Influence of Corrosion Inhibitor in Chemical Conversion Coatings on Corrosion Performance in Scratches in Zinc-Coated Steels

Zinc-coated steel is widely used in the areas of home appliances, construction materials, and automobiles. Chromate-free chemical conversion coatings formed on zinc-coated layer can prevent zinc from corrosion. In this study, with the purpose of establishing guidelines for enhancing corrosion resistance in areas on zinc-coated steels where the underlying steel is exposed to the environment, we analyzed the effect of phosphate compounds used as a corrosion inhibitor on corrosion resistance in areas of exposed steel substrate and examined the mechanism behind the compounds. We demonstrated that adding phosphoric acid to a chemical conversion coating enhances corrosion resistance in scratches that expose the steel substrate, inhibiting zinc corrosion in the coated layer within the scratches. Since compounds consisting of Zn2+ and PO43− covered the steel substrate exposed in the scratches, we think these compounds acted as a barrier against salt water and oxygen. PO43− was eluted from the chemical conversion coating around the scratches by salt water in the salt spray test, and the progress of corrosion was inhibited. We therefore demonstrated that allowing a minimum specific amount or more of PO43− to be eluted from a chemical conversion coating can effectively inhibit the corrosion of zinc-coating in scratches.