Corrosion Inhibition Assessment for 5xxx Series Aluminium Alloy Using Droplet Evaluation Technique
A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering
Karim Gamaleldin Bachelor of Engineering (Honours) – Alexandria university, Egypt
School of Engineering College of Science, Technology, Engineering and Maths RMIT University
March 2022
DECLARATION
I certify that except where due acknowledgement has been made, this research is that of the
author alone; the content of this research submission 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.
In addition, I certify that this submission contains no material previously submitted for award
of any qualification at any other university or institution, unless approved for a joint-award
with another institution, and acknowledge that no part of this work will, in the future, be used
in a submission in my name, for any other qualification in any university or other tertiary
institution without the prior approval of the University, and where applicable, any partner
institution responsible for the joint-award of this degree.
I acknowledge that copyright of any published works contained within this thesis resides with
the copyright holder(s) of those works.
I give permission for the digital version of my research submission to be made available on
the web, via the University’s digital research repository, unless permission has been granted
by the University to restrict access for a period of time.
I acknowledge the support I have received for my research through the provision of an
Australian Government Research Training Program Scholarship.
Karim Gamaleldin
7th March 2022
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Acknowledgments
First and foremost, I am grateful to the almighty Allah for being the most gracious and merciful all
throughout my life. I thankfully acknowledge the funding provided by RMIT University for awarding
me with one of the most prestigious Scholarships – the RMIT Research Stipend scholarship (RRSS) and
the School of Engineering for the top-up scholarship.
I would like to convey my heartiest appreciation and sincere thanks to my supervisors, Drs. Xiao-Bo
Chen, Rou Jun Toh and Paul White for their unstoppable support and guidance throughout my
research journey at RMIT University. A special thanks to Dr Liam Ward, my previous supervisor, for his
support prior to retirement and for providing this great opportunity for me to study at RMIT
University. Thank you all for your invaluable initiatives and input into my work. Especially my principal
supervisor Prof. Ivan Cole has been the real driving force throughout my Master's. I am especially
grateful for his tireless assistance while conducting experiments and thesis editing.
Special thanks are due to Mr Peter Rummel and Dr Billy Murdoch, who have provided me with
technical guidance and support.
I would also like to thank RMIT Micro Nano Research Facility (MNRF) and acknowledge the use of
facilities within the RMIT Microscopy and Microanalysis Facility (RMMF). My sincere thanks to the kind
support technical staff.
I appreciate the ability to work from home and complete the thesis on time without any interruption
during the COVID-19 pandemic lockdown.
Special thanks to my colleagues in the Rapid Discovery and Fabrication team at RMIT University for
their support and technical help. I would also like to extend my thanks to my friends, especially my
closest friends, for constantly being present and looking after my well-being when times were difficult.
Last but not least, there are no words to express my deepest gratitude to my family. I would like to
convey my sincere love and warmth to my mother, Eman El-lethey, for her love, unconditional support
and understanding throughout my life, not only during my research journey. Special thanks to my
father, Mohamed Abbas, for being the kindest father in the world. My parents’ prayers for me are
always the biggest strength of mine.
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List of content
1.1 Background and motivation .............................................................................................................. 3
1.2. Objectives and scope ....................................................................................................................... 5
1.3. Structure of the thesis ..................................................................................................................... 5
1.4. Overview of 5xxx series Al alloys ..................................................................................................... 6
1.5. Corrosion characteristics of Al alloys ............................................................................................... 6
1.5.2. Intergranular corrosion: ............................................................................................................ 7
1.5.3. Pitting corrosion ........................................................................................................................ 8
1.5.4. Influences of microstructure on corrosion: .............................................................................. 9
1.6. The role of inhibitors in corrosion control ..................................................................................... 11
1.6.1. Classification of corrosion inhibitors........................................................................................... 12
Anodic inhibitors ............................................................................................................................... 12
Cathodic inhibitors ............................................................................................................................ 12
Mixed inhibitors ................................................................................................................................ 13
Organic inhibitors .............................................................................................................................. 14
1.6.2. The adsorption of organic inhibitors ........................................................................................... 14
1.6.3. Corrosion Inhibition of 5xxx series Al alloys ............................................................................... 15
Inhibition performance of vanadate salts and rare earth compounds ............................................ 15
Quinoline derivatives ........................................................................................................................ 16
Corrosion protection by azoles and its derivatives ........................................................................... 17
The influence of changing the position of functional groups and molecular structure on inhibitors performance ......................................................................................................................................... 21
Impact of Alkyl chain length with anchor group ............................................................................... 24
New trend in inhibitors: ................................................................................................................ 27
1.7.2. High throughput assessment .................................................................................................. 30
1.8. Discussion and limitations ............................................................................................................. 34
1.9. Conclusions .................................................................................................................................... 35
2.1. Introduction ................................................................................................................................... 36
2.2. Experimental Procedures ............................................................................................................... 38
2.2.1. Sample preparations ............................................................................................................... 38
2.2.2. Experimental environments.................................................................................................... 39
2.2.3. Droplet exposure upon AA5083 surface ................................................................................. 39
2.2.4. Removal of corrosion products ............................................................................................... 40
2.2.5. Quantification of corrosion using optical profilometry .......................................................... 40
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2.2.6. Electrochemical methods ....................................................................................................... 41
2.2.7. Surface analytical methods ..................................................................................................... 41
2.3. Method development .................................................................................................................... 41
2.3.1. Coupon exposure .................................................................................................................... 41
2.3.2. Removal of corrosion products ............................................................................................... 42
2.3.3. Electrochemical techniques .................................................................................................... 44
Open circuit potential measurements (OCP) ................................................................................ 44
Potentiodynamic polarization scans (PDS) ................................................................................... 44
Linear polarization resistance (LPR) .............................................................................................. 44
Electrochemical impedance spectroscopy (EIS)............................................................................ 44
2.3.4. Surface analysis techniques .................................................................................................... 45
Optical microscopy ........................................................................................................................ 45
Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) Analysis 45
2.3.5. Reproducibility of OP data ...................................................................................................... 46
2.4. Conclusions .................................................................................................................................... 46
3.1. Introduction ................................................................................................................................... 48
3.2. Electrolyte preparation .................................................................................................................. 48
3.3. Quantifying corrosion rates through pit counting ......................................................................... 49
3.4 Electrochemical method ................................................................................................................. 50
3.5. Results and discussion ................................................................................................................... 51
3.5.1. Inhibition efficiency based on the droplet technique ............................................................. 51
3.5.2. Inhibition efficiency based on PDS plots. ................................................................................ 53
3.5.3. Comparison between the droplet technique and PDS ........................................................... 54
3.6. Conclusions .................................................................................................................................... 55
4.1. Introduction ................................................................................................................................... 56
4.2. Experimental .................................................................................................................................. 56
4.2.1. Inhibitor solutions ................................................................................................................... 56
4.3. Results and discussion ................................................................................................................... 57
4.4 Conclusion ....................................................................................................................................... 64
5.1 Studies for uninhibited and inhibited solutions over 7 d exposure ................................................ 66
5.5.1. Potentiodynamic scan (PDS) tests .......................................................................................... 66
5.5.2 Linear polarization resistance (LPR) studies ............................................................................ 68
5.5.3. Electrochemical Impedance Spectroscopy (EIS) studies over 2 weeks................................... 70
0.6 M NaCl – uninhibited system .................................................................................................. 70
10-3 M inhibited system (AAT)....................................................................................................... 72
10-3 M inhibited system (3-AT) ...................................................................................................... 74
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5.5.4 a. Surface analysis of ATT inhibited systems over exposure periods of 14 days ..................... 75
5.5.4 b. Surface analysis of 3-AT inhibited systems over exposure periods of 2 weeks ................... 77
5.5.5. Surface analysis using SEM/EDS mapping for uninhibited and inhibited (ATT and 3-AT) systems ............................................................................................................................................. 79
5.5.6. Fourier transform infra-red spectroscopy (FTIR) for uninhibited and inhibited systems over 2 weeks of exposure ............................................................................................................................ 81
5.6. Discussion ....................................................................................................................................... 83
5.7 Conclusion ....................................................................................................................................... 84
6.1. Conclusions .................................................................................................................................... 87
6.2 Recommended future work ............................................................................................................ 88
References ............................................................................................................................................ 89
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List of Figures
Fig 1.1. The galvanic series in seawater from …….………………………………………………………………………………5 Fig 1.2. Dissolution rate of oxide film …….………………………………….………………………………………………………6 Fig 1.3. Mechanism of Pitting corrosion of Al from …..……………………………………………………………………….7 Fig 1.4. Schematic illustration of the role of IMPs on pitting activities of Al alloys from ….………………..8 Fig 1.5. Inhibitors classification from …….…………………………………………………………………………………………10 Fig 1.6. Potentiodynamic polarization diagram: electrochemical behaviour of metal with (a) and without (b) anodic inhibitor……………………………………………………………………………………………………………..10 Fig 1.7. The mechanism of anodic inorganic inhibitor and its effect………………………………………………...10 Fig 1.8. Potentiodynamic polarization diagram: electrochemical behaviour of the metal in a cathodic inhibitor solution (a), as compared to the same solution, without inhibitor (b)……………………………….11 Fig 1.9. Illustration has shown the mechanism of actuation of the cathodic inhibitor………………………11 Fig 1.10. The polarization diagram shows the effect of mixed inhibitors…………………………….……………12 Fig 1.11. The actuation mechanism of organic inhibitors, acting through adoption on the metal surface, where Inh. represents inhibitor molecule…………………………………………………………………………..12 Fig 1.12. Surface film model in NaCl solution: (a) without the addition of rare earth chloride, (b) after addition of rare earth chloride addition …….………………………………………………………………………………….. 13 Figs 1.13. Quinoline derivates structures……………………………………………………………………………………….. 15 Fig 1.14. the proposed mechanism of 8-AQ and 8-NQ on Al surface …….……………………………………….. 16 Fig 1.15 imidazole derivates structures........................................................................................……. 17 Fig 1.16 Imidazole derivates ……………………………………………………………………………………………………………17 Fig 1.17 Triazoles derivates structures…………………………………………………………………………………………….18 Figs 1.18 (a, b, c, and d) Triazoles derivates structures……………………………..……………………………………..19 Figs 1.19 Thiadiazole derivates structures………………………………………………………………………………………..19 Figs 1.20 Thiazole derivate structures………………………………………………………………………………………………20 Fig 1.21 2-Mercaptobenzoxazole structure ……………………………………………………………………………………..20 Figs 1.22 Dihydroxy benzene Isomers…………………………………………………………………………………………..….21 Fig 1.23. Illustration of the structures of standard heterocyclic inhibitors tested. (Note: similar sub- structures are highlighted with coloured circles………….…………………………………………………………………..22 Fig 1.24. Illustration of the inhibitors’ structures, showing structural relationships. (Note: similar sub- structures are highlighted with coloured circles.).…….…………………..………………………………………………..23 Fig 1.25 Schematic diagram of the structural division of the inhibitor into an anchor group and a backbone (left). The anchor group governs adhesion to the surface (indicated by red arrows), whereas the backbone is responsible for cohesive interactions within the monolayer (brown arrows). In order to better disentangle the effects of the two structural components, a standalone adsorbed molecule with a methyl (Me) group as a minimal backbone (right) was used when investigating the effect of the anchor group …….………………………………………………………………………………………………………..24 Fig 1.26. Schematic diagram shows the bonding mechanism of phosphonic groups. ..................…….25 Fig 1.28. Tafel plot……………………………………………………………………………………………………………………………29 Fig 1.29A. Variation of pH in a 1 µL droplet of S1, with addition of WRI, placed on a Zn surface for 6 h…..………………………………………………………………………………………………………………………………………………….31 Fig 1.29B.Variation of pH in a 1 µL droplet of S2, with the addition of WRI, placed on a Zn surface for 6 h …….…………………………………………………………………………………………………………………………………………….31 Fig 1.30A. Angled view of 88 well test apparatus …….………………………………………………………………………32 Fig 1.30B. 96 well plate with the solutions containing dissolved …….……………………………………………….32 Fig 1.31. Multi-electrode assembly showing the layout of wire specimens 1–9 …….………………………..33 Fig 2.1. Photograph of as received sample of AA5083……………………………..………………………………………38 Fig 2.2. The Humidity chamber………………………………………………………………………………………………………..39
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Fig 2.3. The initial humidity chamber……………………………………………………………………………………………….39 Fig 2.4. OP images for a drop of 2 µL……………………………………………………………………………………………....40 Fig 2.6. (A) droplet under digital microscope after 7 days of exposure (B) droplets after 7 days exposure in humidity chamber ……………………………………………………………………………………………………….41 Fig 2.7. (A) OP image, (B) XY profiles and (C) 3D image of a cleaned Al alloy surface after 24 h of exposure………………………………………………………………………………………………………………………………………….42 Fig 2.8. Droplet under digital microscope after cleaning………………………………………………………………….43 Fig 2.9 images of AA5083 after polishing………………………………………………………………………………………….43 Fig 2.10 images of polished AA5083 surface after swabbing with 10% nitric acid…………………………….43 Fig 3.1 A sample of a droplet image using optical profilometry and a line profile for measuring pit depth ………………………………………………………………………………………………………………………………………………50 Fig 3.2. Average of 100 pits’ depth over three droplet replicates…………………………………………………….51 Fig 3.3. Inhibition efficiencies (%IE) based on the mean and maximum pit depth measurements………………………………………………………………………………………………………………………………...52 Fig 3.4. Corrosion current ( icorr) for two different time periods of seven inhibitors compared to NaCl control…………………………………………………………………………………………………………………………………………….53 Fig 3.5. %IE comparison between droplet and PDS results for seven inhibitors after 7 days…………….54 Fig 4.1. keto enol tautomerism………………………………………………………………………………………………………..59 Fig 4.2. Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of dihydroxy isomers after 7 days on left and their efficacies on the right……………………………….59 Fig 4.3. A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of group 2 after 7 days and B) their efficacies …………………………………………………………………60 Fig 4.4 A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of Triazoles after 7 days and B) their efficacies………………………………………………………………..61 Fig 4.5 A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of imidazoles after 7 days and B) their efficacies……………………………………………………………..62 Fig 5.1a PDS of uninhibited and inhibited systems after 4 hours……………………………………………………..67 Fig 5.1b PDS of uninhibited and inhibited systems over 7 days………………………………………………………..68 Fig 5.2. polarization resistance of uninhibited and inhibited systems on AA5083, over 14 days……….69 Fig 5.3a Nyquist plots of uninhibited system over 2 weeks………………………………………………………………71 Fig 5.3b Bode plots of uninhibited system over 2 weeks………………………………………………………………….71 Fig 5.4 Suggested equivalent circuit for 0.6M NaCl system over 2 weeks…………………………………………71 Fig 5.5a. Nyquist plots in the ATT inhibited system over 14 days……………………………………………………..72 Fig 5.5b Bode plots of ATT inhibited system over 2 weeks……………………………………………………………….73 Fig 5.6 Suggested equivalent circuit for the ATT system over 2 weeks……………………………………………..73 Fig 5.7a. Nyquist plots in the 3-AT inhibited system over 14 days…………………………………………………….74 Fig 5.7b. Nyquist plots in the 3-AT inhibited system over 14 days……………………………………………………74 Fig 5.8 Suggested equivalent circuit for the 3-AT system over 2 weeks……………………………………………75 Fig 5.9. Cross-section images of inhibitor layer formed of ATT on AA5083……………………………………….76 Fig 5.10. line scan analysis of aluminium surface after 14 days exposure of ATT inhibited system…..77 Fig 5.11. The overall image of AA5083 after 14 days of exposure of ATT……………….………………………..77 Fig 5.12. 3-AT inhibited-oxide mixed layer on AA5083 surface…………………………………………………………78 Fig 5.13. Analysis of line scans of 3-AT inhibited system on AA5083………………………………………………..78 Fig 5.14A. Elemental composition through EDS after 14 days of saline exposure…………………………….80 Fig 5.14B. Elemental composition through EDS after 14 days of ATT system……………………………………80 Fig 5.14C Elemental composition through EDS after 14 days of 3-AT system……………………………………81 Fig 5.15A. SEM images at lower magnification with minor variation in magnification………………………81 Fig 5.15B. Samples photographs………………………………………………………………………………………………………81 Fig 5.16. FTIR spectra of 3-AT system over 2 weeks of exposure on AA5083………………………………..….82 Fig 5.17. FTIR spectra of 3-AT powder………………………………………………………………………………………………83
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List of Tables
Table 2.1. Percentage of error of pits measurements via OP………………………………………………………………56
Table 3.1. Chemical structures of the inhibitors……………………………………………………………………………….…58
Table 4.1. Compounds tested for corrosion inhibition of AA5083 in 0.6 M NaCl………………………………….68
Table 5.1. Rp of uninhibited and inhibited systems over different time periods………………………………….80
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Abstract
Aluminium alloy 5083 (AA5083) is one of the commonly used metals in marine applications due to its
low-density materials, good mechanical properties, and high resistance to corrosion. On the alloy
surface under normal atmospheric conditions. The oxide film formed on the aluminium alloy surface
is non-uniform, thin and non-coherent. Therefore, it imparts a certain level of protection under normal
conditions. When exposed to environments containing halide ions, of which the chloride is the most
frequently encountered in service, the oxide film breaks down at specific points leading to the
formation of pits on the aluminium surface. This type of corrosion is known as pitting corrosion.
Traditional techniques used to evaluate corrosion and corrosion inhibitors can be time-consuming.
However, high-throughput methodologies can be used to conduct a large number of simultaneous
experiments in a shorter period of time than traditional assessment tests. Consequently, there is an
urgent need to discover and develop new rapid screening methodologies that can be used as a
preliminary screening method prior to supporting traditional electrochemical methods and
computational methods to design new inhibitors.
