Improvement in corrosion resistance of ternary Zn-Fe-Mo plating by additional Mo-oxide coating

Despite the excellent corrosion resistance and the widespread application of Zn-Ni plating in industrial fields, the toxicity of Ni to the human body has caused the demand for alternative plating that does not contain harmful elements. In this respect, ternary Zn-Fe-Mo plating, which includes a third element Mo in binary Zn-Fe plating, has been considered as a promising alternative for Zn-Ni plating. In this study, the corrosion resistance of ternary Zn-Fe-Mo plating was evaluated and compared with conventional Zn and Zn-Fe plating by impedance measurements and cyclic corrosion tests (CCT). Furthermore, formation of a multi-layer structure via additional Mo-oxide coating over the ternary Zn-Fe-Mo plating was attempted to increase the corrosion resistance. Experimental results show that ternary Zn-Fe-Mo plating has higher impedance but lower corrosion resistance for CCT and that the Mo-oxide coating significantly enhances the corrosion resistance of the ternary Zn-Fe-Mo plating selectively by a factor of approximately 10. The role of the formation of multi-layer structure in improving the corrosion resistance of the ternary Zn-Fe-Mo plating is discussed through the structure change of the plating and the generated corrosion products.