Effect of excess negative ions on anodic oxidation in microarc oxidation of titanium

The effects of the electrical bias conditions on oxide film during titanium MAO (microarc oxidation) were investigated. Since DC voltage showed very low stabilized currents but produced oxide films with poor quality, the magnitude of applied voltages, duty cycles, and frequencies were modulated to control the quality of the oxide film. Pulsed voltage showed increased stabilized currents but produced better oxide films. The change in the stabilized currents was attributed to the change of the resistances of the film and local electrolyte adjacent to the anode. Analysis of the film thickness and the resistances of the film calculated from the experiments led to conclusion that the resistance of the electrolyte adjacent to the anode was increased if the negative ions attracted towards the anode were not reacted as fast as the positive ions at the cathode. The increase in resistance was attributed to the excess negative ions repelling other negative ions. In order to relax the negative ions, it appeared that a pulsed voltage source was essential. However, higher off-duty cycle was not the best choice to produce good oxide films indicating that the electrical bias condition depended on the experimental configuration.