Pourbaix Diagrams for Iron, Nickel, and Chromium in Sub-critical and Supercritical Water

Abstract

The Supercritical Water-cooled Reactor presents many challenges in terms of material selection. Because the reactor will operate at temperatures and pressures above the critical point of water, an understanding of how candidate materials behave is essential. Pourbaix diagrams (a plot of electric potential vs. pH) are thermodynamic maps that predict the regions of immunity, passivity and reactivity of a particular metal in an aqueous environment. Pourbaix diagrams have been developed for the near-critical and supercritical regimes for iron, nickel, and chromium. Various sources were used for the necessary thermodynamic parameters, which were extrapolated to the temperatures of interest using the Helgeson-Kirkham-Flowers thermodynamic model and its revisions. The diagrams contain the ionic and aqueous species expected to be present under varying supercritical water density and temperature as well as the oxides expected to be formed under the given conditions for each of the individual metal-water systems.