Relationship between microstructure and formation-biodegradation mechanism of fluoride conversion coatings synthesised on the AZ31 magnesium alloy

The aim of the present work was to evaluate the influence of the microstructure on the formation-biodegradation mechanism of conversion coatings synthesised on the AZ31 magnesium alloy by immersion in HF solutions under different concentrations and treatment times. The experimental results revealed the formation of a magnesium hydroxyfluoride layer, which enhanced the corrosion resistance of the AZ31 alloy in Hanks' solution. Furthermore, the presence of intermetallic particles within the α-Mg matrix favoured the formation of defects that promoted the growth of corrosion products causing the detachment of the fluoride coating, all before it may start to be noticeably biodegraded. •The conversion treatment of the AZ31 alloy in HF produced a layer of Mg(OH)2−xFx.•Increase in HF concentration and treatment time led to coatings with higher fluoride content.•The microstructure played a key role in the corrosion performance of the coating.