Electrochemical deposition of zinc–nickel alloy coatings in a polyligand alkaline bath

The polyligand alkaline bath for Zn–Ni alloy electrochemical deposition containing an aminoacetic acid (AAA) and triethanolamine (TEA) as the ligands to bind Ni(II) cations was developed. The alloy composition was greatly influenced by the [Zn(II)]/[Ni(II)] ratios in the bath, with the nickel content in the resultant deposit being varied in a wide range (from 8 to 75 at.%). X-ray diffraction studies revealed that the alloys consisted of the γ-phase (Ni5Zn21), solid solutions of Zn or Ni in the γ-phase, or of a mixture of the γ-phase and polycrystalline Ni or Zn. The high-quality coatings with the nickel content of 13–20 at.% having the γ-phase or solid solution of Zn in the γ-phase exhibited the highest corrosion resistance in a saline environment. To obtain corrosion-resistant alloys, the optimal Ni(II):АAA:ТEА molar ratio was found to be equal to 0.04:0.65:0.12. The microhardness of these Zn–Ni alloy coatings (1.6–1.9 GPa) was twice as great as the microhardness of zinc coating.