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Electrolytic Deposition of Zn–Ni Alloy from Acidic Sulphate Bath and Their Characterization

R. S. Bhat, J. Nayak, A. C. Hegde

Abstract


A bath containing ZnSO4.7H2O, NiSO4.7H2O, thiamine hydrochloride (THC) and citric acid (CA) has been proposed. Bright Zn–Ni alloy coatings were developed galvanostatically onto mild steel (MS) and their corrosion resistance and other physical properties were studied. THC was found to improve the brightness and homogeneity of the deposit. The dependency of bath composition, current density (c.d.), pH and temperature, agitation on wt.% Ni were tested. Deposition was carried out under different c.d. and molar ratio of [Ni+2]/[Zn+2]. No transition c.d., at which codeposition behaviour changed from anomalous to normal type, was detected. Studies of the factors such as temperature, agitation, pH and [Ni+2]/[Zn+2] which influence the wt.% Ni in the deposit were made. Influence of wt.%Ni on corrosion resistance, hardness, adhesion and glossiness were studied. The wt.% Ni in the deposit was determined calorimetrically and cross-examined by EDX analysis. The morphological variation in deposit patterns with c.d. were examined by Scanning Electron Microscopy (SEM). Good corrosion resistance at optimized c.d. (3.0 A/dm2) is attributed to the capacitive reactance at the interface, evidenced by Nyquist plot having large polarization resistance. The XRD analysis was carried out to identify the phase structures of the deposits.

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