Electrochemical behaviour and analysis of Zn-Ni alloy anti-corrosive coatings deposited from citrate baths

Zinc-Nickel alloy coatings have been widely used to improve the corrosion resistance of steels. Electrodeposition of zinc-nickel alloy films on a steel plate under various deposition conditions is studied. The effects of plating variables such as bath composition, pH and current density on the coating composition, morphology and corrosion property have been systematically investigated. The zinc and nickel ions in plating baths are stabilized using complexing agent potassium citrate. Increasing the molar concentration of potassium citrate extends the pH of nickel hydroxide and zinc oxide precipitation. The morphological properties and crystalline phase structure of the deposited alloy coatings are examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The analyses reveal that the coatings deposited from citrate bath contain γ-NiZn₃ and γ-Ni₂Zn₁₁ alloy phases. These phases improve the corrosion resistance of the steel plate. Electrochemical behaviour analyses of the Zn-Ni film illustrated by electrochemical impedance spectroscopy (EIS) measurement revealed that the sample (a) deposited from 0.0326M citrate bath at acidic pH shown higher impedance modulus than non-citrate bath coated samples, which improved anti-corrosive properties of the coatings. This study confirms that uniform Zn-Ni coating with smaller grain sizes has better anti-corrosive properties. (paper)