Formation of electroless Ni-B on bifluoride-activated magnesium and AZ91D alloy

Previous studies of the authors have reported the formation of electroless Ni-B coatings on magnesium and AZ91D magnesium alloy using a chrome-free process. The substrates were pre-treated by grit-blasting and etching. In the present study, the influence of an activation stage in NH₄HF ₂ solution is investigated. Scanning electron microscopy, X-ray diffraction, Raman spectroscopy and ion beam analysis are employed to determine the morphology, structure and composition of the treated surfaces at different stages of the electroless process. The results indicate that activation has relatively little influence on the growth rate and properties of the final coatings. However, it accelerates the coverage of the surface by Ni-B nodules at the start of the electroless treatment and, in the same period, reduces the electrochemical potential, the latter suggesting inhibition of corrosion of the substrate. Further, activation reduced the oxygen content of surfaces and led to formation of N-H species. Comparison with the earlier studies indicated that activation is of secondary importance to grit-blasting and etching for the generation of uniform coatings.