The Formation of Passive Films of Carbon Steel in Borate Buffer and Their Degradation Behavior in NaCl Solution by SECM

摘要

Passive films formed on carbon steel by different passivation potentials in borate buffer solution andtheir degradation behavior were studied by different electrochemical techniques, such aselectrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) analysis and scanningelectrochemical microscopy (SECM). The film composition was investigated by X-RayPhotoelectron Spectroscopy (XPS). The results of SECM show that the electrochemical reactivity onthe surface of passive film is weakened as the film formation potential increases due to slower chargetransfer rate. The heterogeneous electron transfer rate constant k calculated from approach curvesdecreases linearly with substrate potential increasing, resulting from different electrochemicalreactivity on the surface of substrate. The electrochemical reaction resistance of the film increases withfilm formation potential increasing, while the point defect density decreases, indicating the film3+ 2+ becomes less conductive. The ratio Fe /Fe and the percentage of oxyhydroxide species increasesimultaneously as the film formation potential shifts from -0.1 V to 0.7 V. The SECM and EIS resultsshow that the film formed at 0.3 V has the strongest corrosion resistance in 0.1 M NaCl solutionfollowed by that of 0.7 V which is more defective and porous. The degradation of passive film in NaClsolution is different from the evolution of films in non-aggressive electrolyte.