Therefore, the aim of this research project was to develop a new rapid screening method for metals
susceptible to pitting corrosion and assess the performance of corrosion inhibitors using a droplet-
based technique. The Master's by research project was divided into two parts. A systematic and rapid
droplet-based strategy was used in Part 1 to screen a wide range of corrosion inhibitors for AA5083
and different variations in the inhibitors’ molecular structures. Further characterization of the
performance and associated mechanisms of inhibition, including any adsorption or film-forming
properties, was carried out in Part 2 on selected shortlisted inhibitors.
In Part 1, a series of trials and experiments have been conducted on 21 corrosion inhibitors to figure
out the optimal cleaning methods, surface preparation and condition of using optical profilometry to
assess droplets of the inhibitors. While in Part 2, a series of methods were used, including
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electrochemical and surface characterization, to reveal the long-term performance of two promising
inhibitors that were selected from the droplets’ study.
Overall, the thesis successfully establishes several significant findings over the course of this master’s
by Research project, developing a high throughput droplet-based technique, revealing its advantages
and limitations in the evaluation of pitting corrosion and corrosion inhibitors of AA5083.
The main outcome of this thesis is providing a new high throughput screening tool that will help the
corrosion community in screening new corrosion inhibitors in a rapid and effective way.
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Chapter 1
Introduction and literature review
1.1 Background and motivation
Generally, evaluation techniques for the performance of corrosion inhibitors are tremendously time-
consuming, mainly if various testing methods are used for a wide range of corrosion inhibitors. Testing
periods vary from hours using various electrochemical techniques linear polarization resistance (LPR),
electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scans (PDS), weeks
and months (mass loss measurements), and even years for outdoor exposure field trials. On the other
hand, high-throughput methodologies could be employed to conduct a high number of experiments
simultaneously within less experimental time than traditional assessment tests. Therefore, there is an
urgent need to discover and develop new rapid screening methodologies to support traditional
electrochemical and computational methodologies combined in designing new inhibitors [1,2].
The worldwide costs associated with marine corrosion run into billions of dollars annually [2].
Although 5xxx series (Al) alloys are widely used in a range of automotive, aerospace, cryogenic tanks
and the frame of solar panels, renewable energy and maritime applications, as well as many civil
engineering and industrial applications, these alloys initially have some resistance to corrosion; still,
they are susceptible to both general and localised corrosion when exposed to seawater over long
periods [3,4]. Corrosion-inhibitor utilisation is one of the most sophisticated and effective
methodologies for mitigating corrosion since few paints act as strong and long-lasting barriers to
ingress moisture and salts. As such, using protective paints impregnated with corrosion inhibitors has
become a viable option for providing additional protection [5–7]. In addition, there has also been
considerable interest in developing new eco-friendly inhibitors to apply to various metals [7–9].
While rapid screening techniques for metals have been used to systematically study the performance
of a wide range of inhibitors for 2xxx and 7xxx series Al alloys [10–14], little information is available
on the use of such techniques for studying the performance of a wide range of inhibitors on marine-
grade 5xxx series Al alloys [10-12]. Aerosols and water droplets drive atmospheric corrosion in the
3
marine environment. Therefore, it will be advantageous to replicate droplets in the laboratory
environment to reproduce atmospheric corrosion. Thus, this project aims to develop a rapid screening
technique based on the droplet technique for evaluating a wide range of protection systems for alloys
subject to pitting, to be applied for the marine grade 5xxx series Al alloys, particularly the AA5083
alloy.
There is scope for developing an existing screening system for these alloys. Such studies would
enhance the currently limited body of literature available in developing new inhibitor systems for 5xxx
series Al alloys and assist in understanding associated inhibition mechanisms (adsorption or film-
forming, for example) and longer-term stability and durability. In addition, little work has been done
on elucidating the roles of intermetallic (IMP) formation and the elements on the corrosion behaviour
for 5xxx Al alloys and their function during the inhibition process [13-16].
This Master’s by Research project is divided into two phases. Phase 1 involved a systematic and rapid
droplet-based strategy for screening a diverse range of corrosion inhibitors with different structural
variations to mitigate corrosion of 5xxx Al alloys. Shortlisted inhibitors were then subjected to detailed
electrochemical studies in phase 2 for an in-depth characterisation of the performance and associated
inhibition mechanisms, including adsorption and any film-forming properties. Initial screening studies
were conducted on 21 different inhibitors, categorised into two categories according to the
substitution of functional group and orientation of the chemical functionalities on the same structure.
These sections were classified into four groups, as mentioned above. The rationale for selecting the
inhibitors was made based on two criteria. The main key criterion was to determine how key molecular
features cause differences in different groups over specific families (azoles, thiazole, quinoline etc.)
that have previously been used for 5xxx series Al alloys with good inhibiting performance. The second
criterion was inhibitors previously used for other Al alloys series that share common intermetallic
particles with 5xxx, for example, 2xxx and 7xxx Al alloys, like (Mg2Si). Thus, families with diverse
functional groups other than those previously used for 5xxx were incorporated into this investigation.
4
The impact out of our research is providing the corrosion community with a new high-throughput
technique to rapidly determine the promising inhibitor candidates for metals while saving time and
1.2. Objectives and scope
effort prior to further in-depth analysis.
The specific research objectives of this study are:
1. The development of a high throughput method for evaluation of alloys (5xxx Al as model materials
in this work) subject to pitting;
2. Understanding the roles of variation in functional groups' in determining the inhibition efficiency;
3. Elucidating the inhibition mechanisms; and
4. Unveiling the role of intermetallic present in Al matrix and their contribution to the inhibition
1.3. Structure of the thesis
mechanism, particularly the long-term durability of the inhibitor systems.
Chapter 1 includes the introduction, motivation, scope and objective of this work, followed by a
literature review to understand the issue in need of attention and a summary of previous studies that
have been conducted to the rapid discovery of chemical compounds as corrosion inhibitors.
Chapter 2 lays out an introduction to the experimental techniques used in this thesis, along with the
parameters and steps to conduct a series of experiments to develop a novel high throughput method,
in addition to electrochemical technique and surface characterizations methodologies.
Chapter 3 compares the developed droplet method and traditional electrochemical techniques (PDS)
for seven inhibitors and notices the differences between both.
Chapter 4 includes a series of high throughput droplet-based method results for 21 inhibitors and
evaluates how the structures' differences could affect inhibiting performance.
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Chapter 5 contains a further detailed study of two selected inhibited systems (ATT and 3-AT) for a
better understanding of the inhibition mechanism and bonding with the AA5083 surface
Chapter 6 includes an overall discussion and the main conclusions points drawn from this thesis, and
1.4. Overview of 5xxx series Al alloys
recommendations for future work to build on the results achieved through this study.
5xxx Al alloys are a significant area of interest in a wide range of fields, owing to their outstanding
mechanical properties and superior weldability [3,15,16]. As well as good properties at cryogenic
temperature, the 5xxx series contains alloying elements such as magnesium, manganese and titanium
that provide specific properties such as corrosion resistance and tensile strength [4]. 5xxx series are
used in many applications such as marine structures, shipbuilding and cryogenic tanks (5083, 5454
and 5086). The applications also include the markets where structure and architecture come together,
like highway bridges, commercial buildings, and roadside structures (5052 and 5083). However, heat
treatment or long-time exposure to high temperature is not recommended as it leads to increase
susceptibility to intergranular corrosion (IGC) due to precipitation of intermetallic particles (IMPs) at
grain boundaries, especially if Mg content exceeds 4.5% [17]. Al and its alloys are characterised by
relatively good high corrosion resistance in a wide variety of environments due to the formation of
the natural oxide layer (Al2O3). However, the stability of the protective layer mainly depends on the
1.5. Corrosion characteristics of Al alloys
1.5.1. Galvanic corrosion
characteristics of the environment, such as pH and composition of the alloy itself [18].
This type of galvanic corrosion (also called “dissimilar metal corrosion”) refers to corrosion damage
induced when two dissimilar materials become in contact with a corrosive electrolyte so that a closed
circuit is established. The driving force of galvanic corrosion is the difference in corrosion potential
between the couple. In a galvanic couple, the less noble material becomes an anode and tends to
corrode at an accelerated rate. In contrast, the nobler material will act as the cathode in the corrosion
cell, becoming protected from further corrosion.
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In a galvanic couple involving any two metals in a galvanic series, as shown in Fig 1.1, metals in the
lower region of Fig 1.1 (and to the left) are electrochemically nobler, with the most extreme cases
being graphite and gold. Metals in the upper area (and to the right) are electrochemically less noble,
with more extreme cases including magnesium (Mg) and zinc (Zn). Metals with more positive corrosion
potentials are called noble or cathodic, and those with more negative corrosion potentials are referred
Fig 1.1. The galvanic series in seawater from [13]
to as active or anodic.
The ratio of cathodic to anodic surface area should be considered, as the larger ratio of cathode to
anode causes, the more oxygen reduction or other cathodic reaction can occur and, hence, the greater
the galvanic current. From the standpoint of practical corrosion resistance, the most unfavourable
1.5.2. Intergranular corrosion:
ratio is a very large cathode connected to a very small anode.
IGC damages the mechanical properties of a material, as it follows the grain boundaries in a material.
The susceptibility of Al to this type of attack is attributed to various microstructural factors, such as
precipitation of active particles or depletion of passivating elements. Precipitation and segregation
happen at grain boundaries, making them different from the matrix [15]. Regarding 5xxx Al alloys, Mg
is the principal alloying element in this type of alloy. The presence of Mg poses a good resistance
against pitting corrosion in seawater. However, with content above 4%, the susceptibility of Al-Mg
alloys to IGC increases [20,21] due to Mg precipitation at grain boundaries as a highly anodic beta-
phase (Al3Mg2). This tendency of 5xxx series to IGC is often caused by microstructural change due to
7
heat treatment, called sensitisation. The alloy has become sensitised when subjected to a temperature
1.5.3. Pitting corrosion
range ( 50-220 ˚C) for a prolonged exposure time [19].
Pitting corrosion is a form of aggressive local corrosion which may result from the localised failure of
the oxide layer of a metal, where certain areas corrode, rendering formation of pits or craters. The
resulting pits can become wide and shallow or narrow and deep, which can rapidly perforate the wall
thickness of metal [20,21]. Al and its alloys are very vulnerable to pitting corrosion in environments
when the pH is close to normal, such as seawater with pH 8-8.5, as shown in Fig 1.2. The pH of the
environment determines the rate of oxide layer dissolution. However, pH value is not the only
Fig 1.2. Dissolution rate of oxide film [12].
indicator that must be considered [18].
Pitting of Al is always engaging attention in research because the pits of Al are always covered by white
and gelatinous blisters of alumina gel Al(OH)3 [15,18]. Corrosion reactions can be expressed as follows:
oxidation at anode formed at the pit bottom:
Al → Al3+ + 3e- (1)
+ + 3H2O→ Al (OH) 3 + 3H+ (2)
Al3
Reduction at cathode outside the cavity:
2H+ + 2e- → H2 (3)
O2 + 2H2O + 4e- → 4OH- (4)
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Pitting events can be divided into two steps: initiation and propagation. The initiation is mainly
attributed to the breakdown of the protective layer due to the adsorption of aggressive anions present
in the electrolyte on the metal surface; usually, chloride ions (Cl-) are adsorbed on weak spots of the
surface [15]. Pits are propagated as illustrated in Fig 1.3. The continuous dissolution of metallic Al into
Al3+ ions according to reactions (1) & (2) at the bottom of pits stimulates the migration of chloride ions
toward the bottom to balance the positive charge produced. Reaction (2) leads to increasing acidity
inside the pit cavity. Therefore, pit propagation is accelerated, and alkalinization is generated at the
cathodic sites, where the dissolved Al re-precipitates as a hydroxide species [7,22].
In the case of Al-alloys, two main types of pit morphology have been observed. A ring of attack around
intermetallic particles, this attack appears to be mainly in the matrix phase; this type is called
Fig 1.3. Mechanism of Pitting corrosion of Al from [9].
1.5.4. Influences of microstructure on corrosion:
circumferential pitting. The second type of morphology is intermetallic selective dissolution [23].
Pure Al is relatively soft, so its usage in many industrial applications is limited. Hence, adding alloying
elements to pure Al is important to enhance specific properties, such as mechanical characteristics,
formability and weldability. However, the presence of these elements with Al leads to the formation
of intermetallic particles that significantly influence the properties of the microstructures, such as high
strength-to-weight ratio and abrasion. These intermetallic phases are classified into three main types:
dispersoids, precipitates and constituent particles. They differ in size ranging from nanometres to
9
micrometres. Constituent types are the largest, which are up to tens of micrometres. There is a
diversity in the behaviour of localised corrosion among different sizes of intermetallic [24,25].
Generally, these particles show diversity in electrochemical characteristics from the behaviour of the
matrix, leading the alloy to be subjected to several forms of localised corrosion [23]. They can be
cathodic or anodic relative to Al matrix. The anodic intermetallic is a prime location for nucleation of
pits, as intermetallic corrode, leaving a hole on the metal surface. In contrast, cathodic intermetallic
particles lead to localised corrosion stimulated in the metallic matrix surrounding the precipitates [26].
The intermetallic particles significantly influence the breakdown or reinforcement of the oxide film,
which depends on the morphology, type and distribution of these phases. However, most noble
elements reduce the beneficial properties of the protective film [18,27]. The protective film's
imperfections are generated due to the inhomogeneous structure due to multi-component Al alloys
[24,26]. Such flaws within the oxide layer are generally within the vicinity of the intermetallic particles
and grain boundaries, as the breakdown of the passive oxide layer starts when pit initiation occurs at
Fig 1.4. Schematic illustration of the role of IMPs on pitting activities of Al alloys from [5]
weak (uncovered) spots around intermetallic phases [17].
In 5xxx series Al alloys, two types of common intermetallics are found: Iron-rich intermetallic phases,
which are represented by several phases such as Al-(Fe, Mn, Cr, Si) and Al6(Fe, Mn), these phases are
characterised by relative nobility compared to the matrix, so they promote pitting corrosion and cause
circumferential trenching of the Al-matrix adjacent to these particles. Mg containing intermetallics
10
such as Al3Mg2 (β-phase) and Mg2Si are found to be more active (anodic) than the Al-matrix resulting
1.6. The role of inhibitors in corrosion control
in their preferential dissolution [28,29].
Various corrosion control methods are adopted to minimise corrosion in many industrial applications.
Using corrosion inhibitors is one of the most widely used and practical strategies to prevent corrosion.
There are many definitions proposed for inhibitors, but according to ISO 8044, they are defined as “a
chemical substance that decreases the corrosion rate when present in the corrosion system at a
suitable concentration, without significantly changing the concentration of any other corrosion
agent’’. Due to their high efficiency and low cost, the application of protective coating incorporating
inhibitors arouses great interest from commercial and industrial sectors [30,31].
Regarding coatings systems, very few paints can act as a protective layer to the entree of moisture
and salts. Hence, protective coatings combined with corrosion inhibitors play a pivotal role in
corrosion prevention [10].
Commonly, chromate-based inhibitors were recognised as highly robust and most effective inhibitors.
However, increasing awareness of chromates' toxicity and environmental hazards has led to the
search for environmentally friendly alternative inhibitors of chromates [7,32,33].
Corrosion inhibitors can be categorised based on their chemical nature as organic or inorganic,
according to their mechanism of action as they can be cathodic, anodic or mixed, depending on
whether they retard one or both cathodic and anodic reactions and by adsorption or film-forming, as
Fig 1.5. Inhibitors classification from [33] 11
illustrated in Fig 1.5 [18-–23].
1.6.1. Classification of corrosion inhibitors
Anodic inhibitors
Anodic inhibitors (passivating inhibitors) work by retarding the anodic reaction. As these inhibitors
form insoluble passivating films on a metal surface, usually by reacting with corrosion products such
as an oxide layer, this results in adsorption on a metal surface, forming a protective barrier, therefore
reducing corrosion rate as shown in Fig 1.7 below. They also increase anodic polarization, shifting the
Fig 1.7. The mechanism of anodic inorganic inhibitor and its effect
Fig 1.6. Potentiodynamic polarization diagram: electrochemical behaviour of a metal with (a) and without (b) anodic inhibitor
corrosion potential in the noble direction (to more positive values), as seen in Fig 1.6.
When the concentration of an anodic inhibitor is insufficient, corrosion may be accelerated rather
than inhibited. As the inhibitor will not cover the surface of the metal entirely, leaving spots of the
surface uncovered, thus stimulating localised corrosion which is hard to detect. Hence, concentration
Cathodic inhibitors
plays a vital role when using anodic inhibitors [21-23].
Cathodic inhibitors combat corrosion either by retarding the rate of cathodic reaction or by
precipitating selectively on the cathodic sites, thus enhancing surface impedance and restricting
diffusion of reducible species. The working mechanism of these types of inhibitors is given in Fig 1.9
below. Cathodic inhibitors shift the corrosion potential in the active direction (more negative value);
this shifting is shown in Fig 1.8 below.
12
There are three inhibition mechanisms associated with cathodic inhibitors; namely, they can act as (i)
cathodic poisons; (ii) cathodic precipitates, and (iii) oxygen scavengers. The first mechanism works by
reducing the rate of hydrogen evolution reaction by incorporating salts of arsenic and antimony, which
then deposit on the metal surface and impede hydrogen combination. However, a limitation is that
they can cause hydrogen blisters and stimulate metal susceptibility to hydrogen embrittlement. For
the second mechanism, incorporating calcium (Ca), Zn, or Mg compounds and subsequent
precipitation of an oxide results in the formation of preventive barrier film. Oxygen scavengers such
as sodium sulphite and hydrazine may inhibit corrosion by preventing the cathodic depolarization
Fig 1.9. Illustration has shown the mechanism of actuation of the cathodic inhibitor.
Fig 1.8. Potentiodynamic polarization diagram: electrochemical behaviour of the metal in a cathodic inhibitor solution (a), as compared to the same solution, without inhibitor (b).
