Characterization of the influence of Ni content on the corrosion resistance of electrodeposited Zn-Ni alloy coatings

The electrodeposition of Zn-Ni alloy coatings containing less than 11% Ni was performed in a sulfuric acid bath. We investigated the influence of the Ni content in the electrodeposit on the corrosion behavior of the alloy in detail, focusing mainly on the evolution of internal stress. The initial corrosion rates of specimens before salt spray testing were closely related to the internal stress and the Zn-6 wt% Ni coating showed better corrosion resistance than those of the other investigated coating due to the smallest internal stress. However, it was shown that the amount of residual Ni and a concentration reversal between Zn and Ni at the Zn-Ni/steel interface became more significant as the corrosion proceeded. In addition, closely packed nodular grains with hairline cracks and the formation of electrochemically noble γ-Ni5Zn21 phase were observed at the highest Ni concentration of 10 wt%. And the cross-sectional distributions of alloying elements were more homogeneous with increasing Ni content in the deposit. Electrochemical test results with the resulting specimens with different salt spray testing times were also presented. At the highest Ni concentration, the sacrificial property to steel substrate was maintained with extended testing time, showing minimal corrosion current density. Regarding Zn-Ni alloy coatings for automotive applications, a decrease in corrosion depth with increasing Ni content after long-term exposure to salt spray was observed, indicating improved perforation resistance.