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| Study on Corrosion Failure Mechanism of Microarc Oxidation Coatings Formed on AZ91D Magnesium Alloy |
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Abstract The corrosion resistance of the two types of microarc oxidation (MAO) coatings was evaluated by virtue of total immersion test and salt spray. Then the mechanism of the coating corrosion failure in 5wt.%NaCl aqueous solution was studied through the impedance and open-circuit potential (OCP) measurements. The results showed that both phosphate coating (P-film) and silicate coating (Si-film) could enhance the corrosion resistance of AZ91D alloy significantly and that the later was more effective. Based on the variations of OCP and corrosion morphology of P-film and Si-film immersed in 5wt.%NaCl aqueous solution with different time, the analytic results of electrochemical impedance spectroscopy (EIS) data for Si-film showed the corrosion failure process had experienced the following four steps: the rapid initial penetration of the solution into the porous outer layer; the filling of corrosion products in certain pores; the gradual corrosion of the barrier layer located in the bottom of pores; the stable corrosion process with the failure of barrier layer.
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Received: 22 November 2006
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