A Consideration on the Corrosion Resistance of Zn-Co Based Co-electrogalvanized Steel Sheet

Abstract

The corrosion resistance of Zn-Co based Co-electrogalvanized steel sheet has been investigated, and the effect of the additional elements (Co and Mo) containing in deposit on the corrosion behavior has been considered.
(1) Zn-Co-Mo plating is proved to withstand about 4 to 6 times longer, in the occurrence of red rust by salt spray test, than Zn plating, and has an excellent resistance against the deterioration of PVC adhesion under salt spray test.
(2) After the salt spray test, the corrosion potential of Zn-Co-Mo plating moves slowly to the Fe. On the other hand, that of Zn plating moves rapidly to the Fe potential. Though the initial corrosion current of Zn-Co-Mo plating is larger than that of Zn plating, the corrosion current of Zn-Co-Mo plating tends to be reduced with the corrosion progress, and it becomes smaller than Zn plating after all. The cathodic reaction of Fe exposed by the dissolution of Zn and the anodic reaction of Zn are controlled by the corrosion products containing the composite compounds of basic cobalt chloride and Mo (OH)₃, so that the corrosion of Fe and the occurrence of red rust is restrained. Also, the corrosion resistance of PVC coating on Zn-Co-Mo plating steel sheet is superior to that on Zn plating steel sheet.