Effect of Concentration of Total Metal on Quality of Zn-Ni Alloy Electrodeposits for Automotives

For the high surface quality and corrosion resistance of the passenger cars, the Zn-Ni alloy electrodeposited sheet steel is widely used in many automotive manufacturers. This alloy system is very competitive to the other coating metals in terms of the paintability and corrosion resistance of the carbody. In order to obtain a good quality surface, the control of the concentration of the total metal of Zn and Ni ions in Zn-Ni electrolyte bath is very important in the production line. Thus an investigation on the effect of the total metal concentration on the surface quality and morphology of coating layer was carried out recently in a coating line. The total metal concentration is classified into two groups of 85g/l and 180g/l. The results showed that the striated semi-white color of the surface was increased by the content of the total metal while the edge burning was decreased as it increased. Regarding the surface roughness and morphology, these were also significantly influenced by the total metal concentration. The increase of concentration was also found to contribute to fine grain and surface smoothness. Furthermore the addition of additives improved the quality of electrodeposition of Zn-Ni alloy. The additives consist of the surfactant of ethylenglycol complex and mixed sulfonic-derivative. The former is effective on the higher total metal concentration and the latter is effective on the lower one.