Oxygen and Metal Activities of the Chromium‐Nickel‐Oxygen System Between 900° and 1100°C

Abstract

The equilibrium oxygen pressures for Cr‐Ni alloys existing as the alpha (α) and gamma (γ) phases and coexisting with chromium oxide , nickel oxide , and nickel chromite were determined at 900°, 1000°, and 1100°C using an electrochemical cell with a calcia‐zirconia electrolyte. These pressures were related to compositional determinations of the solid phases and the Cr‐Ni‐O isotherms. The free energies of formation of chromium oxide and nickel chromite were determined. With increasing nickel content of the alloy, the oxygen pressure increases from the dissociation pressure of pure chromium oxide to that of the univariant phase field, , with chromium oxide containing <0.2 weight per cent (w/o) nickel. At higher oxygen pressures, the phase field extends up to the univariant phase field. Nickel chromite was essentially stoichiometric the nickel content of chromium oxide was ∼0.5 w/o, and the γ‐alloy contained only a very small chromium content, ∼10−4 w/o. No variation was detected in the spinel composition across the region but the chromium content of the alloy decreased to ∼10−8 w/o at the univariant phase field, with nickel oxide containing 4.5 w/o Cr.