Localized Corrosion of Corrosion Resistant Alloys in H2S-Containing Environments

Corrosion resistant alloys (CRAs) are widely used in harsh oil and gas production environments where low-alloy steels cannot be used. It is recognized that stress corrosion cracking (SCC) of stainless steels and Ni-based alloys can occur at potentials above the repassivation potential (Erp) for localized corrosion. This paper focuses on experimental work that has been conducted to determine Erp of six CRAs, i.e., super-martensitic stainless steels S13Cr (UNS S41425) and S15Cr (UNS S42625), super-duplex stainless steel 2507 (UNS S32750), and solid solution Ni-based Alloys 2535 (UNS N08535), 29 (UNS N08029), and 28 (UNS N08028) in chloride-H2S environments. A generalized approach is applied to determine Erp values from electrochemical polarization results at temperatures ranging from 85°C to 232°C. The use of completely deaerated solutions and ensuring the stable growth of localized corrosion are both critical to obtain conservative Erp values for CRAs in environments that are similar to downhole conditions. A part of the obtained data set has been used in the development of a mechanistic model for calculating Erp as a function of alloy composition, environment chemistry, and temperature in order to generalize the observations and predict localized corrosion and SCC in a wide range of chloride-H2S environments by using limited experimental tests. Another part of the Erp data set has been used to validate the model predictions. It has been demonstrated that the model predictions agree well with the experimental Erp values. Based on the measurements coupled with model predictions, the effects of H2S, temperature, and alloying elements on the susceptibility of CRAs to localized corrosion and SCC are analyzed.