Influence of structural factor on corrosion rate of functional Zn–Ni coatings

The electrocrystallization processes of Zn–Ni coatings from weakly acid sulfate solutions with the addition of citrate and acetate complexing compounds have been compared. The deposition kinetics and the influence of electrolyte composition on the Zn–Ni alloy composition and on some physicochemical properties (corrosion resistance, texture, phase composition, morphology) were determined. It was found that the properties of coatings obtained under the same operating conditions in electrolytes with the same [Ni(II)]/[Zn(II)] concentration ratio and pH but containing different complexing agents were different. The composition of the deposits and their texture as well as phase structure were found to depend on the complexing species present. Coatings deposited from acetate solutions are richer in Ni than those from citrate solutions. The corrosion resistance of a Zn–Ni alloy depends in practice on the percentage of Ni, but in sulfate–citrate electrolytes, layers containing less Ni were obtained with the same corrosion resistance as Ni-rich coatings deposited from acetate solution. The corrosion current decreases more rapidly with increasing Ni content. This effect is connected mainly with phase composition. In citrate solutions practically single-phase (hexagonal η) deposits were obtained which provide better protection with a lower corrosion rate in comparison with alloys consisting of three phases (hexagonal η, monoclinic δ, regular γ) deposited from acetate baths.