Volume of Activation for the Corrosion of UNS S 30400 in High Subcritical and Supercritical Aqueous Systems

Abstract In this study electrochemical noise analysis (ENA) is used to analyze the effect of pressure on corrosion reactions in high subcritical and supercritical aqueous systems (SCAS). A model is applied to study the effect of pressure on corrosion rates of metals in SCAS, with emphasis on contributions from activation, the degree of dissociations of aggressive species, and the system isothermal compressibility. Experiments were carried out to estimate the pressure dependencies of the electrochemical corrosion rates of UNS S 30400 (Type 304 SS) in HCl at subcritical and supercritical temperatures. The study shows that the volume of activation of the electrochemical corrosion of UNS S 30400 is pressure dependent in SCAS. The contribution from the volume of activation plays a dominant role on the relative corrosion rate at high subcritical temperatures but the degree of dissociation is found to be more important to the relative corrosion rate at supercritical temperatures, due to the high compressibility of the system (a gas-like phase).