Abstract
The electronic properties of the passive films formed on iron in pH6.5 borate solution were investigated by measuring the transient photocurrent. An anodic spike-like photocurrent followed by a small steady one was observed after switching on the illumination onto the passive electrode. Another spike-like current was also observed in the reverse direction after switching off the illumination. The charges and time constants of decay of the spike-like current and the steady state photocurrent were measured as a function of film formation potential and measurement potential. A simulation was made to explain the transient behaviour of the photocurrent by assuming an equivalent circuit with the generation of a photo-induced free carrier pair in the n-type passive film. From the simulation the transient photocurrent behaviour can be explained from the relations among the thickness of the passive film, the recombination rate of the photo-induced free carrier in the film, the density of surface states, and the redox system in the solution which can energetically exchange the hole or electron with the electrode.
