Volume of Activation for the Corrosion of Type 304 Stainless Steel in High Subcritical and Supercritical Aqueous Systems

In this study, electrochemical emission spectroscopy (EES, commonly referred to as electrochemical noise analysis, ENA) was used to analyze the effect of pressure on corrosion reactions in high-temperature aqueous systems. A model was applied to study the effect of pressure on the corrosion activities (and hence corrosion rate) of metals in high subcritical and supercritical aqueous systems (SCAS), where the corrosion activity is defined as the standard deviation of the noise in the coupling current between two identical specimens. On the basis of previous work under similar conditions. the corrosion activity is postulated to be proportional to the instantaneous corrosion rate. Emphasis is placed on the contributions from activation, system compressibility. and degree of dissociation of aggressive species. Experiments have been performed to estimate those contributions to the pressure dependence of electrochemical corrosion activity of Type 304 (UNS S30400) stainless steel and nickel in 0.01 m deaerated hydrochloric acid (HCl) at 350, 450, and 500 degrees C. The model shows that the contribution to the pressure dependence of the reaction rate from activation is more important in high subcritical systems, while the degree of dissociation is more pronounced for supercritical systems. The study also demonstrates that the volume of activation of electrochemical corrosion of metals is pressure-dependent at high subcritical and supercritical fluid conditions. but that opposite pressure dependencies are observed with the volume of activation increasing with increasing pressure in subcritical systems while that in supercritical systems decreases with increasing pressure. The effect of pressure on the volume of activation of the corrosion of Type 304 stainless steel and nickel in 0.01 m HCl at both subcritical and supercritical temperatures can be explained by the decreasing electrostrictive volume loss as the density (pressure) increases.