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.