Film formation in the second step of micro-arc oxidation on magnesium alloys

Magnesium alloys were anodized firstly in a neutral solution and then in an alkaline solution by micro-arc oxidation (MAO). The second step of MAO was a competition between old film dissolution and new film formation. The curves of voltage and mass gain with time showed that in the initial stage, the old film was dissolved. Surface morphologies and EDX analyses indicated that a new film was formed after 1 min. The transition from the process dominated by old film dissolution to that by new film formation took place when the new film had completely covered the old film.