Structural and corrosion characterisation of pulse plated Zn and Zn–Ni alloy coatings

Abstract The effects of pulse plating on the phase and chemical composition, preferred orientation and surface morphology of Zn and Zn–Ni (12 wt-%) electrodeposited coatings have been investigated using atomic force microscopy, scanning electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy and electrochemical techniques. It has been established that pulse current plating with respect to direct current (dc) conditions resulted in ∼50% reduction in the grain size dimensions and in fourfold and twofold reduction in the corrosion current densities of Zn and Zn–Ni coatings respectively. Under all the pulse plating conditions applied the deposited alloy consisted of the single γ-Zn21Ni5 phase, while, according to the XRD measurements both coatings possessed inclusions of the oxide phase. The analysis of the protective properties and the thickness of the oxide films formed on the surface of investigated coatings revealed the fact that the surfaces of pulse plated samples were favourable for the formation of thinner oxide films which at the same time exhibited higher protective properties. Pulse plating increased the amount of non-metallic inclusions in Zn and Zn–Ni coatings, however, these amounts were not high, as the O concentration in the deposits did not exceed 1·5 and 0·2 wt-% for Zn and Zn–Ni respectively.