空気を含む水溶液中におけるニッケルの自己不働態化

Abstract

The corrosion rate of nickel was measured in aerated sulfate solutions of various pH. An abrupt change of corrosion rate was found at aboult pH 5. In acidic region, the corrosion rate was nearly equal to the diffusion rate of dissolved oxygen and the spontaneous potential was less noble than the Flade potential. In alkaline solution, on the other hand, the corrosion rate was undetectablly small and the spontaneous potential was in the passive region. The polarization curve of nickel in acidic solution (pH=3.77) showed that the maximum dissolution current of iMmax in the active state, which can be measured in the solution saturated with pure nitrogen gas, was larger than the reduction current of dissolved oxygen iO. While, in neutral solution (pH=7.53), iMmax was smaller than iO. The critical state of iMmax=iO was found to be at about pH=5. It could be explained from the electrochemical standpoint that nickel is passivated by the reduction current of dissolved oxygen (O2+2H++4e→2OH−) only when |iMmax|<|iO|.