Pulse electrodeposition and corrosion properties of nanocrystalline nickel-chromium alloy coatings on copper substrate

Abstract

Co-electrodeposition of nickel-chromium (Ni–Cr) alloy coating was performed on a copper substrate from a sulfate-chloride bath. The effect of electrodeposition techniques namely direct (DC) and pulse-current (PC) techniques on cathodic efficiency, alloy composition, crystallite size, microhardness, morphology, and corrosion properties of Ni–Cr alloy coatings was investigated. Results show that PC electrodeposition generally produces coatings with desirable properties with respect to DC electrodeposition. Chemical composition measured by energy dispersive X-ray spectroscopy (EDS) and consequently other properties depend on PC parameters including peak current density and duty cycle whose increase rises Cr content of nickel-base alloy coatings. In contrast, PC frequency decreases the Cr content of Ni–Cr alloy coatings. When Cr content increases up to 24 wt% the crystallite size falls down to 66 nm. However, a phase segregation takes place by any further increase of Cr content than 24 wt% and therefore a new phase γˊ appears in the coatings. The surface morphology of the Ni–Cr coatings also changes from large spherical granules with a diameter ranging 27–114 nm to smaller size grain by increasing the Cr content. Above 24%wt. cracks appear on the coating surface. Based on potentiodynamic polarization method, Ni-11.2 wt % Cr alloy coating exhibits the highest corrosion resistance in 3.5 wt % NaCl compared to the other compositions. The corrosion resistance of the alloy coatings is mainly due to the formation of a passive film which in turn is enhanced by increasing the chromium content in the alloy. However, the corrosion resistance drops for high chromium Ni–Cr coatings (>24 wt %) owing to the microcracks developed in the coatings.