Optimization of Bright Zn-Co-Ni Alloy Coatings and its Characterization

Acidic sulphate bath having ZnSO4.7H2O, CoSO4.7H2O, NiSO4.7H2O and thiamine hydrochloride (THC) and citric acid (CA) in combination, was optimized for deposition of bright Zn-Co-Ni alloy coating on mild steel. Bath constituents and operating parameters were optimized by Hull cell method, for highest performance of the coating against corrosion. The effect of current density (c.d.), on deposit characters, such as corrosionresistance and hardness, thickness were studied and discussed. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were used to assess the corrosion behaviors. The composition of deposits were determined by spectrophotometeric method and confirmed by EDX analysis. Surface morphology of the deposits was examined using scanning electron microscopy (SEM). The Zn-Co-Ni alloy, with intense peaks corresponding to Zn (100) and Zn (101) and Zn (110) phases, showed highest corrosion resistance, evidenced by X-ray diffraction (XRD) study. A new and cheap sulphate bath, for bright Zn-Co-Ni alloy coating on mild steel has been proposed, and results are discussed.