A kinetic and electrochemical study of the zincate immersion process for aluminium

Abstract

To plate aluminium, its surface is often first coated with a thin layer of zinc which is formed by immersion in an alkaline zincate solution. This paper describes a kinetic and electrochemical study of the zincate immersion reaction. Using an aluminium sample in the form of a rotating disc, the effects of varying the zinc concentration (0.01–0.5 m), disc rotation speed (66–1380 rpm), temperature (5–72°C), and sodium hydroxide concentration (1.5–9.0 m) on the kinetics were investigated. It was found that the reaction was usually first order. When the zincate concentration was 0.1 m, the reaction was chemically controlled with an activation energy of 35 ± 7 kJ mol−1. At high zincate concentrations, high disc rotation speeds and low sodium hydroxide concentrations, a thin film of zinc metal was formed on the aluminium surface, blocking the subsequent reaction. It was found that the most compact and adherent zinc films were formed at high zincate concentrations. This finding is consistent with industrial practice. These results are explained using mixed potential measurements and Evans' diagrams.