Analysis Of Photocurrent Transient Response Of Oxide Film On Stainless Steel

Photocurrent response (PCR) of the surface oxide film on stainless steel (SS) presents typical transient characteristics. There is still no recommanded theory to give an effective interpretation and quantitative analysis for PCR. Research on PCR could be beneficial to studing in-depth on the composition of oxide film, the electronic structure and the genneration, recombination and other motions of photo-induced carriers, to reveal the mechanism of PCR, to establish the systematical method to analysis PCR qualitatively and quantitatively, to complete the theory about photo-electrochemistry of semiconductor film and metal corrosion.The dissertation was focused on:First, a theoretical model was developed to analyze PCR behaviors. The model derived on the basis of continuity equation for carriers can give a reasonable explaination for PCR patterns. The effect factors of PCR (including bias voltage and the optical absorption coefficient) were analyzed respectively, and their calculation formulas were given. The PCR pattern is mainly dependent on the electric field of oxide film/electrolyte interface. PCR patterns impacted by applied bias and incident light were related by their effects on the electric field.Second, potentiodynamic scanning method was applied to study the corrosion potential and average corrosion current of the oxide film on304L SS in high temperature and high pressure water, then the passive region was obtained. The Mott-Schottky analysis was applied to study the semiconduct properties of oxide film, including the flat-band potential, semiconductor type and carrier concentration. Based on the point defect model, the relationship between the carrier concentration and corrosion was demonstrated.Moreover, the semiconduct properties of oxide film of304L SS in high temperature and high pressure water was investigated in borate buffer solutions, respectively by photocurrent steady response (PCSR) and photocurrent transient response (PCTR). Results of PCSR showed that the oxide film exhibited a p-type semiconductor. And the oxide film was composed of two oxide phase, which band gaps were1.9eV (the iron oxide) and3.7eV (oxides of chromium) respectively. In comparation with PCSR, PCTR presented the same compositon of oxide film, but the oxide film presented positive photocurrent spikes under applied positive bias.Furthermore, according to the mathematical model of PCR, the electric field of the film/solution interface changes from negative to positive, with the applied voltage increasing, and PCR mode changes gradually from mode D to mode F. Then photoelectric chemical parameters of the film/solution interface were calculated, including the electric field, the outer width of the interface and the carrier concentration. The values of electric field calculated by fitting the measured photocurrent data showed a good agreement with the prediction by the model mentioned above. Meanwhile, PCR was simulated on dimension of time, and the simulation was good fit to the experimental results.Third, the semiconductor characteristic of oxide film on304L SS in different kind of water (no zinc and contained zinc) was tested by PCR measurement. Comparing the results of PCSR and PCTR emplyed the sample which no zinc contained, the oxide fim exhibited different semiconductor type which changed from p-type to n-type, with the applied bias moving from positive to negative. The band gaps were obtained as2.2eV and3.5eV, corresponding to the iron oxide and the chromium oxide respectively. The samples which contained zinc showed the nature of the n-type semiconductor. The band gaps were received as2.2eV,2.9eV and3.5eV, which were related to the iron oxides and the chromium oxide respectively.Based on the mathematical model of PCR, it was found that with the internal electric field decreasing, PCR of samples which no zinc contained appeared the trend of mode C-F-D. Meanwhile, considering the co-exit of the electron current and the hole current, the PCR may exhibit as mode G. And PCR of samples which contained zinc acted as mode C because the electric filed was positive. Then, photoelectric chemical parameters of both samples were calculated respectively, including the electric field, the outer width of the interface and the carrier concentration.In summary, PCR tests of the oxide film on SS achieved the expective results, meanwhile it was confirmed the feasibility to employ the model based on continuity equation of the carrier concentration.to analysis numerically PCR. Comparing the PCR experimental results with the simulated value, it was concluded that the value calculated by math model self-established had a good fitting coefficient.