Silicon-containing Solution for Passivation of Zinc Coatings

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 degrees C in the course of 4 min. A warming-up of the solution to 40 degrees 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.