ANODIC-DISSOLUTION OF NICKEL IN CONCENTRATED SULFURIC ACIDIC SOLUTIONS

The anodic dissolution of nickel in concentrated sulfuric acidic solut ions is characterized by two diffusion plateaux related to the active and transpassive regions. For the same speed of rotation of a rotating disc electrode, the two plateaux currents are almost identical. Howev er, the less anodic one gives rise to a rough electrode surface while a polished surface is observed at the more anodic one. For nickel as-r eceived, mass transport influences the current over the whole potentia l range. After heat treatment, only the current for the two plateaux w ere mass transport controlled. Electrochemical impedance diagrams show that the lowest frequency capacitive loop is influenced by mass trans port. Electrohydrodynamic (EHD) impedance diagrams show two different regions. In the low frequency range, the results follow the theoretica l curve corresponding to a uniformly accessible electrode with a very high Schmidt number around 10(7). At high frequency, the EHD impedance may correspond to an interface covered by a gel layer formed from the products of the anodic dissolution.