Probing the effect of microstructural difference of AZ91D magnesium alloys on the corrosion resistance of phosphate conversion coatings

Purpose The purpose of this paper was to compare the electrochemical homogeneity of AZ91D after various heat treatment processes, and its influence on the growth, composition, microstructure and corrosion resistance of phosphate conversion coatings. Design/methodology/approach The electrochemical activity of different heat-treated Mg alloys was evaluated via scanning vibrational electrode technique; the characterization of the microstructure and phase composition of coatings was conducted using a scanning electron microscope and X-ray diffraction. The corrosion resistance was evaluated by electrochemical tests and accelerated neutral salt spray tests. Findings T6 treatment increased the electrochemical homogeneity, while T4 treatment decreased the microstructure homogeneity of AZ91D magnesium alloy, due to the existence of residual Al-Mn impurity phase. The phosphate conversion coating (PCC) on T6 heat-treated Mg alloys showed the most compact microstructure and the best corrosion resistance, while the coating on the T4 heat-treated Mg alloy exhibited the worst microstructure and corrosion resistance. Originality/value The microstructure and protectiveness of coatings are related to the homogeneousness of Mg alloy: an Mg substrate with a more heterogeneous electrochemical reactivity yields a PCC with less protectiveness, which could be explained by the difference of precipitation kinetics at the metal/electrolyte interface.