Effects of Additives in the Bath on the Formation of Chemical Conversion Coatings Using Tungstate As a Substitute for Chromate and Composition Analyses of the Coatings

The chromate alternative conversion coatings were formed on zinc plated steel by immersing it into a chemical conversion bath containing sodium tungstate and phosphoric acid at room temperature. The effects of additives in the bath, such as inorganic and organic acids, on the properties of the coatings were investigated. As a result, it was found that the addition of sulfuric acid made the corrosion resistance of the coatings remarkably improve and that the further addition of borate made the surface of the coatings smooth, fine, and glossy. The bath compositions determined as optimum for both corrosion resistance and the surface appearance of the coatings were 15.0g of Na2WO4·2H2O, 44mL of 85%H3PO4, 2.1mL of H2SO4, and 8.51-26.7g of Na2B4O7·10H2O dissolved in 400mL of water. Also, the optimum pH range of the bath turned out to be 0.9-1.1. The corrosion resistance of the coating was evaluated by the neutral salt spray test, on which the first appearance time of white corrosion products on the surface of the coating was improved to 160-170h, indicating comparatively higher corrosion resistance, suitable as an alternative to chromate conversion coatings formed on zinc plating. The coatings were also examined by glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. Their thicknesses were found to be around 0.5μm. It was suggested that the coatings were composed of zinc ions eluted from the zinc plating, tungstate ions consisting of W6+, W5+, and W4+, and phosphate ions.