Photo-electrochemical study of the passive state

The electrochemical behavior of carbon steel in borate buffer solution under UV illumination has been studied. Strong coupling is observed between the electronic and crystallographic defect structures as a function of applied voltage, light intensity, and photon energy. The photo-electrochemical impedance of carbon steel in pH = 8.5 boric-borax buffer was recorded as function of film formation potential and light intensity. Optimization of the Point Defect Model (PDM) on the electrochemical impedance data for carbon steel in pH = 8.5 boric-borax buffer under 385 nm UV illumination yields numerical values for the various model parameters. The data obtained in this study demonstrate that the passive state on carbon steel under UV excitation is well accounted for by the PDM. The results indicate that super band gap UV light can suppress the electric field in the barrier layer of the passive film, resulting in photo-stimulated anodic oxide film growth. Moreover, we have also detected photo-dissolution of the barrier layer due to the generation of "hot" holes, which significantly complicates the interpretation of the photo-electrochemical data. Finally, the results reveal that the electric field strength in the barrier layer is independent of the applied voltage and hence thickness of the barrier layer, thereby confirming one of the original assumptions in the development of the PDM.