Synchrotron radiation study on structure of atmospheric corrosion products formed on steel surfaces

Abstract

We applied synchrotron radiation to XAFS and diffraction experiments for the structural study of the rust layer on steel and Fe-based alloys, focusing on the protective ability of the rust layer formed by atmospheric corrosion. XAFS analysis is very effective for elucidating the local structural and chemical properties of a specific element in a substance. However, we cannot identify the local structure from a spectrum observed by this technique a priori. It merely allows the evaluation of the reasonability of the structure model being considered. Therefore, it is also necessary to examine the structure model from several aspects using various other experimental techniques. For example, anomalous dispersion X-ray diffraction using the tunability of synchrotron radiation is applicable for judging whether a specific atom is positioned at a certain site of the crystal structure of a material. We would like to apply this technique to determine the crystal site of Ni in the rust layer produced in a saline environment, which is expected to be in the β-FeOOH phase, in the near future. It is difficult to detect the initial state of a corrosion product using the observation technique based on the diffraction method, because the initial corrosion product possesses low crystallinity. In addition, it is necessary to observe a complex ion as a precursor of a corrosion product in order to learn the details of the corrosion process. Thus, an in situ observation technique based on XAFS measurement is required. Energy dispersive XAFS (DXAFS) is developed in many synchrotron radiation facilities. This technique enables the measurement of time-dependent XAFS spectra with a time resolution less than about 50 ms using a white synchrotron radiation X-ray.