Photocurrent Response Analysis Of Passive Films With Duplex Structures

Stainless steels are widely used among the people’s production and life, thecorrosion occurrences are closely related with the structure and composition of thepassive films on their surface. In this paper, the passive films of two austenitic stainlesssteels304L and316L formed in450℃high temperature air and0.5mol/L H2SO4wereinvestigated by a series of electrochemical tests. Meanwhile, the composition andsemiconductor properties of the duplex structure passive films were analyzed. In thepolarization curves, we studied the corrosion potentials, the corrosion currents and thepassivation regions of each sample. And according to the results of Mott-Schottky tests,we got the flat band potential and the variation of the passive films semiconductor typewith applied voltage. Moreover, we calculated the carrier concentrations of passive filmsby the Mott-Schottky equation. Our analysis of the photocurrent responses included twokinds-with and without using the lock-in amplifier. In the photoelectrochemical test withusing the lock-in amplifier, we found that, for each sample, the photocurrent reduced asthe applied bias voltage decreased. And the oxide films froming in the450℃hightemperature air appeared a peak of light, the oxide films froming in0.5mol/L H2SO4,however, appeared two peaks of light. In the (Iphhυ/I0)1/2-photon energy spectrum, wefound three band gap: Eg1(1.7-2.0eV), Eg2(2.7-2.9eV) and Eg3(4.2eV), which wereα-Fe2O3, Cr2O3and FexNi1-xCr2O4respectively. All of these demonstrated that the passivefilms are formed as dual layers, α-Fe2O3and FexNi1-xCr2O4outer layer, and Cr2O3innerlayer. On the other hand, the photoelectrochemical test without using the lock-inamplifier obtained transient photocurrent, steady-state photocurrent and the spike ofphotocurrent. The transient (Iphhυ/I0)1/2-photon energy spectrum and the spike ofphotocurrent-photon energy spectrum showed two band gap-Eg1(2.0eV) and Eg2(2.9-3.0eV). While steady state (Iphhυ/I0)1/2-photon energy spectrum showed band gapswhich were Eg1(1.9-2.0eV) and Eg2(3.6eV) respectively.