Extreme value analysis of carbon steel pitting corrosion under deoxygenated seawater

Severe channelling corrosion of offshore water injection pipelines (WIP) is a major threat for industry. Both microbiologically influenced corrosion (MIC) and under deposit corrosion (UDC) are suspected. A pilot study is reported herein to investigate the contributing factors for the formation of channelling corrosion. This was conducted by exposing half-pipe steel pipelines to different WIP environments. The effect of the presence of deposits, filtration and UV irradiation and nitrate addition were considered. The steel specimens were recovered after 12 days, 180 days and 365 days of exposure. EDS analysis of corrosion products showed evidence of MIC being involved in the corrosion process in natural biotic seawater. The pitting depth was measured and it was found that under deoxygenated seawater environment, the maximum pit depth from the steel specimens covered with deposits is greater than those without deposits. The variability in maximum pit depth changes with increased period of exposure in different test environments. Extreme value statistics were applied to analyse the maximum pit depth. The results show that Gumbel extreme value function are not appropriate for describing the variability of maximum pit depth for all the data from the environments covered with deposits for 180 and 365 days of exposure. For deeper pits, Frechet distribution is more appropriate to model the maximum pit depth. This has important practical implications for predicting long-term pitting governed by anaerobic conditions.