Abstract
The transpassive dissolution of Ni in acidic sulfate media, including the influence of crystallographic orientation, was investigated. The surface plane had low index values, that is (100), (110), and (111). This study was largely based on the analysis of complex impedance. (100) and (110) specimens showed identical electrochemical behaviors, whereas the (111) specimen showed a current density about 20% lower. A reaction mechanism describing the dissolution of the passive film was proposed. This dissolution step was postulated to be catalytic, in the sense that the passive film transformed into a soluble species by anion was not consumed by the transpassive dissolution itself. Calculated polarization curves as well as electrode impedances at various polarization points were in a good agreement with experimental results.