after exposure to to the 2-MBT solution for 7 days, prior to immersion in

the uninhibited solution showed Rp values in the range of 105 Ohms. After exposure to the

uninhibited solution, the Rp value immediately dropped to within the 104 Ohms range,

suggesting immediate breakdown of the film upon immersion. Further analysis shows that

although the Rp can be observed to increase during the first 24 hours of exposure, further

exposure resulted in a reduction in the Rp value and gradually levelling out within the 2-3x104

Ohm range. These results fall within the LPR data range achieved for exposure to an

uninhibited solution. These findings suggest that 7 days of immersion in 2-MBT may not be

sufficient time to form a stable enough inhibitor-induced film over the metal surface to protect

the surface on its own when exposed to NaCl. The large fluctuations observed during the first

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24 hours shows the attempt of the film to continue protecting the surface but is unable to

sustain over long time intervals. However, this could also be due to non-steady conditions due

to high corrosion activities taking place at the alloy surface and thereby an artefact of data

6.1.2 Film breakdown studies after a 30-day pre-treatment period

evaluation.

Based upon these findings and in order to assess the immersion time required to form a stable

film on AA2024-T3 which could withstand prolonged exposure to NaCl, the samples were

Figure 6-2: Polarization resistance after a 30-day immersion period in 2-MBT inhibited system is exposed to an uninhibited solution for 24 hours

subject to inhibited (2-MBT) pre-conditioning for 30 days.

The results obtained for two identical samples after 30 days are shown in figure 6.2. Here the

plots show change in Rp values with exposure time for two identical samples. Analysis of the

curves reveal significant variations in how Rp changes with time for the two samples. Sample

1 shows an immediate reduction in Rp upon exposure to the uninhibited solution (1.2 X 105

Ohm compared with 5 X 105 Ohm) for the sample prior to exposure, as shown by the black

dot. Gradual reduction in Rp can be observed with further exposure achieving a value of 1.2

x105 Ohm after 16 hours. Rp values remained constant for further exposure up to 24 hour. In

contrast, sample 2, showed a slightly higher value than sample 1 upon immediate exposure

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to the uninhibited solution (3.55 X 105 Ohm). The Rp remained within the 3 - 4 X 105 Ohm

range, up to 9 hours, after which a significant reduction in the Rp can be observed, dropping

to 2.0 x 105 Ohm after 14 hours and then levelling out at this value up to 24 hours exposure.

The results indicate that after 30 hours pre-treatment with the inhibitor, the ability of samples

to retain to a protective corrosion inhibition layer after immersion in the uninhibited solution

and the overall susceptibility to breakdown of this protective layer, varies significantly.

For sample 1, although the polarization resistance, Rp seems to drop immediately upon

immersion from 5 X 105 Ohm to 1 x 105 Ohms, the sample maintained this Rp value for about

3 hours, serving as an indication of the existence of a protective layer remaining to some

extent. However, the gradual reduction in the Rp value beyond 3 hours and finally levelling

out at the value of 2 x 104 Ohms after 14 hours, suggests further breakdown of this layer

For sample 2, increased susceptibility to maintaining a protective layer can be observed by

the higher Rp values over the full duration period, particularly during the first 9 hours of

exposure, where the Rp values fluctuates between 3 and 4 x 105 Ohms. However, the rapid

drop in Rp beyond 9 hours, signifies a breakdown of the protective film. Beyond 14 hours

exposure, Rp values fall well within the 104 Ohms range indicating that the sample may not

have any passivating layer over its surface.

As the Rp vs exposure time plots vary significantly between samples, it was concluded that

30 days was not sufficient exposure to ensure the formation of, and maintenance, of a stable,

protective layer. Subsequently, pre-conditioning treatments were carried out for longer periods

6.1.3 Film breakdown studies after a 60-day pre-treatment period

(60 days of immersion).

To evaluate if longer pre-conditioning period would contribute to better film stability, samples

were treated for 60 days and the results from these studies are shown in figure 6.3. Here two

samples were exposed to identical conditions as for the shorter immersion period studies

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Figure 6-3: Polarization resistance after a 60-day immersion period in 2-MBT inhibited system is exposed to an uninhibited solution for 24 hours

Similar to the plot after 30 days of immersion, the black dot indicates the Rp in inhibitor after

being exposed to a 2-MBT induced NaCl solution after 60 days. From figure 6.2, it was

observed that within a 24-hour time period the protective layer was possibly degraded and the

Rp values fell below the 105 Ohms range. From figure 6.3 however, it can be seen that the Rp

values drops immediately upon immersion by a significant portion reaching a Rp value of about

1.5 x 105 Ohms which could imply that the system was still able to retain the protective layer

to a certain extent.

For sample 1, after the initial drop in the resistance, there is an increment over the next two

hours which then remains relatively constant up until the treated sample has been exposed to

the uninhibited solution for about 9 hours at a Rp value of 3 x105 Ohms. A slight increment

can then be observed which is followed by a reduction with a very low gradient over the next

10 hours.

For sample 2, a slightly different trend was observed. Similar to sample 1, there is an increment

in the Rp after the initial drop which then shows some fluctuations. In addition, between

exposure times of 1 hour and 3.5 hours, the Rp show some increment in value which is then

increased at a higher rate over the following 2 hours reaching a Rp value of around 4 x 105

Ohms. The Rp then follows an overall increasing trend which reaches the initial polarization

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resistance obtained before being exposed to the uninhibited system after about 7 hours. After

the 10-hour exposure mark, polarization resistance appears to rise at a fairly low rate, after

which it appears to stabilise between 4 - 4.5 x 105 Ohms.

For both samples, the drastic drop observed initially could be due to the change in chemistry

in the system or a potential breakdown of the passivating film. However, in both cases, the

polarization resistance continued in the 105 Ohms range and as indicated in Chapter 5, this

implies that the sample has managed to retain the protective film over the surface. However,

the two samples have displayed results with a notably high deviation which could be an

indication of the varying self-repairing process with different samples.

In comparison to the sample pre-treated for 7 days and 30 days, these specimens have shown

promising results without any indication of a complete breakdown of the passivating film.

Further analysis was done to determine the performance of these systems over longer time

periods. Therefore, the samples pre-treated for 7 days, 30 days and 60 days were left exposed

6.1.4 Effect of the pre-treatment period on the film stability

to an inhibitor free solution for 168 hours ( 7 days ) to understand the longer term performance.

The above experiment conducted over 24 hours was extended to 7 days to understand the

film break down over longer time periods. The graph below (figure 6.4) shows the effect of

immersion time (7 to 60 days) in inhibitor-induced NaCl solution on the film stability in the

absence of inhibitor over a 7-day period.

104

Figure 6-4: Effect of immersion time period in an inhibitor induced NaCl solution on film degradation

Here, the sample after 7 days pre-conditioning continues to remain constant in the low 104

Ohm range. For sample pre-conditioned for 30 days, although the Rp was initially high within

the 105 Ohm range, a marked reduction over 24 hours was observed, attaining values similar

to the 7 day pre-conditioning sample, remaining constant at this value thereafter. This

suggests that any protective layer on the surface was broken down within 24 hours (1 day)

exposure.

However, for samples exposed to 60 days pre-conditioning, After an initial rise in Rp over the

first 12 hours, a gradual reduction could be observed, levelling out after 72 hours exposure.

However, it should be highlighted that despite the decrease in Rp, the values still remain well

above the 1 x 105 Ohms. After 72 hours, the system displayed an increase in resistance, over

the next 24 hours. Thereafter the polarization resistance remained steady for 24 hours, after

which a slight decrease was observed, which then started to stabilize and remain steady at

around a Rp value of 1.84 x 105 Ohm.

