Study of Rust Structure Change in Wet and Dry Conditions Using XAFS Measurement

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Steel products used in social infrastructures such as bridges and tunnels are required to have a long corrosion life for a safe, secure and sustainable society. In general, it is well known that the corrosion behavior of steel varies depending on the corrosive environment, added elements, metallurgy, and rust. In particular, the rust affects corrosion electrochemically, so it is very important to clarify the type and distribution of rusts. Various types of measurement and analysis have been applied to understand the microstructure and distribution of rust layers. In particular, imaging X-ray absorption fine structure (XAFS) is a very effective method for analysis of the generation and growth behavior of rusts that include many amorphous components, because it is possible to measure the valence and bonding distance of iron rusts in two dimensions. Although direct observation of the corrosion progression process by dry-wet cycles has been studied in the past, all of them are limited to structural analysis of only the steel surface under a thin water film in order to suppress the effect of X-ray absorption by the solution. In this study, we attempted in-situ XAFS observation of rust growth and structural change in the depth direction in a dry-wet condition using rusted steel under a controlled water film environment. Rusted steel specimens were corroded by atmospheric exposure, the rust layer was protected by resin coating, and the specimens were thinned to a thickness of about 0.01 mm using a microtome. Imaging XAFS measurements were performed at SPring-8 BL16B2 (SUNBEAM BM) at the same location in the order of pre-test, immersion in salt water, and drying. Noise2Noise, which learns and processes images based on acquired data alone, was used to remove noise from the images, enabling high-resolution analysis. XAFS spectrum were extracted from the denoised data, background removal and normalization were performed. The normalized XAFS spectrum fit the curve obtained by the weighted addition of the spectra for magnetite, goethite and lepidocrocite, and the rust existence state was analyzed from each component and its score value. From the measurement images, the distribution of each rust is clearly observed, and in particular, the amount of Fe3O4 decreases from the steel surface to the surface layer of the rust. In the wet condition, the amount of Fe3O4 increases significantly compared to that in air conditions, and goethite decreases in the vicinity of the steel, while lepidocrocite decreases throughout the rust layer. The reason is assumed to be that oxygen and magnetite react to form goethite and lepidocrocite in the surface layer of the rust, consuming oxygen, and that oxygen does not reach the inner rust layer, causing magnetite to increase as it moves toward the interior.