Mixed inhibitors
caused by oxygen and removing it from the solution [22,25,34]
These inhibitors work by retarding both cathodic and anodic reactions involved in the corrosion
process by indirectly blocking anodic and cathodic spots that have resulted from the formation of the
precipitates on the surface. Therefore, they provide the highest inhibition efficiency as they affect
both reactions. Fig 1.10 below shows the influence of mixed type on current density (icorr) and
Fig 1.10. The polarization diagram shows the effect of mixed inhibitors.
corrosion potential (Ecorr). Using mixed inhibitors is more secure than anodic inhibitors alone [34].
13
Organic inhibitors
Organic inhibitors are widely used in corrosion protection; their main mechanism is through
adsorption on the metal surface. They provide a blanketing influence over the whole surface, as shown
below in Fig 1.11. When organic inhibitors are adsorbed on the metal, they limit water access and
oxygen diffusion to the metal surfaces, thus impeding the reactions at the metal surface; therefore,
organic inhibitors can act as cathodic, anodic or mixed inhibitors.
The adsorption behaviour of the organic inhibitors may occur by electrostatic interaction of the
inhibitor molecules and charged metal surface, known as (physisorption) or by forming covalent
bonds by sharing free electrons of inhibitors with the metal surface, known as chemisorption
Fig 1.11. The actuation mechanism of organic inhibitors, acting through adoption on the metal surface, where Inh. represents inhibitor molecule
1.6.2. The adsorption of organic inhibitors
[7,22,37,38]
Many parameters could influence the adsorption efficacy, such as chemical composition, functional
group and inhibitors’ structure, as the presence of donor atoms with high electronic density enhanced
the adsorption process. Also, alkyl chain length affects the efficiency of inhibitions, as the alkyl chain
may be responsible for lateral cohesive interaction between species of adsorbed inhibitor; these
hydrocarbon tails form a hydrophobic network which provides further protection. The primary role of
this network is to keep water molecules and aggressive ions away from the metal surface. Either
repulsive or attractive interactions may occur [37–39]. It has been reported that the efficient
adsorption performance of inhibitors has relied on the presence of a heterocyclic compound, π-
electrons and heteroatoms like oxygen, nitrogen and sulfur. As they were found to act as adsorption
centres as well as having multiple bonds in their molecular structure capable of forming a strong
coordination bond to the metal surface, they have shown a good correlation to effective corrosion
14
inhibition behaviour, as their presence strengthens the adsorption ability of organic corrosion
1.6.3. Corrosion Inhibition of 5xxx series Al alloys
Inhibition performance of vanadate salts and rare earth compounds
inhibitors [11-12, 32, 39].
Special attention is devoted to replacing chromate-based and other toxic inhibitors for Al and its alloys,
and thus there is a search for alternatives. Kharitonov et al. evaluated [40, 41] sodium metavanadate's
and orthovanadate's inhibition performance on AA6063-T5 in 0.05 M NaCl. Polarization
measurements showed that the sodium metavanadate inhibitor blocked active metal dissolution.
Both sodium metavanadate & orthovanadate were found to act as a mixed inhibitor with a maximum
efficiency of > 95% and 98.3%, respectively. The proposed inhibition mechanism was due to
polymerised vanadate film formation on active sites and decreasing the cathodic reaction rate as
vanadate ions are absorbed on noble sites [45]. Ward et al. studied sodium metavanadate's inhibition
ability and orthovanadate exposed to Al alloys 5083 and 6061. Both compounds showed a promising
potential, although the sodium metavanadate showed slightly better performance [46].
Rare earth chlorides compounds have been widely used as corrosion inhibitors for various Al alloys
[47]. Bethencourt et al. [45] studied the inhibition performance of LaCl3 and CeCl3 in a marine
environment when exposed to 5083 Al alloy. The inhibition process was thought to be due to
lanthanide hydroxide precipitating on cathodic sites on Al alloy surface, as shown in Fig 1.12. SEM and
EDS confirmed the formation of a protective film on the metal surface, which acts as a barrier by
blocking cathodic sites, thus, reducing the corrosion rate. LPR studies suggested a cathodic behaviour
for the inhibition process [48].
1.12. Surface film model in NaCl solution: (a) without the addition of rare earth chloride, (b) after the addition of rare earth chloride addition [47]
Fig
15
Quinoline derivatives
Quinoline compounds are among some of the most widely used inhibitors for many metals, especially
for Al and its alloys. 8-Hydroxyquinoline (8-HQ) is one of the benchmarks as a corrosion inhibitor for
Al and its alloys. It has been investigated in detail for its inhibition effectiveness on AA2024-T3 and
AA5083 in 0.6M NaCl/acidic and alkaline solutions with pH values of 2 and 12, respectively. Using PDS
and EIS electrochemical corrosion testing methods, the results from the tests confirmed that the
inhibition mechanism was associated with the adsorption of 8-HQ onto active sites on the alloy
surface. 8-HQ was thought to act as a mixed inhibitor with a predominantly cathodic action which
prevents alumina oxide layer deterioration. The authors attributed the effective inhibition ability of 8-
HQ to its ability to form insoluble aluminium-chelate Al(HQ)3, which restricts the adsorption of the
chloride ion [36,28]. Another investigation was conducted by Lamaka et al. [47] using a range of
organic inhibitors, including 8-HQ, to study the inhibiting effect on the corrosion of 2024 Al alloy. The
effectiveness of the 8-HQ was due to the formation of a thin organic protective layer on the surface
of the Al alloy. The passivation of intermetallic zones due to the restrained dissolution of Mg and Al
Fig 1.13A. 8-Hydroxyquinoline
was the main reason for the inhibiting action[50].
The inhibition performance of two quinoline derivatives, namely 8-aminoquinoline (8-AQ) and 8-
nitroquinoline (8-NQ), in the presence of AA5052 in 3 wt.% NaCl was assessed using weight loss (WL),
PDP and EIS electrochemical corrosion tests [51]. The electrochemical measurements showed that
both acted as anodic inhibitors by retarding the anodic reaction. The maximum inhibition efficiencies
of (8-AQ) and (8-NQ) were 87.1% and 89.5%, respectively, when their concentration was 0.02 mM.
The formation of the protective layer was observed by SEM/EDS analysis. Strong hybridisation
occurred between the p-orbital of the inhibitors and the sp-orbital of Al atoms, as revealed by Density
Functional Theory (DFT) calculations [51].
16
Fig 1.13B. 8-aminoquinoline Fig 1.13C. 8-nitroquinoline
Fig 1.14. shows the inhibition mechanism of these inhibitors [51]. Strong coordination bonds were
formed between inhibitor molecules and Al atoms due to the movement of the inhibitors in the
solution to the damaged corroded regions. Therefore, chloride ions have been prevented from
Fig 1.14. the proposed mechanism of 8-AQ and 8-NQ on Al surface [49].
Corrosion protection by azoles and their derivatives
attacking the Al surface by forming a barrier layer.
Over the past two decades, N-heterocyclic compounds & azole families have been the most tested
organic compounds as corrosion inhibitors for metals such as copper, mild steel and Al and its alloys.
Several researchers attempted to find efficient and powerful inhibitors in aggressive environments for
Al and its alloys [52]. Researchers have attributed the improvement in inhibition efficiency to several
factors, including increasing molecular weight, the number of heteroatoms and the presence of a
phenyl ring in the structure.
Azoles are a category of 5-membered heterocyclic compounds containing a nitrogen atom and at
least one more non-carbon atom as a part of the ring. As corrosion inhibitors, azole compounds,
typically the derivatives of imidazole, triazole, thiadiazole and thiazole types, appear very attractive to
use with Al alloys. However, they have not been extensively explored [40,41]; however, imidazoles
have shown good corrosion inhibition, primarily for copper-based materials, and precisely, purine and
17
adenine [53], 4-methyl-2-phenyl imidazole [54] and 2-aminobenzimidazole [55]. Benzimidazole, which
contains two nitrogen atoms, has the first atom located at position one in the structure. This atom is
a pyrrole type nitrogen atom whose unshared electron pair participates in imidazole π-electron
sharing. The second atom located at position three in the molecule, a pyridine type nitrogen atom,
whose unshared electron pair is free, was studied for the corrosion of inhibition of mild steel, where
benzimidazole (BI) and 2-mercaptobenzimidazole (2-MBI) are found to be suitable inhibitors as their
efficacies increase within increasing concentration in this order 2-MBI > BI, where their maximum
efficiencies were 73.1% and 90.4% obtained at 250 ppm for BI & 2-MBI, respectively [56].
Furthermore, several imidazole compounds have been used as corrosion inhibitors, mostly for pure Al
Fig 1.15C. 4-Methyl-2-phenylimidazole
Fig 1.15B. 2-Aminobenzimidazole
Fig 1.15A.Purine
Fig 1.15F. 2-Mercaptobenzimidazole
Fig 1.15E. Adenine
Fig 1.15D. Benzimidazole
and the AA2024 Al alloy [36,57,58].
According to He et al. (2014), studies of the inhibition effect of imidazole (IM) and 2-phenyl-2-
imidazoline (2-PI) for AA5052 in 0.1 M HCL using WL & polarization techniques were investigated, and
the inhibition efficiency was observed to increase with increasing concentration. At concentrations
greater than 3.9 mM, 2-PI showed higher efficiencies than IM, indicating its potential use in
commercial applications. The polarization results suggested that the two inhibitors act as mixed
inhibitors with a prevalent cathodic effect. The adsorption follows a Langmuir isotherm. The surface
morphology was studied by SEM & contact angle, and both showed that IM and 2-PI could be adsorbed
onto the surface and prevent the dissolution of AA5052. However, considering that the efficiencies of
both inhibitors were not high enough, the authors compared the effect of the single inhibitors and the
effect of both with the same concentration. The synergistic effect of both was studied, and the highest 18
efficiency obtained was 97.7%, which indicates that both are promising candidates for commercial
Fig.1.16B .2-phenyl-2-imidazoline
Fig. 1.16A. Imidazole
applications of corrosion inhibition of Al, only when used together to produce a synergistic effect.
Triazole, a five-membered cyclic compound with three nitrogen atoms, has also been widely used for
corrosion inhibition of many metals and their alloys in the last decade. Several studies have
investigated corrosion inhibition of copper and 6082 Al alloy using benzotriazole and its derivatives
[60,61]. Recently, attention has focussed on examining structures containing 1,2,4-triazole moiety for
the corrosion inhibition of metals. These compounds can be seen as environment-friendly inhibitors
because of their low toxicity and strong chemical activity characteristics. These derivatives are
naturally amphoteric, forming salts with acids and bases, and have a special attraction toward metal
surfaces displacing water molecules on the surface. Furthermore, they have the availability of π-
electrons and unshared electron pairs on the nitrogen atom that can interact with d-orbitals of any
Fig 1.17A. Hydroxybenzotriazole
Fig 1.17B.Benzotriazole
Fig 1.17C .3-Amino-1H-1,2,4- triazole
metal, providing a protective barrier film [62].
Four 1,2,4 triazole inhibitors were tested using high throughput methods for inhibition of 5083 and
6061 Al alloys, revealing adequate performance. However, further studies are required to determine
how these inhibitors protect the alloys and their effectiveness in more detail [46]. The inhibition
efficiency of 3-Amino-1,2,4-triazole-5-thiol in controlling corrosion of Al was investigated in 0.6M
Arabian Gulf seawater. The performance was evaluated by EIS and CP techniques. Both agreed that
inhibition efficiency was improved by increasing inhibitor concentration [41]. Furthermore, 3-Amino-
1,2,4-triazole-5-thiol has been used for corrosion control of copper, where it was found to be an
effective inhibitor with optimal efficiency of 93.5%, around 6 × 10–5 M according to three different
19
testing methods. The adsorptive behaviour of this inhibitor belongs to chemical adsorption, and
Fig 1.18B .5-(3-Pyridyl)- 1H-1,2,4-triazole-3-thiol
Fig 1.18D. 4-Amino-3-mercapto-5- phenyl-4H-1,2,4-triazole
Fig 1.18A .5-(4-Pyridyl)- 1H-1,2,4-triazole-3-thiol
Fig 1.18C. 3-Amino-1,2,4-triazole-5- thiol
according to polarization studies, it acts as a mixed inhibitor [62].
Thiadiazole is characterised by polar groups having two nitrogen atoms and one sulfur atom, which
enforce its inhibiting effect as a result of adsorption onto the metal surface, decreasing the corrosion
rate [63]. Ward et al. investigated 5-methyl-1,3,4-thiadiazole-2-thiol and 5-Amino-1,3,4-thiadiazole-2-
thiol as corrosion inhibitors using a rapid screening method to determine their initial potential for
resisting corrosion for two types of Al alloys: 5083 and 6061. Comparing the two inhibitors, whereby
the main difference is an amine group replaced by a methyl group, both showed good potential as
corrosion inhibitors with a slight advantage over the inhibitor-containing methyl group [46]. The
authors recommended further studies should be done to delve deeper into understanding the
Fig 1.19B. 5-methyl-1,3,4-thiadiazole-2-thiol
Fig 1.19A. 5-Amino-1,3,4-thiadiazole-2-thiol
mechanism of these inhibitors.
Thiazole compounds are expected to show excellent inhibition performance as organic compounds
containing a ring with heteroatoms like oxygen, sulfur, or nitrogen are known to be efficient corrosion
inhibitors [7,32,34,36]. Many thiazole derivatives were used for corrosion inhibition of mild steel and
copper, showing reasonable corrosion control.
2-aminobenzothiazole (ABT) and 2-amino-6-bromobenzothiazole were investigated for copper
corrosion inhibition in 3 wt.% NaCl. Both were shown to act as cathodic type inhibitors [64]. Thiazole
inhibitors were used for both Al and Al alloys; AA2024 and AA7075 [41]. 6-Amino-2-
mercaptobenzothiazole and 2-mercaptobenzothiazole were investigated for 2024 and 7075 Al alloys
20
utilising the weight loss method to discuss how different structures could affect the inhibition
performance. Diverse structures resulted in variation in efficiency. For AA2024, 2-
mercaptobenzothiazole efficiency was about 95 %, whereas 6-amino-2-mercaptobenzothiazole was
about 89 %. In contrast, for 7075 alloy, 6-amino-2-mercaptobenzothiazole showed better
performance than 2-mercaptobenzothiazole. However, in general, both showed good corrosion
Fig 1.20D 2-mercaptobenzothiazole
inhibition for the two alloys [1].
Oxazole is one of the azole families. It shows relatively good corrosion resistance due to one oxygen
and one nitrogen atom in its structure, which act as active centres for adsorption. The effect of 2-
mercaptobenzoxazole {MBOH} was evaluated on Cu, where the results of EIS indicated that over 100
hours, the surface layer of MBOH underwent various rearrangements and needed a relatively long
immersion time for the most protective layer to be built as Rp increased with immersion time. MBOH
was tested for corrosion of 6082 Al alloy, and the initial electrochemical tests showed at 50 C that
MBOH was not effective compared to 2-mercaptobenzimidazole and 3-Amino-1,2,4-triazole, which
were tested in the same study [62]. However, there is a lack of information on MBOH for Al alloy 5083
Fig 1.21. 2-Mercaptobenzoxazole
The influence of changing the position of functional groups and molecular structure on inhibitors' performance
in NaCl solution [40,41].
Corrosion inhibition efficiency is influenced by not only the type and number of functional families in
the structure but also changing the position of the same functional group may have a substantial
impact as well. This topic is often overlooked but is no less critical. As those features are related to
some characteristics of atoms in the structure, such presence of lone pair of electrons on atoms on
21
the functional group in the structure or/and electron sharing through drawing or donating, which leads
to bidentate or tridentate binding on the metal surface. Researchers have investigated the effect of
the position of functional groups on corrosion inhibition behaviour of different compounds [65–67],
where they noticed that increasing the amount of carbonyl group on chemical compound structure
resulted in enhancing and increasing charge transfer resistance due to increased electron sharing
between the inhibitor and Al surface. The inhibiting effect and mechanism of different Dihydroxy
benzene Isomers (resorcinol, catechol and quinol), as shown in Fig 1.22 (a, b &c), were investigated on
5754 Al alloy in bicarbonate solution buffered to PH = 11 [68]. This study suggested that the resorcinol
protection mechanism was due to interaction with the insoluble Al corrosion products layer.
On the contrary, catechol and quinol followed the ligand exchange model of adsorption. There was a
substantial increase in the inhibition performance provided by catechol at 10 mM after 24 hours of
exposure in the electrolyte from 63 to 98%, with only minor changes in inhibitor efficiency being
observed for resorcinol for the same duration. The researchers indicated how the location of the
Fig 1.22B. Resorcinol
Fig 1.22C. Quinol
Fig 1.22A. Catechol
hydroxyl group affects corrosion protection and inhibition mechanism towards 5754 Al alloy.
The impact of the structure of the inhibitor has long been a focus of attention when considering
corrosion protection by using inhibitors. Harvey et al. [1] studied a wide range of organic structures
with diverse molecular structures on their ability for corrosion control of aerospace alloys AA2024-T3
and AA7075-T6 in 0.1 M NaCl solution. Data of the study were measured using the weight loss method
as a performance measure; the substrates were held in the solution for four weeks. As shown in the
illustrated Figs 1.23 and 1.24, the authors investigated the influence of the structure of the compounds
on the inhibition performance. They showed how disparity in the molecular structure, whether
orientation or change in functional groups, could affect the inhibition performance of the inhibitors.
Positive values are an indication of corrosion retardation, while negative values indicate that corrosion
was stimulated. This study highlighted the importance of functional groups regarding different types
22
of these groups and their positions in the structures on inhibitor's performance for corrosion control.
The results obtained from the analysis showed that the thiol group, different positions of a
carboxylate, and substitution of N for C with particular orientation strongly retarded the corrosion, as
shown in Fig 1.24 when 1X (Na-acetate) and 1V (Na-mercapto acetate) were compared.