From figure 6.4, it is apparent that the sample which was treated for 30 days had managed to

maintain a protective film over the first 12 hours which then started to show signs of breaking

105

down, whereas for the sample treated for 60 days, the formation of a more stable film was

evident, retaining its protective layer with evidence self-repair of the layers over extended time

periods, as observed by fluctuations in Rp during the 72 to 168 hours exposure period .

6.2 Electrochemical impedance spectroscopy (EIS) Studies

Electrochemical impedance spectroscopy was another technique use to understand the

mechanism behind the film-degradation process. EIS studies were conducted on an AA2024-

T3 sample after 60 days of immersion in a 2-MBT induced 0.1 M NaCl solution. The first EIS

scan was conducted after 60 days immersion in the inhibitor (0 hours in the uninhibited

solution), as a reference point. Upon exposure to a 0.1 M NaCl only (uninhibited) solution, EIS

scans were conducted after 1 hour, 24 hours (1 day), 72 hours (3 days) and 168 hours (7

days). These results are presented as Nyquist plots and Bode plots, as shown in figures 6.5

Figure 6-5: Nyquist plot after being exposed to an uninhibited system at different time intervals (60-day treatment period in 2-MBT)

and 6.6 respectively.

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Analysis of the Nyquist plots (figure 6.5) reveal after 1 hour of exposure to the NaCl only

solution, the capacitance has reduced from that observed before exposure. In contrast, the

Nyquist plots after 24 hours, 72 hours and 168 hours exposure show increasing capacitance

at lower frequencies. Further analysis of the Nyquist plots shows, at a frequency of 10 mHz,

the polarization resistance, Rp, prior to exposure was around 325 kOhm, which then drops to

275 kOhm after 1 hour exposure to the uninhibited system. The Rp was then observed to

increase over the next 24 hours, which continued to increase further up to 350 kOhms after

72 hours. After 168 hours exposure, the polarization resistance at a frequency of 10 mHz

doesn’t show any significant difference other than an increase in the capacitance in

Figure 6-6: Bode plot after being exposed to an uninhibited system at different time intervals (60-day treatment period in 2-MBT)

Figure 6-7: Suggested equivalent circuit for the film degradation process

comparison to that after 72 hours.

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Table 6-1: The fitted parameters from the equivalent circuit (figure 6.7) for the film degradation process

1 hour 24 hours 72 hours 168 hours Before exposure (1 day) (3 days) (7 days)

74.86 Ohm 89.9 Ohm 87.77 Ohm 88.37 Ohm 82.28 Ohm

R1

16 671 Ohm 18 238 Ohm 10 536 Ohm 16 580 Ohm 15 682 Ohm

R2

25.31E-06 F 24.86E-06 F 22.48E-06 F 15.35E-06 F 28.97E-06 F

C2

254 230 Ohm 206 678 Ohm 221 035 Ohm 323 408 Ohm 360 486 Ohm

R3

15.06E-06 F 15.7E-06 F 14.19E-06 F 15.35E-06 F 14.79E-06 F

C3

53 144 Ohm.s-1/2 40 913 Ohm.s-1/2 97 018 Ohm.s-1/2 102 908 Ohm.s-1/2 137 527 Ohm.s-1/2

W

The Bode plots in figure 6.6 present the phase (Z) data and log (|Z|) over a frequency range

between 100 kHz and 10 mHz. The phase signal in the Bode plot in figure 6.6 for all time

intervals show two distinct time constants and based on this a simple equivalent circuit has

been utilized (figure 6.7) to fit the plots at different time intervals. A simple equivalent circuit

has been fitted to fit the plots at different time intervals where R1 is the electrolyte resistance,

R2 and R3, the polarization resistance and C2 and C3, the capacitance of the potential layers.

A Warburg component, W3 was incorporated into a R-C cycle to accommodate for the

increased porosity and the diffusion-limited process. This equivalent circuit was chosen as this

was the best fit model to accommodate two time constants with a deviation below 2 %. To

avoid overfitting the data, an equivalent circuit with two R-C cycles was chosen to

accommodate for the inhibitor/oxide layer and the interfacial activities. Only limited work has

been conducted on the film degradation which limits the use of references from previous work.

At high frequencies, the impedance hasn’t varied over the different exposure times. However,

after 168 hours of exposure, the impedance during this frequency range was reduced although

the same gradient was observed as for shorter exposure times. At lower frequencies between

100 mHz and 10 mHz, the magnitude of the impedance has dropped by a small yet significant

amount after 1hour exposure compared to the unexposed sample. Further increment in the

impedance was apparent after 24 hours and 72 hours. However, after 168 hours, a slight

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decrement could be observed. With respect to the phase diagram, up until a frequency of 10

Hz, the phase remained unchanged after exposure to the uninhibited system although the plot

before exposure shows a slight but noticeably lower phase angle. The phase angle diagram

obtained in the presence and absence of inhibitor, includes a time constant with fairly low

phase angle variations. With inhibited solutions, between the frequency range from 10 Hz to

100 mHz, two very subtle peaks are observed Similarly, after 1 hour exposure, the phase

angle diagram shows two subtle peaks but with a relatively lower phase angle than that of the

inhibited system with reducing frequency. Furthermore, after 24 hours, 72 hours and 168

hours, the phase angle diagrams demonstrate two more prominent time constants, one at ~

10 Hz and the other peaking around ~ 100 mHz. It is quite evident that longer testing periods

have a very negligible effect on the time constant correspondent to R2 and C2 which are

observed at high frequencies.

By analysing the fitted data obtained, it could be seen that the dominant resistance is R3 which

represents the porosity of the system. However, it can be seen that R3 undergoes an initial

reduction after 1 hour and 24 hours which then increases up until 7 days. Even though the R3

shows a significant difference, the double layer capacitance, C3 has only shown a slight

variation Moreover, whilst it can be seen that R2 and C2 relatively remains stable throughout

the 7 day period, the Warburg component has shown a continuous increment in value.

6.3 Potentiodynamic scanning (PDS) Potentiodynamic scans were conducted on samples having been previously pre-conditioned

in a 2-MBT inhibitor environment for 60 days and then exposed to the uninhibited environment

for 15 min and 168 hours (7 days). A 15 minute period was selected as a minimum time to

observe any effect the uninhibited solution may induce to the protective layer formed

immediately after removal from the inhibited system. The results are shown in figure 6.7.

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Analysis of figure 6.8 reveals that potentiodynamic scans show that, during cathodic

polarization, the cathodic slope or the rate of change in current with voltage within a 300 mV

range from OCP is slightly higher for the 168 hours (7 days) exposure compared with the 15

Figure 6-8: Potentiodynamic scanning after being exposed to an uninhibited system at different time intervals (60-day treatment period in 2-MBT)

Table 6-2: icorr and Ecorr values obtained from tafel extrapolation (60-day treatment period in 2-MBT)

min exposure.

After 168 hours After 15 min in NaCl (7 days) in NaCl

0.352 0.114 icorr / µAcm-2

-691 -653 Ecorr /mV

During anodic polarization, both curves show significant noise starting at around a potential of

-0.45 V, indicating possible metastable periods due to repeated passivation breakdown and

re-passivation. The anodic slopes or the rate of change in current up to this point are almost

identical for both cases, however beyond this potential of -0.45 V, the 168 hours (7-day)

exposure curve displayed a significant increase in the anodic slope, resulting in increased rate

110

of change in current. The gradient of the slope or the rate of change in current during

polarization taken about ± 200 mV from OCP.

In addition, it should be noted that extended exposure to a free-inhibitor system has caused a

positive shift in the Ecorr value in comparison to that obtained at 15 min of exposure by about

38 mV.

