Semiconductive behavior of passive films formed on pure Cr and Fe-Cr alloys in sulfuric acid solution

Photoelectrochemical current response and capacitance were analyzed for passive films formed on pure Cr and Fe-Cr alloys in sulfuric acid solution. The photocurrent was plotted as a photoelectrochemical action spectrum that could be separated into two components, which were mainly derived from Cr₂O₃ (E_g=3.6-3.7 eV) and Cr(OH)₃ (E_g=2.5 eV). The band gap energy, E_g, of each component was almost constant for various applied potentials and polarization periods. The photoelectrochemical response showed negative photocurrent for most potentials in the passive region, which means that the photocurrent apparently exhibited p-type semiconductor behavior. On the other hand, Mott-Schottky plots for pure Cr exhibited positive slope, which means that passive films on Cr behave as n-type semiconductors. Furthermore, the passive films on Fe-18Cr alloy exhibited both positive and negative slopes, which means that passive films on Fe-18Cr in sulfuric acid solution have both p- and n-type semiconductive properties. These apparently conflicting results can be rationally explained, if we assume that passive films on Cr and Fe-Cr alloys formed in the sulfuric acid solution are composed of an outer hydroxide layer with an n-type semiconductor property and of a p-type inner oxide layer.