Effect of NH_4HF_2 on deposition of alkaline electroless Ni-P coatings as a chromium-free pre-treatment for magnesium

摘要

An alkaline electroless treatment that forms a low-phosphorus Ni-P coating on magnesium is described. The treatment is shown to provide a suitable base for deposition of a high-phosphorus, electroless nickel coating using a slightly acidic bath. The composition of the alkaline bath was selected to limit the corrosion of the substrate during coating and to avoid the utilization of chromate ions or hydrofluoric acid. An optimal amount of NH_4HF_2 is identified that provides a relatively thin Ni-P coating, with the subsequently-deposited, high-phosphorus coating enhancing the corrosion resistance of the magnesium in sodium chloride solution.