Effect of Pretreatment Process on Performance of Hot Dip Galvanized Steel with Chrome-free Coating

Spray method was used for pre-treating, such as cleaning and phosphating, the hot dip galvanized steel with chrome-free coating. SEM, EDS and low surface resistance measuring instrument were used to study the surface morphology, chemical composition and adhesion of the chromate-free coating after spray treatment. The results showed that when the pH of alkaline cleaner was lower than 11, chromate-free thin coating might form some limited swelling and dissolving caused by raactrion of the coating with alkaline solution, and the electrical conductivity of the sample was improved by more exposed zinc peaks. During the phosphating process, with the decrease of zinc phosphate solution pH, the amount of phosphate increased, but it was difficult to form fan rib shape phosphate chip, and the pH of phosphate solution should higher than 3.5.