In the development research of stainless steel, the stabilization mechanism of passive film on the stainless steel has not yet been elucidated. The reason for this is that the structural analysis of the passive films is difficult because its thickness is too thin (a few nm) for a standard X-ray diffraction study. In order to develop the technique of the structural analysis of the passive film, we applied the grazing incidence X-ray scattering (GIXS) and total-reflection XAFS to the passive films on SUS304L and SUS316L using SPring-8. The results of GIXS suggested that their atomic structures have the characteristic of spinel-type structure. From the results of total-reflection XAFS, it was indicated that the component of Cr was enriched in the passive films. This paper will review the history of the research of the passive film using synchrotron radiation at first, and report the results of our investigation of the structural analysis method of the passive films on the practical stainless steel mentioned above.
X-ray photoelectron spectroscopy and X-ray absorption fine structure using synchrotron radiation are powerful tools for chemical state analysis for surfaces of steel products. These methods provide us important information, which is difficult to be obtained using ordinary surface analysis techniques, for material designs. In this article, these methods are introduced with some applications for practical materials.
Corrosion of SUS304, SUS316L and SUS430 stainless steel has been investigated for 200 h in concentrated aqueous solution of mixed LiBr-LiNO3-KCl-LiOH salts at elevated temperature as a function of temperature. The structure and composition of the surface films were analyzed by XRD and XPS. These stainless steels were passivated in the solution up to 463 K, and localized corrosion as pits was not observed. At 383 K, oxide films (without very thin passive film) of these stainless steels were hardly formed. However, the amount of oxide film (WF) increased with temperature. By addition of Li2MoO4, WF of SUS304 and SUS430 stainless steels increased at 463 K , while the amount of metals dissolved into the solution (WD) did not depend on the addition of Li2MoO4. WF and WD of SUS316L did not depend on the addition of Li2MoO4. Outermost parts of oxide film on the stainless steels were mainly composed of γ-Fe2O3. The role of surface film-γ-Fe2O3 in the corrosion behavior was discussed in terms of repair function of the imperfections in the film.
This research proposes an effective inverse analysis technique for solving an inverse problem to detect corrosion in reinforced concrete non-destructively from potentials on a concrete surface. The technique can detect the number, the shape and the location of corrosion without particular assumption with GA (genetic algorithm), and increases accuracy of solution gradually if necessary. The following three strategies achieve the proposed technique: (1) Control of analysis precision, i.e. coarseness of BEM (boundary element method) meshes, depending on required precision of solution. (2) Use of the net elements for BEM for decreasing the number of elements for steels. (3) Analysis with higher precision only within detected corrosion area on lower precise analysis. We performed a numerical simulation to solve an inverse problem for detecting steel corrosion in concrete with the proposed technique. The result showed that the proposed method could detect corrosion with high accuracy. The calculation time of the proposed method was about 1/100 in comparison with that of the previously proposed method.