|Budget Amount *help
¥2,900,000 (Direct Cost : ¥2,900,000)
Fiscal Year 1998 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1997 : ¥2,200,000 (Direct Cost : ¥2,200,000)
Extraordinarily thick and porous passive film can be obtained on austenitic stainless steels, Fe-18Cr alloy, and Ni-18Cr alloys by square wave potential pulse polarisation in hot sulphuric acid.The material was polarised in 5 kmol / m^3 H_2S0_4 solution at 50 - 80 ﾟC with applied potential modulated as square wave.The transmission electron microscopy revealed that the film consists of nano-crystalline oval-shape grains of around 1-4 nm in its size.The film has large amount of open pathways linking with each others to be penetrated easily by ions or water molecule.The film formation mechanism is classified into two processes according to the applied potentials.Electrochemical impedance and photoelectrochemical properties of the porous films were also examined.For the anodic type film, the capacitance is fairly larger than that of the substrate steel, and is nearly proportional to the total thickness of the film.On the other hand, the capacitance for the cathodic type film was almost equivalent to that of the substrate steel, and was independent of thickness, structure, and other properties.Therefore, the capacitance arised only at the substrate / solution interface, and porous structure of the thick film has no electrochemical connection with substrate.Photoelectrochemical response was measured for the anodic type film.The photo current revealed the p-type semiconductor response, and the photo current spectrum is similar to that for ordinary thin passive film formed in sulphuric acid.The photo current efficiency is fairly larger than that for thin passive film.That is, the photo current increased with increasing film thickness, revealed a maximum at the thickness of about 10 nm, then decreased.The electrochemical impedance and the phoelectrochemical response revealed that the thick porous film has various electrochemical properties depending on the structure, which is possible to be ordered by electrolysis conditions.