Electrodeposition of nanocrystalline Zn–Ni coatings with single gamma phase from an alkaline bath

Nanocrystalline Zn–Ni (grain size about 26nm) alloy coatings were electrodeposited on carbon steel substrates in alkaline bath with 5,5′-dimethylhydantoin (DMH) as the complexing agent. The coatings with 13wt.%–16wt.% Ni content and the electrolyte with high current efficiency (>85%) were achieved at the optimized parameters, i.e., Ni2+/(Zn2++Ni2+) ratio 0.32, current density 2A·dm−2, temperature 50°C and agitation speed 1000rpm. The electrochemical behavior of the bath has been studied by cyclic voltammetry (CV), chronopotentiometry (CE) and cathodic polarization. Results show that the deposition of Zn–Ni alloys occurs at moderate overpotential and the rotation speed and bath temperature have a strong effect on cathodic polarization curves. The effects of Ni2+/(Zn2++Ni2+) ratio, current density, temperature and agitation speed on Ni content and cathode current efficiency were investigated. Studies indicate that the phase structure, grain size, microhardness and corrosion resistance of deposits are directly dependent on Ni content in deposits. The phase structure of deposits changes from a mixture of η-phase and γ-phase to single γ-phase with (411) plane orientation. Furthermore, the grain size decreases with the increase of Zn content in deposits and the increase in current density can also lead to the decrease in grain size. The increase in microhardness of deposits is strongly affected by the increase of Ni content in deposits rather than the Hall–Petch relationship. Zn–Ni alloy coatings with about 13wt.% Ni content present best corrosion resistance and the bath has a good stability. •DMH was used as the complexing agent in the Zn–Ni alloy alkaline bath.•A higher current efficiency (>85%) was achieved compared to zincate alkaline bath.•Nanocrystalline Zn–Ni coating with single γ phase was obtained.•Phase structure, grain size and hardness were related to Ni content in deposits.•Zn–Ni alloy coatings with 13% Ni content presented best corrosion resistance.