【正文】 m. The corrosion process of anodized sample without sealing was also detected and analyzed by EIS ( Fig. 5 ). The equivalent circuit of anodized AZ31 magnesium alloy was present in our previous study [12] . Based on the equivalent circuit and EIS patterns, the nonlinear fit curve of resistances was obtained and shown in Fig. 6, in which Rs , R c , R po and R ct were resistances for solution, film, pores and corrosive reaction, respectively. Since the film without sealing was porous, the corrosive ions penetrated into the film and then reached the interface between anodic film and base metal. As shown in Fig. 6 , the corrosive resistances Decreased rapidly within 10 h, and then remained relatively stable. The results implied that the corrosion process rapidly initiated within a short period, and thereafter this process slowed down because of anodic film formation.. Effect of sodium silicate concentration on anticorrosion properties of anodic films Comparison of corrosion resistances of anodic films formed in solutions containing different concentrations of sodium silicate is shown in Fig. 7. The corrosion potential was positively transferred with increasing concentration when concentration was lower than 90 g l ˉ. The largest positive transfer of corrosion potential was observed at 90 g l ˉof Thereafter, corrosion potential was negatively transferred. The results indicated that the addition of 90 g lˉcould get the best improvement of anticorrosion properties for anodic films. However, when Na2SiO3concentration was over 90 g l1, the corrosion resistance decreased. The Xray diffraction patterns of anodic films formed in solutions containing different concentrations of sodium silicate are shown in Fig. 8 . When the concentration of sodium silicate in anodizing solution was 10 g lˉ , a large amount of MgO could be found in the anodic films. However, with the increasing of sodium silicate concentration, the peak of MgO gradually diminished, and then finally disappeared. As seen in Table 1, the results of EDS revealed an increased of siliconCentration con species but a constant atomic ratio of Mg to O for the films formed in electrolytes with more sodium silicate addition. From substance phase analysis in XRD patterns ( Fig. 8 ), it was indicated that the species with element silicon or oxygen were amorphous. The possible explanation was that the molten productions of anodizing, which was caused by high temperature produced by sparking, were cooled down suddenly when touching the solution. And then the cooling was too prompt for the atoms of magnesium, silicon and oxygen to be arranged regularly according to the lattice structure. Effect of applied current density on anticorrosion properties of anodic films The different applied current densities led to the obvious variation of the experimental phenomena in this study. With increasing applied current density, sparking moved more rapidly. Moreover, the little white sparks changed into large, yellow bright one with current density increasing. The anticorrosion properties of anodic films formed at different applied current densities are presented in Fig. 9. The results revealed that with the increasing of applied current density, better corrosion protection of anodic film could be obtained. The voltage transients observed during anodizing processes conducted at different applied current densities are shown in Fig. 10. It was found that the voltage increased almost linearly at the initial stage of anodizing, then reached up a plateau and remained constant. The increasing of voltage with time was caused by the increment of coverage percentage to substrate and thickness of anodic film. Together with the voltage decreasing caused by the change of anodic film structure and property under sparking, the voltage was kept almost constant. To ensure the wellbalanced growth of anodic film, there should be simultaneity of destruction of old film and appearance of new film. The destruction processes of anodic film included the breakdown, physical fusion and chemical dissolution. The extensive oscillation of voltage revealed that the growth of anodic film may be the petitive process of the three steps including destruction of old film, reparation of destroyed film and formation of new film. The latter two should be domi。 and (2) study the effects of some process parameters such as oxysalt concentration, applied current density and temperature on the anodization of magnesium alloy AZ31. The outes can provide information on the technological improvement of the anodization of magnesium alloy, AZ31.2. Experimental The specimens are AZ31 magnesium alloy plates, which contain 3 mass% Al and 1 mass% Zn. The specimens were left 1 cm 2 of the surface exposed. After polishing to lm alumina powder, the electrodes were carefully degreased by water and acetone. An electrochemical system was constructed with Pt coil as a counter electrode and Ag/AgCl saturated with KCl as a reference experiments were performed in a bath containing 40 g l ˉof NaOH and 30 g l ˉ of sodium borate with agitation. The anodic polarization curve was measured by a potentiostat with 60 mV s ˉ of scan rate, and anodizing process was carried out at constant cur rent density. 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