In situ analysis of pitting corrosion in artificial crevice of stainless steel by X-ray absorption fine structure

A new technique by XAFS (X-ray Absorption Fine Structures) and XANES (X-ray Absorption Near Edge Structures) has been developed for in situ observation of metal corrosion. XAFS and XANES spectra were obtained with a special electrochemical cell to elucidate pitting for stainless steel. A sheet of stainless steel was attached below a reservoir of a specific aqueous environment with thin films. The solution in the reservoir attached to the metal thorough the film, and the solution inside the film corresponds to an artificial pit. X-ray beams pass through the film containing the solution, and XAFS and XANES measurements were performed in a transparent geometry with keeping the specimen at a pitting potential. Measurements were performed for Cr-K, Mo-K, Cl-K and Br-K edges, and changes of concentrations and coordination states of ions were successfully obtained as a function of the distance (d(z)) from the metal/solution interface. Concentrations of chromium and bromide ions inside the artificial crevice of Fe-18Cr-12Ni-2Mo (mass%) alloy shows a linear dependence on the distance d(z). Structures of bromide ion were changed with positions; the distance between bromide ion and the nearest-neighboring ion at a position near the interface was shorter than that at a position far from the interface. This indicates the formation of hydrobromo-complex near the metal/solution interface. The state of molybdenum inside the artificial crevice of Fe-18Cr-20Ni-5Mo (mass%) alloy was investigated using two solutions: LiCl and LiBr. The formation of the [MoO4(H2O)(2)](2-) octahedra was observed in both solutions, but there was observed a significant difference in networking of the octahedral. In LiCl solutions it was similar to that of molybdate ions (MoO42-), but in LiBr it was rather different form that of molybdate ions. This shows that the favorable effects of molybdenum can be attributed to the formation of MoO42- 4 network near the interface.