Corrosion behavior of Zn-Co alloy coating from acid sulphate bath

Abstract

Optimization of sulphate bath for deposition of a smooth and uniform Zn-Co alloy over mild steel is discussed in the present work. Electrodeposition was done using Thiaminehydrochloride (THC) and citric acid (CA) as additives in combination. The bath followed anomalous codeposition with preferential deposition of Zn over noble Co. The experimental results reveal that a bright Zn-Co alloy having ~1.06 wt. %Co was showing peak performance against corrosion in compliance with other physical properties like reflectance, hardness, thickness and adhesion. The dependency of bath composition, current density, partial current density, pH and temperature on deposit properties like reflectivity, corrosion resistance were discussed. Deposition was carried out under different condition of c.d. s and molar ratio of [Co+2]/[Zn+2]. No transition c.d. s at which codeposition behavior changed from anomalous to normal type was observed. Cyclic polarization measurement was performed to study the nature of corrosion taking place in the corrosion cell. An electrochemical behavior of Zn-Co alloy electrodeposition in alkaline solutions was studied using cyclic voltammetry technique. The increase in the corrosion resistance of coatings attributed to the formation of n-type semiconductor film at the interface was confirmed by Mott-Schottky (M-S) plot with straight line having positive slope. The composition of deposits were determined by colorimetric method and confirmed by EDX analysis. Surface morphology of the deposits was examined using scanning electron microscopy (SEM). The phase content was examined by XRD analysis at different current densities. A stable acid sulphate bath has been proposed for bright and uniform deposit of Zn-Co over mild steel and discussed. 2014 by CEE.