Effects of Electrolyte pH on the Electrodeposition of Nickel on Zincated Aluminium Surfaces

Abstract

A nickel coating was successfully deposited on zincated aluminium by electrodeposition. A detailed study was made on the effect of the pH of electrolytes containing 0.1 M nickel sulphate (NiSO4) by cyclic voltammetry (CV). A constant potential of -0.8 V (pH 2), -1.1 V (pH 6) and -1.5V (pH 10) was chosen from different regions of each CV for the deposition of nickel using chronoamperometry (CA). The surface morphology, chemical composition and crystallographic structure of the nickel coating were investigated by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDAX) and X-ray diffraction (XRD), respectively. The adhesion strength of the nickel coating was analysed using  Scotch® tape. It was found that the nucleation potential for nickel deposition on zincated aluminium increased with increasing electrolyte pH. Notably, pH 2 produced the highest current during the deposition of nickel as compared to pH 6 and pH 10, due to the simultaneous hydrogen evolution reaction (HER). Because of HER, a lot of pits were present on the zincated aluminium surface and only a few nickel particles were formed, as shown in FESEM images. Meanwhile, deposition using a pH 6 solution (without HER)  produced an almost fully-covered nickel coating with uniform nodular morphology. However, when the pH 10 solution was used, the coating morphology appeared as cauliflower-like agglomerates with less surface coverage. EDAX analysis showed that the nickel coating deposited at pH 6 had the highest nickel content with 73.63 wt%. XRD analysis indicated that all nickel deposits had a cubic crystalline structure. Nickel deposited with the pH 6 solution was the most well-adhered coating. Thus, electrolyte pH has a strong influence on the deposition of nickel on a zincated aluminium surface.