Tafel extrapolation analysis was carried out and the Ecorr and icorr values determined from

both scenarios are presented in table 6.2. Here, an icorr of 0.35 µAcm-2 and an Ecorr of -690.6

mV were recorded after being exposed to NaCl solution for 15 minutes and an icorr of 0.11

µAcm-2 and Ecorr of -652.8 mV after being exposed to NaCl solution for 168 hours (7 days). In

2 and the icorr of AA2024-T3 in a 2-MBT induced system after 30 min of exposure was recorded

Chapter 4, the icorr of AA2024-T3 in an inhibitor-free system was evaluated to be 4.58 µAcm-

to be 0.372 µAcm-2. When compared with these data, it is apparent that by the performance

displayed by these pre-treated samples in an inhibitor-free solution, these samples have

managed to retain the protective layer or undergo a self-repairing process which has allowed

it to possess a high corrosion resistivity even in the absence of any inhibitor.

The Tafel extrapolation results, showing reduction in icorr with exposure time, are consistent

with those obtained from LPR and EIS analysis, whereby an increase in polarisation

resistance, and hence a reduction in the corrosion current value, was observed after 168 hours

(7 days) exposure compared to immediate exposure.

In general, for both cases, excellent corrosion resistance, as indicated by the relatively low

icorr values recorded, is evident.

Figure 6-9 shows a photograph of the pre-treated sample before and after exposure to an

inhibitor-free solution for 168 hours (7 days) taken to demonstrate that the sample had

managed to protect itself even in the absence of the inhibitor.

111

Before exposure to NaCl only solution

After exposure to NaCl only solution for 7 days

Figure 6-9: Image of the sample which was treated for 60 days and then was exposed to a NaCl only solution for 7 days

The extent of corrosion that takes place even after 24 hours can be observed in the OM images

shown in figure 5.7. Therefore, when comparing the area which was exposed to NaCl solution

for 168 hours (7 days) and the area which wasn’t exposed, there is no significant difference to

the naked eye, implying the ability to retain a protective passive layer even after 7 days

exposure.

112

7. CHAPTER 7: DISCUSSION

7.1 Effectiveness of inhibitor-induced film formation and associated mechanisms 7.1.1 Preliminary evaluation of performance of inhibitors

The corrosion inhibitors 2-mercaptobenzothiazole and 3-mercaptopropionic acid were chosen

for this study as both these inhibitors have been previously studied and have shown

particularly good performance with the metal alloy AA2024-T3. Further, these two inhibitors

have distinctive and quite different structures; 2-MBT has a heterocyclic ring with a thiol group

attached to it whereas for 3-MPA, it has a linear structure with a thiol group. Therefore, a main

objective of this study was to systematically study variations in the mechanism of film-

formation as a result of the quite different structures.

Preliminary evaluation of the inhibitor systems in terms of optimising the conditions using a

range of electrochemical methodologies was conducted in Chapter 4. As outlined in chapter

2, previous studies confirm the excellent corrosion inhibiting properties provided by 2-

mercpatobenzothiazole for aluminium alloy, AA2024-T3 and these suggestions were

confirmed via the preliminary experiments conducted in this study [7, 9, 48, 49, 74, 94].

Potentiodynamic scans were conducted at different concentrations and pH values at room

temperature to optimise conditions for further performance assessment. The rationale for

conducting all studies at room temperature was based upon the premise that aluminium alloy,

AA2024-T3, widely used in aircraft fuselages and other components, is susceptible to

corrosion during take-off, landing and parking (room temperature) and to a much less extent

during flight.

From figure 4.2 it can be seen that during cathodic polarization, the current density recorded

is relatively stable for both uninhibited and inhibited systems. This could be a result of the

113

presence of an oxide layer in both cases and in addition, an inhibitor induced film on the

inhibited system which is known to form immediately upon exposure and this is in agreement

with the findings of Zheludkevich [9], Ryan [48], Monticelli [95] and Balaskas et al. [7] . The

2-MBT system displayed a reduced current density of 0.37 µA/cm-2 which is an order of

magnitude less than that of the uninhibited system 4.58 µA/cm-2, after only 30 minutes of

exposure. Analysis of the cathodic regions reveals suppression of the cathodic current density

in the presence of an inhibitor, is likely to have occurred as a result of the diffusion-limited

cathodic reaction. Due to the presence of a passivating layer, kinetics of the oxygen-reduction

reaction is affected as the oxygen diffusion towards the metal surface is minimized. Plateauing

of the anodic regions, particularly for the mid-range and higher inhibitor concentrations,

indicates that re-passivation processes may have occurred. At the initiation of the anodic

polarization the sample appears to be already passive which then undergoes active dissolution

during the measurement which then passivates again. However, this process of re-passivation

is then followed by the breakdown of this passivating layer around Ecorr value of -0.5 V.As

this wasn’t prominent at lower 2-MBT inhibitor concentrations, this could imply that the re-

passivation is triggered by the presence of sufficient inhibitor in the environment.

The 3-MPA (figure 4-3) system showed similar results to the 2-MBT system. The cathodic

region of the lower inhibitor concentration system demonstrated a current density which was

relatively stable, implying a diffusion-limited process. However, the observed increase in

cathodic current density with increasing inhibitor concentration could be due to the increased

acidic nature of the solution due to its low pH of 3.5. This could be accelerating corrosion and

thus at these pH’s this inhibitor could be considered to be a corrosion-accelerator at the high

concentrations. In contrast to the 2-MBT system, 3-MPA shows immediate film breakdown

with no re-passivating feature during the anodic polarization indicating reduced corrosion

protection. For both inhibitor systems, the results obtained via Tafel fitting and linear

polarization resistance trends represent a convolution of both inhibitor film and the oxide film.

Therefore, the term “film” will refer to any combination of inhibitor and oxide layers. However,

114

due to the lack of previous studies conducted on this inhibitor, limited background information

is available to understand the results observed.

It should also be noted that by initiating the cathodic sweep from -1.2 V to OCP, this could be

affecting the anodic sweep significantly. If there is heightened oxygen reduction taking place

at the metal surface, the surrounding solution may become highly alkaline. Such pH change

could affect the oxide surface charge as per the Pourbaix diagram [8]. Therefore, high

alkalinity could promote corrosion to a certain extent, and this should be taken into

consideration. In the case of the results obtained for both inhibitors, 2-MBT and Na-MPA, this

may have caused the plateauing of the cathodic current and also shift the curves more

negatively in comparison to the uninhibited system.

The relatively poor performance of the 3-MPA system, particularly at the higher concentration,

as observed from the Tafel extrapolation data, was attributed to the acidic nature of the

environment (low pH of 3.5). Consequently, improvements were observed through

modifications to the system allowing for consistency in the data. Here, the pH of 1 mM 3-MPA

solution was buffered to obtain a pH of 7 by using NaOH, leading to “in-situ” synthesis of Na-

MPA. This compound showed a current density one order of magnitude lower than that of 3-

MPA at the same concentration. These results confirm that the acidic nature of 3-MPA could

be influencing the low corrosion resistivity.

