Passivation Studies using Group VIA Anions: II. Cathodic treatment of tin

Abstract

The passivation of tin by cathodic treatments in aqueous solutions of chromate, molybdate and tungstate has been compared for a range of conditions: pH 5–9, 20–60°c, 0·01–1·0 M ion concentrations. The process has been assessed using polarisation and coulometric techniques and the films analysed using ESCA. The chromium content is shown to increase with increases in solution ion concentration, process pH, process charge, treatment time or a decrease in temperature. Chromate films are thinner than molybdate and tungstate films and consist mainly of Cr^III with small amounts of metallic chromium and Cr^VI. The molybdate and tungstate films contain metal only as Mo^VI or W^VI. Tin exists both in the metallic state and oxidised state - a combination of Sn^II and Sn^IV.