experimental STUDY OF PITTING CORROSION OF STAINLESS STEEL SERIES 316 IN 0.1m SODIUM CHLORIDE with polished and rough surfaceRan Dy Ooi | 8563481

Ran Dy OoiAbstract Corrosion could badly effects on materials. Pitting is one of the localized corrosion that will occur on material surface. Usually, pitting corrosion make a small holes on the surface material. In this study, a piece of stainless steel series 304 have been tested in two different solutions; one is in the sodium chloride with neutral pH while the other solution is in the sodium chloride with pH 2 solution. The experimental study is performed by scanning the potential using a sweep generator connected to a potentiostat. It is found at the end of the study that stainless steel series 304 in pH2 have lower pitting potential compared with in neutral solution. Thus, the corrosion rate is higher.KeywordsCorrosion, pitting, polarization curve, stainless steel series 304, pH, neutralIntroductionCorrosion or simply known by people as rust is a gradually destructive situation usually on metal. This is because the tendency of metal reacts with the environment mostly with atmospheric gases through chemical reaction. In the most common use of term, this means chemical reaction whereby oxidation of metal with oxidant.

The pitting corrosion is one of the localized corrosion that leads to the formation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to localized corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions.

In this respect, the present work is aimed to study the corrosion behaviour of the stainless steel series 304 in sodium chloride solutions, mainly its pitting corrosion and the morphology of pits. The effect of the pH of the solutions on the localised corrosion of the stainless steel 304 series will be considered. MethodologyThe workflow of study is divided into two phases. Initially, the condition of electrolyte is in neutral. Next, the condition of electrolyte is in pH 2. In the end, the polarization curve is being plotted after the data is being logged using data-logger device.

Figure 1: Experimental Setup

Figure 2: Experimental Setup. Computer shows the data logger software interface.DataGraph 1: Graph Applied Potential versus Logarithm of Current Density for Stainless Steel Series 304 in Sodium Chloride Neutral Solution.

Graph 2: Graph Applied Potential versus Logarithm of Current Density for Stainless Steel Series 304 in Sodium Chloride pH 2 Solution.

Graph 3: Combine Graph of Applied Potential versus Logarithm of Current Density for Stainless Steel Series 304 in Sodium Chloride Neutral Solution and pH 2 Solution.

Results and DiscussionGraph 1 shows the polarization curve of Stainless Steel Series 304 behaviour in Sodium Chloride neutral solution. It is found that as the applied potential increase, the curve undergo passivity first. In the passivity region, it is shown that noise occurs. This happened because of disturb from electronic device while experimental study is being conducted. As the applied potential reach its pitting potential level, the curve immediately spike up with constant value recorded.

As for in pH 2 solution, it is observed from polarisation curves that the metastable region behaves moderately compared to neutral solution. As it reached the pitting potential, it goes same as well as neutral solution behaviour whereby the curve immediately spike up with constant value recorded.

In comparison between neutral solution and pH solution, it can be seen from the graph that there is a difference pitting potential value. In neutral solution, the pitting potential is higher compared with in pH 2 solution. Nevertheless, the metastable region for pH 2 solution is moderate in noise compared with the neutral solution.

The reason of difference pitting potential value between neutral solution and pH 2 solution is because the reaction takes place in the solution itself. As the pH decrease, the passive current increase, and it goes the same with the pitting potential. In the end, the corrosion rate increase.

Figure 3: Morphological View for Sample of Stainless Steel Series 304 in Sodium Chloride Neutral Solution #1

Figure 4: Morphological View for Sample of Stainless Steel Series 304 in Sodium Chloride Neutral Solution #2

Figure 5: Morphological View for Sample of Stainless Steel Series 304 in Sodium Chloride Neutral Solution #3

Figure 6: Morphological View for Sample of Stainless Steel Series 304 in Sodium Chloride pH 2 Solution #1

Figure 7: Morphological View for Sample of Stainless Steel Series 304 in Sodium Chloride pH 2 Solution #2

Based on the sample shown on figure 3, 4 and 5 using microscope, it is found that the corrosion is less severe compared to sample shown on figure 6 and 7. Figure 3, 4 and 5 is the condition of stainless steel series 304 in sodium chloride neutral solution while figure 6 and 7 is the condition of stainless steel series 304 in sodium chloride pH 2 solution.

It can be deduce that when stainless steel series 304 in sodium chloride neutral solution, it is more severe compared to the pH 2 solution. The corrosion behaviour in pH 2 is less severe compared to in the neutral solution. In figure 6 and 7, the hole or pit is being visualised clearly while in figure 3, 4 and 5, the pit not severe as in figure 6 and 7.

Conclusion And RecommendationIn conclusion, it is found that as the pH value decrease, the passive current increase and the general corrosion rate increase. But, the value of pitting potential is decrease. As for the neutral condition, the passive current decrease and the general corrosion rate lower compared with in the pH 2 solution. The pitting potential for neutral solution is higher compared with the pH 2 solution.

Based on the sample morphology observed via microscope, the stainless steel series 304 in sodium chloride neutral solution has severe pitting on the metal. In contrary, the stainless steel series 304 in sodium chloride pH 2 solution act oppositely whereby it more and less like general corrosion on the metal.AcknnowledgementThe author would like to thanks Corrosion Protection Centre, Dr Stuart Lyon, Dr Drik Engelberg, Dr Peter Skeldon, Dr Fabio Scenini and Mr Paul Jordan, the faculty members and classmates in order completing the experimental study.

References

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