Regarding the position of chemical functionalities on the ring structure, the study proved that the
orientation has a significant impact on the inhibition performance. The efficiencies changed as the
position of the functional group changed, as indicated by the following transitions: by changing the
location of the thiol group (-SH) in the structures 1L, 1Q and 1S, which is probably associated with
structural geometry, which in case of 1L the coverage of the surface will be full-on metal surface, the
efficiencies for 2024/7075 were (97/76%), (16/-22%) and (88/80%), respectively, while by changing
the position of nitrogen in structures 1N, 1O and 1P, found that the efficiencies were (-107/-91%), (-
12/-45%) and (58%,14%). On the other hand, the efficacies of structures 1K and 1R were (-34/-56%)
and (-175/-89%) by altering the location of hydroxyl. The hydroxyl showed a little inhibition behaviour
as by comparing 1G (-80/-62%) within 1K, and 1R, noticed that the inhibition declined after the
addition of the -OH group, while the carboxylate group provided low or no corrosion inhibition, as
shown in Fig 24. In several cases, the researchers found that some inhibitors showed diverse impacts
on the different alloys; for instance, compounds that contained thiol have shown higher efficiency for
Fig 1.23. Illustration of the structures of standard heterocyclic inhibitors tested. (Note: similar sub-structures are highlighted with coloured circles.) [1]
AA2024 than for AA7075.
23
Fig 1.24. Illustration of the inhibitors’ structures, showing structural relationships. (Note: similar sub-structures are highlighted with coloured circles.) [1]
Impact of Alkyl chain length with anchor group
The influence of the length of the alkyl chain (responsible for lateral interactions between adsorbed
inhibitor species) and anchor group (adsorbed on the metal surface) was studied by Milošev et al. [69]
for thirteen alkyl-based organic compounds on a superhydrophobic, hydroxylated Al surface. In this
study, several anchor groups were tested (azide, imidazole, thiocyanate, amino, disulphide, thiol,
phosphonic, carboxylic, and benzoic) with two alkyl chain lengths (eight and eighteen carbon atoms).
The superhydrophobic properties and the corrosion resistance were observed to rely on the anchor
group, which determined the adhesion to the surface and alkyl chain, which were associated with
lateral cohesive interactions.
24
Fig 1.25 Schematic diagram of the structural division of the inhibitor into an anchor group and a backbone (left). The anchor group governs adhesion to the surface (indicated by red arrows), whereas the backbone is responsible for cohesive interactions within the monolayer (brown arrows). In order to better disentangle the effects of the two structural components, a standalone adsorbed molecule with a methyl (Me) group as a minimal backbone (right) was used when investigating the effect of the anchor group [69]
According to the electrochemical testing results, the researchers divided anchor groups into three
categories: (1) strong inhibition behaviour, (2) either no, or weak to moderate inhibition behaviour
and (3) corrosion accelerators. The authors determined that the phosphonic group was found to be
protective in all cases regardless of the chain length. In comparison, the carboxylic group appeared to
be protective when bonded to a longer chain, the octadecyl alkyl chain (C18-R). In contrast, thiol and
imidazole were not effective regardless of alkyl chain length; All inhibitors consist of an alkyl chain
with eight (designated as C8) or eighteen carbon atoms (C18) and an anchor group. However, only
four anchor groups were tested for their inhibiting effect on the C18 alkyl chain. XPS analysis was used
to study the adsorption of organic inhibitors and the establishment of dense molecular layers on the
Al surface. The result revealed that only two anchor groups were strongly bonded to the etched Al
surface, but anchor groups containing nitrogen or sulfur were not. ToF-SIMS fragment analysis was
used to deduce the binding mechanism. The study clarified that the phosphonic acids were bonded to
either two or three Al on the surface whilst carboxylic acids were bonded to either one or two Al on
25
the surface, as shown in Fig 1.26 below. In comparison, the remaining anchor groups were not present
Fig 1.26. A schematic diagram shows the bonding mechanism of phosphonic groups.
Alternative inhibitors for 5xxx series Al alloys
on the Al surface in sufficient concentrations to be detected by XPS [69].
This section of the literature is focused on reviewing a selection of alternative inhibitors which have
been used previously for 5xxx series alloys, including organic inhibitors, green compounds and
inorganic inhibitors. However, they were not used in this study as they are not linked to the selection
Polyoxyethylene (20) sorbitan monooleate (Tween 80)
criteria for this research.
The inhibiting effect of Tween 80 was studied for 5754 Al alloy in 0.5 M NaOH using polarization
measurement and quantum chemistry method. The inhibition efficiency increased with increasing
concentration but decreased with temperature, with the maximum efficiency value obtained at 300
PPM at 298 K. Polarization curves showed that Tween 80 acted as a mixed inhibitor and the adsorption
isotherm followed a Langmuir isotherm with negative values of Δ𝐺o ads. The authors attributed the
inhibition action of Tween 80 to its adsorption on the surface of the metal, forming a barrier and thus
Inorganic Inhibitors
blocking the corrosion process [70].
− 4 and MoO2
− 4 as corrosion
A study was conducted for A5754 and A5052 to test the impact of WO2
inhibitors in 0.3 M H2SO4. Pure Al was examined in sulfuric acid through PDS as a reference. The
investigation was done by using PDS, EIS, scanning electron microscope (SEM), and energy-dispersive
X-ray (EDX) methods. The results showed that surface coverage increased with increasing inhibitor
concentration but was found to decrease with increasing temperature and electrolyte concentration.
− 4 was found to be more efficient. For all three systems, the corrosion rate in H2SO4 was found
MoO2
to decrease in the following order: Al > A5754 > A5052 [71].
26
A new trend in inhibitors
Environmental concerns require eco-friendly corrosion inhibitors to be developed. Efforts are now
underway for the industry to come to terms with the environmental impact of certain previously
accepted corrosion inhibitors when present in discharge water, for example. The conventional
inhibitors used may be satisfactory concerning corrosion mitigation, but their ecological implications
are not fully understood. Thus, efforts have been directed toward the development of cost-effective
and non-toxic corrosion inhibitors. Plant products and some other sources of organic compounds are
rich sources of eco-friendly acceptable corrosion inhibitors. Green corrosion inhibitors are
biodegradable and do not contain heavy metals or other toxic compounds like some of the synthetic
ones. In addition to being environmentally friendly and ecologically acceptable, plant products are
inexpensive, readily available and renewable. Some research groups have reported the successful use
of naturally occurring substances to inhibit the corrosion of Al and its alloys in an acidic and alkaline
L-cysteine
environment [57].
The inhibition ability of L-cysteine was investigated for 5052 Al alloy in 4M NaOH through polarization
measurement and quantum chemical calculation. The inhibition efficiency increased with increasing
concentration but decreased with temperature. L-cysteine was selected as it contains a mercapto
group, apart from an amino group and a carboxylic group in its molecule, as they were considered
active adsorption sites. The maximum efficiency of 93.3% was obtained at 30 mM concentration.
Polarization curves showed that L-cysteine acted as a cathodic inhibitor and the adsorption isotherm
Henna (Lawsonia inermis)
followed a Langmuir-type isotherm [72].
The inhibition effect of Henna was studied for 5083 Al alloy in seawater with different concentrations
of henna, namely 100, 200 & 500 PPM for 60 days of immersion. Using WL and EIS Techniques. FTIR
was used to determine the functional group in the henna powder. The inhibition efficiency increased
with increasing concentration. The highest efficiency reached was 88% at 500 PPM. The inhibition
27
potential of Henna was due to adsorption by forming a protective layer for the charge and mass
Honey
transfer between alloy and the surrounding [3].
Natural honey has been used as an effective corrosion inhibitor for different metals such as copper &
steel [73]. For Al alloys, natural honey has been studied for its corrosion resistance for two different
alloys (AA6061 & AA5052) using PP, LPR and EIS. The existence of –C=C– groups, N, S and O atoms in
the structure of its molecule, therefore, leads to adsorption of the chemical compound on the metal’s
surface. Honey was found to act as a mixed inhibitor from polarization data. For AA5052, five types of
natural honey were used as corrosion inhibitors in 0.5 NaCl. The inhibition efficiency increased with
Natural oils
increasing concentration as well [3].
A few natural oils were investigated as corrosion inhibitors for Al and its alloys. Natural oils are
concentrated hydrophobic liquids containing monoterpene, sesquiterpene hydrocarbons and
oxygenated compounds. The right combination of these components makes natural oils appropriate
for use as green inhibitors, combined with their availability and low toxicity. Laurus nobilis L. oil
inhibition performance was investigated for Al and 5754 Al alloy in 3 wt.% NaCl using WL, EIS and PDP.
The electrochemical results showed that the corrosion resistance of 5754 in NaCl was better than that
of Al. However, inhibition of pitting corrosion of oil for Al was more effective than for the 5754 Al alloy.
SEM analysis confirmed the obtained data and offered corroborating evidence that the passive layer
1.7. Corrosion monitoring
1.7.1. Electrochemical techniques
is adequately protected by adsorption of the active molecules present in L. Nobilis L. Oil [22,74] .
The measuring techniques used to determine corrosion rates can be broadly classified into two
categories: non-destructive and destructive. The measuring method is destructive if it alters the
corrosion process during the measuring process (e.g., PDS). Non-destructive techniques include LPR
and EIS.
28
Since corrosion is a process involving electrochemical oxidation and reduction reactions, it makes
sense that electrochemical methods can be used to study and measure corroding systems. More
specifically, when a metal is immersed in a solution, electrochemical reaction characteristic of the
metal-solution interface occurs at the surface of the metal, causing the metal to corrode. These
reactions create an electrochemical potential, called the corrosion potential or the open circuit
potential (OCP, measured in volts), at the metal-solution interface. Since the specific chemistry of the
system determines the corrosion potential, it is a characteristic of the specific metal-solution system.
The potentiometric methods include OCP, named Ecorr, as well as LPR and PDS. The measurements use
three standard electrodes; the working electrode (the material being tested and will vary), the counter
electrode (to supply the current flowing at the working electrode during the test) and the reference
electrode.
The three electrodes are immersed in the electrolyte (corrosive media). A difference in potential
between the microcells of metal is registered after reaching the OCP value (equilibrium). The
reduction and oxidation reactions occur at that potential, which allows evaluation of whether the
metal is resistant to corrosion in the tested electrolyte or not. The more positive the free corrosion
potential is, the more thermodynamically stable the system will be toward corrosion. PDS results are
represented graphically by Tafel slopes. The plot is between the applied potential (E, V) versus current,
which is calculated as the current density (i, A/cm2). This graph is a straight line. However, the
measured current (black lines) needs to be extrapolated from the straight sections (green dots) to give
the corrosion current (icorr, red dots) [75].
29
Fig 1.28. Tafel plot [75]
The cathodic and anodic behaviour of the applied inhibitors will be identified by using PDS, besides
determining the inhibition efficiencies of the inhibitors. LPR technique will be used to measure
corrosion rate via polarization resistance (Rp). In this technique, small perturbations in potential are
applied of approx. 25mV about open circuit potential (OCP) to calculate the potential of the corrosion
resistance of the Al alloy, where the data will be recorded at different time intervals that could reveal
changes in the inhibition efficiency over time. It is a powerful tool for providing information about the
long-term performance of an inhibitor via measuring Rp over a specific period of time, where data of
Rp can be directly associated with film formation. Thus, increasing Rp over time could be an indication
of film building up over the surface. For film build-up confirmation, EIS can be used as an additional
technique for providing data about film formation over the surface. The electrochemical methods will
1.7.2. High throughput assessment
be recorded for inhibited and uninhibited conditions to determine the mechanism and film build-up.
There is always a need to develop and discover more inhibitors for specific applications to replace
chromate-based ones. However, the traditional testing methods can be extremely time-consuming,
mainly if several techniques are applied to assess a wide range of inhibitors, such as electrochemical
and weight loss experiments. Much concern surrounds the development of methods to evaluate a
wide range of inhibitors in a short period of time, and consequently, many new rapid screening
methods have been developed to address this issue. The various rapid screening methods are
discussed in this section.
30
Previous studies have based their work on corrosion in marine environments using traditional
techniques for measuring mass loss to indicate corrosion rates. These long-term techniques have
many advantages, but the cumulative nature of these testing methods makes it hard to separate
environmental parameters individually. This challenge might also be said of other corrosion data
methods. The conventional testing techniques are linked with a deficiency in experimental controls,
Droplet method
lack of duplicability, and requiring a long exposure time [76].
In this method, the droplets are placed on sample surfaces of different sizes to represent coarse-sized
marine aerosols within various concentrations and pH. In this letter, the corrosion rate is determined
by volume loss (∆V) through optical profilometry (OP). This technique can quantify the role of
individual variables and allows for exploring the possible interactions between different parameters.
Azmat et al. [76] have systematically used the droplet method to study individual variables involved
in atmospheric corrosion. The evaluation of atmospheric corrosion measurements has been done
through OP because it was proved to be an appropriate tool due to its rapidity, efficiency and
reproducibility and experimental control [77]. Several factors are related to the capability of predicting
atmospheric corrosion. Muster et al. have studied the influence of droplet chemistry (electrolyte
concentration), size and shape on atmospheric corrosion [78].
The droplet method provides results and analysis in a short time compared to electrochemical
techniques. Azmat studied atmospheric corrosion of zinc under various simulated marine aerosols
using the droplet method. Once droplets were placed on the Zn surface, several interactions were
observed, such as the development of pH, O2 gradients, the establishment of electrochemical cells,
secondary spreading and precipitation of corrosion products. Four droplets (S1-S4) were prepared to
investigate the effect of different acidified marine droplets. The testing time for the droplets was 6
hours, which is reasonably short compared to traditional corrosion testing methods. Figs 1.29A and B
show the effect of the variations of PH with the addition of a wide range pH indicator (WRI) after
placing droplets on Zn surface for six hours. Fig 1.29A shows droplet S1 (0.6M NaCl) and Fig 1.29B
31
shows droplet S2 (0.6M NaCl solution acidified with HCl). These figures reveal that when droplets are
placed on a metal surface, a sequence of events is stimulated, rendering the formation of three distinct
zones (centre, periphery and secondary spread region), which impacts oxide growth and corrosion
Fig 1.29B.Variation of pH in a 1 µL droplet of S2, with the addition of WRI, placed on a Zn surface for 6 h [79].
Fig 1.29A. Variation of pH in a 1 µL droplet of S1, with addition of WRI, placed on a Zn surface for 6 h [79]. A similar approach can be used to discover and test
[79].
a wide range of inhibitors in droplets [77], where a wide range of variables can be assessed
simultaneously with considerable experimental control and relatively short exposure time. This novel
high throughput method was built on studies carried out in the corrosion inhibitors field. So, this
method can be conducted in a short time by placing several droplets with inhibitors and different
electrolyte concentrations. However, there are limitations to this method as it is sensitive to metals
Wells testing method
subject to uniform corrosion but not to metals’ surfaces that are subject to pitting.
White et al. developed a rapid screening technique, which involved conducting 88 simultaneous semi-
quantitative tests of various inhibitors on a sheet of metal [80]. Wells testing technique has the
advantage of being rapid, correlating with the classical corrosion testing methods, short period of
experiment preparation and reduction of analysis time. Additionally, satisfying the previous
objectives, the wells testing method also has a reduced environmental impact as it uses fewer
materials in the procedure as well as reduces the volume of solution to 17.6ml (88 x 0.2 ml), as shown
in Figs 1.30A and B below. This method utilises image processing to analyse the corrosion. The
processing equipment classified the corrosion between 0 - 10, where 0, indicating the least corrosion
32
product present to 10, is a great deal of corrosion. In 2012, White et al. conducted a wells testings
method to investigate corrosion of AA2024-T3 in NaCl and dichromate solutions of varying
concentrations (10-1–10-6 M) and (10-3–10-7 M), respectively. This method showed good agreement
with standard immersion tests over the same 24h period and could be used as a corrosion predictor
for a 28-day standard immersion test.
In 2016, Ward et al. studied the corrosion behaviour of marine-grade alloys, particularly 5083-H321
and 6061-T6, using high throughput well testing on a wide range of inhibitors [35]. The corrosive media
of this study was NaCl with 0.1 M (5.8 wt.%) concentration to assure corrosion of these alloys occurred.
The test was successfully applied to rapidly screening 125 commercial corrosion inhibitors. A shortlist
of high-performing inhibitors was made, and the authors found that strong inhibiting characteristics
were associated with i) the presence of two or more nitrogen molecules connected either by
coordinate bond or nearby π-bonding; (ii) the existence of 5-6 membered heteroaromatic rings; and
Fig 1.30B. 96 well plate with the solutions containing dissolved [35]
Fig 1.30A. Angled view of 88 well test apparatus [71]
Multielectrode
(iii) the presence of at least one sulfur atom connected to the aromatic ring.
Multielectrode arrays have been demonstrated by Taylor and Chambers as a high throughput
electrochemical testing method on corrosion inhibitors. This approach was used to accelerate the
discovery of potential inhibitors and their synergetic effect for Al alloy 2024. Direct current
polarization was applied between two 2024 alloys wire electrodes and multiple electrode testing
systems to evaluate the inhibition characterisation of 50 different chemistries within nine hours. A
33
path for rapid discovery and inhibition performance of effective inhibitors in an array of conditions
was provided by this experimental approach [81].
An adaptation of the multielectrode array was developed by Muster et al. with numerous
developments to previous studies, such as the ability to evaluate various metals simultaneously in one
solution, as shown in Fig 1.31. This method reduced the experimental time by using a blank sample to
confirm the validity of the experimental setup before each inhibitor evaluation [10]. The validity of
this method to evaluate the inhibition performance of novel chemicals over a pH range and
Fig 1.31. Multi-electrode assembly showing the layout of wire specimens 1–9 [17]
1.8. Discussion and limitations
optimization of their concentration was later studied by the same authors [82]
According to the literature on high throughput techniques for evaluating a wide range of chemical
compounds with the aim of stopping corrosion on metals, a need was found for developing a new
rapid screening method for metals subject to pitting. The literature has shown that there are many
corrosion inhibitors that have not been used before on Al alloy 5xxx series, particularly AA5083, which
is prone to pitting when exposed to droplets or rain aerosols, which is important as it is being used in
a wide number of industries such as construction and shipbuilding. Also, limited work has been done
on determining inhibitors' mechanisms on that alloy. According to the literature, it has been observed
there are special features that make chemical compounds act as promising corrosion inhibitors; lone
pairs of electrons from S, N or O-rich ligands as they act as adsorption centres or by the same elements
in the ring, attachment of additional functional group to the structure may affect electron density on
the lone pairs which could be withdrawing or donating, the position of a functional group on the
chemical structure has shown that a great effect on the performance where the geometry of the shape
34
influence the settling of inhibitor on metal surfaces may lead to steric hindrance. Also, little work has
been done on testing different features of inhibitors on many metals, particularly Al and its alloys,
which triggered the curiosity about understating how the different inhibitors within the same class
work and interact on metal surfaces.
