Comparison of biocorrosion behavior of stainless steel 316 l and mild steel induced by slime producing bacteria

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The present study investigated the biocorrosion behaviour of Stainless steel 316L (SS 316L) and mild steel coupons exposed to a medium containing slime producing bacteria, Escherichia coli (E.coli).

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp. 404-411. Article ID: IJMET_10_03_041 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=3 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed COMPARISON OF BIOCORROSION BEHAVIOR OF STAINLESS STEEL 316 L AND MILD STEEL INDUCED BY SLIME PRODUCING BACTERIA Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni Rooshde School of Ocean Engineering, Universiti Malaysia Terengganu, Terengganu, Malaysia Mohd Sabri Mohd Ghazali School of Fundamental Science, Universiti Malaysia Terengganu, Terengganu, Malaysia ABSTRACT The present study investigated the biocorrosion behaviour of Stainless steel 316L (SS 316L) and mild steel coupons exposed to a medium containing slime producing bacteria, Escherichia coli (E.coli). Corrosion analysis was performed to observe the progression of corrosion process on tested steels induced by E.coli and its metabolite activity for 7 days. The findings revealed that both SS 316L and mild steel experienced severe pitting corrosion. The colonization of E.coli and secretion of exopolymeric substances or slime have accelerated corrosion process on mild steel surface and damaged the passivation layer on SS 316L. As a result, the tested mild steel specimen exhibited 25% higher corrosion rate compared to that of the SS 316L. Keywords: Biocorrosion, SS 316L, Mild steel and E.coli. Cite this Article: Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni Rooshde, Mohd Sabri Mohd Ghazali, Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime Producing Bacteria, International Journal of Mechanical Engineering and Technology, 10(3), 2019, pp. 404-411. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=2 1. INTRODUCTION Steels are one of the major class of engineering materials and they are widely applied in multiple sectors including construction, transportation, energy as well as the manufacturing industry. A steel is generally an iron-based alloy containing some amount of alloying elements such as Ni, Cr, Mo, Mg, Co, Nb, B, C and many more. Different grades of steel http://www.iaeme.com/IJMET/index.asp 404 editor@iaeme.com
Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime Producing Bacteria ranging from stainless steel to carbon steel are made with different selections of mechanical strength, malleability and corrosion vulnerability. Stainless steel 316L (SS 316L) is a Mo-bearing austenitic steel with excellent corrosion resistant. Due to the presence of Cr in its alloying formulation, SS 316L is self-passivated by layer of Cr oxides whenever it is exposed to corrosive and oxygen rich environment. Mild steel on the other hand is a steel with approximately 0.05 – 0.25% carbon. It is easy to form due to its high malleability and ductility. The mild steel has relatively lower mechanical strength and lower tolerance to corrosion. Both types of steels have significant roles in the development of marine structures. The SS 316 L is mostly selected for obtaining structures with high tensile strength and must be used under direct exposure to chloride-containing condition. Meanwhile, the mild steel is needed where the structure has to be deformed or welded. Biocorrosion of steels is a great challenge for numerous engineering applications. In this process, the general metal corrosion process is accelerated by attachment and colonization of microorganism on metal surfaces [1-3]. Metabolic activity of microorganisms adhering to the surface of metal particularly the steels can damage the passivation layer, inhibit its development or alter the rate of redox reaction at cathode or anode terminal [4-6]. Eventually, the unintended event promotes the occurrence of localized corrosions such as pitting, crevice or grain boundary corrosions [6,7]. For that reason, the biocorrosion can lead to costly economic loss and give rise to many environmental issues [8,9]. Recent studies reported that slime producing bacteria is one type of corrosive bacterial community that has great potential to influence the rate of steel corrosion [10-12]. This bacteria adheres to steel walls and secretes gel-like exopolymeric substances which contain polysaccharides, proteins, lipids, and nucleic acids [11]. The roles of this slimy layer of EPS on biocorrosion behaviour of steels are still unclear. In certain condition, EPS layer increases the corrosion rate of metals while in other conditions, the layer sometimes serves as additional protective layer on top of metal surfaces, thus reducing the corrosion rate [4]. In order to identify the best strategy for corrosion prevention and control, an understanding on the biocorrosion behavior of different types of steels is required. The present study investigated the biocorrosion behaviour of Stainless steel 316L (SS 316L) and mild steel coupons upon immersion in medium containing slime producing bacteria, Escherichia coli (E.coli). 2. MATERIALS & EXPERIMENTAL PROCEDURES 2.1. Materials Two types of metallic specimens studied are made of mild steel and stainless steel 316L. Table 1 indicates the composition of specimens used in this work. http://www.iaeme.com/IJMET/index.asp 405 editor@iaeme.com
Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni Rooshde, Mohd Sabri Mohd Ghazali Table 1 Compositions of the mild steel and SS 316L specimens Elements Stainless Steel 316L Mild Steel Composition Composition (%) (%) C 0.03 0.33 Mn 2.00 0.76 P 0.045 0.05 S 0.03 0.05 Si 0.75 0.19 Cr 18.00 0.12 Ni 14.00 0.11 Mo 2.00 - N 0.10 - Cu - 0.003 Fe Balance Balance Escherichia coli (E.coli), the Gram negative bacteria was selected and cultured as the model for slime producing bacteria. Bacteria culture was obtained from frozen bacteria stock stored in 15% glycerol at -20 oC. The bacteria was grown in 100 ml of nutrient broth before further use. 2.2. Methods 2.2.1. Preparation of Specimens The metal specimens were prepared according to the procedures suggested in ASTM G31-72. The SS 316L and the mild steel sheets were were cut into smaller rectangular coupons having 25 mm width x 25 mm length x 1.5 mm thick. The specimens were polished with abrasive paper sized P320, P600, P800, P1000, P1200, P1500 and P2000 using a polishing machine (Mecapol P225U) until a mirror surface was obtained. 2.2.2. Preparation of Medium and Bacteria Culture The nutrient agar was prepared by mixing 20 g nutrient agar powder for every 1 L of distilled water. The nutrient agar was autoclaved at 121oC for 15 minutes and cooled down to approximately 35 oC. It was then poured into a plate and left to solidify in a chiller for 24 hours. By using the streak plate technique as suggested in [13], the pure culture of E.coli was inoculated onto the plate containing nutrient agar. 2.2.3. Bacterial Attachment Test Determination of bacterial attachment on substrate surface can provide useful information regarding the affinity of the bacteria species to settle on a certain metal substrate before analyzing the subsequent corrosion process. This study adopted the procedures suggested in [6,14,15] with some modifications. The metal specimen was placed on nutrient agar which has been streaked with bacteria and incubated for 24 hours. The progress of corrosion process on tested specimen was recorded for 7 consecutive days. The inspection was completed using metallurgical microscopy and scanning electron microscopy techniques. The observations were also tabulated in term of scoring damage as suggested by Smirnov et al., [16]. 2.2.4. Determination of Weight Loss and Calculation of Corrosion Rate Determination of weight loss is a procedure to measure the metal loss after metal substrates immersion test at specific intervals. The technique involves weighing of the mass of metal substrates before and after the immersion test and calculation of the net metal loss. Equation (1) asserts the calculation to find percentage of metal loss. http://www.iaeme.com/IJMET/index.asp 406 editor@iaeme.com
Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime Producing Bacteria Initial mass of metal specimen % Weight loss   100% (1) Mass loss of metal specimen after immersion Initial mass and mass loss both are calculated in gram and multiply by hundred percent to get percentage of mass loss. The standard corrosion rate calculation was done according to the ASTM G31-72. The average corrosion rate in millimetres per year (mm/y) can be calculated by the following Equation (2). (K W ) Corrosion rate per year (mm/year)  (2) ( A  T  D) where K is a constant, T is the time exposure in hours, A is the area in cm2, W is the mass loss (g) and D is the density (g/cm3). The K constant value used in this experimental work is 8.76 x 104 while the density for stainless steel and mild steel is 7.98 g/cm3 and is 7.86 g/cm3, respectively. 3. RESULTS & DISCUSSION The evolution of corrosion on SS 316L and mild steel specimens exposed to medium containing E.coli was monitored for 7 consecutive days and the observations are tabulated in Table 2 whereas the scoring damages on the specimen surface are listed in Table 3. Tarnishing of both SS 316L and mild steel specimens was observed after Day 1 and it was more rapid on the SS 316L. At this stage, the passivation layer has started to develop on SS 316L due to oxidation of Cr with dissolved oxygen in the medium. Early signs of corrosion product deposition was observed as early as on the Day 3 for both SS 316L and mild steel. After the 4th day, the mild steel indicated many signs of accelerated corrosion including tarnishing, appearance of black spot and rusting at the back of surface and corroded slimy surface. The deposition of red-brown corrosion product on the surface of corroded mild steel could be associated to formation of -hematite or Fe2O3.H2O or Fe(OH)3 whereas the blackish corrosion product was probably Fe2O3 or FeO. Further inspection was performed using a metallurgical microscope and the scanning electron microscopy technique in order to analyze the corrosion effects at microscopic scale. Table 2 Evolution of corrosion process on SS 316L and mild steel specimens induced by E.coli Metal Specimens Day SS 316L Mild Steel Day 1 http://www.iaeme.com/IJMET/index.asp 407 editor@iaeme.com
Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni Rooshde, Mohd Sabri Mohd Ghazali Metal Specimens Day SS 316L Mild Steel Day 3 Day 5 Day 7 Table 3 Scoring damage observed in SS 316L and mild steel specimens exposed to E.coli Scoring Damage Exposure day SS 316L Mild steel 0 A A 1 D B 2 D B 3 D, E F 4 G E, F 5 G E, F 6 G G, H 7 G, I I, J Scoring damage: A: no changes, B: Minor tarnish on surface, C: A transparent layer easily removed on the surface, D: Tarnish cover 25% of surface, E: Surface at the back (adhere on agar) become brownish rust, F: Black spot at the back of surface, G: Tarnish 50% on surface, H: Black spot on surface become bigger, I: Corroded slimy surface, J: Back surface of metal 100% rusting. http://www.iaeme.com/IJMET/index.asp 408 editor@iaeme.com
Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime Producing Bacteria Based on Figure 1, the surface of the immersed SS316L specimen was obviously affected and the corrosion produced attacks in the form of spots or pits. The occurrence of pitting corrosion implies that the passivation layer has weakened or damaged at specific points on the SS 316L surface due to metabolite activity by E.coli. The corrosion impacts on the mild steel surface were even more severe. After the 7th day of immersion in E.coli containing medium, a combination of corrosion processes took place on the surface of the specimen. Figure 2 depicts that larger area are covered with corrosion products and the corrosion attacks at grain boundary region and pits have been detected. SEM images in Figure 3 confirm that the surfaces of both SS 316L and mild steel specimen were covered with dense layer of E.coli cells. The appearance of exo-polymeric substances (EPS) can be clearly observed on the surface of mild steel. In accordance to the data in Table 4, the SS 316L and mild steel specimens lost 2.83 and 3.63% or its initial weight, respectively in just 7 days. In terms of corrosion rate value, the mild steel specimen suffered slightly 25% more severe corrosion as compared to the SS 316L. Figure 1 The surface morphology of SS 316L (a) before exposure and (b) after 7 days of exposure to E.coli containing medium Figure 2 The surface morphology of mild steel (a) before exposure and (b) after 7 days of exposure to E.coli containing medium http://www.iaeme.com/IJMET/index.asp 409 editor@iaeme.com
Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni Rooshde, Mohd Sabri Mohd Ghazali Figure 3 SEM images of E.coli adhesion on (a) stainless steel and (b) mild steel surfaces after 7 days Table 4 Percentage of weight loss and the corrosion rate for stainless steel and mild steel after exposure to E.coli containing medium Specimen % weight loss Corrosion Rate (mm/year) SS 316 L 2.83+ 0.01 2.20 + 0.01 Mild steel 3.63+ 0.01 2.74 + 0.01 The findings from this study indicates that slime producing bacteria like E.coli has great potential to cause harm on both SS 316L and mild steel even in a short period of time. A Gram negative bacteria, E.coli adheres to the steel surface to obtain carbon nutrient sources for supporting its growth [12]. In addition, colonization of dense E.coli cells on steel surface produces biofilm layer that may capture moisture and inhibit oxygen at the same time. In the case of SS 316L, both conditions may have inhibited formation of passivation layer or weakened it at certain spots. As a result, pitting corrosion progressed rapidly on its surface. The corrosion induced by E.coli has relatively greater impacts on mild steel as it is not protected by self-passivation mechanism. Exposure to moisture and colonization by E.coli initiate corrosion process on its surface. Detection of abundant EPS on mild steel may suggest the preferential colonization of bacteria on mild steel over the SS 316L. As supported by [4], excretion of EPS promoted the cathodic reaction, thus encouraged the dissolution of iron from steel. Deposition of corrosion products, the formation of pits and grain boundary cleavage increased the surface roughness on mild steel, thus encouraging further attachment of E.coli and its settlement. As the network of EPS grew, the mild steel surface was constantly in contact with slimy and wet layer or biofilm. For that reason, corrosion process on its surface became accelerated and more deterioration can be observed. 4. CONCLUSION This study investigated the biocorrosion behavior of Stainless steel 316L (SS 316L) and mild steel induced by slime producing bacteria, E.coli. The findings revealed that the corrosion resistance of SS 316L deteriorated by E.coli and its metabolite activity. The SS 316L subjected to medium containing E.coli has suffered from severe pitting corrosion due to weakening or damaging of passivation layer at localized sites. Meanwhile, the results showed the preferential colonization of E.coli on the mild steel surface. The subsequent corrosion process has caused attacks on pits, grain boundary and larger surface area on mild steel. In conclusion, microbially induced corrosion can occur rapidly on steels and self-passivation alone could not fully protect steel from getting affected by the slime producing bacteria. ACKNOWLEDGEMENTS The authors would like to thank for the financial support from research grant provided by Ministry of Higher Education under Fundamental Research Grant Scheme (FRGS), Grant No: 59436 and Universiti Malaysia Terengganu for providing the research facilities. REFERENCES [1] Kato, S. Microbial extracellular electron transfer and its relevance to iron corrosion. Microbial Biotechnology, 9, 2016, pp. 141-148. [2] Kryachko, Y. and Hemmingsen, S.M. The role of localized acidity generation in microbially influenced corrosion. Current Microbiology, 74, pp.870-876. http://www.iaeme.com/IJMET/index.asp 410 editor@iaeme.com
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