Corrosion and corrosion fatigue performances of micro-arc oxidation coating on AZ31B cast magnesium alloy

In this work, the corrosion performances of micro-arc oxidation (MAO) coating on AZ31B cast alloy were examined. MAO coating synthesized at a low current density of 34 mA/cm(2) has a uniform and compact microstructure with very fine micro-pores and few structural defects; thus, robust protection is provided for the AZ31B substrate in corrosive solutions. The long-term corrosion and corrosion fatigue behaviors of the MAO coatings formed at the optimal current density for 5 and 10 min were characterized. The results show that the MAO coating formed for 10 min provides greater protection in a salt fog environment than the coating formed for 5 min does because it is thicker. The fatigue life and fracture analysis of bare, MAO-coated and E-paint MAO-coated AZ31B indicate that the MAO coating reduces corrosion fatigue life by 55% due to the micro-pores and micro-cracks on its surface providing initiation sites under stress conditions and accelerating crack propagation. However, E-paint MAO-coated specimens show improved corrosion fatigue strength in 10(7) cycles at similar to 60 and similar to 50 MPa under the same conditions due to the sealing of the micro-pores and micro-cracks in the MAO coatings.