Buffering of the original pH of a 1 mM 2-MBT solution from 4.5 to 7.0, revealed no significant

difference in icorr values but a slight increment, implying that the change in pH doesn’t pose

much effect on the inhibitor film formation process or protection of the metal surface. [68]. It

could be interpreted through the Pourbaix diagram (figure 2.3) that at pH levels between 5 and

8.5, there is a more stable thicker oxide layer over the aluminium surface [8, 17]. Therefore,

the acidic nature of a solution contributes to the breakdown of the passivating oxide layer over

the alloy surface. However, in the case of 2-MBT, as the alloy surface would be positively

charged at a pH of 4.5, film-formation may be favoured with the presence of negatively

charged or deprotonated 2-MBT anions and the presence of an inhibitor film could also be

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minimizing the dissolution of the oxide layer by blocking any pores [68]. At neutral pH, the

passive layer is less likely to undergo dissolution. In addition, an alumina surface would have

a positive charge as the isoelectric point of an aluminium surface with an oxide layer is at 9.5

which is higher than the neutral pH of 7 which again promotes film-formation [68]. Similarly, in

the case of 3-MPA, rather than the protonated molecule, the deprotonated molecule which is

Na-MPA is more likely to interact with the alloy due to the positive charge on the surface. This

could be the reason behind Na-MPA at a pH 7 performing better than 3-MPA at a pH of 3.5

whilst 2-MBT at a pH of 4.5 and 7 showing no significant difference. However, there is a lack

of previous studies conducted with Na-MPA or 3-MPA as a corrosion inhibitor for the substrate

AA2024-T3 to gather any substantial supporting information from the literature.

Surface analysis techniques were utilized in conjunction with the electrochemical techniques

to support the data obtained in each case. EDS analysis of the 2MBT inhibited alloy surface

(figure 4-13) confirmed the presence of particles linked to two Cu-rich S-phases and a

relatively Cu-poor AlCuFeMn phase. Sulphur and carbon were significantly more prominent

over the S-phase particles, indicating the presence of inhibitor over these intermetallic

particles. Previous studies have implied that the thiol group in the inhibitors is the cause of

corrosion inhibition due to the high affinity of Cu towards sulphur, thus providing support to the

presence of the inhibitor over Cu rich IMP’s. study here.

Upon exposure in Na-MPA for 168 hours, (figure 5.13), EDS results showed that the periphery

area of the pits formed is below the limit of detection for oxide/inhibitor layer whilst the far outer

region has displayed a layer with the presence of oxide as well as inhibitor elements. This

could be due to a range of possibilities with one being the pH of the environment around the

corroded intermetallic particles could be preventing coating formation or the coating which was

formed may be unstable which as a result then spalled away. In addition, the re-precipitation

of Cu over the surface could be triggering the formation of the mixed inhibitor/oxide layer in

the outer area. However, it should also be noted that the coating may have “flaked” away

during preparation and observation in the SEM.

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The presence of visible trenching around the S-phase particles which is absent in the other

intermetallic particles can be explained through the mechanisms as proposed in figure 2.6 [4].

S-phase particles undergo trenching before the AlCuFeMn intermetallic particles due to Mg

dissolution leaving a Cu-sponge and therefore trenching is more prone to take place over S-

phase particles [4, 19]. In the present study, the oxygen elemental map reveals the outline of

the IM particles shows a denser presence of oxygen which is a result of trenching as corrosion

products are formed during the trenching process. The simultaneous presence of sulphur over

the outline of the particles suggest the formation of inhibitor-induced corrosion products or a

film formed with an inhibitor/oxide complex.

Further analysis which revealed both the presence of, and absence of, trenching around S

particles (figure 4.14) provides important information regarding the mechanisms, of trenching,

particularly inhibitor film formation. This was discussed by Catubig et al. [46] in his study on

the necessity of a limited amount of de-alloying of S-phase intermetallic particles required to

initiate the film-forming process. The presence of inhibitor, which was more prominent over

the particle that had undergone some trenching in comparison to the un-trenched samples, is

an excellent indication of how initial trenching could induce and promote inhibitor film

formation. Similar results obtained for the Na-MPA system with inhibitor elements, C and S,

observed to be present over several S-phase intermetallic particles which had undergone

trenching to a certain extent, suggest similar mechanisms may be present. In the case of 2-

MBT, under acidic conditions, the naturally occurring oxide layer undergoes dissolution

releasing Cu which then interacts with the inhibitor molecules to form a Cu-MBT complex and

thereby initial trenching can aid the film formation process. In the case of Na-MPA, although

some initial trenching helps, the competitive behaviour of the deprotonated

mercaptopropionate inhibitor molecules circumvent the Cl- and OH- reaching the alloy surface

and instead bonds to the positively charged alloy matrix forming a film.

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7.1.2 Effect of presence of intermetallic particles on inhibitor-induced

film-formation

As per previous literature, pure aluminium is known to have excellent corrosion resistance

properties. By analysing the inhibitors with pure aluminium demonstrates the presence of

oxide layer in conjunction with a possible inhibitor/oxide layer but by comparing these results

with that achieved with the alloy, AA2024-T3 demonstrates the effect of the presence of

intermetallic particles. As the main alloying element of AA2024-T3 is Cu, the performance of

inhibitors on Cu could provide a good insight to seeing if the results obtained could be

consistent with the cause of the difference observed between pure aluminium and AA2024-

T3 results.

Potentiodynamic scans conducted on pure aluminium revealed icorr values obtained (figure

4-16) for the inhibited system were 3 orders of magnitude lower than uninhibited AA2024-T3

and one order of magnitude lower than inhibited AA2024-T3, is a clear indication of the

presence of homogeneous oxide layer on pure aluminium, providing a higher level of

protection compared to the AA2024. Introduction of inhibitors to the system showed no

significant improvements in corrosion resistance, yielding similar icorr values all falling within

the same range of 10-2 µA/cm2. Slight improvement in icorr was observed for the Na-MPA

system, while the 2-MBT yielded slightly higher icorr value. These findings are valuable in

terms of the additional protection that the presence of an inhibitor and the resultant film can

have on the alloy system containing intermetallic particles compared with the pure Al system.

Both inhibitors resulted in a cathodic shift which indicates that the cathodic behaviour of the

inhibitors is more dominant. The lower rate of change in current during anodic polarization of

the aluminium system, particularly for the Na-MPA system, indicates the breakdown of the

inhibitor/oxide layer occurs at a lower rate and the fluctuations could be a cause of repeated

passivation breakdown and re-passivation. This is a representation of a metastable behaviour

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and this occurs due to the presence of inhibitor which is continuously attempting to maintain

a passivating layer.

The polarization resistance monitored over a 3-day period (figure 4-17) shows that upon

immersion of pure Al for the uninhibited system, there is evidence of the passivation layer

breakdown which then re-passivates over time and continues to rise, as shown by the drop in

polarisation resistance and then subsequent rise. The continued rise in polarization resistance

implies that the oxide layer continues to undergo a self-repairing process. The Na-MPA

system, which starts at a high Rp value and shows further increase after a 24 hour period,

implies continued protection by an oxide/inhibitor film over the metal surface. On the contrary,

although the Rp values for 2-MBT lies within the 105 Ohm range which signifies continued

protection of the surface, these values are one order of magnitude lower than that recorded

for the uninhibited and Na-MPA system. Therefore, the uninhibited system and the Na-MPA

may have a thicker passivating layer whilst the presence of 2-MBT could be hindering the

formation of the oxide layer over the surface. As the 2-MBT inhibitor is most commonly known

to work well due to the high affinity of Cu-rich intermetallic particles towards the sulphur and

given that pure aluminium is absent of Cu intermetallic particles, the inhibitor maybe unable to

form a film over the surface. Instead, the acidic nature of the 2-MBT solution at a pH of 4.5,

could essentially be dissolving the passivating layer and thereby resulting in a relatively

reduced polarization resistance. These results were compared with the Tafel extrapolation

results obtained with pure Al and they were found to be quite comparable and consistent with

the trends.

