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

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 gamma' 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 % NaCI 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. (C) 2020 Elsevier B.V. All rights reserved.