Electrode Impedance Diagrams of SUS316L Stainless Steel at Transpassive Potentials in H<sub>3</sub>PO<sub>4</sub>-H<sub>2</sub>SO<sub>4</sub> Electro-polishing Solution

Electrochemical impedance diagram measurements were taken at transpassive potentials on SUS316L stainless steel in a 63.8%H3PO4－24.5%H2SO4－11.7%H2O solution to elucidate the dissolution mechanism of the steel during electro-polishing in the solution. The frequency change in electrode impedance was measured at 10 μHz － 100 kHz under electro-polishing at transpassive potentials. Nyquist plots of impedance showed diagrams of five types: a capacitive hemicycle ①, which is attributable to charge transfer resistance and an electrical double layer for a transpassive dissolution reaction; a capacitive hemicycle ③, which is attributable to the formation of transpassive film; an inductive hemicycle ④, which is attributable to the change in the coverage of intermediate adsorbates from dissolution of the transpassive film; a straight line ⑤, which is attributable to infinite diffusion of dissolution products; and a curved line ⑥, which is attributable to finite diffusion of dissolution products. The consecutive appearance of ①-③-④-⑤ was observed at potentials lower than the O2 evolution potential. However, the consecutive appearance of ①-③-④-⑥ was observed at potentials higher than the O2 evolution potential. The addition of CH3SO3H into the solution increased the resistance of finite diffusion ⑥.