Abstract
The microstructure and chemical state of Zn-Co-Cr-Al2O3 composite coating before and after the cyclic corrosion test (CCT) were investigated by electron microscope, X-ray photoelectron spectrometer, and electron probe micro analyzer. The coating layer consisted of a Zn matrix containing super-saturated Co and dispersed particles of cohesive Al2O3, Cr2O3, Cr(OH)3 coordinating SO4. The dispersed particles existed uniformly in a regular matrix in coatings formed in electrolytes with lower concentrations of dispersed Al2O3 particles and Cr-ions, while the matrix was dendritic and the dispersed particles concentrated along the dendrite arms in coatings formed in electrolytes with higher concentrations of Al2O3 particles and Cr-ions. The main corrosion products of the composite coating after the CCT were ZnCl2·4Zn(OH)2 and ZnO, which is similar to that with conventional Zn coatings, but the amount of the Zn rust is less than that with conventional Zn coatings. Fine networks of ZnCl2·4Zn(OH)2 grew in the coating in the vicinity of the steel substrate during the CCT. A region of highly-condensed dispersed particles existed between the ZnO layer and the residual coating layer. It is considered that ZnCl2·4Zn(OH)2 formed uniformly by the local cathodic reaction of Co, and the existence of the protective region of dispersed particles gave high corrosion resistance to the composite coating.
