A polarization study of alloy 625, nickel, chromium, and molybdenum in ammoniated sulfate solutions

A polarization study of Alloy 625 (UNS N06625), nickel, chromium, and molybdenum, in a solution of ammonia (NH3) and sulfuric acid (H2SO4) at pH 10, is presented. The effect of air, oxygen, hydrogen peroxide (H2O2), and variation in pH is examined. Pourbaix diagrams for the Ni-NH3-H2O and Cr-H2O systems are presented. The formation of metastable oxides is suggested by the observed passivation of both the alloy and Ni outside their thermodynamic fields of stability. The alloy's polarization curves are similar to those obtained for Cr. Ni is seen to have the highest passive current density while Mo does not passivate. Selective dissolution of Ni from the surface regions of the alloy is predicted. Corrosion rates, which are low (1.2 x 10(-3) mmpy at pH 10), increase with decreasing pH. Passive current densities are found to be higher in solutions of sulfate alone. Highly oxygenated solutions do not result in an increase in corrosion rate. However, the addition of H2O2 to the ammoniated solution results in the alloy's direct activation at transpassive potentials.