Influence of applied stress on the corrosion resistance of micro-arc oxide coatings on AZ31B magnesium alloy

A micro-arc oxidised (MAO) protective coating was formed onto AZ31B magnesium alloy samples which were then loaded into 'C' rings to apply compressive and tensile stresses. The influence of these applied stresses on the protective ability of the coating in simulated body fluid was assessed by surface characterisation and electrochemical testing. It was found that the undulating surface and pore structure of the MAO ceramic coatings was affected by the stress during immersion with a greater number of closed pores under compression and more open pores, with additional cracks, under tension. During the earlier stages of immersion, compressive stresses enhanced the corrosion resistance whilst tensile stresses accelerated corrosion. However, at later stages of immersion both compressive and tensile stress caused enhanced corrosion with tensile stresses consistently worse than compressive stresses.