Almost all metal materials are in the state of thermodynamic instability because of aggressive service environments, and their corrosion failures even cause many serious accidents such as fire and explosion etc. For improving the corrosion resistance of metal materials, passive metal surface tends to form a layer of stable, adherent, tenacious, and permanent solid oxide film to achieve steady state. However, the evolution mechanism of passivation on atomic scale is still unclear. Therefore, a team from national center for materials service safety of University of Science and Technology Beijing has gained insight into the structure and corrosion resistance of titanium passive film by AES, XPS, XAFS and electrochemical measurements. And their research has been published on July 20^th, 2016 in ACS Applied Materials & Interfaces.

A series of XANES and EXAFS spectra at Ti K-edge were collected from the passive films using synchrotron radiation at BSRF. Quantitative X-ray absorption near edge structure analysis of the local electronic structure of the topmost surface shows that the chemical states of oxides in passive film changes greatly with the applied potential, because the ratio of [TiO₂]/[Ti₂O₃] is consistent with that of passivation/dissolution of electrochemical activity, which is validated by XPS measurements. Theoretical calculation and analysis of extended X-ray absorption fine structure spectra indicate that the increase of average Ti？O coordination causes the electrochemical passivation, and the dissolution is due to the decrease of average Ti？Ti coordination. Besides, the local fine structure variation affects the distribution and migration of the micro defects in the passive film, which further changes the corrosion resistance of the applied metal in severe media. The results are in good agreement with the prediction of point defect model (PDM).

Figure 1. Schematic representation of local fine structure of titanium passive film