No abstract

Zinc coated steel is widely utilized for home appliance, construction, automobile, and so on. Chromate-free chemical conversion coating, which is set on zinc coated layer, can inhibit white rust generated by zinc corrosion. When zinc coated layer and chemical conversion coating formed above the layer are strictly damaged, steel can be exposed on corrosive environment. The effect of phosphate acid included in chemical conversion coating, which is generally used as corrosion inhibitor of zinc, on corrosion protection at planes is well known. However, information on parts, where steel is exposed, is not significantly clear. In this paper, in order to improve corrosion protection of zinc coated steel where steel is exposed, the influence of phosphate acid was investigated on corrosion performance at scratches of zinc coated steel by use of salt spray test. It was clarified that addition of phosphate acid into chemical conversion coating reduced corrosion of zinc coating layer at scratches and steel exposed on corrosive environment was covered with a compound composed of phosphate and zinc. It was considered that phosphate and zinc moved from chemical conversion coating and coating layer near scratches during the salt spray test and the compound protected zinc coated layer from corrosion factor such as salt, water and oxygen. In addition, it was clarified that the larger elution amount of phosphate from chemical conversion coating is, the superior corrosion protection at scratches is.