Micro-beam XRF and Fe–K Edge XAFS on the Cross Section of the Rust Layer Formed on a Weathering Steel

Abstract

Fig. 3. Optical microscopic image with polarized light (a), XRFintensity ratio image Cr-Ka/Fe-Ka measured by microbeam XRF with an energy of 7.8 keV (b), and Cr-Ka Xray intensity mapping measured by EPMA (c) for the cross section of the weathering steel exposed for 38 years. The images in (a), (b), and (c) were obtained from the identical area. EPMA mapping area is denoted by a dashed rectangle in (b). Points and line where XRF and XAFS spectra were recorded are denoted in figure (b).the difference of Cr concentration between the two points. The net intensity ratio of the Cr-Ka peak to the Fe-Ka peak was estimated to about 5 times (point 1) and about 0.4 times (point 2) of that for the steel region. XAFS spectra obtained for the points 1 and 2 are shown in Fig. 4(b) with spectra for the rust powder scraped off from the surface of the specimen and for commercial materials; a-FeOOH and g-FeOOH. X-Ray absorption nearedge structure (XANES) spectra for the rust specimens in Fig. 4(b) are more similar to that for a-FeOOH rather than that for g-FeOOH. This result shows that the chemical states of the main corrosion products are present at the analytical point close to that of a-FeOOH. By comparing the spectra at the photon energies from 7150 to 7350 eV, the spectral shape for point 2 is close to that for the rust powder. The spectral shape for point 1 is slightly different from those for point 2 and the rust powder. This suggests that there are some modifications in the a-FeOOH structure at point 1 where Cr is enriched.Figure 4(c) shows the Fourie transform (FT) derived from k 3 -weighted EXAFS oscillations in the k range of 3-11 (1/Å) for points 1 and 2. The data for the rust powder specimen and the commercial a-FeOOH and g-FeOOH are also shown. The FT corresponds to the radial distribution function (RDF) obtained by XAFS in which the horizontal axis is not corrected by the phase shift. Peaks at about 1.5 Å are attributed to the nearest neighboring oxygen atoms around Fe atoms. The second and third peaks at about 2.7 Å and 3.2 Å have been attributed to the mixture of the first neighboring Fe atom groups and the second neighboring oxygen atom groups.6) The peak shapes of the second and third peaks for the points 1 and 2 are similar to each other and are also similar to those for a-FeOOH. However the relative heights of the second and third peaks normalized to the first peak are significantly different; the peak heights for point 1 with enriched Cr are lower than that for point 2 with depleted Cr. The crystal structure of a-FeOOH consists of ordered edge-sharing and point-sharing of Fe-(O,OH) 6 octahedrons. We denote this characteristic as the short-range ordering of a-FeOOH in this paper. The lower peak height for the first neighboring Fe atoms and second neighboring oxygen atoms (but the relative intensity between them is not changed) indicates that the short-range ordering of the Fe-(O,OH) 6 network of a-FeOOH is more distorted in the Crenriched rust than in the Cr-depleted rust.In Fig. 4(c), the peak heights of the ...

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