Electrochemical Study of the Austenitic Stainless-Steel Susceptibility to Sulfide Stress Cracking in H2S-Containing Brines

Abstract Austenitic stainless steels are widely in use in oil & gas production environments providing effective protection for mitigation of general corrosion due to the presence of CO2 in the produced fluids. However, there is little agreement on the service limits when exposed in environments that contain H2S, particularly in relation to the susceptibility to sulfide stress corrosion cracking (SSC) assisted by H2S. The objective of the study is to evaluate the effects of temperature, H2S, and Cl- concentration on the pitting onset of pitting and the likelihood of SSC. Using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by UNS S31603 stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H2S (bal. CO2). The results obtained indicate that the decrease of the pitting potential is consistent with the passive layer formed on the S31603 stainless steel increasing the electron donor carrier density at constant temperature. In this context, the Cl- content in the brine exerts a larger effect than the H2S activity. The increase in the susceptibility to SSC was found to be proportional to the H2S content in the brine at constant temperature and chloride level; however, the threshold H2S content to induce SSC is shown to increase with the brine ionic strength. This behavior is consistent with the observed reduced SSC susceptibility at higher chlorides and the presence of metastable pitting which is intensified by the activity of H2S.