Electrodeposited Zn-Ni coatings are known to possess a high corrosion resistance in many aggressive environments. Apart chemical composition, the protective ability of Zn-Ni coatings depends mainly on their microstructure and phase composition. In the present study the Zn-Ni alloys (with Ni content up to 20 wt.%) were electrodeposited on steel substrates from a sulphate bath of different [Ni]/[Zn] concentration ratio with the addition of sodium citrate or ammonium sulphate. The influence of operating variables (electrolyte composition, pH, cathode current density) on the chemical and phase compositions of Zn-Ni coatings was determined.
The results of investigations indicate that the composition of the phases occurring in electrodeposited Zn-Ni alloys varies greatly with the deposition conditions. Moreover, owing to a change in deposition conditions, various phases of the phase diagram appear with the same mean Ni content. On the contrary to some authors we have reported that the phases contained in the Zn-Ni deposits do not correspond to those obtained at thermodynamic equilibrium.
It was established that with the increase of Ni content in Zn-Ni alloys electrodeposited in presence of acetate ions the phase composition is changed: in deposits with Ni content up to 13 % the distorted η-phase crystallizing hexagonally is dominating, whereas alloys with the highest Ni content consist mainly of the cubic γ-phase with composition Ni3Zn14.On the contrary in citrate solutions practically single-phase (hexagonal h) deposits were obtained in whole range of Ni content (0 to 16%).
It is shown that the substitution of zinc by nickel is responsible for the formation of distorted h-phase corresponding to the “supersaturated” hexagonal h-phase of Zn-Ni alloy. Moreover, thermodynamically, the Zn-Ni h-phase only appeared up to about 1 wt % Ni and d-phase (Ni3Zn22 intermetallic compound) exists in a wide composition range.