| Plasma electrolytic oxidation (PEO) technology provides a new platform for surface modification of light-metallic materials. The films formed by PEO combine the performance of both anodic oxide films and ceramic films, which broadens the application field of light-metallic materials. Many researches related to the PEO films'properties have been down around the world. The discharge phenomenon, the basic process of PEO, is one of the most important characteristics of the PEO technology. In this paper, the AZ31 magnesium alloy was used as the working electrode. The influence of dielectric barrier layer's (DBL) on the micro-arc discharge, and the mechanism of discharge was investigated.(1) The general growth process of PEO was studied. The PEO process was optimized in the common alkaline silicate electrolyte. It is found that the optimum process conditions are 20 g·L-1 Na2SiO3 and 4 g·L-1 KOH at 300 V for 40 min. The morphology, composition and corrosion resistance of the PEO ceramic coatings formed in the optimal process conditions were studied. The characteristics of breakdown discharge in the entire PEO process were systemically investigated under escalating voltage waveform. In addition, the morphology, composition and corrosion resistance of coatings formed at different PEO stages and the amount of the dissolved magnesium ions in the electrolyte were also studied.(2) The chemical conversion films were acted as the dielectric barrier layer. First, the chemical conversion films, which exhibite different characteristics, were produced on the surface of the AZ31 magnesium alloy. Then, the influence of different dielectric barrier layer's (DBL) characteristics on the breakdown discharge and coatings'property were investigated. A new chrome-free phosphate-permanganate solution was chose. The technology of the films formed in the phosphate-permanganate solution was optimized. These formed films were used as the DBL. In order to investigate the influence of different dielectric barrier layer on the PEO process, several chrome-free conversion films formed on magnesium alloy were obtained by changing the pH of phosphate-permanganate solution. The morphology, composition, the corrosion resistance of the coatings, the amount of dissolved magnesium ions in the electrolyte and the current-voltage characteristics of PEO process were examined.It was found that: (1) The PEO process can be divided into four stages: the conventional anodic oxidation stage, the spark oxidation stage, the micro-arc discharge stage and the arc discharge stage. When it is low voltage, there will occur anodic oxidation and produce oxide films on the surface of magnesium alloy. If add voltage, the dielectric barrier layer will be discharged, occur spark oxidized and get ceramic coatings. Further boost, micro-arc discharge or arc discharge will reach and generate uniform and dense ceramic coatings. (2) the influence of dielectric barrier layer's (DBL) characteristics on the breakdown discharge and the follow-up discharge characteristics of PEO are very outstanding. The different properties of DBL, such as compactness, thickness, electrical insulation, etc., will directly affect the breakdown discharge current density, the stable equilibrium current density, the dissolution of the substrate, the migration of anions, the electrode/electrolyte interface, and so on. Ultimately, it will affect the morphology, the composition and the corrosion resistance of ceramic coating.
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