Studies On The Growth Process And Corrosion Behavior Of Micro-arc Oxidation Coatings On Magnesium Alloy

In recent years, micro-arc oxidation (MAO) technique is the most favorable method in the surface treatment of magnesium alloys. This method combines the electrochemical oxidation with a high voltage spark treatment in an aqueous electrolytic bath. With the introduction of high voltage spark treatment, the wear resistance, corrosion resistance, mechanical strength, and electrical insulation of the metals and their alloys can be effectively increased. It is of great importance to study the MAO procedure, to investigate the growth characteristics and the corrosion behavior of the MAO coatings. In this work, MAO coatings were prepared on AZ91D Mg alloy in alkali silicate electrolyte. The growth process and the corrosion behavior of the MAO coatings were studied by electrochemical and surface analysis method, and corresponding mechanisms were discussed. At the same time, the relations between the structure characteristics and the electrochemical properties of the coatings were also established. It was found that the micro-arc oxidation process could be divided into three stages, the passive film stage, the micro-spark ceramic coating stage and the spark ceramic coating stage. These oxide layers presented significant differences in structure, composition characteristics and the electrochemical properties. Spark discharge was the representative phenomenon of MAO method distinguished from other electrochemical oxidation methods. Under the spark discharge treatment, the structure of the anodic layer was significantly changed, the enrichment of silicon and oxygen was enhanced, and the surface potential was ennobled. Moreover, it seemed that intensive spark discharge appeared at oxygen abundant region, and oxygen played a vital role in determining the size and intensity of the spark discharges. Investigations were also carried on the corrosion behavior of MAO treated Mg alloy in corrosive medium. The results showed that the entire corrosion process was composed of two periods, the MAO coating deterioration period and the Mg alloy corrosion period, and the specimen exhibited different electrochemical behaviors at different period. In addition, the corrosion behaviors of MAO treated and untreated Mg alloy with a NaCl droplet were compared in atmosphere. The corrosion zone could keep for a long time on Mg alloy surface. Whereas, for the MAO treated Mg alloy, the corrosion zone expanded gradually due to the external loose layer. Finally,two kinds of new electrolyte were developed, and the properties of the corresponding MAO specimens were also compared.