In contrast, in figure 4-18, the similar icorr values obtained for uninhibited system and Na-MPA

system for pure copper, suggests that Na-MPA inhibitor doesn’t have any significant impact

on achieving better corrosion resistivity. However, the presence of 2-MBT, resulted in a greatly

reduced icorr and also the formation of a re-passivating region. Tafel extrapolation analysis

confirms that the improved performance of 2-MBT with pure Cu and the poor performance of

Na-MPA. The inhibitor 2-MBT has been previously studied with pure Cu and excellent

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inhibition was exhibited due to high affinity of Cu towards sulphur [61]. It should also be

highlighted that although small, a slightly higher icorr was recorded for Na-MPA which could

mean that this inhibitor could be aiding corrosion. These results were simply confirmed via the

Rp studies over 3-days (figure 4-19) where the 2-MBT system showed continued growth in

Rp values whereas Na-MPA Rp values remained within the same region as the uninhibited

system. After 30 hours of exposure, the Na-MPA system exhibited lower Rp values than the

inhibitor-free system which is consistent with the PDS results. These findings have shown that

2-MBT works well with pure copper, Na-MPA works better over pure aluminium. This is

extremely important in understanding which elements of AA2024-T3, whether in the matrix or

as an alloying addition present in the IMP’s, triggers the film formation. For the 2-MBT system,

the Cu-rich intermetallic particles could be aiding the film formation process whereas with Na-

MPA, the film formation may be occurring more efficiently over the aluminium matrix through

the carboxylate groups that are present. The elemental maps from the EDS results for Na-

MPA, which show more pronounced presence of sulphur and carbon over the intermetallic

particles in conjunction with oxygen, suggest that the mechanism of interaction of the inhibitor

with the various regions of the alloy surface are more complex, and hence the presence of a

complex oxide/inhibitor film.

While these findings provide opportunities to explore synergistic effects of these two inhibitors

with AA2024-T3, such a study is beyond the scope of this program and is a promising venture

that should be explored as part of future investigations.

7.2 Inhibitor-induced film stability and growth 7.2.1 Electrochemical analysis of inhibitor-induced film formation and its stability Linear polarization resistance conducted over 168 hours (7 days) (figure 5-1) in 2-MBT

inhibited solution revealed that the monitored polarization resistance falls within the 105 Ohm

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range, which indicates the presence of a protective layer over the AA2024-T3 alloy surface.

Polarization resistance can provide an indication of the thickness of the film that has developed

over the metal surface as well as the resistance developed to corrosion. A system displaying

a higher polarization resistance is an indication of lower corrosion rates due to the presence

of a thicker and/or denser film, provided that any porosity remains constant.

Large fluctuations in the polarisation resistance can be observed during the first 2 days for the

inhibitor-free system, which then stabilized thereafter. These fluctuations could be due to the

creation of a non-steady state system due to the heterogeneous nature of the alloy surface,

formation of an unstable oxide layer and/or constant change in chemistry during localized

corrosion.

For the 2-MBT inhibited system, as observed for the uninhibited system, the fluctuations during

the initial 24 hours could be associated with the presence of a non-steady system due to the

heterogeneous nature of the alloy surface, changing chemistry at the metal surface due to film

formation and any initial trenching that has taken place. The irregular fluctuating behaviour

observed during the first 72 hours could be attributed to breakdown of passivating layers that

may have initially formed, initiation of trenching and subsequent re-passivation due to the

formation of an oxide-inhibitor layer over the metal surface. Stabilization of the film thereafter,

is an indication of a stable film existing over the metal surface. It should be emphasized that

the polarization resistance obtained from the inhibited systems are one order of magnitude

higher than that of the inhibitor-free system. If the uninhibited system is utilized as a reference

point, for any system where Rp falls within the 104 Ohm range implies that a protective layer

is non-existent over the substrate and that the alloy surface is no longer protected. Given that

polarization resistance serves as an indication of the thickness and/or stability of a protective

layer, higher polarization resistances observed for the inhibited systems suggests the

existence of a more protective mixed oxide/inhibitor film compared to oxide layer formed under

uninhibited conditions.

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After 72 hours, the 2-MBT system shows relatively more stable Rp values, suggesting the

formation of a thicker and/or stable protective layer. The range of polarization resistance

values obtained in this study using LPR, for both uninhibited and inhibited systems here were

consistent with those obtained by Zheludkevich et al. [9] via EIS. However, the polarization

resistance, Rp obtained via LPR is essentially an overall resistance which accounts for the

electrolyte resistance, inhibitor/oxide layer resistance and the resistance of the oxide-covered

alloy surface whilst EIS analyses these resistances individually. This is discussed in further

detail later.

For the Na-MPA inhibited system, the graph shows significant fluctuations over the complete

time period it was monitored, which could be due to the non-steady state behaviour of the

system as mentioned previously. It is quite likely that these fluctuations and spikes are

associated with charge-transfer and diffusion-control processes that occurs in the inhibited

systems [96]. Despite these fluctuations, the overall trends indicated by the data show a

gradual lowering in the resistance values followed by plateauing to a constant level after 72

hours, while still maintaining values within the 105 Ohm range. This suggests the formation of

a protective oxide/inhibitor layer present over the AA2024-T3 surface.

The linear polarization resistance data was followed up by a potentiodynamic scan after 30

min and 168 hours exposure (figure 5-2), to determine the current density immediately upon

immersion and the effects imposed by longer exposure periods. When comparing the Tafel

extrapolation data, from potentiodynamic scans conducted after 168 hours, compared with

those conducted at 30 min (refer Chapter 4), it is quite evident that the cathodic current has

dropped by more than one order of magnitude suggesting that with longer immersion times,

the effect of the inhibitors is stronger. Further, for the 2-MBT system, not only has the icorr

reduced by an order of magnitude after 168 hours exposure compared to that after 0 hours,

but two point of passivation breakdown and re-passivation can observed. The primary

passivation potential observed at around -0.6 V and the start of the primary passive region, is

then followed by a breakdown potential at around -0.5V, leading to a transpassive region being

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observed. A secondary passive region can be further observed soon after where re-

passivation takes place. These findings suggest, after a longer exposure period, the 2-MBT

system is capable of successful re-passivation, even after initially breaking down.

In contrast, the cathodic currents for the Na-MPA system have increased after 168 hours

exposure. Combined with the higher cathodic slope, this suggests a higher rate of change in

current during cathodic polarization. During anodic polarization, the absence of any

passivation and the high rate of change in current values, imply the immediate breakdown of

any passivating layer over the metal surface and the inability to re-passivate thereafter.

In terms of the shifts in the Ecorr, the significant cathodic shift for the 2-MBT system compared

with the significant anodic shift for the Na-MPa system after 168 hours exposure suggest that,

over longer periods of time, the cathodic behaviour of the 2-MBT system appears to be more

prominent whereas with Na-MPA the anodic behaviour becomes more dominant. However,

both inhibited systems have resulted in lowered current densities after 168 hours which could

be the result of an already existing more stable and thicker oxide/inhibitor layer protecting the

metal surface.

Electrochemical impedance spectroscopy (EIS) results for the 2-MBT system (figure 5-3)

presents several key aspects. The phase signals for the 2-MBT system at all time intervals

have displayed the presence of two-time constants. The time constant at high frequency is

associated with the inhibitor/oxide complex layer and represented with Rinh/ox (R2) and Cinh/ox

(C2), the one at low-frequency correspondent to the interfacial corrosion activities and

represented by the charge transfer resistance, RCT (R3) and double layer capacitance, Cdl

(C3).The EIS results were fitted to chosen equivalent circuit (figure 5-4) as the two time

constants are observed in the phase angle plots which is consistent with the layers observed

via the FIB/TEM captures of the cross-sectional area (figure 5-17).

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Figure 7-1: Schematic diagram of the equivalent circuit for the inhibitor system As there is no significant variation in the Rinh/ox in comparison to RCT, it appears that the

inhibitor/oxide structure has only contributed slightly to the polarization resistance of the

system. However, a significant variation in RCT could be observed at different time intervals.

