| New anodic films which were protective, adhesive and chromate-flee had been developed for magnesium alloy, AZ31, surface at constant applied current density. The two processes differed in the anodizing electrolyte bath compositions and continuous sparking existed in one process, but not in the other. Both of them were free of the poisonous ingredients, such as chromate, phosphate, fluoride which do harm to environment and human beings. And they could improve the surface properties of magnesium alloys significantly, especially corrosion protection. Then, the effects of all kinds of process parameters on the formation and properties of anodic .films were investigated. At the same time, the mechanisms on the formation of anodic film as well as the corrosion and protection of anodized samples were studied. The results are of great significance for richening the theory on the corrosion and protection of magnesium alloys, forming a novel technology of anodic oxidation and promoting the application of magnesium alloys.By comparing the anti-corrosion properties using polarization, curves in corrosive solutions, by observing the surface morphologies using SEM and by analyzing the structures and components of anodic films using XRD and EDS, the effective ingredients of anodizing electrolyte solution were decided. The effects of concentrations of ingredients in electrolyte, applied current densities, temperatures and anodizing time on properties of anodic films and processes of anodizing were investigated to obtain the optimum parameters.Research of the two totally different processes suggested the electrolyte solution played a derminant role in anodizing of magnesium alloys. Besides electrolyte solution, the anodization of magnesium alloys was also affected by some other factors, including solution temperature, treatment time, and electrical parameters, etc. Such as, higher solution temperature could improve anti-corrosion properties and surface morphologies of anodic films in non-sparking process, however the negative effects occurred in sparking process. The corrosion protection provided by anodic film was determined not only by thickness of anodic film, but also the structure, component and surface morphology.In non-sparking anodizing process, one of the additives, Na2C2O4, could change anodizing process greatly and improve anti-corrosion properties of anodic film. During sparking anodizing process anodic film with better anti-corrosion properties could be formed in electrolyte solutions containing Na2CO3 or phenol. Phenol could also made film smoother and more compact.In analysis of mechanisms on the formation of anodic film, The region in which anodic film could exist stably was obtained in thermodynamical analysis firstly. Then from the view of electrical behavior, accumulation of elements in film, variations of surface morphologies and kinetics of growth, formation of anodic film were studied and growth model was put forward. In non-sparking process, the pH value must be higher than 11.475 for stable existence of anodic film. Preferential dissolution occurred in some regions on base metal and film was formed. For the Pilling Bedworth Ratio was 1.77 for hydroxide, the formation of Mg(OH)2 inside the film could lead to the development of cracks. In hypothetical occasion, that was current efficiency 100% and all Mg(OH)2 in film, anodic film grew according to Faraday's law of electrolysis. The growth rate could be described by Ti =0.5776it, where i was the applied current density. The combination of Mg2+ and OH was the main reaction during formation of anodic film, and sodium borate played a role like catalyst, which could .accelerate reaction of film formation, but substances with boron did not exist in products of reaction.The sparking anodizing process could be divided into two stages. The first stage was before the occurrence of sparking, in which the bath voltage increased greatly and very thin, compact film was formed. The second one was after the occurrence of sparking, in which the sparking scanned over the surface of the specimen. Anodic film continuously increased its coverage percentage to substrate and then it increased its thickness towards the direction that was vertical to anode surface. The growth was simultaneity of destruction, restoration of old film by sparking and formation of new film.In the first stage, the colloid layer, generated by deposition of SiO32- and Mg(OH)x, was compressed, concentrated and dehydrated for large quantity of heat provide by. high current density and compact film was formed. In the second stage, the bath voltage increased continuously until it reached the value of breakdown and sparks occurred. Then anodic film grew in cross and vertical direction for movement of sparking.The concept of irregular dimension was used to deal with the SEM image of the interface of substrate/anodic film in magnesium alloy anodizing films. According to principles of lattice method, program was developed and important conclusion can be drawn. That was growth of anodic film was controlled by chemical reaction in both of non-sparking and sparking anodizing processes.EIS (Electrochemical Impedance Spectroscopy) is a very powerful tool to investigate corrosion behaviors and corrosion protection mechanisms. The corrosion of anodized magnesium alloy AZ31 was divided into two stages, and every step of process was analyzed. Then different equivalent circuits were put forward to describe the two stages. The mathematic representation of impedance was obtained by complicated derivation and Nyquist plot were drawn out. The impedance. spectroscopy, which was obtained by monitoring corrosive process of anodized samples, had similar shape with theoretical model. Then data fitting was conducted to calculate parameters in equivalent circuits and the change of EIS could be used to explain the structure of film further.
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