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Abstract
A Zn passivating bath based on a sodium metasilicate solution as an alternative to a highly hazardous Cr(VI) chromating solution has been investigated. The effect of composition of passivating solution on formation, characteristics and properties of silicate conversion coatings on Zn has been studied. The silicate conversion coating forms on the Zn surface in an acidic sodium metasilicate solution only in the presence of an oxidising agent. The addition of Ti3+ ions and carboxylic acid to the sodium metasilicate–oxidising agent solution prolongs its stability and improves the appearance and corrosion resistance of silicate conversion coatings. Infrared measurements have indicated the formation of Si–O–Ti linkages in the surface film on addition of Ti3+ ions in the coating solution. It was demonstrated that polymerisation degree of Si–O–Si network in the conversion coating increases on addition of carboxylic acid to the sodium metasilicate–oxidising agent solution containing Ti3+ ions. Higher polymerisation degree of Si–O–Si linkages in conversion coatings provides their enhanced decorative look and higher corrosion resistance.