The rise in RCT after 24 hours and the drop after 72 hours which then shows an increment

again could be an indication of an unstable system, where the porosity of the double layer is

varying based on the extent to which the inhibitors are blocking the pores. The pores in the

film allows active species in the corrosive medium to reach the alloy surface. Therefore, the

charge transfer resistance is essentially a measure of free active surface where less porosity

indicates less charge transfer and thereby higher resistance (RCT). It is evident that over longer

exposure periods, the charge transfer resistance is the dominating factor as this is being

controlled by the porosity of the layers. Subsequently, inhibitors blocking these pores

increases the resistance by hindering the migration of active species to the alloy surface and

thereby minimizing the corrosion activity at the surface to ensure continued protection. Despite

the fluctuating Rp values, the total polarization resistance calculated through EIS also falls

within the lower 105 Ohm range which are consistent with that observed through the LPR

results. Both the capacitance associated with the inhibitor/oxide complex layer and the double

layer capacitance appears to remain relatively stable with minute variation which could be an

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implication that the film thickness remains quite similar over the course of 168 hours, whilst a

more stable compact layer is formed. It should also be noted that the noise observed after 1

hour of exposure is absent over longer periods of exposure. This may be because, at lower

frequencies, the time taken in between data point readings is significantly high and if the

system is not at a steady state the OCP tends to keep shifting. Eventually, the change in OCP

in between data points in EIS leads to irregular plots or noise. This has been circumvented

over longer exposure times as the measurements are made on a more stable system with a

potentially more stable film over the alloy surface.

The Na-MPA system displayed quite different results (figure 5-5), with the EIS data providing

two time constants. Similar to the 2-MBT system, the resistance, R1 is associated with the

electrolyte resistance where as R2 and R3 are for the inhibitor/oxide complex layer and R3 for

the charge transfer resistance. The parameters, C2 is the capacitance of the inhibitor/oxide

layer present over the metal surface and C3 is a representation of the double layer

capacitance. The equivalent circuit, as shown in in figure 5.5, was chose in order to

accommodate the inhibitor/oxide layer similar to 2-MBT, taking into account the phase angle

plots displaying two time constants. From the evaluated data, it can easily be seen that the

Rinh/ox and Cinh/ox show insignificant variation over the 168-hour period. Therefore, the

inhibitor/oxide layer which is initially formed, remains relatively stable over time.

However, the charge transfer resistance changes, showing a large drop in the resistance

between 24 hours and 72 hours of exposure. Along with the resistance, the capacitance of the

double layer has also increased by an order of magnitude. The increased capacitance may

be a reflection of thinning of the double layer and the lowered resistance could imply a

heightened porosity of the layers. With the increasing porosity and the thinning of the film, the

larger area of the alloy surface is likely to be exposed to the corrosive medium, increasing the

rate of corrosion. However, the total polarization resistance, which is a combination of R1, R2

and R3 still remains in the 105 Ohm range even after 168 hours which is consistent with the

results obtained via the LPR technique (figure 5-1) and exhibits continued protection.

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However, the presence of insufficient concentration of the inhibitor could be a cause of the

reduced pore blockage and film thickness. Similar to the 2-MBT system, the charge transfer

resistance appears to be the dominating factor along with the double layer capacitance for

long exposure periods.

The above discussions have been confined to film stability for the various systems for periods

up to 168 hours (7 days) exposure. For longer immersion periods, it was found that the 2-MBT

system over the 168 – 336 hour exposure period, showed a steady rise in Rp whilst

maintaining its Rp values within the 105 Ohm range, suggesting the formation of a protective

inhibitor film / oxide layer (figure 5-6). One contrast, over longer immersion periods of up to

336 hours, the Na-MPA system started to degrade as evidenced by LPR monitoring over this

period, as shown in figure 5.6. Polarisation resistance for Na-MPA system showed a steady

decline, entering the lower end of the 104 Ohm range after 168 hours immersion, suggesting

there may not be any additional protection offered by the film formed, when compared with

the uninhibited system. As seen from the potentiodynamic scans, the anodic behaviour of Na-

MPA was more pronounced, suggesting it is essential to have a high enough concentration to

ensure the full coverage of all the anodic sites to provide sufficient inhibition. According to

previous literature, insufficient concentration could even accelerate corrosion, and this could

be the case with Na-MPA system where over longer time periods, there is a lack of an

appropriate amount of inhibitor for effective performance [97].

The schematic diagram in figure 7.1 demonstrates the potential physical basis for the EIS

results based on the equivalent circuit correspondent to the 2-mercaptobenzothiazole system

as per figure 5.4. The “film” over the metals surface includes a combination of porous inhibitor/

7.2.2 Surface analysis of film growth and stability

oxide layer. The presented diagram is in consistency with previous studies carried out [7, 70]

Surface topography analysis of the sample exposed to the inhibitor-free system (figure 5-7)

showing severe localised (pitting) corrosion after 24 hour exposure, and even after 1 hour

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exposure, was used as a baseline reference for comparison of the surface topography for

alloys exposed to inhibited environments. High resolution SEM images presented in figures

5.8 and 5.10, confirm the electrochemical results and inhibition of corrosion in the presence of

2-MBT and Na-MPA, with no significant corrosion or pitting being observed. The presence of

inhibitor strands populating the metal surface after 168 hours exposure is more likely due to

dried up remnants (dehydration) of inhibitor deposited over the surface and /or due to the high

vacuum conditions of the scanning electron microscope. While these strands are not thought

to be associated directly with mechanisms of film formation itself the EDS results of the

composition of these strands showing the presence of oxygen, and the absence of Al or Cl

does suggest that these strands could be an oxide-inhibitor complex which has been formed

during the process.

For the Na-MPA system (figure 5-12), the presence of pits surrounded by corrosion products,

observed on the alloy surface after 7 days of exposure, suggests that although there is a

protective layer over the surface, evidence of localised corrosion can be attributed to film

spallation or incomplete film coverage. The formation of corrosion product rings away from

the pits suggests that the peripheral area of the pits themselves is a cathodic region which is

protected. EDS results shown in figure 5-13 have confirmed this, the immediate surrounding

area of the pit not showing any presence of oxygen, sulphur or carbon. In contrast, the cracked

film surrounding the peripheral does contain these elements. Indicating that this could be an

oxide-inhibitor combined layer.

Due to the 2-MBT exhibiting continued protection over a 336 hour (14 day) period, immersion

studies were extended to 1,440 hours (60 days) to ascertain the development of a more stable

film over the metal surface. Similar to that observed after 168 hours (7 days), inhibitor strands

were observed to cover the sample surface, as for 60 days of exposure, although these

strands were more uniformly distributed, as shown in figure 5-14. Such distribution of these 2-

MBT strands can be an indication of a more uniform film formed over the metal surface with a

longer exposure period. As the samples are being exposed to high vacuum conditions, and

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various beam currents, it is also possible that the film formed shrinks down to form such

strands or these strands could simply be excess dried remnants deposited over the metal

surface.

Focused ion beam and transmission electron microscopy studies revealed the presence of a

dark layer between the alloy surface and a lighter oxide layer (figures 5-15 to 5-18), which was

more prominent over the intermetallic particle than over the aluminium matrix. While this layer

could not be identified due to the thickness being around 20 nm, it is quite likely to be

associated with the inhibitor as previous compositional scans conducted (figure 4-13 and

figure 4-14) showed the presence of sulphur over the intermetallic particles was more

pronounced than over the aluminium matrix. The presence of sulphur suggests the presence

of an inhibitor film layer. The presence of a single layer, possibly associated with an oxide-

inhibitor complex layer, over the aluminium matrix, would suggest that while distinct dual layers

of oxide and inhibitor film form over the IMP regions and to a certain degree over the transition

region, a mixed, diffuse layer is formed over the matrix surface. These findings suggest that

mechanisms associated with oxide and/or inhibitor film formation are different over the matrix

compared with the IMP and is an area for future investigation.

