Role of Cl− in breakdown of Ni Passivity in aqueous NaOH solutions

Repetitive potentio- and galvanokinetic traces of Ni rods yielded anodic peaks (or arrests) corresponding to probable formations of α and β Ni(OH)₂ and NiO(OH) prior to O₂ evolution. Cathodic half cycles consistently showed only two reduction processes that are attributed to NiO(OH)→ Ni(OH)₂åNi. Potentiostatic polarization resulted in multi-peaks i-t traces, in [NaOH] ? 1 moll^?1; the main peak exhibiting the general form characteristic of instantaneous nucleation processes. The maximum current intensity, i_p, of the potentiokinetic traces as well as the parameter i_mt_m of the i-t curves showed negligible dependence on NaOH in the range 0.01–1 moll^?1. In the range of 1–200 mVs^?1 scan rate, i_p ∝ v. Employing an aqueous media of mixtures of different concentrations of NaCl plus a constant [NaOH] resulted in: (a) significant increases of both i_p and i_mt_m and (b) nearly linear i-v^{$\text{sol1}\text{2}$} relation. Achievement of electrode passivity was retarted by Cl^? but eventually attained. As [Cl^?]/[OH^?] exceeded ～ 3 passivity showed signs of breaking down. The passivation current showed continous rise with increasing [Cl^?]. A mechanism of Cl^? attack involving surface adsorption, increasing solubility of an intermediate Ni hydroxide species that nucleates into passive film and peptization of the deposited oxide, by Cl^?, is discussed.