The effect of pulse electroplating on Zn–Ni alloy and Zn–Ni–Al2O3 composite coatings

Abstract

Zn–Ni alloy and Zn–Ni–Al2O3 composite coatings were electrodeposited on mild steel substrate by direct and pulse current in sulphate bath. The effect of pulse electroplating on Ni content, phase structure, microhardness, surface morphology and corrosion resistance of coatings were investigated. Applied pulsed current increased the Ni content for both Zn–Ni alloy and Zn–Ni–Al2O3 composite coatings, which also increased the microhardness of the coatings. XRD results showed that the phase structure of all direct current (DC) and pulsed electroplated coatings was single Ni5Zn21-γ phase. Surface of coatings was smoother and more uniform with smaller nodules’ size for coatings deposited by pulse current. Pulse frequency did not have significant effect on the chemical and mechanical properties of deposits. Corrosion resistance of pulse plated Zn–Ni alloy coatings were improved significantly compared to the DC electroplated coatings. However, this effect was less pronounced for Zn–Ni–Al2O3 composite coatings.