An EDS analysis could not be conducted on the TEM lamella due to the very low thickness of

the dual layer. In addition, sulphur also tends to decompose under high vacuum, and this

causes difficulty in identifying the presence of inhibitor in any layer accurately. However, this

hypothesis of the dark layer observed being a potential inhibitor layer should be further

investigated to confirm with more plausible evidence.

In summary these studies have shown that for both inhibited systems, the 2-MBT system

shows continued protection from the polarization resistance data, Na-MPA only managed to

retain the protective layer for about 8-9 days. Therefore, this provided the basis for further

electrochemical and surface analysis on the 2-MBT system to investigate the persistency of

the inhibitor-induced film in the absence of inhibitor (film durability and film breakdown

studies). This is discussed in more detail below.

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To conclude, from the results demonstrated in this study, the inhibited systems showed

excellent short-term performance in comparison to the uninhibited system. This was presented

through the lowered current density as well as increased polarization resistance. The results

achieved through electrochemical techniques and surface analytical techniques appears to be

consistent in regard to the development and maintenance of a protective film over extended

time periods. 2-MBT film-formation may be triggered by the Cu remnants formed through initial

trenching and its high affinity towards For the Na-MPA system, both the thiol and carboxylate

groups may have a significant contribution to the film-formation process through the aluminium

matrix. During relatively longer-time period exposures up to 14 days, whilst the 2-MBT system

exhibited continued protection with a stable polarization resistance, the Na-MPA exhibited

film-failure after about 8-9 days due to its predominant anodic nature and thereby the reducing

inhibitor concentration over time may not be sufficient enough to continue protection.

7.3 Inhibitor-induced film degradation studies This phase of the study involved exposing samples to 2-MBT inhibitor for periods of 7, 30 and

60 days (168, 720 and 1440 hours respectively) to allow the protective film to develop (referred

to as pre-conditioning). Samples were subsequently removed and placed in uninhibited NaCl

solutions and film degradation / breakdown in the absence of a continuous supply of inhibitor,

was monitored using LPR, EIS and PDS over a 15 min to 7 day (15 min to 168 hours) period.

A similar study was conducted by Tan et al. [55] where the alloy AA2024-T3 was treated for

48 hours and the sample was then exposed to a corrosive medium with diluted inhibitor

concentration by an order of magnitude. However, although they demonstrated self-repairing

properties, the corrosive medium wasn’t inhibitor-free. This study demonstrates the self-

repairing ability of the film in the complete absence of inhibitor.

For samples exposed to the inhibited solution for a period of 168 hours (7 days) (figure 6-1),

film breakdown was observed to take place immediately upon exposure via the significant

drop in polarization resistance within a 15-min period. This indicates that a 168-hour exposure

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period was considered to be insufficient for a stable film to be formed over the metal surface.

Samples which were pre-conditioned for 30-days (figure 6-2) resulted in very inconsistent

results with some sample showing the breakdown of the film immediately or after a few hours,

whereas other samples were able to retain the protective layer between 12 – 24 hours, after

which they then showed signs of breaking down. Two examples, as shown in figure 6.2,

representing both scenarios, confirm the inconsistency in these results and such data not

being reliable enough to conclude that the inhibitor-induced film was able to provide continued

protection after 30 days pre-conditioning. Continued exposure of the 7-day and 30-day pre-

conditioned samples for 168 hours (7 days) confirmed the inability for the film formed to

provide any further protection after breaking down, as evidenced by the Rp values falling within

the 104 Ohm for the full duration, and the samples corroding quite freely.

In contrast for 60 days pre-conditioning (figure 6-3), although the Rp values showed an

immediate drop upon exposure to the inhibitor-free solution, though to be associated with

sudden change in chemistry at the metal surface, an increase in the resistance over the next

few hours was attributed to self-repairing behaviour of the film. Although some differences

were observed, both samples showed consistent Rp values within the 105 Ohm range, even

after 168 hours exposure to uninhibited solution indicating the retention of the protective layer

over this period. The fluctuating behaviour over the full 7 day period, showing a decrement in

Rp over 0.5 to 4 days, followed by an increment in Rp after 4 days confirms the self-repairing

behaviour of the system.

130

As the sample treated for 60 days showed promising results, further experimentation was

conducted with these pre-treatment conditions and EIS tests were conducted to understand

the system and the processes taking place during this degradation. For reference purposes,

the EIS scan conducted in the presence of inhibitor has been included and this was fitted into

an equivalent circuit with two time constants. The plots achieved after exposure to an inhibitor-

free solution at different time intervals was also fitted into an equivalent circuit with two time

Figure 7-2: Schematic diagram of the equivalent circuit for the degradation of the passivating layer in the absence of inhibitor

constants and is presented in the schematic diagram below (figure 7.2).

The schematic diagram in figure 7.2 demonstrates a hypothesis for the physical basis EIS

based on the equivalent circuit correspondent to the 2-mercaptobenzothiazole film

degradation system as per figure 6.7. The “film” over the metals surface includes a

combination of porous inhibitor/ oxide layer. Only limited work has been done previously in

degradation of the film.

Similar to that for the inhibited system, as the two time constants at high to mid frequency

range are not distinctive enough, an equivalent circuit with two R-C cycles was chosen to

avoid overfitting of data. Therefore, whilst one cycle is correspondent to the inhibitor/oxide

layer, the other time constant at low frequency is associated with interfacial corrosion activities.

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The polarization resistance is essentially a sum of the R1, R2 and R3 values. The capacitance

of the inhibitor layer at high frequencies shows no significant variation although the polarization

resistance seems to have changed significantly and can be observed through the Nyquist plot.

This could imply that the structure of the inhibitor film remains with no significant changes

occurring. However, with the time constant at the mid-range frequency, the capacitance still

remains unchanged, but the phase angle shows significant variation over different time

intervals. A Warburg component was incorporated into a R-C cycle to accommodate for the

increased porosity and the diffusion-limited process. The initial breakdown of the film could be

seen with the reduced polarization resistance when compared to that before exposure to the

inhibitor free solution. In addition, the film self-repairing process could be observed from the

Nyquist plot after 24 hours where the polarization resistance has recovered after 24 hours of

immersion in comparison to that after 1 hour. Therefore, the results achieved via EIS (figure

6-5 and 6-6) has shown that upon exposure to a NaCl only solution, the system has managed

to retain the protective layer and has shown self-repairing behaviour between 1 hour and 24

hour which is consistent with that observed via LPR results.

In terms of double layer capacitance measured, this could have been a measurement obtained

from the actual active surface. When there is a porous oxide layer over the metal surface, the

measurements are taken from the exposed or uncovered pores where the surface area is

much smaller than the actual measurement area and this should be taken into consideration.

The increment in R3 and Warburg component over time could be due to the pores of the

combined passivating layer being blocked off with the aid of high insoluble corrosion products

and thereby the pore sizes are decreasing, and the resistance measured increases as the

active species in the corrosive medium reaching the alloy surface is hindered. Therefore,

during the degradation process, the inhibitor film remains significantly unaffected whilst the

porosity of the oxide layer is affected and thereby a drop in R3 is seen. However, with the

presence of inhibitor, it assists in blocking off the pores and thereby the active surfaces for

cathodic and anodic reactions in a self-repairing process.