1- No high throughput droplet technique for metals subject to pitting.
2- A low number of corrosion inhibitors are being used for marine-grade alloys, particularly
Therefore, here are the highlighted gaps that were missing in the literature:
3- Very few studies have been done on the long-term durability and stability of inhibitors on
AA5083.
1.9. Conclusions
AA5083.
At present, a range of major research initiatives in the broad “materials genomics” space is progressing
where high throughput experimentation, traditional electrochemical methods and computational
materials are being combined to allow the rapid design of new inhibitors. New high throughput
methods are urgently needed to support these studies. The droplet methods reported by Azmat have
proved useful for rapid studies but do not capture all the information required for metals and alloys
that corrode primarily by pitting.
So, this masters project will present
1) A new high-throughput method for alloys that suffer a local attack.
2) Testing a wide range of inhibitors from 4 different classes for a better understanding of
molecules/ metal interaction
3) Provides a comparison of the degree of attack and the efficiency of inhibitions between
corrosion under droplets and in bulk solution.
35
Chapter 2
Methodology
2.1. Introduction
This chapter focuses on developing the rapid screening method using the droplet strategy, starting
with adjusting the environment of the tests, followed by surface preparations to be sure the surface
is ready for exposure and surface cleaning after exposure, in addition to the methodology for
electrochemical methods and surface analytical techniques that also will be used in this study. Here,
there are features that should be presented in the new method, which are as follow.
1. Reasonably rapid to save time and effort
2. Generating measurable results that can be used for other metals.
3. Limited error with higher accuracy and reproducibility
4. Useful for metals and alloys that are subject to localized attack, particularly pitting.
5. It can be correlated to atmospheric corrosion as AA5083 alloy.
6. High sensitivity for inhibited solutions.
The current rapid screening droplet method was developed by Azmat [76] recently. In this method,
multiple drops are placed on a polished and flat test piece for a fixed time. The developed corrosion
products are removed, and the volume loss of the metal is determined by optical profilometry. The
main principles of this method can be applied in our study, but the practical protocols and procedures
will be modified for a substrate with a different corrosion mechanism. Thus, as the high throughput
droplet studies (using Azmat’s approach) have never been applied to the system used in our study,
considerable work has been undertaken to ensure that the method is accurate and reproducible.
Before developing and applying the technique, there were main issues that needed to be addressed
to develop standardised operating procedures: to achieve the above requirements.
1) How to ensure the preparation of the AA5083 surfaces renders the surface stable, consistent, and
reproducible. The surface must be flat and free from scratches and artificial pits, and other surface
36
abnormalities. This is to ensure that the profilometric of pits depth post-exposure does not include
the volume of any pre-exposure defects.
2) How to expose the optimal number of drops on each plate to maximize data acquisition by ensuring
post-exposure droplet loss has minimum errors, and the drops remain stable throughout the exposure
time. Given that this is a high throughput methodology maximising the number density of drops on a
single plate is the key. However, several challenges remain. Firstly, the drops must neither interfere
with adjacent ones nor the plate edges. Secondly, there might be some inherent sources of errors in
the pit depth methodology. The procedure used in the cleaning of corrosion products might affect the
loss of underlying metal. Thus, a comparison with the volume of mass loss of a blank benchmark is
required. For an accurate cleaning method, the post-exposure pit depth of a tested sample should be
much greater than the pit depth of the blank. Thirdly it is important that the drop remains stable and
does not evaporate during the exposure time period. This is achieved by placing the entire setup in a
> 90% RH chamber with minimum airflow.
3) How to clean the specimens after exposure in such a way as to maximize the removal of corrosion
product while minimizing attack on the underlying non-corroded substrate. A procedure based on
removing oxides with 10% nitric acid was used in this study [83]. However, the time of acid cleaning
and the
procedures before and after cleaning with acid must be optimized to ensure oxide removal without
metal attack. The risk and extent of the metal attack are measured using a “blank”. A blank is a coupon
of the same metal material, prepared in the same manner but not subjected to droplet exposure. The
coupon was cleaned with the same steps used for the tested material, and the blank depth of the pits
was determined. An effective cleaning method should ensure the removal of corrosion products with
a minimum effect on the bulk metal.
In addition to optimizing the procedures for droplet tests, protocols for electrochemical
measurements were optimized. The standard procedures that have been used in our group may need
to be tailored and optimized for each material by considering two important steps:
37
1) Surface preparation technique that ensures the removal of any contamination of the surface prior
to electrochemical examinations
2) Determining the appropriate holding time prior to undertaking PDS. When a surface is placed in an
electrolyte, it will adapt to the electrolyte (typically by the surface hydrating or changing its oxide
layer). This initial surface reaction will lead to instability of the surface potential. It is critically
2.2. Experimental Procedures
2.2.1. Sample preparations
important that electrochemical techniques are made after this initial adjustment.
The main objective of this part is to identify a method for efficient surface preparation through which
a stable, consistent, hydrophilic and reproducible Al alloy surface can be prepared. AA5083-
H116AA5083 plates with dimensions (150 x 100 mm x 3 mm) were received from (Calm Aluminium,
Minto, NSW) as shown in Fig 2.1. These plates were machined into 15 mm x 15 mm x 3 mm coupons
to fit in the polishing machine. The samples were mounted with epoxy resin, followed by mechanically
grinding with SiC papers 1200 grit and progressive polishing with diamond cloths of sizes 9, 3 and 1
µm. Analytical grade ethanol was used for cleaning the surface after each grinding/polishing step, and
finally, the samples were ultrasonically cleaned in ethanol and dried by a N2 air flow at room
temperature.
m m 0 5 1
Fig 2.1. Photograph of as-received sample of AA5083.
100mm
38
2.2.2. Experimental environments
The droplet method was conducted in an appropriate humidity chamber with relative humidity (RH)
above 90% at room temperature that ensuring the drops would not evaporate during exposure time
Fig 2.2. Potassium sulphate (K2SO4) saturated solution was used to keep the RH between 96-98 % at a
Fig 2.2. The Humidity chamber
2.2.3. Droplet exposure upon AA5083 surface
range from 15 to 40 °C. [84,85].
Initially, four replicates of 0.6M NaCl droplets with a size of 2 μL were deposited on AA5083 coupons,
as shown in Fig 2.3. Droplets were placed at least 10 mm apart to avoid overlapping moisture layers
resulting from secondary spreading [86]. After droplet deposition, the samples were immediately
moved to a holder in a humidity box. Fig 2.3. shows the initial humidity box made using a plastic
container with K2SO4 in another small box inside it. The K2SO4 was left overnight in the humidity
Fig 2.3. The initial humidity chamber
chamber to maintain a high humidity environment prior to the droplet corrosion events.
39
2.2.4. Removal of corrosion products
In order to get an accurate depth measurement, corrosion products should be removed thoroughly
and carefully. Suitable cleaning protocols should be chosen to maximize the removal of corrosion
products without affecting the base metal. Different post-cleaning methods were trialled. The first
one (M1), Al alloy coupons, were ultrasonically cleaned with concentrated nitric acid for a minute,
followed by washing with distilled water and ethanol. The second method (M2), post-cleaning, was
carried out by washing with distilled water and ethanol twice before and after swabbing with 10%
2.2.5. Quantification of corrosion using optical profilometry
nitric acid for a minute and was dried using a nitrogen gun.
Azmat used an optical profilometer (OP) to measure corrosion rate in terms of volume loss [87]. OP is
a rapid, effective, and reproducible machine to conduct measurements of atmospheric corrosion
under droplets.
The corrosion rate of AA5083 under droplets was quantified in terms of measuring the pits. Pits
counting and measuring were determined after cleaning the Al surfaces, as removing corrosion
products is necessary to determine an accurate depth and produce a surface amenable (reflective) to
OP by interferometry. That was done using an OP (Bruker) in vertical scanning interferometry (VSI)
operating mode, in coordination with Vision software and the aid of user-interactive software-assisted
Fig 2.4. OP images for a drop of 2 µL
thresholding. Fig 2.4 shows 3D images of AA5083 corroded under a droplet of 2 μL.
40
2.2.6. Electrochemical methods
Electrochemical methods were compared to results obtained from the droplets method. In addition,
OCP, PDS and other electrochemical techniques indicate the required time for system stability and
film formation of inhibitors. The Al alloy 5083-H116 substrate was ground with 2500 grit SiC paper
2500 grit and then rinsed with ethanol before immersion in the electrolyte. Aerated NaCl solution at
room temperature was used as corrosive media with a concentration of 0.6 M, which is similar to
2.2.7. Surface analytical methods
seawater concentration.
Surface analytical techniques used in this study were scanning electron microscopy (SEM), optical
microscopy, energy dispersive spectroscopy (EDS), and focused ion beam (FIB), which were used to
determine inhibitor's coverage of the metal surface along with oxide development and identify the
2.3. Method development
2.3.1. Coupon exposure
area of corrosion attack.
The modified humidity chamber provided the required RH, as after 7 days of exposure and deposition
on the AA5083 surface, droplets did not evaporate, as the 2.2 mm diameter remained the same after
the exposure and showed good stability as a comparison between before and after as shown in Fig
2.6.
Fig 2.6. (A) droplet under the digital microscope after 7 days of exposure (B) droplets after 7 days of exposure in the humidity chamber
B A 2.2 mm
41
2.3.2. Removal of corrosion products
using method 1 (M1) to remove corrosion products was found to dissolve the Al alloy substrate leaving
no signs of droplet sites, as shown in Fig 2.7. Concentrated nitric acid caused some damage to the base
plate of AA5083 when used to remove corrosion products. In contrast, method 2 (M2) removed most
corrosion products but left some tiny traces. Fig 2.8 shows how the droplet looks like after cleaning
using M2. The cleaning time using the M2 method was adjusted to maximize removing corrosion
products. Time adjustment was achieved using acid cleaning on a blank substrate without exposure
to droplets. The blank was prepared in the exact way of the exposed samples. The effect of cleaning
on the depth of pits after the evaluation was determined by comparing M1 and M2. It was found that
when the specimens were exposed in a humidity chamber for 7 days under a saline drop, the average
pit size in blank exposure in the humidity chamber case was 900 nm, which was found to be equal to
the average pit’s depth in the case of cleaning with M1. Thus, it could be concluded that nitric acid
cleaning after exposure does not substantially increase pit depth, as both depths do not change after
cleaning a blank coupon.
A
C
B
Fig 2.7. (A) OP image, (B) XY profiles and (C) 3D image of a cleaned Al alloy surface after 24 h of exposure
42
Fig 2.8. Droplet under the digital microscope after cleaning
After surface preparation and polishing process without any exposure, surface roughness and depth
of pits have been measured using OP. The maximum surface roughness that was observed in 1 x 1 mm
was 52 nm, and the depth of the pit was 450 nm. Fig 2.9 shows images of surfaces after polishing, with
and without exposure to the dilute nitric acid cleaning solution. On the other hand, AA5083 polished
surface after swabbing with 10 % Nitric acid was found to raise marginally surface roughness to 1.5
microns, the depth of pits at maximum and the mean was between 550 to 600 nm. Thus, the cleaning
technique has caused only a minor (around 20%) increase in the average pit depth. Therefore, the
depth that was observed in that case was automatically deducted from our calculation after applying
an aspect ratio range from 1:1.3 to 1:2.4.
Fig 2.9 images of AA5083 after polishing
Fig 2.10 images of polished AA5083 surface after swabbing with 10% nitric acid
inhibition systems. It was observed that the depth was 1.3 to 2.4 times longer than the width, within
43
2.3.3. Electrochemical techniques
Open circuit potential measurements (OCP) OCP technique was conducted with inhibited and uninhibited solutions over 5 hours to determine the
stability of the system. Data obtained from OCP was used to determine the breakdown stability. At 2
hours, the voltage drift had stabilized sufficiently; hence, this chosen time was used before conducting
PDS, LPR, and EIS; found to be 2 hours prior to conducting PDS, LPR and EIS these tests. There were
some minor initial fluctuations before the system stabilized. However, these minor errors will not
Potentiodynamic polarization scans (PDS)
affect the results.
To reveal the effect of inhibitors on cathodic and anodic reactions, PDS was conducted at a scan rate
of 5 mv/s over a range of -200mV vs OCP to 400 mV vs OCP from cathodic to anodic. Prior to every
full cathodic/ anodic scanning, the system was left at OCP measurements were conducted for 2 hours
to ensure system stability before starting PDS. The tests were conducted at least in triplicates to
ensure reproducibility. Obtained data were plotted E (VAg/AgCl) vs i (A/cm2) using OriginPro 8.5
software, followed by Tafel fitting to determine corrosion rates through calculating icorr values. All
Tafel fittings were conducted by using EC-lab software. Extrapolation was fitted across ±50 mv
Linear polarization resistance (LPR)
around the OCP point for each scan.
LPR scans were conducted using the corrosimetry (CM) technique in EC Lab software. LPR was used to
calculate polarization resistance (Rp) over the time of the test as an indication of inhibitor film
formation over the surface of AA5083. Before starting the scan, the AA5083 specimen was allowed to
stabilize for 2 hours. This step reduced noise in scans. LPR experiments were conducted over 2 weeks
Electrochemical impedance spectroscopy (EIS)
to determine the long-term stability of shortlisted inhibitors.
EIS was conducted using the potentio-electrochemical impedance spectroscopy (PEIS) technique in EC
Lab to obtain impendence data. The impedance data were collected every 24 hours for 2 weeks. The
measurements were collected over a frequency range of 100 kHz to 10 MHz with Nd = 7 points per
44
decade and signal amplitude, Nd Va of 15 mV. EC-Lab V 11.31 software has been used for fitting the
model and EIS processing.
For the fitting of a suitable circuit model, Z fit was used, which is the impedance fitting tool in the
software for fitting the electrical circuit for an inhibitor-containing system. The Nyquist plot and the
2.3.4. Surface analysis techniques
minimization tool were used as an initial parameter as a starting point in the software.
Post-surface analysis techniques were used to confirm the data obtained from electrochemical
techniques. They also help in revealing the surface chemistry of samples after immersion in inhibiting
Optical microscopy
solutions. Different analytical imaging techniques used were optical microscopy, SEM/EDS and FIB.
An upright materials and metallurgical microscope, Leica DM2700 M, was used with the Leica
application suite X (LAS X) as the software system for initial imaging of the surface after being
Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) Analysis
immersed in uninhibited/inhibited solutions at magnifications ranging from 2.5 to 40 ×.
SEM and EDS analysis were carried out using FEI Quanta 200 with Gatan Alto Cryo stage; the operation
was in high vacuum mode. Before placing the samples inside the machine, samples were cleaned using
a nitrogen gun to remove particles that could disturb the imaging process. The bigger the spot size, the
clearer the signal, while the resolution is better for smaller spots. Thus, the spot size used was adapted
depending on the imaging purpose. A working distance was 10 mm, and the voltage was between 10 kV
and 30 kV.
In EDS analysis, AztecEnergy software (Oxford Instruments) was employed for data acquisition and
analysis. EDS was used to identify elemental composition over the surface and revealed intermetallic
phase content, structures, and distribution. For image processing, the accelerating voltage was reduced
to the range of 5 to 10 kV, as a lower accelerating voltage guarantees better accuracy in detecting a wide
range of elements bonded with the surface. Elemental compositions on the surface were determined
using a point, line, or map scan. The advantage of using EDS is to get visual evidence that would help in
45
understanding different corrosion and inhibition processes that happen on the AA5083 surface and the
2.3.5. Reproducibility of OP data
intermetallic particles.
Table 2.1 shows the percentage error of optical profilometer measurement to the depth of pits, where
the measurement was taken for the same pit’s depth 20 times. From four pits that have been measured,
Table 2.1. Percentage of error of pits measurements via OP
the error associated with each depth value was within 15% for most droplets.
# pit Pit depth (nm) % Error Actual range (nm)
1 1450 11% 1291-1610
2 3400 15% 2890-3910
3 2700 14% 2322-3078
2.4. Conclusions
4 4593 15% 3904-5282
In summary, the AA5083 surface was prepared by being ground with SiC paper 1200 grit followed by
progressive polishing with a diamond suspension of sizes 9, 3 and 1 micro. SiC grinding papers are
routinely used for surface preparation of several materials prior to corrosion tests, although
contamination with SiC and the possible impact on subsequent surface interactions is usually
disregarded. To ensure the decontamination, surface polishing will be done with diamond finishing
and examined under the microscope. The humidity chamber was modified by bringing the humidity
reservoir closer to the samples to provide an appropriate RH for the test and stabilize drops during
the exposure time. In order to get accurate measurements, corrosion products were removed after
the exposure period with an appropriate cleanser. The following protocol (wash with distilled water,
then ethanol, swabbing with 10% nitric acid on droplets sites for 60 seconds followed by washing with
distilled water and ethanol to stop the acid attack) was shown to be effective, as it removed most of
46
the corrosion product. However, some tiny traces were found with this method, and the time duration
was adjusted appropriately. Pit measurements were conducted using OP operating in vertical scanning
interferometry (VSI) mode. Subsequently, pit measurements were estimated using a user-interactive
software-assisted manual thresholding and masking routine to make the surface flat. OP was
employed to characterise and analyse the number of pits, depth and height differences and volume
measurements.
