In Situ Analysis of Pitting Corrosion in Artificial Crevice of Stainless Steel by X-ray Absorption Fine Structure.

Abstract

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 [MoO 4 (H 2 O) 2 ] 2Ϫ 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 (MoO 4 2Ϫ ), 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 MoO 4 2Ϫ network near the interface.KEY WORDS: crevice corrosion; X-ray absorption fine structure; X-ray absorption near edge structures; stainless steel; bromine; chlorine; chromium; molybdenum; ion coordination.are easily expected to be susceptible to experimental conditions, and in situ observation would be essential for understanding the effects of molybdenum.However, because of experimental difficulties, there have been few reports on direct observation of structures of ions inside pits. Recently X-ray absorption and fluorescence measurement has become one of powerful methods for the in situ study of chemical and physical changes during electrochemical and corrosion processes. It was observed dissolved metal ion concentration inside artificial pit and content of salt films for Type304 SS by using in situ X-ray microprobe technique. 7,8) In this study, we have tried to develop an in situ observation technique by XAFS (X-ray Absorption Fine Structures) and XANES (X-ray Absorption Near Edge Structures) measurements conducted on a special electrochemical cell to elucidate pitting in order to measure a change of concen...

Effects of S Content and Surface Finish on Pitting Corrosion of Austenitic Stainless Steels Containing Mo in Chloride and Bromide Solutions

“…Whereas the structures near the interface showed a siginificanto difference near the corroding interface; in LiCl, Mo-O bonding similar to MoO 6 octahedra in MoO 3 oxide or MoO 4 2Ϫ ions was dominant, but in LiBr, the Mo-O-Br complex formed. 18,19) Sugimoto et al analyzed state of Mo on stainless steels in hydrochloric solutions and Mo was considered to exist in passive film improving corrosion resistance.…”

A Corrosion Resistant Sintered Stainless Steel: Type 304L Containing Mo-Rich Phases

“…It is well-known that Mo improves the pitting corrosion resistance of stainless steels. 13) This has been mainly attributed to the existence of Mo-species in passive films, 2,49) and to the inhibition effect of the dissolved Mospecies. 1014) For instance, Montemor et al proposed that Mo exists as Mo(IV), Mo(VI), and/or metallic Mo in the oxide films on stainless steels.…”

“…4) Kimura et al demonstrated that the formation of MoO 4 2¹ networks near the dissolving metal interface enhances the pitting corrosion resistance of stainless steel. 13) There is no unified theory about the role of Mo.…”

The Interaction Between Alloyed Molybdenum and Dissolved Bromide in the Pitting Corrosion of Stainless Steels

“…In particular, alloyed molybdenum provides much less extra resistance to pitting corrosion in bromide solutions than in chloride solutions. 1 The reason for this behavior was sought by Kimura et al 2 using X-ray absorption spectroscopy performed in situ in 1 M solutions of LiCl and LiBr on artificial "crevice" electrodes made from metal foil mounted in resin ͑since these had one-dimensional transport they were really more like artificial pits͒. The potential used was fairly oxidizing, 0.8 V vs Ag/AgCl, but not unreasonably so.…”