132

Tafel extrapolation results of the 60-day pre-conditioned sample (figure 6-8), immediately after

exposure to the inhibitor-free solution, and after 7 days exposure yielded current density

values typical of those expected after exposure to an inhibited environment. Further, the lower

icorr values observed after 7 days exposure to the uninhibited environment further confirms

not only the ability for the alloys to retain a protective film, but to develop further this protective

film even in the absence of an inhibitor. These findings were confirmed by surface microscopy,

as shown in figure 6.8.

Through the film degradation studies conducted it was concluded that after pre-treating an

AA2024-T3 alloy surface over a 60-day period, the sample was able to preserve its film over

a period of 7 days even in the absence of an inhibitor. The variance in polarization resistance

over the 168-hour period displayed the disruption on the layers and the self-repairing ability of

the film through LPR and EIS technique. After a 168-hour period, the observed current

densities, which were two orders of magnitude less than that of the uninhibited system,

indicate the presence of a well-protected surface. The EIS data analysis demonstrated the

influence of the porosity of the layers whereby, although the film thickness and the

inhibitor/oxide layer structure remained significantly unaffected, the pores within the

passivating layer became blocked by corrosion products, thus increasing the resistance to

corrosion. The display of self-repairing abilities of the film to maintain itself provides a very

promising aspect to inhibitor film stability and its applications in the industry, to understand

how well the film is able to protect the sample, in the absence of an inhibitor.

133

8. CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

This study has outlined the effectiveness and performance of inhibitors, 2-

mercaptobenzothiazole (2-MBT) and 3-mercaptopropionic acid (3-MPA) in 0.1 M NaCl

solution for AA2024-T3. This chapter concludes the work conducted on the key findings in

terms of the film forming behaviour of the inhibitors and interactions with oxide layer formation,

longer-term stability of the protective films formed and the film durability and failure. This is

followed by suggestions for future work to build-up on the results achieved through this study.

8.1 Inhibitor-induced film formation . The following key conclusions were made in terms of inhibitor induced film forming behaviour.

• The effectiveness of unbuffered inhibitor 2-mercaptobenzothiazole (2-MBT) increased

with increasing concentration whilst that of unbuffered 3-mercaptopropinoic acid (3-

MPA) decreased due to the high acidic nature of the solution.

• The sodium salt of 3-mercaptopropionic acid, Na-mercaptopropionate (Na-MPA)

showed improved corrosion inhibition due to the deprotonated anions being more likely

to interact with the positively charged alloy surface. In addition to the thiol group, the

carboxylate group also may be interacting with the alloy surface for better film-

formation.

• The surface properties of the alloy particularly the presence of intermetallic particles

(IMP’s), influenced the film forming behaviour of the inhibitors.

• In the case of the 2-MBT system, the dissolution of the oxide layer appears to be

promoting some initial trenching to take place in the vicinity of the intermetallic particles

(IMP’s) and subsequent re-deposition of Cu remnants. Due to the high affinity of Cu

134

toward sulphur, the thiol groups maybe the trigger for film-formation over the metal

surface.

• Energy dispersive spectroscopy (EDS) analysis for both inhibitors confirmed the high

affinity of Cu towards S with the more pronounced presence of inhibitor elements, S,

C and N over the Cu-rich intermetallic particles in comparison to the Cu-deficient

intermetallic particles.

• Initial trenching of the S-phase particles to a certain extent supports the film-formation

process due to the molecular bonding and interactions with the trenched regions, as

the presence of an inhibitor film over the trenched S-phase particles was more evident

than over the un-trenched particles.

8.2 Long-term stability of the inhibitor-induced film The key findings from the long-term immersion experiments, based upon optimisation of the

systems, and their corresponding stability behaviour are presented below.

• Both the 2-MBT and 3-MPA inhibitors showed continued protection over a 168-hour

period, with a reduction in the rate of corrosion after 7 days compared to immediately

upon exposure. This was associated with the formation of a more stable film with time.

• For the 2-MBT inhibitor, over a 168-hour period, EIS data evaluation suggested no

significant structural variations for the dual layer inhibitor/oxide complex film formed.

However, the EIS data have indicated that the porosity of the layer may have

minimized over time due to blockage effects induced by the inhibitor and this appears

to be the dominating factor over time.

• For the Na-MPA, inhibitor, over a 168-hour period, as per EIS data evaluation, the

inhibitor/oxide complex layer showed no significant structural changes although the

double layer thickness was reduced. In contrast to the observations for the 2-MBT

inhibitor, the EIS results displayed a reduced charge transfer resistance which is an

135

indication of increased porosity of the oxide layer was observed to increase over longer

exposure periods.

• 2-MBT continued to show protection over the extended time period of over 60 days

whilst Na-MPA showed increased corrosion rates after 8-9 days of exposure. As Na-

MPA displayed predominant anodic behaviour, this can be attributed to the lack of

sufficient inhibitor concentration and availability to provide effective corrosion

inhibition.

• Upon 60 days of immersion in 2-MBT system, a combined protective film with a

thickness of about 20 nm was observed through transmission electron microscopy over

the alloy surface.

8.3 Film degradation studies Based upon the film persistency, durability and breakdown studies conducted on the 2-MBT

system, once the inhibitor-induced film has been developed for different time periods and then

exposed to an uninhibited corrosive medium, the following conclusions were formulated.

• To establish a stable enough inhibitor-induced film in a 0.1 M NaCl only solution, a

minimum of around 60 days of pre-treatment was required. The developed film showed

self-repairing abilities and continued to protect the alloy surface even after 168 hours

exposure and this was confirmed via electrochemical techniques such as PDS, LPR

and EIS.

• When exposed to a free-inhibitor solution, it was observed that the overall structure

and thickness of the complex inhibitor/oxide film formed was not affected. However,

the resistance corresponding to the porosity of the layer was again the more dominant

factor and, in the absence of inhibitor, the corrosion products formed may be blocking

the pores resulting in an increased resistance as observed through LPR and EIS

studies.

136

In general, the 2-MBT inhibitor continued to protect the alloy surface for extended periods

even up to 60 days whereas Na-MPA only provided protection up to about 8-9 days after which

severe corrosion was initiated. A lower corrosion current observed for both inhibitor systems

after 7 days of exposure in comparison to that immediately upon immersion provides testimony

to the presence of a more stable film over longer periods. Further, in the absence of a

continuous supply of inhibitor, 2-MBT, after a 60-day pre-treatment period, still retained an

inhibitor induced film which provided protection even up to 7 days, through the films self-

repairing abilities.

8.4 Future work and recommendations Based upon the findings from this study, a number of suggestions are provided for future

investigations into this area of research, as detailed below.

• Further in-depth TEM analysis of the complex film formed over the alloy surface. Such

studies would be of significant value, particularly to identify the interaction between the

inhibitor and oxide layer and provide further support to the information gained from the

equivalent circuits fitted through EIS.

• Studies to investigate the possibility of dimerization with the inhibitor 3-

mercaptopropionic acid. The ability to undergo dimerization could assist or restrict the

film-formation process and provide further understanding of the bonding that takes

place between the alloy surface and the inhibitor layer as well as the bonding in

between inhibitor molecules.

• Further in-depth characterisation using surface analytical techniques such as Raman

spectroscopy and attenuated total reflection (ATR) to determine the chemical

composition in terms of bonding between the oxide layer and the inhibitor layer as well

as the bonding with the alloy surface. This would provide valuable insights into the

interactions between the naturally occurring oxide layer and the inhibitor layer.

137

• As the intermetallic particles play an important role in the film-formation process,

further investigations should be conducted to understand the spatial evaluation of the

film-formation process utilizing techniques such as SECM. This would provide a

deeper understanding of film-formation, both localised around the IM particles and/or

forming uniformly over the entire alloy surface. 3D topographical image studies would

provide insights as to how the placement of IM particles influence film formation.

138

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