Anodic oxide films at nickel electrodes in alkaline solutions—II. pH dependence and rate determining step

Room temperature growth kinetics of thin anodic films of the β-Ni(OH)2 phase is determined in alkaline solutions of different pH's. The kinetics follows the Cabrera—Mott formalism of the high field assisted growth. The potential at which the hydroxide films begin to grow, Vo, depends on pH according to Vo(pH) = 0.377 - 2.3(RT/F)pH [V vs she]. The exchange current density, io, also is pH dependent. It is first order with respect to OH−. For the high field assisted growth it is usually assumed that the rate determining step is either a process at the metal—oxide film interface or a process within the oxide film itself. It is difficult to see how ia, for either of these processes, could depend on pH. It is suggested that the rate determining step is a process at the oxide film—solution interface. The pH dependence of the rate for this step is discussed in relation to the structure of the double layer.