Mechanism Researches On The Anodic Oxidation Film Of Magnesium Alloy Based On The Semi-conductor Electrochemistry

For these issues of the pollution problem of anodizing electrolyte including F, Cr and P and the difficulty of controlling the process conditions, this paper developed the environment-friendly anodizing oxidation electrolyte and process based on magnesium alloy of AZ31.With the theory of semiconductor electrochemistry, the formation mechanism of AZ31 was discussed and evaluated the corrosion performance and galvanic corrosion behavior of the anodic film formed on AZ31.This research will be promote the practical applications.After a lot of trial, a kind of environment-friendly anodizing oxidation electrolyte and process was developed. And the electrolyte consists of NaOH, Na₂SiO₃,Phytic acid and sodium of polyaspartic acid（PASP）.With the optimal process, the anodic film with micro-cracks was grey white and porous. And the composition of the anodic film is MgO and Mg₂SiO₄.As a scale inhibitor, PASP barely is used in anodizing magnesium alloy as the organic agent. With the regime of PASP from 0g/L to 28.8g/L, the cell voltage, the formation rate and the content of the constituents were improved obviously. But over 28.8g/L, PASP decreased the development of the anodic film. Thus, a plausible model we propose that the anodizing process is regulated by two main plausible adsorption orientations of PASP at the surface anode. Furthermore, the quantum chemistry calculation was adopted to explain this model including flat-on configuration and end-on configuration.From the thermodynamic analysis, the constituent was decided on the concentration of NaOH, Na₂SiO₃ in anodizing electrolyte. From the experimental phenomenon and the variation of the thickness, there were three stages in anodizing process and the growth curve was in keeping with the tendency of parabolic curve. With the structure and composition test, the anodic film has two layers of porous lay and metallurgical coating. And the amount of substance of MgO and Mg₂SiO₄ was increased. Mg₂SiO₄ was the main constituent in outer coating and MgO was the main constituent in inner coating. These results implied that in the initial stage, the anodic film was formed with epitaxial growth with the control of electrochemistry reaction and in the later stage, the anodic film was formed with topotaxy with the control of diffusion process. From the surface and cross-sectional morphologies, the dimension of micro-pore became smaller and the dimension of cavities became larger. Finally, based on the semi-conductor electrochemical theory and PMD model, the electrochemical mechanism about energy band model and defect model was proposed. The bending of energy band of Mg alloy in anodizing electrolyte is very important reason for the discharge of sparks. The anodic film formed on AZ31 originated from the migration of Oxygen ion vacancy and the dissolution of Mg alloy originated from the Magnesium ion vacancy.We have used semiconductor electrochemistry technique, a correlation of the formation conditions and semiconductor properties was established. In 3.5% NaCl solution, the anodic film formed on AZ31 was n-semiconductor. With the increasing of anodizing time, the carrier concentration decreased gradually and the flat band potential shifted to the negative direction, while the characteristics of the semiconductor declined. With the increasing of current density and temperature, the carrier concentration almost increased and the flat band potential almost shifted to the positive direction, while the characteristics of the semiconductor declined. The anodic coating formed on AZ31 showed stronger semiconductor characteristics and good corrosion performance.Finally, using electrochemical technique and immersion experiment, the corrosion performance of the anodic film was better than that of Mg ally AZ31.And the galvanic effect of the sealed anodic film and the aluminium alloy and low-carbon steel treated with organic coating declined evidently.