Effect of NH4HF2 on deposition of alkaline electroless Ni–P coatings as a chromium-free pre-treatment for magnesium

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 NH4HF2 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.

► A chromium-free, alkaline electroless nickel pre-treatment is developed for magnesium.
► The bath composition is optimized with respect to coating kinetics and substrate corrosion.
► The pre-treatment provides a base for subsequent deposition of electroless nickel in acid baths.
► Formation of a corrosion-resistant, high-phosphorus, duplex coating is demonstrated.