Influence of the Current Density of Deposition on the Properties of Zn–Ni Coatings

Abstract

Zn–Ni coatings were deposited under galvanostatic conditions at current densities varying within the range 20– 60 mA ⋅ cm –2 . The influence of the current density of deposition on the surface morphology, chemical and phase compositions, and corrosion resistance was investigated. Structural investigations were performed by the X-ray diffraction (XRD) method. The surface morphology and chemical composition of deposited coatings were studied by using a JEOL JSM-6480 scanning electron microscope. The studies of general electrochemical corrosion resistance were carried out in a 5% NaCl solution by using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. The local corrosion resistance was determined by the Scanning Kelvin Probe (SKP) method. On the basis of the research, the possibility of deposition of Zn–Ni coatings containing 14–18 at.% Ni was demonstrated. It was stated that the surface morphology, as well as the chemical and phase compositions of these coatings depend, to a small extent, on the current density of deposition. However, current density of deposition determines the quantity of zinc which is codeposited with nickel and is bounded in the form of an intermetallic compound or a solid solution. Small differences between the chemical composition and the uneven distribution of the Zn(Ni) and Ni2Zn11 phases on the surfaces of the coatings may cause differences in the local Kelvin potential. As a result, the Zn–Ni coatings exhibit corrosion resistances varying depending on the current density of deposition. The optimal values of the current density from the viewpoint of corrosion resistance were found to be j = 30– 40 mA ⋅ cm –2 .