Photo electrochemical response of passive films formed on pure Cr and Fe-Cr alloys in sulphuric acid solution

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 kmol.m(-3) H(2)SO(4) solution. Photo electrochemical action spectrum could be separated into two or three constituents. These components were considered to be derived from Cr(2)O(3) (E(g)(opt)similar to 3.6 eV) and Cr(OH)(3) (E(g)(opt)similar to 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(2)O(3) was about 700 mV for Cr and about 500 mV for Fe-Cr alloys. E(fb) of Cr(2)O(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(2)O(3) for pure Cr, and mixture of Cr(2)O(3) and Fe(2)O(3) for Fe-Cr alloys. Properties of Cr(OH)(3) is almost constant with increasing polarization time and potential. The quantum efficiency of Cr(2)O(3) of Fe-Cr alloys increased with increasing Cr content of the substrate alloy. Therefore, the photo response of Cr(2)O(3) seems to correspond to Cr content and state in the oxide layer.