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

Abstract

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 O 2 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 m t 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 m t m and (b) nearly linear i-v sol1 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.