Effect of sulfate and dissolved hydrogen on oxide films on stainless steel in high-temperature water

In the present paper, we focus on the influence of sulfate ion impurity and dissolved hydrogen on the protective ability of the oxide film on stainless steel in high-temperature water. For the purpose, the electrical and electrochemical properties of oxide films formed on AISI 316L NG in simulated boiling water reactor crack conditions (10 ppm sulfate purged with N₂ + 0.01 or 0.5% H₂) were characterized by in situ electrochemical impedance spectroscopy (EIS). In addition, the surface and in-depth composition of the oxide films has been estimated by ex situ Auger electron spectroscopy (AES). The quantitative assessment of the protective ability of the oxide is based on an interpretation of the electrochemical impedance data per the mixed-conduction model. A novel procedure for the estimation of kinetic and transport parameters that involves comparison of the model equations to both EIS and AES results is proposed and tested. Based on the parameter values, the effect of sulfate and dissolved hydrogen on the processes of film growth and dissolution is discussed in view of an approach to the initiation of stress corrosion cracks.