Study of the Passivity Limits of Austenitic Stainless Steel in H2S-Containing Brines Using Mott-Schottky and Point Defect Model Analysis

The objective of the study is to correlate the effects of H2S and Cl− concentration on the passivity limits and the onset of localized corrosion in UNS S31603 stainless steel by evaluating the changes in the semiconducting behavior exhibited by the passive layer. The study is accomplished experimentally by using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H2S (bal. CO2) at 25°C. The results obtained using the Mott-Schottky analysis indicate that the decrease of the passive layer stability formed on the UNS S31603 stainless steel is consistent with the increase in the electron donor carrier density. This is observed as the consequence of the effect of Cl− and H2S. In this context, the Cl− content in the brine was found to exert a larger effect than the H2S activity. The correlation with the evaluation of the passive layer using the Point Defect model suggest that both the polarizability and the rate of annihilation of the cation vacancies at the metal/film interface increase with the H2S content in the environment. This behavior can explain the increased content of Cl− and sulfides as main electron donor species, also the observable increase in the passive layer susceptibility to both stable and metastable pitting.