Abstract：To study the requirements of the power supply in micro-arc oxidation (MAO) of magnesium alloy, many experiments were performed under the DC, unipolar, and ambipolar pulse output modes. Based on the experimental results and electric arc theory, the separate local arc discharge mechanism was put forward. It is considered that magnesium MAO process consists of three stages including anodic oxidation, micro-arc oxidation, and large-arc discharge in turn with increasing source voltage. The MAO film is composed of metal oxides that resulted from numerous discrete local arc discharges, which accumulate the non-equilibrium structure after undergoing sudden heating and cooling cycles. Separate local arc discharge is caused by the process in which the oxygen-based gas is ionized in the conduct channel bearing electric field intensity, changed from insulator to conductor that presents sharp negative resistance effect, and produced partially high temperature to ignite locally metal oxidation. The local arc discharge model is described as four courses: gas created from electrolysis, arc discharge, metals oxidization, and cooling and shrinking of oxides. The purpose of pulse supply is to inhibit the large-arc discharge by intervening proper cooling time, which cannot be actualized by a unipolar pulse mode because of the strong capacitive load characteristics but can be reached by an ambipolar pulse supply because the negative pulse period acts as cooling time. Using a discharge loop to remove the influence of load capacitive, a new type of pulse power supply for MAO is developed, so that the large-arc problem is resolved effectively, the film-forming efficiency is improved, and the pollution of the film and electrolyte caused by negative voltage is avoided.
. Local arc discharge mechanism and requirements
of power supply in micro-arc oxidation of magnesium alloy[J]. Front. Mech. Eng., 2010, 5(1): 98-105.
Ming CHEN, Yuezhou MA, Yuan HAO, . Local arc discharge mechanism and requirements
of power supply in micro-arc oxidation of magnesium alloy. Front. Mech. Eng., 2010, 5(1): 98-105.
Gray J E, Luan B. Protective coatings on magnesiumand its alloys. A critical review. Journalof Alloys and Compounds, 2002, 336(1,2): 88―113 (in Chinese)
Yerokhin A L, Nie X, Leyland A, Matthews A, Dowey S J. Plasma electrolysis for surfaceengineering. Surface and Coatings Technology, 1999, 122(2): 73―93 doi: 10.1016/S0257-8972(99)00441-7
Zhang R F, Shan D Y, Han E H, Zeng Z L. Status andprospect of anodization on magnesium and its alloys. The Chinese Journal of Nonferrous Metals, 2006, 16(7): 1136―1148
Wei T B, Tian J. Technique of surface modificationby plasma electrolysis deposition method. Journal of Materials Science & Engineering, 2003, 21(3): 450―452
Rudnev V S, Vasileva M S, Lukiyanchuk I V. On the surface structure of coatings formed by anodicspark method. Protection of Metal, 2004, 40(4): 352―357 doi: 10.1023/B:PROM.0000036956.77006.7d
Xue W B, Deng Z W, Lai Y C. The rules of magnesium alloy films grow by micro-arcoxidation. The Journal of Metal Heat Treatment, 1998, 19(3): 42―44
Lang B L, Zhang X F. Growth process and corrosionresistance of ceramic coatings formed by micro-arc oxidation on magnesiumalloy. Journal of Chinese Society for Corrosionand Protection, 2005, 25(2): 97―101 (in Chinese)
Chen F, Zhou H, Yao B, Qin Z, Zhang Q F. Corrosion resistance property of theceramic coating obtained through microarc oxidation on the AZ31 magnesiumalloy surfaces. Surface and Coatings Technology, 2007, 201(9―11): 4905―4908 doi: 10.1016/j.surfcoat.2006.07.079
Long B Y, Wu H H, Long B H. Effects of electrolyte solution on phase component andelementary composition of MAO coatings on aluminium alloy. Journal of Jilin University (Science Edition), 2005, 43(1): 68―72 (in Chinese)
Zhang R F, Li M S, Long X L. Research progress in effects of electric parameters onproperties of anodic coatings on magnesium alloys. The Chinese Journal of Nonferrous Metals, 2006, 16(11): 1829―1837
Wang N Z, Long B Y, Wu H H. Effects of unipolar pulse current density on the phasecomposition and microstructure of alum in aluminium alloy MAO coating. Journal of Jilin University (Science Edition), 2005, 34(5): 645―649 (in Chinese)
Wang L S, Cai Q Z, Wei B K. Magnesium alloy double pulse micro-arc oxidation surfacefilm formation and structural analysis. Special Casting and Nonferrous Alloys, 2005, 25(3): 187―190
Lv X Y, Jin Z S, Wu H H. Effects of cathodic /anodic current density on propertiesof MAO coating on aluminium alloy. Journalof Jilin University (Science Edition), 2005, 43(1): 64―67 (in Chinese)
Krysman W, Kurze P, Dittrich K H. Process characteristics and parameters ofoxidation byspark discharge (ANOF). Crystal Researchand Technology, 1984, 19(7): 973―979 doi: 10.1002/crat.2170190721
Wu H H, Yu F R, Li J J. Properties of aluminium alloy ceramic coating preparedby microarc oxidation. Journal of InorganicMaterials, 2004, 19(3): 617―622
Kuang Y F, Hou Z H, Liu J P. Development of electric breakdown theory of anodic oxidation. Electroplating & Finishing, 2000, 19(3): 38―42