47
Chapter 3
Development of a high-throughput screening tool
3.1. Introduction
This chapter presents a new high-throughput droplet technique used to evaluate pitting corrosion and
its mitigation under conditions similar to the marine environments. For this purpose, seven common
inhibitor compounds were selected for investigation: catechol, resorcinol, quinol, 3-amino-1,2,4-
triazole-5-thiol, benzotriazole, 3-amino-1H-1,2,4-triazole and imidazole. Al alloy 5083 (AA5083) was
employed as a model material for proof-of-concept given the high vulnerability of the AA5xxx series
to pitting, which is ascribed to the inclusions of intermetallics such as Al6(Fe, Mn) and Mg-Si phases
[28]. Moreover, to verify the results from the proposed droplet method (based on pits depth
measurements), they were compared to that of PDS, a conventional electrochemical method used for
3.2. Electrolyte preparation
corrosion evaluation and inhibitor characterization.
In this study, seven chemical compounds were used as corrosion inhibitors for AA5083. The seven
chemical compounds are given in Table 3.1. All compounds were purchased from Sigma Aldrich. They
were chosen to represent a range of functional groups at different positions that can act as active
centres for interaction with the metal surface [40,57,74]. Aqueous sodium chloride solution (0.6M)
was used as an electrolyte, and all inhibitors had a concentration of 10-3 M.
Table 3.1. Chemical structures of the inhibitors
Inhibitors’ structure
Name of inhibitor
And its identifying letter
Inhibitors’ structure Name of inhibitor And its identifying letter
Catechol (1A) Benzotriazole (2F)
3-Amino-1H-1,2,4-triazole (2G) Resorcinol (1B)
48
Quinol (1C)
Imidazole (4D)
3.3. Quantifying corrosion rates through pit counting
3-Amino-1,2,4-triazole-5-thiol (2C)
All droplets were 2 µL in volume (2.1 -2.5 mm in diameter), and five droplets were placed on each
polished AA5083 substrate using a micropipette (Eppendorf). There was at least a 10 mm distance
between each droplet to avoid overlap of moisture layers from secondary spreading [86]. Polished
samples with droplets were placed in a sealed humidity chamber to prevent evaporation. The
experimental time was seven days, and relative humidity (%RH) was kept above 90% at temperature,
e.g., T = 22 ± 2°C temperature, using an internal dish of potassium sulphate (K2SO4) saturated solution.
No evaporation of the droplet was observed by visual inspection before and after the exposure after
7 d of exposure.
After the exposure period, AA5083 samples were rinsed with high purity water and ethanol and,
according to ISO 8407,10% nitric acid (HNO3) for one minute to remove corrosion products. For pit
evaluation, the mean of 100 pits and the maximum depth of pits were counted and measured for all
droplets by using a 3D optical profilometer (OP) (Bruker Contour GT-X 3D Optical Profiler with 20X
zoom) operating in vertical scanning interferometry (VSI) mode using Vision software and the aid of a
user interactive software-assisted masking routine. Measurements were conducted on at least 3
droplet replicates.
The visualisation and measurements of pits depth were shown in Fig. 1, where the image on the left
shows a pit that occurred inside a droplet. On the right is a line scan measurement of the pit of depth
4.4 µm, and below is a photograph of droplets after 7 d exposure.
49
Fig 3.1. A sample of a droplet image using optical profilometry and a line profile for measuring pit depth
3.4 Electrochemical method
PDS was used to detect localized damage and instantaneous corrosion. However, it is usually confined
to bulk solutions [65,88–90]. Therefore, it is beneficial to evaluate and compare corrosion rates from
the droplet and bulk electrolytes. Electrochemical tests were conducted on uninhibited and inhibited
AA5083 exposed substrates using a Biologic VMP-300 potentiostat in conjunction with a standard 3-
electrode electrochemical cell arrangement. Saturated Ag/AgCl was used as the reference electrode,
platinum mesh as the counter electrode, and the specimen as the working electrode. AA5083 samples
were immersed in uninhibited and inhibited solutions for a week before conducting tests. The open-
circuit potential (OCP) was monitored for 1.5 h prior to the PDS, and the exposed area of the working
electrode (1 cm diameter) was 0.78 cm2. Cathodic/anodic PDS were scanned over the voltage range
of -200 mV to 400 mV vs OCP at a constant voltage scan rate of 5 mV/s. All tests were replicated at
least three times to ensure reproducibility.
50
3.5. Results and discussion
3.5.1. Inhibition efficiency based on the droplet technique
The intention of this study was to compare the droplet method with bulk volume electrolytes. Two
different methodologies were assessed from the droplet: the average pit depth and the maximum pit
depth. The results for the seven inhibitors used are presented here. In comparison to uninhibited
0.6M NaCl droplets (benchmark), inhibition efficiencies (%IE) were calculated as follows:
𝑷−𝑷′ 𝑷
, Where P is the pit average or maximum depth exposed to NaCl, and P’ is for inhibited %IE=
solutions.
In order to determine how many pits needed to be measured after the droplet exposure, average pits
depth for 10 to 100 pits was measured for each droplet. The image of the droplet circle was quartered,
and 25 random pits above 2-micron pit depth were measured for each quarter, and then all the 100
pits were sorted from largest to smallest. The mean was then determined for each of the first 10 pits
and then for every cumulative group of 10 pits afterwards. The determining step was repeated for
eight replicates of droplets of 0.6M NaCl. It was found at around 40 pits, the value of the mean pit
depth began to stabilize, and the standard deviation started to decrease. Hence, counting 40 pits was
chosen as a compromise between accuracy and speed of analysis, as shown in Fig.2 below for eight
replicates. Hughes et al. [91] measured intermetallic density on AA2024, and assuming a similar
density on our alloy, the droplets in these tests would cover approximately 11,000 to 15,000 IMPs.
Thus, we propose the droplets contain sufficient intermetallic particles and surface chemistries for
reproducibility. In addition, the five droplet replicates will further enhance reproducibility.
51
Fig 3.2. Average of 100 pits’ depth over eight droplet replicates
Results for inhibitor solutions based on counting the average depth of 40 pits, converted into %IE for
each droplet replicates, are shown in Fig 3.3. Results are also shown for the deepest pit depth (Max).
Using average pit depth, three of the triazole inhibitor family (2C, 2G, and 2F) showed promising
performance between 83-88% IE. The dihydroxy benzene isomers did not show good performance
toward AA5083, as only the meta position (1B) hydroxyl groups (-OH) showed moderate efficiency
(74%), while the ortho and para positions (1A and 1C respectively) had poor performance (29% and
53% respectively). Imidazole (4D) with 64 %IE was in the mid-range performance.
For five of the inhibitors, there was an agreement between the mean pit depth and maximum pit
depth measurements with only minor variations, with the triazole group (2C, 2F, 2G) still showing the
best inhibition performance among the seven inhibitors. Of the dihydroxy inhibitors, 1B showed a
moderate performance similar to the mean pit depth, followed by 1C. 1A had the lowest and negative
performance indicating it acted as a corrosion accelerator. Imidazole (4D) again showed mediocre
inhibition, with efficacy below 50%.
52
Fig 3.3. Inhibition efficiencies (%IE) based on the mean and maximum pit depth measurements
3.5.2. Inhibition efficiency based on Tafel scans.
In order to compare the droplet method, two different PDS time period experiments were conducted
to see inhibition over time. icorr is one of the key indicators of corrosion kinetics of any given system.
Though not ideal for pitting corrosion, it is a competent tool for understanding the initial corrosion
responses of metal in the presence of inhibitors.
As shown in Fig 3.4, at 4 h, the control (0.6M NaCl) showed a low corrosion current density (icorr) of
0.075 µA/Cm2, lower than all the inhibitors at the same time period. However, after one week, the icorr
of 0.6M NaCl control increased to 0.307 µA/Cm2, indicating an increase in the corrosion rate over a
longer period. The inhibitors, however, all had reduced their corrosion current density over the
corresponding time period to below that of the 0.6M NaCl control. The triazole group (2C, 2F and 2G)
showed promising inhibition performance on the AA5083 surface over time [62], ranging from 0.0168
to 0.029 µA/cm2, while the dihydroxy isomers showed significant improvement over time, where
catechol (1A - ortho) had the lowest corrosion current (0.023 µA/Cm2) of the three structural isomers.
This is opposite to that described by Ryl et al.[68]. This may possibly be due to their different surface
preparation (mirror-finished to 0.05 µm), electrolyte (bicarbonate buffer solution) and the alloy
(AA5754). Imidazole (4D), however, showed an improved icorr of 0.021 µA/cm2, on a par with the
triazole group.
53
The graph shows how the performance of the inhibitors varied between 4 h and 7 d exposure, where
the performance of the NaCl control changed from the lowest corrosion current after 4 h to the
highest corrosion current after a week. The corrosion current of all the inhibitors decreased, and hence
Fig 3.4. Corrosion current ( icorr) for two different time periods of seven inhibitors compared to NaCl control
3.5.3. Comparison between the droplet technique and potentiodynamic polarization scans
the corresponding inhibition efficiency significantly increased.
In order to validate the droplet technique, the values obtained should be consistent with values
obtained from PDS, which is a technique commonly used for determining the efficiency of corrosion
inhibitors in bulk solution. Therefore, a comparison of the results from the droplet tests to the
traditional electrochemical method (e.g., PDS), expressed as %IE, is shown in Fig 3.5 for the 7
inhibitors. Results of five inhibitors in both techniques were in relative agreement with each other.
But results of droplets of the remaining two (1A and 4D) were contrary to PDS results, with the %IE of
both at 93% by PDS measurement but considerably lower by droplet tests. It is expected that the
extent of oxygen diffusion into a droplet will be higher than for the bulk solution (PDS) test, and thus,
the droplet will maintain higher oxygen levels [92,93]. This may be one reason for the different results
in some tests [68,87]. Further, it has been observed that after the initial stage of pit growth, a small
number of pits tend to dominate and substantially increase their depth. This concentrated mass loss
in a few pits changes the alignment of our damage indicators, with the maximum pit depth being much
54
higher and thus %IE for this measure being lower when compared to mean pit depth or PDS, which is
Fig 3.5. %IE comparison between droplet and PDS results for seven inhibitors after 7 days
3.6. Conclusions
consistent with what is seen in inhibitor 1A [15,94].
A high-throughput droplet method for evaluating corrosion inhibitors is proposed that gives consistent
results for defining pit growth both in terms of variability and in comparison between maximum pit
depth and average pit depths. Seven common inhibitors were tested using both these methodologies
and PDS, and for five inhibitors, %IE results were in agreement between the droplet and PDS
methodologies. However, for the two inhibitors, they are markedly different, and this may be because
of the difference in oxygen saturation between the two experimental methodologies, as oxygen is
limited in bulk solution, allowing for a small number of pits to grow significantly deeper. The newly
developed high-throughput method has strong implications for the investigation and determination
of the best inhibitors for atmospheric corrosion of metals that undergo pitting corrosion.
55
Chapter 4
Study of droplet-based technique
4.1. Introduction
Increasing awareness of the toxicity and environmental hazards associated with chromates has led to
the search for alternative inhibitors that are less toxic, affordable, and effective at low concentrations.
Traditionally, the development of an inhibitor system for a particular application can be extremely
time-consuming. This is particularly true if a systematic study utilising various techniques is applied to
a large, diverse range of potential inhibitors [6,10,14]. Thus, there is a clear need to develop rapid
screening techniques, particularly for evaluating a wide range of inhibitors to suppress atmospheric
corrosion on AA5083. This chapter discusses the application of the high throughput droplet method
for 21 inhibitors, differences in inhibitor performances and how changes in chemical structures can
4.2. Experimental
4.2.1. Inhibitor solutions
have an influence on inhibition effectiveness.
All chemical compounds tested were obtained from (Sigma Aldrich) and used as 10-3 M solutions in
0.6 M NaCl. Twenty-one compounds were used in this study, split into four groups. The different
groups were labelled from 1 to 4, and every inhibitor in each group was labelled with a letter; for
example, group one was 1A, 1B, etc.
The four groups that have been selected are dihydroxy isomers, triazoles, thiazoles and imidazoles.
They were chosen as it has been shown that compounds containing oxygen, nitrogen or sulfur could
be chemisorbed onto an Al surface through the bond formation by p-valence orbitals of Al. Also,
adsorption can also be via π-electrons on the ring. In group 1, as shown in Fig 4.2, the main aim is to
evaluate the effect of hydroxyl (OH) group structural position (meta, ortho and para) has on inhibition
performance. While in group 2, as shown in Fig 4.3, triazoles (three nitrogens in a five-membered ring)
will be evaluated by substitution of a functional group for another, i.e., where changing the methyl
group (2B) with an amino group (2C) or the effect of additional functional groups to the structure such
56
as an extra benzene ring (benzotriazole, 2F) and then the addition of a hydroxyl group (2E) to the
benzotriazole. The group 3 Fig 4.4, thiazoles are five-membered rings with sulfur and nitrogen in the
ring. Starting with the mercaptobenzothiazole (2B) and replacing the ring sulfur with oxygen to give
the oxazole (3A), replacing the thiol group with amine (3D) and then adding bromine to the phenyl
ring (3E). Finally, the addition of an amino group to the phenyl ring of the original
mercaptobenzothiazole. Similarly, in group 4, the family of imidazoles will be evaluated by testing the
influence of adding functional groups (amine, thiol) or extra benzene or pyrimidine rings to the base
imidazole ring (4B) to give the benzimidazole (4F) with amino (4A) or thiol (4C) substituents. The
addition of the pyrimidine ring gives purine (4D), followed by the addition of an amine to give adenine
(4E).
In addition to the intragroup comparisons above, there exists the possibility of intergroup
comparisons between structurally related compounds. For example, benzotriazole (2F) and
benzimidazole (4F) are different by only one ring of nitrogen. Similarly, mercaptobenzothiazole (3B)
and mercaptobenzimidazole (4C), as well as aminobenzothiazole (3D) and aminobenzimidazole (4A),
4.3. Results and discussion
have a ring sulfur (group 3) replaced with ring nitrogen (group 4).
Inhibitor efficacies were calculated as in the previous chapter. Herein, the aim is to study and evaluate
a wide range of chemical compounds that are structurally related to test the influence of different
structures on the inhibition performance of Al alloy 5083. As atmospheric corrosion in the marine
environment is triggered by aerosols and rain droplets [21], the droplet method was selected as it is
advantageous to be tested under similar conditions rather than bulk volume.
The results of droplet tests using the mean and maximum depth of pits are given in Table 4.1, and the
relationship between structures is illustrated in Figs 2-5.
57
Table 4.1. Compounds tested for corrosion inhibition of AA5083 in 0.6M NaCl
Identifying letter Chemical compound
Catechol Resorcinol Quinol 5-(4-Pyridyl)-1H-1,2,4-triazole-3-thiol 4-Methyl-4H-1,2,4-triazole-3-thiol 3-Amino-1,2,4-triazole-5-thiol 4-Amino-3-mercapto-5-phenyl-4H-1,2,4-triazole 1-hydroxybenzotriazole Benzotriazole 3-Amino-1H-1,2,4-triazole 2-Mercaptobenzoxazole 2-mercaptobenzothiazole 6-Amino-2-mercaptobenzothiazole 2-Aminobenzothiazole 2-amino-6-bromobenzothiazole 2-Aminobenzimidazole Imidazole 2-Mercaptobenzimidazole Purine Adenine Benzimidazole Inhibitor efficiency %, Max -3 72 49 56 71 87 75 78 90 87 87 92 4 80 82 51 37 48 51 20 47 Mean 29 74 53 46 57 84 73 71 88 84 78 83 57 83 74 50 64 38 46 36 50 1A 1B 1C 2A 2B 2C 2D 2E 2F 2G 3A 3B 3C 3D 3E 4A 4B 4C 4D 4E 4F A number of these compounds were found to be promising. Moreover, various effects of different
functional groups on inhibition performance were recognized. The inhibition activity of each group
• Group 1 (Dihydroxy) - effect of changing substituent position
may be summarized as follows:
According to a study by Ryl et al. [68], the position of a functional group on the chemical compound
plays a pivotal role in enhancing inhibition performance. As shown in this work, the inhibitors’
efficacies are determined by the changing position of the hydroxyl group. Resorcinol (meta, 1B)
showed the best performance rather than ortho and para positions (catechol (1A) and quinol (1C)),
respectively). It has been observed that resorcinol molecules undergo keto-enol tautomerism, where
chemical compounds are able to resonate between two forms by exchanging a hydrogen atom
between the ketone (keto) and the alkene-alcohol (enol), as shown in Fig 4.1. An increased colour
change of the solution was seen over time as the keto isomer increased in concentration. This
58
phenomenon did not appear for the ortho catechol (1A) or the para quinol (1C), which decreased their
performance for a period longer than 24 hours and have no associated colour change. The relative
performances of the three isomers are partly in agreement with the order found by Ryl et al., where
resorcinol was shown to have varying inhibitor performance depending on the concentration and pH
compared to the other isomers. The experimental conditions used in the work of Ryl et al. (AA5475,
24h, bicarbonate buffer at pH 11), however, are sufficiently different from those used herein (AA5083,
droplet for 7d, 0.6M NaCl) and, as such, are not directly comparable. Fig 4.2 shows inhibitors
Fig 4.1. Keto enol tautomerism [95]
Fig 4.2. Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of dihydroxy isomers after 7 days on the left and their efficacies on the right
• Group 2 – Triazoles
performance of 1A, 1B and 1C after seven days in 0.6M NaCl.
Triazoles are five-membered heterocycles containing three nitrogens and are well known and
established as effective inhibitors among those that contain two or more N atoms. They have been
used as inhibitors to stop corrosion of Al and its alloys [14,62,68].In this work, seven chemical
compounds were tested via the droplets method, and the efficacy increased marginally from 2A (mean
68%/ max 83%) to 2B (73% /83%) and 2C (84%/ 87%) by substituting pyridyl with methyl and amino
groups respectively. These results are in agreement with results from previous work [1]. Moreover,
59
the efficacy remained unchanged from 2C to 2D (85% / 88%) when a phenyl ring was added as a
substituent on the ring. Similarly, adding a phenyl ring directly to make the bicyclic benzotriazole (2F)
has no real effect on the inhibiting performance (88%, 90%). The addition of OH group to 2F was
comparable to that without an extra group (81%, 78%). Likewise, 2G (84%, 87%) showed similar
inhibiting performance as well.
