In Situ XAFS Observation of Chemical Species Near Solid/Liquid Interface in a Model Reaction of Pitting Process
Authors: Kimijima Ken'ichi , Niwa Yasuhiro , Kimura Masao 
Published: February 2020

The corrosion of metal and alloy under a wet environment includes a series of process: ionization of constituent elements by electron transfer reaction (oxidation) at the solid/liquid interface between a solution and a metal surface, and dissolution of metal into the solution, diffusion of the dissolved ions, and formation of hydroxides. These reactions progress depending on the environments near the interface, which change with the progress of the corrosion reaction itself. One of the most important environments is the concentration distribution of chemical species near the solid/liquid interface that is different from that of uniform corrosion. This is the case of pitting corrosion, and it is difficult to understand its mechanism by only using ex situ observation technique. Therefore, an in situ measurement technique with time and spatial resolution is required. In this study, an unique cell for an in situ electrochemical-XAFS measurement was developed to observe diffusion behavior of dissolved ions and reactions among them near the solid/liquid interface.
Figure shows the schematic diagram of an in situ electrochemical cell [1], which can detect the state of dissolved metal ions in a minute space simulating pitting corrosion. The X-ray absorption spectrum (XAS) measurement was performed using the cell. The change in concentrations and structures of dissolved ions were measured at different distances from the solid/liquid interface as a function of reaction time. A sheet of SUS304 (Cr 18 %, Ni 8 %) with a thickness of 200 μm was used as a sample, and worked as an electrode. LiBr aqueous solution (1.0 M) was used as an electrolyte. A typical measurement of corrosion was carried out by applying potential of 0.8 V vs. Ag/AgCl on the specimen. The size of an X-ray beam was set to 500 μm (horizontal) × 90 μm (vertical) (FWHM). As the corrosion progressed, the interface shifted to –Z direction (Fig.), but the shape of the interface, the metal surface shape, along the horizontal direction remained sufficiently smooth compared with the beam size. XAS for Fe, Cr, and Br K-edges were measured by the transmission geometry using QXAFS technique at BL-9A of Photon Factory (PF), High Energy Accelerator Research Organization (KEK).
The X-ray absorption spectrum changed according to the progression of the corrosion reaction. From the spectrum, the change of absorption (Δμt) at the energy of white line at different distances from the solid/liquid interface as a function of the reaction time: t was obtained.These results showed that the absorption of Fe rapidly increases at the early stage of reaction and as the reaction advances, the change in the absorption becomes small and reaches an equilibrium state. It was also shown that the absorption change also heavily depends on the distance from the interface. The same measurements were also performed for Cr and Br. These results will be discussed by paying a special attention to the relationship of the concentration change of Fe, Cr, and Br and their coordination structures.
References
[1] M. Kimura, et al., J. Synchrotron Rad., 8, 487 (2001).

Figure 1