Effects of fluoride ions as electrolyte additives for a PEO/Ni-P composite coating onto Mg alloy AZ31B

Plasma electrolytic oxidation (PEO) and a subsequent direct electroless nickel (EN) plating were carried out on magnesium (Mg) alloy AZ31B. The influences of additives of fluoride ions in the electrolyte on the growth of PEO/Ni-P composite coating were investigated. Results show that addition of fluoride ions in the electrolyte plays a dual role in the formation of PEO coating, i.e. uniformly increasing the thickness of PEO coating and the compactness of inner layer. Fluoride is enriched in the inner PEO layer as an amorphous phase. For the subsequent EN plating, extension of PEO time is beneficial to the deposition of Ni–P layer, whilst fluoride ions in the PEO electrolyte are detrimental to the direct EN plating. It is evident that Ni–P nodules selectively nucleate and grow in the micro-pores of the PEO layer. Electrochemical tests reveal that the PEO/Ni-P composite coating provides superior corrosion protection to the underlying Mg substrate. A nucleation mechanism of the Ni–P nodules on the PEO layer was proposed.