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

Abstract

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 ∼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 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 ofthese metal and alloys consist ofat 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