Silicon-containing Solution for Passivation of Zinc Coatings

Abstract

A process was developed for chromate-free passivation of zinc-plated surfaces via formation of conversion silicon-containing coatings on these surfaces. The passivation performed in a solution containing 20–35 g L⁻¹ sodium metasilicate, 0.2–0.5 g L⁻¹ phosphonic acid, 20–30 mL L⁻¹ H₂O₂, 200–500 mg L⁻¹ saccharine, and 4–10 mg L⁻¹ potassium pyrophosphate at pH 2.0–2.5 and temperature of 18–25°C in the course of 4 min. A warming-up of the solution to 40°C is allowed. The developed silicon-containing passivating coatings on zinc-plated steel details are comparable in corrosion resistance and protective capacity with iridescent chromate coatings, but, in contrast to the latter, sustain a thermal shock without deterioration of their protective characteristics and are more wear resistant.