Pit Propagation of Carbon Steel in Sour Conditions

Pitting corrosion is the dominated failures cause of pipelines transporting wet sour gas. Pitting corrosion is described by three stages. They are pit initiation, pit propagation and repassivation. Some pits may grow causing leak of a pipeline after initiation stage, but some may stop growing by repassivation. The growing pits posts a serious thread to integrity of pipelines operated in sour gas environment.  To annualize a pitting corrosion rate from the corrosion testing data obtained in a laboratory, one extrapolates the testing duration to a year. Errors are introduced due to the nonlinear pit propagation rate with the time. This paper is to study the effect of the testing duration on the passivation and repassivation, as well as effect on the pitting corrosion rates. The results in this paper will also show that H₂S/CO₂ ratio, temperature and acidic acid content affects the repassivation process, and therefore, influences the pitting corrosion rates. Pitting inhibition of two corrosion inhibitors was studied in the paper. With the Surface Profiler, the authors are able to measure the pit depth with high level of accuracy.