Pulse plating effect on composition and corrosion properties of zinc alloy coatings

Abstract

The influence of pulse plating parameters on the chemical and phase composition, surface topography and corrosion resistance of Zn–Ni (∼12%Ni), Zn–Co (<1%Co) and Zn–Fe (<1%Fe) alloy coatings has been studied. Pulse plating of Zn alloys resulted in different outcomes for Zn–Ni and low alloyed (Zn–Co and Zn–Fe) coatings. Non-stationary crystallisation resulted in formation of single phase γ-Zn21Ni5 coatings, while two different phases of Zn were produced during Zn–Co and Zn–Fe deposition. Pulse plated Zn–Ni coatings possessed from two to three times smaller grain size and at the same time, corrosion currents half those of the direct current deposits. However, the reduction in the grain size of Zn–Co and Zn–Fe coatings was not accompanied by any reduction in the corrosion current of these alloys. It is suggested that the dual phase formation was the principal reason why pulse plated low alloyed Zn coatings did not exhibit improvement in their corrosion resistance.