Effect of near-surface pH on electrodeposition of nickel on a rotating fine mesh electrode

Abstract

Near-surface pH was measured during Ni electrodeposition from NiCl2 solutions. As NiCl2 concentration was increased, the usually observed increase of surface pH was suppressed at low cathodic potentials, due to hydrolysis of Ni+2, and at high cathodic potentials due to precipitation of hydroxides. The effect of agitation on surface pH during Ni electrodeposition was also investigated. Under vigorous agitation conditions the surface pH remained close to the bulk pH value and increased only at very negative potentials. Under mild agitation, however, the surface pH curves contained a plateau which appeared at lower pH values than in an non-agitated system. When Ni is electroplated from simple NiCl2 salts, the deposit appearance is strongly depended on the value of pH reached at the surface. In vigorously agitated electrolytes, bright Ni deposits were obtained without addition of buffers, because strong agitation keeps the surface pH to low values. Under low agitation conditions, similar results are obtained only with addition of buffers, which act to keep a low surface pH.