Characterization and formation mechanism of grey micro-arc oxidation coatings on magnesium alloy

• Grey MAO coatings with K2TiF6 addition into developing electrolyte had been formed.
• Grey MAO coatings had improved mechanical and corrosion properties.
• Mechanism of grey MAO coatings had been discussed, which was different from the ones reported previously.

Grey micro-arc oxidation (MAO) coatings were prepared on AZ80 Mg alloy using DC electrical source in the electrolytes with different addition of K2TiF6. Microstructure and properties of the coatings were comparatively studied by SEM, XPS, Nano-indentation and electrochemical experiment. The results showed that with the increase of K2TiF6 into the developing electrolyte, the size and distribution of micropores on the grey coatings were obviously improved. The roughness decreased and the density of coating surface increased. The grey MAO coatings mainly contained MgO and MgF2 phases. Ti element was only found on the coating after surface etching. The grey colour of MAO coatings was caused by the interaction of MgF2 phase and Ti element on the coating surface, which was different from the traditional preparation of MAO coatings. The nano-hardness and corrosion resistance of these grey MAO coatings could be enhanced as adding 10 g/L K2TiF6 into the base solution. Furthermore, the reason of corrosion resistance variance of this grey MAO coating was discussed.