POTENTIAL PH DIAGRAMS FOR ALLOY WATER-SYSTEMS UNDER LWR CONDITIONS

Potential-pH diagrams, which present the regions of stability of metallic species with water, are generally known as Pourbaix diagrams. They are especially useful to corrosion scientists because the diagrams indicate the areas of potential and pH in which oxides of the metal are stable and thus the regions in which an oxide corrosion layer can form on the metal, protecting it from corrosion. Potential-pH diagrams are generally available for the metallic elements, even at high temperature (up to about 600 K). But rarely is a pure metal used as a construction material. Stainless steel, which is an alloy of iron, chromium, and nickel plus some trace elements, is often used as a construction material for water systems because of its good corrosion resistance. The oxides in equilibrium with the water-Fe-Cr-Ni system are generally mixed metal oxides. In this paper, Pourbaix diagrams for this system are calculated from thermodynamic values at room temperature and at typical LWR (light water reactor) temperature. Diagrams for the ancilliary systems Fe-Cr-water, Fe-Ni-water and Cr-Ni-water are also given.