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.

For a hybrid chromium plating film produced by impregnation and polycondensation of silicon compounds, formation and corrosion resistance of cracks expanded by anodic etching of chromium plating were investigated. During anodic electrolytic etching of chromium plating film, crack widths and numbers increased along with the electrolysis time. The plateau area decreased after electrolysis duration of 600 s. The entire surface became porous after electrolysis duration of more than 900 s. The amount of silicon compound impregnated into the expanded cracks increased from a crack width of 0.3 μm. The hybrid chromium plating film was a gradient film in which the silicon compound decreased from the surface side to the substrate. Its maximum thickness was inferred as ca. 30 μm. The order of corrosion resistance in the CASS test results was as follows: hybrid chromium plating >> nickel undercoat plating / chromium plating >> nickel plating > chromium plating. The hybrid chromium plating which had been optimized by controlling the expanded crack showed no sign of pitting, corrosion or discoloration after 336 h during the CASS test.

Diamond-like carbon（DLC）coatings are widely used for cutting tools due to their superior mechanical properties, which contribute to longer tool life. In this study, we developed a pulsed plasma arc method（PPAM）that generates highly activated plasma through periodic arc discharges and synthesized DLC by varying the discharge voltage. The sp³ bonding ratio, characteristic of DLC, reached a maximum of 73%, which enhanced carbon atom diffusion and promoted a densified microstructure. As a result, the microhardness and elastic modulus increased to 68.0 GPa and 325.3 GPa, respectively. This paper introduces the novel deposition method（PPAM）and presents results on the chemical bonding states, mechanical properties, and microstructures of DLC synthesized under various discharge voltages.