Anodic Polarization Behavior Of Nickel-Chromium Alloys In Sulfuric Acid Solutions

Abstract

Abstract Anodic polarization behavior of annealed high-purity Ni, Cr and selected Ni-Cr alloys in hydrogen-saturated, 1, 5, 10 and 20N sulfuric acid solutions was investigated at 25 ± 1 C (77 F) using a potentiostatic technique. All specimens except pure Ni in 20N acid had an active-to-passive transition. Decreasing acid concentration increased passive potential range of pure Ni while acid concentration did not appreciably affect the passive region of pure Cr or the Ni-Cr alloys. Increasing acid concentration and decreasing chromium content increased the secondary passivation tendency in the transpassive region. No oxygen evolved in the transpassive region except for pure Ni and the higher nickel alloys. Tafel slopes for anodic dissolution, early transpassive region and oxygen evolution were determined. Corrosion potentials for pure Ni and Ni-Cr alloys containing up to 90 w/o Cr behaved as a reversible hydrogen electrode over the pH range 1.63 to −0.44. This linear relationship did not exist for the higher acid concentrations. Slope dependence for pure Cr and the 99Cr-1Ni alloy was −0.110. A minimum in critical current density versus percent Cr curves was established at 90 to 95 w/o Cr. Passivation potentials were determined. Passive current density decreased with increasing chromium content over the composition range 0 to 33 w/o Cr. It was established that increased specimen purity decreases the magnitude of critical current density and shifts corrosion and passivation potentials in the noble potential direction.