EFFECTS OF ZINC RICH UNDERLAYER ON STONE CHIPPING RESISTANCE AND CORROSION RESISTANCE IN A Zn-Ni ALLOY ELECTRODEPOSITED STEEL SHEET FOR AUTOMOTIVE BODY

摘要

As the current density, pH, the concentration of impurities and metal ions in electrolyte except flow velocity increased, Zn-Ni coatings had a lager amount of nickel content. from the result of relationship between red rust initiation time and nickel content in the film, resistance to corrosion in salt spray was a maximum at about 10.5 approx 11.5 wt percent nickel. It was shown that Zn rich under layer in the Zn-Ni coatings was very effective in the improvement of stone chipping resistance. But, in the case of cyclic corrosion test, corrosion resistance was inferior to the specimens having higher nickel content in under layer. In the in-line tests, the chipping resistance was enhanced by increasing a HCl concentration in pickling process and applying a low current density in the initial electrolyte cells. The products electroplated at high current density in the last electrolyte cell show good phosphatability and paint adhesion.