Improved corrosion resistant properties of electrochemically deposited zinc-nickel alloys utilizing a borate electrolytic alkaline solution

Zinc-nickel γ-phase alloy coatings were electrochemically deposited from a borate electrolytic bath on stainless steel substrates as protective coatings for corrosive environments. Alkaline conditions were used with a working pH range of 9.3-9.5 resulting in uniform deposits and corrosion protection. Strongly adhering deposits were obtained with smooth and even overall coverage of the substrate. The effect of borate addition was examined in relation to zinc, nickel and zinc-nickel alloy deposition. The presence of borate did not affect the anodic stripping potential of zinc in solution but did result in an anodic shift in the stripping potential of nickel with increasing borate concentration. Borate was adsorbed onto the electrode surface and shifted the onset of hydrogen evolution towards more cathodic potentials as borate concentration was increased. X-ray diffraction and atomic absorption spectroscopy confirmed the presence of the γ-phase alloy with a nickel content range of 8-15%. Pulse potential and direct potential processes were examined as deposition methods. The pulse potential method resulted in smaller grain sizes and uniform coatings as confirmed in scanning electron microscopy. The overall corrosion potential for the zinc-nickel γ-phase coatings was improved as compared to the pure nickel and zinc coatings.