Ternary Zn-Ni-Co alloy: anomalous codeposition and corrosion stability

Abstract

Electrochemically deposited Zn-Ni-Co alloys under various deposition conditions were investigated using atomic absorption spectroscopy for the determination of the chemical composition, atomic force microscopy for surface morphology analysis, and electrochemical impedance spectroscopy for determination of corrosion properties. The influence of the ratio of the alloying elements, [Co2+]/[Ni2+], in a chloride deposition solution, as well as the deposition current density, on the chemical content, morphology and corrosion properties of the ternary alloys was studied. The plating solutions used were with the same overall metal concentration, free of additives and the deposition was realized at room temperature. It was shown that the deposition of the ternary alloy coatings was of anomalous type and that the ratio of nickel to cobalt ions in the plating solution, as well as the deposition current density, had a significant influence on the chemical composition and corrosion stability of these coatings. The presence of both Ni and Co was beneficial for their corrosion stability. It was shown that morphology and porosity were important factors in the corrosion stability of these coatings. The coating deposited from the solution with higher [Co2+]/[Ni2+] ratio at 8 A dm(-2) was made up of the smallest crystalline aggregates, had a small surface roughness and porosity, and was with quite uniform distribution of agglomerates and it exhibited the best corrosion stability.