Investigation of chromate-free and fluorine-free pretreatments for electroless NiP deposition on Mg from alkaline electrolytes

Electroless or electrodeposition of NiP on Mg requires the formation of a pretreatment layer on the Mg surface before deposition, due to the susceptibility of Mg to corrosion. Historically, chromate and fluorine ions have been used to pretreat the Mg surface to facilitate coating deposition. Unfortunately, both chromate and fluorine ions pose environmental issues, which should be avoided whenever possible. In this work, four environmentally friendly recipes to pretreat Mg for NiP electroless deposition from an alkaline electrolyte are explored. A single pretreatment layer is shown to be inadequate at generating a well adhering NiP coating to Mg, which is deposited from an alkaline electrolyte. Instead, two pretreatment processes combining vanadate and manganese-phosphate conversion layers are needed to provide a good conversion coating for electroless deposition of uniform and adherent NiP coatings to Mg substrates. Cross-hatch testing indicated that the best NiP coating achieved the highest adhesion designation of 5B. Preliminary assessment of corrosion and mechanical property behavior was done through potentiodynamic polarization and hardness measurements. The NiP coating fabricated with the combined vanadate/manganese-phosphate conversion pretreatment had better corrosion resistance, with a lower corrosion current density and higher corrosion potential, than Mg and significantly higher hardness.