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• The corrosion failure progress and mechanism of MAO coating has been studied. • The surface roughness affects the bonding strength of the MAO coating significantly. • The high surface roughness could dramatically improve the corrosion resistance of MAO coating. In this study, micro-arc oxidation (MAO) coatings were formed on ZM5 Mg alloy substrate with different surfaces roughness. Therefore, the influence of surface roughness on the corrosion failure progress and mechanism of the MAO coated ZM5 Mg alloy has been investigated. The microstructure, bonding strength, and corrosion resistance of the MAO coatings were studied by SEM, LSCM, bonding strength measurement, and electrochemical test. The results showed that the surface roughness affects the porosity and bonding strength of the coatings. While it had little effect on the thickness and chemical composition of the MAO coating. Due to the high surface porosity, the coating of the grooved sample had poor corrosion resistance in the initial immersion. But, after a long-time immersion, the grooved sample was still protected by the coating, while the smooth sample showed severe damage. The strong bonding strength between the coating and groove sample was the key factor for enhancing the corrosion resistance of MAO coated ZM5 Mg alloy. [ABSTRACT FROM AUTHOR]