Prediction of critical environments for active-passive transition of corrosion resistant alloys in sour environments.

Abstract

The corrosion behavior, in particular passivation, of corrosion resistant alloys (CRAs) in sour environments was investigated intensively by using a duplex stainless steel and an austenitic stainless steel as representative CRAs. It was found that hydrogen sulfide significantly retarded the passivation of CRAs, whereas carbon dioxide did not. The changes in corrosion morphologies due to the changes in environmental aggressiveness were classified on the basis of the immersion test results. A new criterion for predicting the critical environments for active to passive transition was proposed. The critical environments for the transition of a given alloy can be determined by comparing the pH of environments with the depassivation pH (pHd) of the material which is a function of environmental factors such as temperature and partial pressure of hydrogen sulfide. The locus of the points having pH=pHd gave a good estimation for the critical environments. It was confirmed that the prediction was in good agreement with the long-term immersion test results for 6 months. Since this criterion is based on the corrosion mechanism, it has many advantages; the prediction is accurate; the results for CO2-free environments can be extended to CO2-containing environments; and the prediction can be performed very quickly.