Photo Electrochemical Response of Passive Films Formed on Pure Cr and Fe-Cr Alloys in Sulphuric Acid Solution

摘要

Photo electrochemical current response of passive film was investigated for pure Cr and Fe-xCr (x =8, 14, 18) alloys polarised potentiostatically in 0.1 kmolm~(-3) H_2SO_4 solution. Photo electrochemical action spectrum could be separated into two or three constituents. These components were considered to be derived from Cr_2O_3 (E_g~(opt)～3.6 eV) and Cr(OH)_3 (E_g~(opt)～2.5 eV), and possibly CrO OH. The optical band gap, E_g~(opt), of each component was almost constant for various applied potentials, polarisation periods, and substrate materials. Flat band potential E_(fb) at which the polarity of photo current changes from negative to positive with increasing potential was determined for each phase. E_(fb) for Cr(OH)_3 on Cr and Fe-Cr alloys was about 250 mV_(Ag/AgCl). E_(fb) for Cr_2O_3 was about 700 mV for Cr and about 500 mV for Fe-Cr alloys. E_(fb) of Cr_2O_3 for Fe-Cr alloys slightly shifted in noble direction with increasing Cr content for Fe-Cr alloys. This means a film shows both p and n type conduction properties simultaneously for some potential region. Passive films of these metal and alloys consist of at least two layers, that is, on the top there is Cr(OH)_3 layer, and underneath Cr_2O_3 for pure Cr, and mixture of Cr_2O_3 and Fe_2O_3 for Fe-Cr alloys. Properties of Cr(OH)_3 is almost constant with increasing polarization time and potential. The quantum efficiency of Cr_2O_3 of Fe-Cr alloys increased with increasing Cr content of the substrate alloy. Therefore, the photo response of Cr_2O_3 seems to correspond to Cr content and state in the oxide layer.