Effects of different plating modes on microstructure and corrosion resistance of Zn-Ni alloy coatings

The aim of the work is to study the effects of different plating modes on the microstructure and corrosion resistance of Zn-Ni alloy coatings. Zn-Ni alloy coatings were deposited by direct current (DC), pulse current (PC) and pulse reverse current (PRC) methods, respectively. The surface morphology and the grain size were investigated by SEM and XRD. The corrosion resistance of the alloy coatings was evaluated in 3.5% NaCl solution by electrochemical impedance spectroscopy (EIS) technique and polarizing curves method. The results showed that the crystal grains of the alloy coatings prepared by DC, PC and PRC, respectively, became smaller and smaller in order, and the coating surface became compacter and smoother correspondingly. The alloy coatings were composed of a large number of gamma-Zn-Ni alloy and a small quantity of eta-Zn-Ni. The relative intensities corresponding to crystal face gamma (4 1 1, 3 3 0) and eta (1 1 2) of alloy coatings deposited by DC, PC and PRC, respectively, were decreased gradually, while the half width of peaks were increased, which illustrated that the crystal grains of the alloy coatings were fine. This was consistent with the surface morphology of the alloy coatings. The results of EIS and polarizing curve tests showed that the corrosion resistance of Zn-Ni alloy coatings deposited by PRC method was the best, and then by PC and DC in sequence. The corrosion resistance of the alloy coatings depended on the alloy structure and composition, the best corrosion resistance of the Zn-Ni alloy coatings deposited by PRC was attributed to its fining of the crystal grains and the dense structure. (C) 2008 Elsevier B.V. All rights reserved.