Comparative study of fluoride conversion coating structure on two types of bulk magnesium materials

This paper deals with the structural and composition differences of two types of fluoride conversion coatings on two types of bulk magnesium materials. The bulk magnesium materials used were powder metallurgy cold compacted magnesium and sintered magnesium materials. One type of coating was prepared by dipping the magnesium samples in hydrofluoric acid and the second one by immersing the samples in Na[BF 4 ] molten salt. Microstructure and elemental composition were examined by scanning electron microscope equipped with an EDS analyzer. The phase composition was analyzed using an XRD. The fluoride conversion coating prepared from Na[BF 4 ] molten salt grew significantly deeper into the cold compacted magnesium than to the sintered material. Also, traces of fluorine were found inside the bulk of cold compacted material, while no fluorine traces were found inside the sintered material. The coating prepared from hydrofluoric acid appeared the same in both magnesium materials, with similar depth and fluorine traces found inside the bulk of both of the materials. Furthermore, the coating prepared from Na[BF 4 ] molten salt grew deeper into the material than the coating prepared from hydrofluoric acid. The differences between the two coatings in creation depth, structures, and layout of fluorine inside the material suggest a difference in the creation mechanism of the two coatings.