【正文】 constantly and rather low, about 3 V. During the whole procedure, no sparking occurred and the formation of anodic film was stable. Anticorrosion properties of anodic films formed in different bath solutions were pared in this study (Fig. 2). Compared with basic alkaline electrolyte solution, the addition of sodium silicate resulted in the positive transfer of corrosion potential in the curve of potential against current density. But those phenomena were not found in an electrolyte containing sodium phosphate, sodium aluminate and sodium molybdate. The result indicates that the corrosion resistance of anodic film was greatly improved by the addition of sodium silicate. The surface morphology of anodic film was also observed in this study. The film formed in an electrolyte containing sodium phosphate or sodium molybdate appeared to be corroded, and there were some corrosive productions on surface (Fig. 3 ). From the SEM image of the anodic film surface formed in a solution containing sodium aluminate, holes of different sizes caused by sinter appeared on the surface of film and were connected with cracks, which may contribute to the inferior corrosion resistance. While the anodic film formed in solution containing sodium silicate was rather smooth with little cracks (Fig. 3). From the above mentioned results, sodium silicate could be chosen as the optimum addition of the secondary oxysalt. The crosssection morphology o anodized sample was shown in Fig. 4 . From this figure, it can be seen that the anodic film was bined with base metal tightly and the thickness of the anodic film was 200 181。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 do