A
Fig 4.3. A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of group 2 after 7 days, and B) their efficacies
B
• Group 3 – Thiazoles
Thiazole derivatives have been proven to be effective inhibitors with various metals, particularly on Al
and its alloys[8,96]. This is because their structures contain some special properties such as a pi bond,
hetero atoms and aromatic ring in the structural geometry, and these properties are associated with
strong inhibiting characteristics [46]. 2-mercaptobenzothiazole (3B) showed good inhibition
performance measured by both methods (mean and maximum) as its efficacy was (75%, 84%)
60
respectively, which is in good agreement with literature values of performance on Al alloys 2024 and
7075 determined using weight loss for a longer period of time [1]. Substitution of the sulfur atom in
the ring with oxygen from 3A to 3B appeared to decrease the inhibitor performance as the efficiency
decreased (68%, 75%). The addition of an amino group to the benzyl ring of 3B structure (3C) appeared
to have decreased its performance (57%, 4%). However, the additional amino group on the 6-
membered ring provides an alternative binding site to the thiol, which might disrupt inhibitor film
formation. The replacement of the thiol on 3C with amino (3D) restores the performance (82%, 80%)
and is comparable with the original thiol (3B), whereas the addition of bromo group to 3D did not
show a significant influence on the performance (73%, 80%) as indicated by the relative performance
of 3E and 3D. Inhibition efficacies are shown in Fig 4.4
Fig 4.4 A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of Triazoles after 7 days and B) their efficacies
B
61
• Group 4 – Imidazole
Due to the existence of nitrogen atomic functional group and free electrons of nitrogen atoms in their
structures which make them able to interact with the metal surface and thus protect the surface from
corrosion attacks, imidazole and its derivatives have a promising potential to stop corrosion[11-14].
Starting from imidazole (4B), as the baseline of this group, its efficacy was average (64%, 37%), while by
adding benzene ring (4F), the efficiency was similar (50%, 47%), then by adding the amino group to 4F as
in 4A, no significant effect was observed as performance remained the same (50%, 51%), however, that
addition of thiol to 4F, as in 4C, resulted in an inhibition reduction (38%, 48%). On the other side, from
imidazole (4B) to 4D (Purine), where a pyrimidine ring was added to the imidazole ring, performance
remained moderate as others in the same group, as efficiency was (46%, 51%), while the addition of
amino, as in 4E, resulted in very poor performance as it was (36%, 20%), that was the lowest among the
group 4 compounds. The chemical structures of group 4 are shown in Fig 4.5.
A
Fig 4.5 A) Inhibition efficiencies by maximum pit depth (max) and by mean pit depth (mean) with error bars of imidazoles after 7 days and B) their efficacies
B
62
A summary of some active functional groups from Table 4.1. that showed promising performance for further analysis is as follows.
(a) Meta position of the hydroxyl group was found to be the most effective among dihydroxy isomers
from group 1. This is consistent with the literature on different metals, steel and various techniques
used, such as PDS and EIS, as it is associated with film formation on the metal surface, providing
protection from corrosion [97].
(b) The thiol functional group showed good performance in all inhibitor families. This may be
associated with monodentate, or bidentate ligands promoted by lone pairs of electrons on sulfur
from the mercapto group and other functional groups linked to the structure. As a result, electron
density on the ring may be influenced except in the case of 4C (38%, 48%) and 3C (57%, 4%), which
had a competing amino group.
(c) Imidazole and its derivatives (including 4C) did not show a good performance, contrary to expected,
as they were previously reported to show good inhibition influence [53,54,57] on Al and other
metal surfaces. This may be due to variations such as time of tests, used evaluation methods and
materials with different topography and a much higher oxygen concentration in the droplet tests.
(d) The addition of an extra nitrogen atom on the structure (imidazole to triazole) enhanced the
performance of inhibitors as observed for benzimidazole (2F) and benzotriazole (4F). The efficiency
increased by 40% with the added nitrogen, which may be explained by increased centres of
adsorption of a chemical compound on alloy surface due to the increasing number of lone pairs
ligands to form covalent bonds with metal ions, thus, enhancing binding between the inhibitor and
AA5083 alloy.
(e) Triazoles in group 2 showed very promising performances, which may be associated with three
nitrogen atoms as it may end on the surface by forming bidentate or tridentate bonding, including
the possibility of π- electrons from the ring which leads to enhanced stability of inhibitors on the
surface.
(f) The effect of ligand position is demonstrated by the comparison of all compounds in group 1, where
changing the position of the -OH functional group was evaluated on inhibition performance.
63
(g) The effect of steric hindrance may be demonstrated by a comparison of 2C and 2D, where it shows
that the performance remained the same after the addition of a phenyl group. The negative effect
of steric hindrance was observed between 4B to 4D and 4F when it comes to the mean depth
measurement. However, the efficiency increased for the maximum depth, giving an inconclusive
trend.
(h) The effect of additional groups affecting the charge density in the ring and on the lone pair can be
seen in the addition of hydroxide from 2F to 2E, where the activity (88%, 90%) decreased after
addition (81%, 78%). While in comparison between 2C with 2D, it can be seen that the performance
is essentially the same, with no effect of the additional phenyl group.
(i) The addition of an amino group to the phenyl ring resulted in a deterioration in performance from
3B (75%, 84%) to 3C (57%, 4%), where the amino group is thought to compete with the thiol for
4.4 Conclusion
metal bonding.
A wide range of commercially available, structurally related chemical compounds were tested using a
developed droplet high-throughput method in 0.6M NaCl to inhibit corrosion on marine Al alloy 5083.
It was found that some structures have strong inhibitive behaviour, particularly the triazoles family
with different functionalities, which were mostly all strong inhibitors. Little work has been done on
these corrosion inhibitors on AA5083 to date.
In several chemical compounds, there were variations between mean and maximum pits’ depth
regarding the efficiencies. This could be a reflection of how inhibitors work, some prevent more
pitting, and others stop the propagation of pits into the metal. When inhibition is good, there is a
reasonably good correlation between the mean and max pit depth measurements. However, over
time when inhibition is failing, the larger pits tend to dominate, and the max pit depth overshadows
the mean pit depth, and the two values diverge from each other. It was found that the features that
make an effective inhibitor is as follow; the presence of lone pair of electrons on heteroatoms
attached to the ring, the addition of an extra-functional group which can lead to steric hindrance,
64
which increases surface area coverage of inhibitor on the metal surface and addition of extra
heteroatoms both in the ring and attached to it, which may affect electron density on the structure
by increasing adsorption centres.
This droplet method has been successfully applied on AA5083 with various chemical compounds and
is recommended to be applied on metals that are suspectable to pitting corrosion. This work is
preliminary for more detailed future studies to enable more understanding of how the most effective
corrosion inhibitors in this study work. Herein, the most effective inhibitors in this study are 3-Amino-
1,2,4-triazole-5-thiol (2C), 3-Amino-1H-1,2,4-triazole (2G), 2-Mercaptobenzoxazole (3A), 2-
mercaptobenzothiazole (3B), 2-Aminobenzothiazole (3D), 2-amino-6-bromobenzothiazole (3E).
65
Chapter 5
Long-term performance and stability of the shortlisted inhibitors
It is crucial to understand the durability and stability of inhibition systems that rely on film formation,
particularly for the long-term effect of corrosion inhibitors. However, there is a lack of knowledge in
the current literature about the performance and behaviour of inhibitor-containing systems, 3-
Amino-1,2,4-triazole-5-thiol (ATT) and 3-Amino-1H-1,2,4-triazole (3-AT), over an extended period of
time. A limited amount of work has been conducted in order to understand film formation over metal
surfaces for extended time periods. Hence, it is not only important to monitor the time that the
inhibitor film takes to form on the metal surface and to develop sufficient protection, but also the
stability of the film over an extended period of exposure.
In this chapter, the long-term performance and stability of two inhibitors that were short-listed using
the high throughput droplet method are investigated, namely 3-Amino-1,2,4-triazole-5-thiol (ATT) and
3-Amino-1H-1,2,4-triazole (3-AT) by studying the durability of the film formed over a period of two
weeks. Two electrochemical techniques (LPR) and (EIS) were conducted to monitor corrosion rate and
corrosion inhibition over two weeks, with analysis over two weeks at 24-hour intervals. PDS was taken
after 4 h and 7 d exposure to monitor inhibitors’ performance from the start and after a longer period.
For PDS tests, multiple samples were used to present each period of exposure in inhibited solution
5.1 Studies for uninhibited and inhibited solutions over 7 d exposure
5.5.1. Potentiodynamic polarization scan tests
due to the destructive behaviour of the technique.
PDS tests were conducted over the first 4 h and 7 d of exposure for uninhibited and inhibited solutions.
Fig 5.1a shows the Tafel curves for both systems at the beginning of the exposure.
For the uninhibited system, 0.6M NaCl, a passivation region was observed in the anodic arm, which
extended about 0.1 VAg/AgCl until it reached 0.7 VAg/AgCl, where the Epit started. For the 3-AT system, it is
well noted from the Tafel curve in Fig 5.1a that the value of Ecorr compared to the uninhibited system
66
shifted cathodically with icorr 0.14 µA/Cm2 almost was doubled of uninhibited, which is an indication
of poor performance in the initial period, that could be due to of delay of film formation over the
surface which provides the protection.
In contrast, for the ATT system, in the initial period, the system shifted toward an anodic value
compared to a saline solution with icorr 0.106 µA/Cm2, which is lower than the current density for the
3-AT system, which is a good indicator of better performance, however, both inhibitor-containing
systems, do not show promising performance in the first hours of exposure, which may indicate that
the inhibitors take time to build up a film over metal’s surface. In addition, oxygen reduction
reaction could have an influence in the first period of exposure due to natural aeration before
Fig 5.1a. PDS of uninhibited and inhibited systems after 4 hours
exposure of the systems.
Over a 7-day period, the performance of uninhibited and inhibited systems changed. For the
uninhibited system, Ecorr value shifted towards more cathodic values as it changed from -0.8 to -0.9
VAg/Agcl. In addition, a significant passivation region appeared in the anodic sweep, which may be
attributed to the high amount of corrosion products on the surface caused by severe corrosion. The
high icorr of 0.307 µA/cm2 indicates a high initial rate of Al dissolution, which could promote the
formation of a protective oxide.
67
For the 3-AT system, compared to 4 h exposure period, an anodic shift was observed equal to 0.05 V.
Thus, the inhibitor acts as an anodic inhibitor with a passivation region due to the formation of barrier
film on the surface by the adsorption process as the icorr value dropped significantly to 0.0168 µA/cm2
after 7 d compared to 4 h exposure, which indicates that the inhibitor is providing corrosion protection
and a lower corrosion rate compared to uninhibited system. Some variations in current have been
monitored in the anodic arm as it could be associated to the passivation process and localised actions.
In contrast, in the ATT system, the corrosion potential shifted to a more negative value, which
indicates a cathodic behaviour of the inhibitor in the system. A shift was about 0.07 VAg/AgCl after 7 d
of testing; the potential after shifting is -0.77 VAg/AgCl with a significant reduction in icorr value (0.106
µA/Cm2 to 0.024 µA/ Cm2) can be observed after a week as shown in Fig 5.1b. Moreover, the slope
and shape of the anodic arm have been changed with a significant passivation region, which appeared
Fig 5.1b. PDS of uninhibited and inhibited systems over 7 days
5.5.2 Linear polarization resistance studies
between -0.55 and -0.4 VAg/AgCl.
LPR testing on both uninhibited and inhibited systems was conducted over 336 h. A rest time for 2 h
was used for system stability before commencing long-term measurements.
Fig 5.2 shows polarization resistance (Rp) over 2 weeks of exposure for AA5083 in both uninhibited
(0.6M NaCl only) solution and inhibited systems, ATT and 3-AT at concentrations 0.01M and pH 7.
68
Fig 5.2. polarization resistance of uninhibited and inhibited systems on AA5083, over 14 days
For the uninhibited solutions, in the first 15 h, a drop in the polarization resistance was observed that
could be associated with oxide layer instability that naturally formed over the AA5083 surface, which
could be linked to non-homogeneity of the surface and the existence of intermetallic particles, Rp
dropping until it reached 1.8 x 103 Ohms. After the first 15h, there was a gradual increase with obvious
fluctuations in the Rp until it reached the peak of 126 x 103 Ohms at 142 h. Between 142 and 145 h,
there was a big drop in Rp, as it reached 57 x 103 Ohms, then a slight increment happened after 3
hours at 148 h. However, after about 209 h, the Rp seems to be quite stable around 10 x 103 Ohms,
with a slight upwards curve toward the end of the experiment. Furthermore, the resistivity of the
solution, build-up of oxide layer on the surface and large quantities of corrosion products were
observed during the experimental time.
Regarding the first inhibited system, 3-AT showed variation in Rp over 2 weeks (336 h). In the first 12
h, a drop occurred in Rp value, which could be associated with the breakdown in the initial oxide film
prior to the build-up of an inhibitor-induced barrier on the surface. After that, it started to increase
gradually over the entire period with minor drops in the middle. Increasing Rp over two weeks is a
69
good indication of film formation over the surface of AA5083. The data for 3-AT demonstrated that
the slow build-up of a film over the metal’s surface through the absorption of the inhibitors is
consistent with the work of Zheludkevich et al. [91] and Sherif et al. [95], who found that triazole
derivate inhibitors suppressed the process of dealloying of the intermetallic particles by film
formation. The Rp value increased gradually during the whole period from 22 x 103 to 47 x 103 ohms,
as shown in table 5.1. The magnitude of the resistance in the case of 3-AT is higher than the
uninhibited system at many periods of time, in addition to the endpoint.
For the ATT inhibited system, as shown in Fig 5.2, a significant increase in polarization resistance was
observed in the first 50 h, where it started from 18 x 103 ohms and increased to 70 x 103 ohms, whether
it stayed or the remaining time (14 days) with some fluctuations, which could be caused by formation
and reformation of the barrier layer on AA5083 surface. Overall, the polarization resistance was higher
compared to 3-AT. Therefore, the magnitude of the ATT polarization resistance is higher than
uninhibited system and double the magnitude compared to 3-AT, which could be associated with the
presence of an additional functional group (thiol), which enhances the adsorption process onto the Al
Table 5.1. Rp values of uninhibited and inhibited systems over different periods
surface due to the presence of sulfur [52,98,99].
10-3M ATT Time 0.6M NaCl Rp / ohm 10-3M 3-AT Rp / ohm Rp / ohm
4 h 65 x 103 22 x 103 17 x 103
7 d 26 x 103 39 x 103 78 x 103
5.5.3. Electrochemical Impedance Spectroscopy (EIS) studies over 2 weeks
0.6 M NaCl – uninhibited system EIS measurements have been recorded for all systems after 1 h, 24 h, 7 d and 14 d. All results were
14 d 0.98 x 103 47 x 103 72 x 103
represented using two plots; Nyquist and Bode, as shown in Fig 5.3 for uninhibited system.
70
Fig 5.3a Nyquist plots of uninhibited system over 2 weeks
Fig 5.3b Bode plots of the uninhibited system over 2 weeks
Fig 5.4 Suggested equivalent circuit for 0.6M NaCl system over 2 weeks
Nyquist plots attained from EIS data over two weeks, as in Fig 5.3. (a), have revealed increasing Rp
with time until 7 d of exposure, then started to drop at the end of 14 d. A capacitive loop followed by
a linear region were observed at two-time constants were presented, one at high frequency, which
may be due to oxide layer over AA5083 surface, and the linear part at middle-frequency range which
71
may be due to diffusion process on the metal surface. Nyquist plots also revealed that at 7 d of testing,
an oxide layer formed on metals surface with the presence of pores which appears in the linear region
where diffusion process probably of Cl ions or oxygen molecules on the metal surface, also it was
confirmed by the presence of Warburg resistance (W) in the equivalent circuit of the system, where
was associated with diffusion process through metal- electrolyte interface as linear section after the
first loop was an indication for a beginning of a new process on the metal’s surface [55].
Bode plots (Fig 5.3b) indicated a high phase angle after 1 h and 7 d of exposure which could be
associated with a more capacitive response, which is probably related to the presence of protective
film (oxide layer) on AA5083. However, with increasing exposure time at 14 d, a decrease in phase
angle was observed due to the breaking of oxide due to the diffusion process [99].
The equivalent electric circuit that has been fitted to fit the plots at various time intervals is shown in
Fig 5.4, where R2 represents charge transfer resistance, W represents limited diffusion process,
porosity exists in oxide layer over AA5083 surface, C1 is double-layer capacitance, and R1 is related to
10-3 M inhibited system (AAT)
Fig 5.5a. Nyquist plots in the ATT inhibited system over 14 days
solution resistance [99].
72
Fig 5.5b Bode plots of ATT inhibited system over 2 weeks
Fig 5.6 Suggested equivalent circuit for the ATT system over 2 weeks
The results of AA5083 exposed to the inhibited system (3-AT) over different time intervals, 1h, 1d, 7d
and 14d, have been recorded using EIS and are represented in Nyquist and Bode plots. Nyquist plots
have shown in Fig 5.5, increasing semicircle diameter with time. However, in the first two cycles,
where time intervals were 1 h and 1d, they both showed a tail at the end of the circle, which could be
associated with the ionic diffusion process at the beginning of the exposure process. However, it was
not the same case after a longer two periods of time, 7d and 14 d, where the diameter marginally
increased after 7d, and an obvious change occurred at 14 d, where a significant increment of semicircle
diameter happened, and the Rp value was 2.7x105 ohm, therefore, indicating the presence of a
protective layer [99]. A single time constant was observed, was associated with the capacitance of
coating with no defect was merged with an oxide layer of AA508 [58].
