| Micro-arc oxidation is the most favorable method of surface treatment for magnesium alloy at present. The morphology, composition and structure of micro-arc oxidation coatings on AZ91D were analyzed by stereomicroscopy, scanning electron microscopy and X-Ray diffraction. According to oceanic and inshore environments, corrosion resistance of the protection system of micro-arc oxidation on AZ91D magnesium alloy was investigated by environmental corrosion experimentation and electrochemical impendence spectroscopy. The static tension and fatigue properties of micro-arc oxidized AZ91D before and after pre-corrosion were studied. Foundation was established for further application of micro-arc oxidation technology, especially, theoretical gist was supplied for feasibility of micro-arc oxidation application in magnesium alloy structure of aviation equipment.The behavior of corrosion to AZ91D was studied. It was indicated that the corrosion behavior was determined by phase distribution.αphase corrosion occurred along grain boundary at first and extended inside along the grain boundary. The form of corrosion was corrosive pitting primarily, and it developed to local corrosion based on the corrosion source of corrosive pitting. The corrosion process was a comprehensive process of multi-corrosion forms. The corrosion resistance of AZ91D by different heat treatments was different. The corrosion resistance of AZ91D heat treated by solution and aging (T6) was better than that treated by solution and uniform heat (T4). The reason was thatβphase of AZ91D precipitated along grain boundary after treated by T6, which acted as barrier against corrosion and was in favor of holding up general corrosion on AZ91D.The results show that the salt spray corrosion resistance of micro-arc oxidation treated AZ91D was better than that treated by chemical oxidation, which was determined by the structure and stability of coatings. The micro-arc oxidation ceramic coating was made up of two layers with special structures. The outer layer was porous loose, while the inner one was imporous and compact. Owing to the metallurgical bonding, the compact layer had good bonding strength with the parent metal. The bonding strength was high. The electrochemical impendance of micro-arc oxidation coatings was better than that of chemical oxidation coatings. By 168h neutral salt spray test, the corrosion form of micro-arc oxidation coatings was corrosive pitting, the protection grade was nine, while the primary corrosion form of chemical oxidation coatings was mainly slot corrosion and uniform corrosion, and corrosive pitting occurred at the same time, the protection grade was three.The corrosion resistance of micro-arc oxidation ceramic coatings was related to the thickness of coatings. Under certain technological parameters, there existed one micro-arc oxidation coating with best corrosion resistance. The best thickness was decided by the effective thickness of coatings and was independent of total thickness.The protection system with repainted paintcoats after micro-arc oxidation had good corrosion resistance. After 23 cycles of environmental corrosion experimentation (one cycle contains 3h ultraviolet ray aging test plus 36h neutral salt spray test), no corrosion occurred. The reason was that the thin and porous surface micro-morphology of micro-arc oxidation coatings was in favor of forming the interface of high binding force, which increased the painting quality of the paintcoats. The compact structure of the inner layer could reduce the disfigurement, which held up corrosion mediums.Micro-arc oxidation treatment had no influence upon the tensile strength of AZ91D. Pre-corrosion decreased the tensile strength of specimens treated by different means, which was not obvious. It indicated that the stress concentration created by corrosive pitting had little influence upon the tensile strength of AZ91D.The fatigue test condition was as follows, Mean Value Sm was 4.72 KN, Amplitude Value Sa was 3.15KN, Stress Ratio R was 0.2, Load was constant amplitude spectrum and Frequency was 10Hz. The average fatigue life of AZ91D treated by micro-arc oxidation was 54148 cycles, corresponded to that without micro-arc oxidation. The pre-corrosion by 168h neutral salt spray test made the fatigue life of AZ91D treated by different means decline evidently. But the corrosion resistance of micro-arc oxidation coatings was better than that of chemical oxidation, slot corrosion and uniform corrosion didn't exist, only a few pits occurred. The effective cross-sectional area of specimens didn't decrease and the properties of materials didn't degrade evidently. So the pre-corroded fatigue life of specimens was higher than that treated by chemical oxidation. The thickness of micro-arc oxidation coatings had no influence upon the pre-corroded fatigue life of the specimens pre-corroded 432h by neutral salt spray test. The reason was that the fatigue life of the specimens would not be sensitive to the dimension of pits when the pits spreaded to a certain extent.
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