Anodic oxidation of a pure Al plate was carried out in aqueous solutions containing sulfuric acid (H 2 SO 4 ), oxalic acid (H 2 C 2 O 4 ) and a percarboxylic acid-based additive at temperatures between 273 and 293 K for 0.6 to 9 ks under a constant current density of 300 A·m ¹2 (3 A·dm
¹2). The microstructures and the mechanical properties of the obtained anodic oxide film were investigated to determine the optimum electrochemical condition. In the H 2 SO 4 + additive bath, the film with a maximum thickness of approximately 100¯m was formed at 293 K for 7.2 ks. However, the surface hardness of the film decreased with increasing processing time and film thickness. To enhance the film hardness, the Al plate was anodized in the H 2 SO 4 + H 2 C 2 O 4 + additive bath at lower temperatures, which resulted in higher Vickers hardness of 400500 and excellent wear resistance for the thick film formed at 278 K for 7.2 ks. The film formed at 278 K in the H 2 SO 4 + H 2 C 2 O 4 + additive bath had a typical cell structure with a nano-sized pore at the center of each cell. Compared with the film formed in the H 2 SO 4 bath, the cells were almost uniform in size and the pore size in the cell was smaller. In addition, the occupation ratio of the pores in unit area of the film formed in the H 2 SO 4 + H 2 C 2 O 4 + additive bath was smaller than that of the film formed in the H 2 SO 4 bath. Such microstructural features are thought to be responsible for the high hardness of the anodic oxide thick film.
Titanium plates covered with anodic oxide films with thicknesses of approximately 10¯m were embedded in a mixture of iron, graphite, and alumina powders, and then heated in the temperature range of 10731373 K for 3.6 ks in a nitrogen flow. We refer to this heat treatment method as "iron-powder pack (IPP) treatment", and its ability to reduce the anodic oxide film was examined inclusive of a diffusion phenomenon of carbon and nitrogen into the film. From X-ray diffraction results, the film consisting of rutile and anosovite was gradually converted to titanium nitride with increasing heating temperature. The diffusion of carbon was also confirmed in the film after the IPP treatment. However, such a remarkable change was not achieved by heating without the powder mixture. This indicates that the powder mixture has an important role in reducing and carbonitriding the anodic oxide film. A porous structure in the film formed by anodic oxidation was retained regardless of the heating temperature in the IPP treatment. On the other hand, the peeling of the film from the titanium plate occurred through the IPP treatment at 1373 K. This would be caused by the accumulation of carbon monoxide gas, which was generated by the reduction of the oxide film.
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