Bode plots in Fig 5.5b show the θmax for ATT inhibited samples are about 77.4°, 78°, 80° and 83° for
1h, 1d, 7d and 14d periods of exposure, respectively. Θmax was moving to a higher frequency range
along with time. A gradual shift occurred for phase angle to the left side with more exposure period
of time.
73
Evidence of film formation presence on the metal surface has been determined by Rp values
improvement over the time that were observed from Nyquist and Bode plots. The suggested EIS
electrical circuit has been fitted to fit the plots at four different time intervals, as shown in Fig 5.6,
where R1 is solution resistance, R2 and R3 represent Rp and C1, and C2 are capacitance of the
10-3 M inhibited system (3-AT)
Fig 5.7a. Nyquist plots in the 3-AT inhibited system over 14 days
Fig 5.7b. Nyquist plots in the 3-AT inhibited system over 14 days
potential films.
74
Fig 5.8 Suggested equivalent circuit for the 3-AT system over 2 weeks
EIS measurements were recorded for inhibited system 3-AT after 1h, 1d, 7d and 14 d of testing and
represented in Nyquist and Bode plots, as seen in Fig 5.9 above.
Similar to the inhibited system AAT, the suggested electric circuit has been selected based on the best
fitting that matches all plots, where R1 is solution resistance, R2 and R3 represent Rp and C1, and C2
are capacitance of the potential films. Nyquist plots have shown an improvement and build-up of a
protection layer on AA5083 alloy over time, as it is noticed that an increment in semicircles diameters
with longer periods as at 7 and 14 d. capacitive loops are associated with film capacitance. There was
not a huge difference in diameter between the last two-time intervals, 7 and 14d, which is probably
attributed to the stability of film on metals surface after 7 days.
Surprisingly, regarding the Bode plot, the phase angle of 1h period was the maximum degrees at 87.33,
which could be possible due to the settling or stability of the oxide layer on the surface. However, θmax
of the initial periods of exposure were stable within a narrow frequency range, contrary to a longer
5.5.4 a. Surface analysis of ATT inhibited systems over exposure periods of 14 days
period that was stable within a wider range.
The visual observation of the surface has confirmed the formation of a mixed layer between the
natural oxide layer and inhibitor molecules on AA5083 over 14 days of exposure. The study was
conducted after the exposure period and then analysed using the focused ion beam (FIB) machine. FIB
was used to capture a cross-section of the film formed on metals’ surfaces.
Fig 5.9 shows the formation of a layer on the alloy surface after two weeks of immersion in the
inhibited system, ATT at 10-3M. The presence of a surface film was supported using Energy-dispersive
X-ray spectroscopy (EDS) to reveal the elemental composition of the white-black mixed layer on the
alloy surface. The orange line scan in the images revealed the presence of sulfur in the black-coloured
75
layer, which makes a confirmation about the bonding of sulfur molecules in the structure of ATT with
Fig 5.9. Cross-section images of inhibitor layer formed of ATT on AA5083
AA5083 molecules on the surface.
As suggested, the analytical data from the line scan showed that sulfur exists above the AA5083
surface, particularly between the black-white mixed layer, which enhances the theory of a mixed
barrier layer to protect the surface from being corroded and damaged. As shown in Fig5.10, the counts
per second (cps) of sulfur was 2400 and shows a depth profile of 1 micron of sulfur exists in that region.
Al also was monitored to count 70000 per second as the main element, with Mg around 4000 cps as
the second major element in that alloy, and thus the observed intermetallic may be Mg2Al3 [100].
Oxygen existence was confirmed through line scan as well at 3k cps, as its content increased above
the black layer and before the white layer. Thus, the possibility indicates the formation of Al2O3 film
mixed with the inhibitor layer.
76
Fig 5.10. line scan analysis of Al surface after 14 days exposure of ATT inhibited system
Fig 5.11. The overall image of AA5083 after 14 days of exposure of ATT.
5.5.4 b. Surface analysis of 3-AT inhibited systems over exposure periods of 2 weeks
Inhibitor - oxide mixed layer Al surface
Fig 5.12 shows the FIB/SEM images of 14 days of exposure of 3-AT at concertation 10-3M. An
incomplete layer in black colour on the matrix surface can be seen with white filling between the
pores. The pores in the layer may be associated with the influence of Cl- ions which attack the surface
and may lower Rp in case of LPR compared to ATT inhibited system.
77
Pt Layer
Al matrix
Fig 5.12. 3-AT inhibited-oxide mixed layer on AA5083 surface
Inhibitor-oxide mixed layer
LPR results, along with PDS data and the FIB image, imply that a continuous protective layer forms on
the Al surface. However, line scans in Fig 5.13 for this layer showed the presence of nitrogen in small
counts per second, around 200, which confirms N atoms on the surface and possibly from the bonding
Fig 5.13. Analysis of line scans of 3-AT inhibited system on AA5083
of the amino-functional group of the chemical structure of 3-AT.
78
5.5.5. Surface analysis using SEM/EDS mapping for uninhibited and inhibited (ATT and 3-AT) systems
Figs 5.14 (A, B and C) show the elemental compositions on the surface for uninhibited and inhibited
systems after an exposure period of two weeks.
For uninhibited sample after two weeks of immersion in saline solution, Fig 5.14A displays elemental
mapping, which interprets the presence of intermetallic in the centre of the image. Two types of
intermetallics were found, one containing Fe and Mn, which are likely to be attributed to Al6(Fe, Mn)
and another with the presence of Si could be associated to IMP Mg2Si. However, no existence of Mg
was detected as it may have dissolved during the corrosion process [28]. Moreover, it was noticed
that corrosion and pitting occurred around the intermetallic, where the Al matrix exhibited some
trenching.
Regarding ATT inhibited system, as shown in Fig 5.14B, it appears different to the uninhibited sample
as no evidence for Si appeared in the elemental mapping. However, less corrosion has occurred in this
case, although damage in the form of pitting appeared in the middle of the image around Al6(Fe, Mn),
where some trenching also took place. The trenching that occurred using that inhibitor is less than the
case of uninhibited sample for the same intermetallics, which may be associated with the presence of
inhibitor around intermetallic and providing some protection against corrosion.
Lastly, similar observations could be made for inhibited system 3-AT as shown in Fig 1.14C;
intermetallics containing Fe and Mn appeared in many spots in the image, where most of them were
un-trenched particles as only pitting and trenching occurred in the intermetallic particles (circled in
yellow). Therefore, compared to uninhibited SEM/EDS images, the protection around intermetallics,
in this case, was stronger. However, it is risky to view SEM images in isolation and hence the actual
surface corrosion at a lower magnification is shown in Figs 5.15A and 5.15B for the lower
magnification. SEM images and sample photographs, respectively, where the reduced corrosion on
the metal exposed to inhibitors is more obvious.
79
Al Fe
Fig 5.14A. Elemental composition through EDS after 14 days of saline exposure
Mg Mn Si
Al Mg
Fig 5.14B. Elemental composition through EDS after 14 days of ATT system
Fe Mn
80
Al Mg
Fig 5.14C Elemental composition through EDS after 14 days of 3-AT system
Fe Mn
Fig 5.15A. SEM images at lower magnification with minor variation in magnification
3-AT Saline ATT
Fig 5.15B. Samples photographs
5.5.6. Fourier transform infrared spectroscopy (FTIR) for uninhibited and inhibited systems over 2 weeks of exposure
3-AT ATT Saline
FTIR was used to characterise the presence of any bonding type attributed to the inhibitor system or
chemical composition on the AA5083 surface. FTIR was conducted on both uninhibited and inhibited
systems. However, regarding the uninhibited system using saline solution only, in addition to one of
81
the inhibited systems (ATT), no FTIR peaks could be attributed to either the inhibitor film formed on
the surface is too thin and below the detection limit of the ATR-FTIR (0.1 wt.%) or the inhibitor, the
film is only adsorbed to small regions (i.e., the intermetallics) which were not in significant quantities
in the area detected by the FTIR laser.
For recognition of functional groups and any linking between inhibitor molecules and metal surface,
FTIR was also used on inhibited system 3-AT over two weeks. For 1 h and 7 d, there was no evidence
of an inhibitor layer formed on the AA5083 surface. However, after two weeks of exposure to 3-AT,
as shown in Fig 5.16, there were two bands at 2858 and 2924.6 cm-1, which reveal possible C-H
stretching on the AA5083 surface, which without additional inhibitor peaks is inconclusive. Replicate
scans of the metal surface gave similar weak peaks. Fig 5.17 shows FTIR peaks of the inhibitor powder
as received. It showed a large number of peaks, particularly in the range between 2500 to 3500 cm-1,
where it was expected for C-H and N-H stretching to appear. Bonding of the inhibitor to the surface of
AA5083 would be expected to involve the amino group, and this would certainly change the values of
the N-H stretching and also possibly the adjacent C-H stretching, as can be seen when a comparison is
made between two FTIR spectra.
2858
T %
2924. 6
cm-1
Fig 5.16. FTIR spectra of 3-AT system over 2 weeks of exposure on AA5083
82
Fig 5.17. FTIR spectra of 3-AT powder
5.6. Discussion
Two promising inhibited systems (ATT and 3-AT) have been selected for further detailed studies and
analysis to enhance understating of their inhibition mechanisms and bonding on the AA5083 surface.
Electrochemical studies were in agreement regarding revealing the effectiveness of those two
inhibited systems, where PDS showed a protective layer after ageing for a week. LPR was performed
over two weeks, and an increment of Rp was observed for the sample immersed in saline up to 7 days
before it started to decrease and reached its lowest value after two weeks. For the 3-AT inhibited
system, which has an amine as an additional functional group in the structure, a higher Rp was
gradually reached compared to the saline sample. However, the Rp of the other inhibited system (ATT)
was higher than both, which could be associated with the presence of two additional functional
groups, amine, and thiol, which may promote more absorption on the surface due to the presence of
sulfur and nitrogen molecules.
Moreover, EIS data enhanced the results of PDS and LPR as EIS results for uninhibited showed a
protection layer in the initial stages of exposure. However, similar to LPR, after a week of exposure,
the diffusion (where the corrosion process started) of Cl- ions or oxygen molecules to the AA5083
surface was evident. Also, for inhibited systems, ATT and 3-AT, the electrochemical results were in
agreement with each other as Rp for both increased gradually over two weeks. However, Rp values
obtained from EIS were higher than calculated from LPR, but both showed that ATT has better
83
inhibition performance. When it comes to surface characterization to determine bonding or
interactions between inhibitors molecules and AA5083 surface, no strong evidence has been detected.
However, FIB/SEM revealed a depth profile of a 1-micron thick layer containing sulfur in the region of
inhibitor/oxide layer of ATT inhibited system, while FTIR detected organics on AA5083 surface from 3-
5.7 Conclusion
AT inhibited system, where two peaks have been noticed at 2858 and 2924.6 cm-1 for C-H bonding.
Two inhibited systems have been evaluated for their protection performance against corrosion of
AA5083. Electrochemical testing methods have shown consistency of the performance for those
systems through using three different techniques (PDS, LPR and EIS), where they showed high Rp
values and lower icorr value compared to uninhibited solution.
Surface characterisation analysis has revealed the presence of films formed mixed with oxide layer
over the surface of AA5083. However, no conclusive evidence of the nature of surface bonding groups
was obtained.
84
Chapter 6
Conclusions and recommendations for future work
The work in this master’s project was focused on addressing the challenges and research gaps and
answering research questions to develop a new rapid screening droplet method to evaluate various
chemical compounds as corrosion inhibitors for metals subject to pitting. This thesis describes a new
high throughput method for metals susceptible to localised corrosion to assess the inhibiting
performance of inhibitors, most of them new to AA5083, with subsequent in-depth analysis for lead
inhibitors identified by the rapid screening method. The developed droplet technique has a profound
impact on the corrosion community as a rapid discovery method for identifying lead inhibitor
candidates for further analysis.
The droplet method was chosen as a rapid screening technique as an approximation for atmospheric
corrosion, which is driven by the deposition and removal of droplets and aerosols, particularly such as
those occurring in a marine environment. A total of 21 chemical compounds were selected to be used
in this study as corrosion inhibitors, where the basis of the selection was to evaluate how changes to
the chemical structure affect the inhibitor performance. The chemical compounds can be categorised
into four groups: dihydroxy benzene isomers, triazoles, imidazoles and thiadiazoles, which have been
shown as good inhibitors for other Al alloys.
In the first part of the thesis, a comparison was conducted with seven inhibitors to analyse differences
between the droplet technique and a basic PDS electrochemical test using the best performing
inhibitors in the droplet tests. While most of the results are in agreement with each other, two
inhibitors highlighted differences in the testing environments between bulk solutions and droplets,
where it was thought the unimpeded availability of oxygen in the case of the droplet test was
highlighted the diffusion-controlled oxidation in the bulk solution electrochemical cell. The full study
of 21 inhibitors was conducted using two different comparison measurements: (1) the mean of 40 pits
and (2) the maximum pit depth. The study revealed differences in each group of inhibitors and the
recurring features of promising inhibitors. The best performing inhibitors were 5 membered
85
heterocycles with 3 nitrogens in the ring (triazoles) or with one nitrogen and one sulfur (thiazoles) or
one nitrogen and one oxygen (oxazole). The inhibition efficiency decreased when only two nitrogens
were in the 5-membered ring (imidazoles). There were some inhibitor pairs where a direct comparison
could be made between the groups (benzotriazole (2F) and benzimidazole (4F); 2-
mercaptobenzathiazole (3B) and 2-mercaptobenzoxazole (3A) and 2-mercaptobenzimidazole (4C); 2-
aminobenzimidazole (4A) and 2-aminobenzathiazole (3D)) and in all cases the thiazole, oxazole and
triazole groups were superior to the imidazoles. The addition of either a thiol or amino group to the
ring increased the inhibitor performance, whereas the addition of both groups had a synergistic effect
if they were adjacent or near to each other but a negative or antagonistic effect if they were opposite
each other or further away on a second ring where they may have been competing for surface binding.
This is well demonstrated in the comparison between 2-mercaptobenzothiazole (3B), 2-
aminobenzothiazole (3D) and 6-amino-2-mercaptobenzothiazole (3C), where having either the thiol
or amino group between the sulfur and nitrogen on the 5-membered ring gave good inhibition but
having both thiol and amino groups at either side of the molecule caused the inhibition to drop
markedly. In the case of the dihydroxybenzene isomers, the hydroxy’s meta to each other (resorcinol,
1B) gave the best of adequate performances suggesting electronic effects trumped steric effects in
this case, where having the groups next to each other (ortho, 1A) otherwise might be assumed to give
bidentate and hence stronger bonding to the metal surface, leading to better inhibition.
Further detailed studies were performed using two triazole inhibitors selected as promising
candidates from the droplet study. In-depth analysis was conducted using electrochemical tests and
surface characterization to investigate the interaction between inhibitor molecules and the AA5083
surface either by understanding the inhibition mechanism or adsorption type. Electrochemical tests
(PDS, LPR and EIS) were mostly in agreement with each other. However, the surface analysis showed
no strong evidence for inhibitor film formation as described in the electrochemical methods except in
the FIB/SEM analysis, which showed evidence for sulfur on the metal surface. However, this was only
in cross-section cuts where the beam was parallel with the metal surface and where the penetration
86
depth of the electron beam was penetrating further along into the inhibitor film. When the electron
beam was perpendicular to the metal surface, no evidence for sulfur could be seen. In addition, using
FTIR to detect any bonding on the AA5083 surface had inconclusive evidence of very weak peaks when
3-amino-1,2,4-triazole-5-thiol (2C)) system was used, and no peaks were observed for the 3-amino-
1H-1,2,4-triazole (2G), which may be because of the limited sensitivity of the FTIR method to detect
6.1. Conclusions
thin inhibitor film on the metal surface
This master's project was driven by the need to create or develop a new droplet method for rapid
discovery of corrosion inhibitors for metals that are susceptible to pitting to complement the current
method for metals that undergo general corrosion. The project concluded that
• A novel high throughput method was developed for evaluating corrosion and corrosion
inhibitors for metals that are vulnerable to pitting. A comparison of inhibitors with different
interactions showed discrepancies between bulk solution methods and droplets.
• Features of promising inhibitors were observed through the droplets’ method and detailed
analysis, which agreed with what was mentioned in the literature. The inclusion of molecules
containing O, S, and N in the structure plays an important role in the adsorption process.
Moreover, the structural geometry plays a key role and can have either a negative or positive
influence on inhibition performance based on the bonding between the inhibitor and the
metal’s surface.
• Two shortlisted inhibitors from the initial droplet tests had further analysis over two weeks
using the electrochemical method and surface characterization. Electrochemical tests
detected high performance over a long period, but the films took time to build up a layer over
the AA5083 surface, as shown by PDS, LPR and EIS.
• FIB/SEM revealed inhibitor film formation of one inhibitor by detecting sulfur in the film/oxide
only parallel but not perpendicular to the AA5083 surface. FTIR analysis of the surface was
inconclusive, suggesting the inhibitor film was too thin for detection by these methods.
87
6.2 Recommended future work
• As the marine corrosion mechanism, in reality, involves the deposition on the metal surface
with droplets and aerosols of different sizes, evaporation and rewetting of the droplets and
the washing of the surface by rainwater, the droplet method needs to be compared to the
actual corrosion observed and, if necessary, modified and improved.
• Further detailed analysis of successful inhibitors identified by the droplet method is needed
•
to understand the protection mechanism.
It would be beneficial to use the identified molecular features of successful inhibitors
identified by the droplet method to select similar compounds and test these selected
compounds to see if they improve inhibitor efficiencies for AA5083.
• Investigate the possibility of synergies from combining two or more good inhibitors in the
same droplet
• Further Thinning the SEM specimen to a TEM specimen. Cross-sectional TEM characterization
is helpful in detailing the microstructure of the passive film, particularly the passive film on
the Al matrix and the passive film on top of the constituent particle.
88
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