Pourbaix Diagrams for the Ternary System of Iron-Chromium-Nickel

Pourbaix diagrams (potential-pH diagrams) for the ternary system of Fe-Cr-Ni at 25°C to 300°C were calculated. Extrapolation of thermochemical data to elevated temperatures was performed with the revised model of Helgeson-Kirkham-Flowers, which also allows uncharged aqueous complexes to be handled. The large stability of the bimetallic spinel oxides (trevorite [NiFe2O4], chromite [FeCr2O4], and nichromite [NiCr2O4]) is shown by their predominance areas on top of those for the single metal Pourbaix diagrams. NiFe2O4 had the largest stability area of the spinels, and it covered the entire potential range for the stability of water at intermediate pH. FeCr2O4 had a stability area located around the hydrogen line. NiCr2O4 had the smallest stability area and was the least stable of the bimetallic spinels. Results were discussed in connection with the different chemistries used in nuclear power reactors of the boiling water type.