Pitting corrosion of mild steel in long-term contact with particulate media in seawater

Steel infrastructure in contact with particulate media is at risk of failure due to pitting corrosion. For reliability analysis it is important to understand the progression of pit depth with time and the associated extreme value statistics. An analysis was conducted on an extensive data set of pit depth observations, made on mild steel coupons buried in different particulate media and immersed in natural Pacific Ocean seawater. It showed that the trends for the deepest pits with time were consistent with the bimodal model, and that pitting was deepest under seawater and increased with particle size when under particulate media. Gumbel plots of the data showed that the deepest pits plotted as a straight line, indicating a good fit. The plots also showed small, regular deviations from this straight-line, and these permitted interpretations for pit depths developing in incremental steps. These step sizes were largest for surfaces under seawater and increased with increasing particle size under particulate media. These findings allowed for new conceptual models for the process of pit depth growth under rust and particulate media. They also allow for new insights into metastable pitting for steels. Opportunities for further research are also presented.