Electrochemical Formation of a Failure Preventing Film on Nickel Surface in Borate Solutions

Abstract

Cyclic voltammograms (CVs) of the Ni electrode are traced in Na2B4O7 solutions as a function of electrolyte concentration, voltage scanning range, and rate in order to determine the nature of failure protective species formed in the slightly alkaline media. The species formed on the Ni electrode are found to depend on the sweep number due to changes in the activation state of the electrode surface. The voltammograms are characterized by a pronounced anodic peak due to the formation of NiO and a protective passive film corresponding to the formation of β-Ni(OH)2 before the evolution of oxygen. An additional anodic peak in the vicinity of oxygen evolution potential appeared in the advanced cycles that is attributed to the transformation of β-Ni(OH)2 to β-NiOOH. The cathodic branch shows only one peak corresponding to the reduction of β-NiOOH to β-Ni(OH)2. The current density flowing along the anodic oxidation peak varies with the concentration of the electrolyte according to: \( \log\,i_{\text{p}} = a + b\log\,C_{{{{\text{B}}_{4} {\text{O}}_{7}^{}}^{2 - } }} \) where a and b are constants. An increase in the scan rate increases markedly the current density flowing along the whole range of the CVs. As the concentration of borate anions increases, the anodic peak potential is shifted toward more positive values, whereas the cathodic peak potential is shifted in the negative direction, indicating the irreversibility of formation of the passive film formed on the electrode surface. A correlation is made between the anodic oxidation processes and their corresponding cathodic one. The failure-protecting film in borate solutions is assumed to be caused by the formation of a sandwich oxide having the form: NiO/β-Ni(OH)2/β-NiOOH.