Effect of Molybdenum on Pit Initiation Rate and Pit Growth Using Electrochemical Noise and Its Correlation with Confocal
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JMEPEG https://doi.org/10.1007/s11665-020-04920-9
Effect of Molybdenum on Pit Initiation Rate and Pit Growth Using Electrochemical Noise and Its Correlation with Confocal Laser Scanning Microscopic Studies Namrata Upadhyay, A. Ravi Shankar, B. Anandkumar, R.P. George, M.G. Pujar, John Philip, and G. Amarendra (Submitted November 6, 2019; in revised form May 22, 2020) The effect of Mo concentration on pit initiation rate and pit growth in austenitic stainless steels (SS) were investigated by electrochemical noise (EN) and confocal laser scanning microscopy (CLSM) techniques for the first time. We used 304LN, 316LN and 317LN containing different concentrations of Mo (0.02, 2.53 and 3.58 wt.%) for our studies. Using EN technique, initiation of pits and growth of pits were analyzed using Weibull and Gumbel distribution function, respectively. Pit depth was obtained using CLSM and the correlation between pit aspect ratio and Mo concentration were studied. Weibull probability plots showed that the Mo present in the alloy reduces the pit generation rates and improves the passivity. The plot for the pit size distribution using Gumbel distribution function showed the lowest metastable pit in 317LN and the highest in 304LN, suggesting the improvement in the pitting corrosion resistance due to Mo addition. The CLSM imaging showed maximum pit depth for specimen 304LN and minimum for specimen 317LN SS. Specimen 316LN SS showed intermediate pit depth. The depth of the pits observed in 304LN, 316LN and 317LN ranged from 80-100 lm, 30-40 lm and 20-30 lm, respectively. Alloy 317LN containing highest Mo concentration (3.58 wt.%) showed the lowest pit aspect ratio values followed by alloy 316LN with 2.53 wt.% Mo. These results indicate that Mo present in the stainless steel helps in arresting the pit growth and improve the resistance to pitting corrosion. Keywords
confocal laser scanning microscopy, electrochemical noise, Gumbel distribution function, pit aspect ratio, pitting corrosion, Weibull distribution function
1. Introduction Austenitic stainless steels (SS) find wide applications in many industries such as nuclear power plants, chemical industries, and in marine field due to its high ductility, high mechanical strength, and high corrosion resistance properties (Ref 1). These materials form a strong passive oxide film on its surface which is responsible for providing high corrosion resistance. The resistance of passive film depends upon the environmental condition in which these steels are exposed to and on the alloy composition. Although showing strong passivity in most of media, these materials suffer extensive pitting attack in highly aggressive chloride ions. Pitting corrosion of SS is one of the main catastrophic failures of structural material in chloride medium. The two main processes involved in the pitting corrosion are pit initiation and pit growth. Pit initiation occurs due to rapid breakdown of passive film. Once the pit has nucleated, it may repassivate immediately or may grow for some time before repassivation, lead
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