A study of the behavior of bromide in artificial pits using in situ X-ray microprobe analysis

Abstract

An in situ x-ray microprobe analysis of Type 316 stainless steel amficial pits has been canid out with bromiddchloride solution. A high intensity 8 pm diameter polychromatic x-ray beam was scanned across the steellsolution interface within the artificial pit. The resulting x-ray fluorescence was analyzed using an energy dispersive x-ray detector. In contrast to the light Cl atom, Br could be detected, making it possible to monitor the behavior of halides in the amficial pits and in the salt layer at the interface. It was found that Br was more active than c1. At high potentials, elemental Br was produced as an oxidation product, whereas without added bromide, chloride only formed a salt layer. Br also mcentrated at the salt/steel interface at potentials below where it was oxidized. INTRODUCIIONMajor changes in solution composition occurs when pits and pores are produced. It is the purpose of this paper is to describe an in situ x-ray fluorescence technique for solution analysis that has been used to measure changes taking place during pitting of stainless steels.(l,2) The technique has many similarities to scanning electron microprobe analysis, but uses x-rays rather than electrons, to excite the elements. The particular advantage of the technique was the in situ electrochemical control maintained during measurements. The technique offers a method for determining, in detail, the concentfation or depletion of elements at the metavsolution interFace with a sensitivity difficult to achieve using other techniques. Cl under wuditions associated with pitting. X-rayThee have been few direct investigations of the solution in pits and cracks mainly owing to difficulties in analyzing the small volumes of solution involved (1,2,6-9).Consequently, few models take into consideration the importance of the solution chemistry in pitting. Local changes in solution composition are the dominant factors in the initiation and propagation of pits in stainless steels (10-15). In contrast, many theories have been proposed for the breakdown of passivity but these are usually developed only up to the point where passivity is disrupted and assume that corrosion will continue because of active dissolution.(l4-17) However, in most solutions under conditions where pitting can develop, surfaces generally repassivate following mechanical removal of the passive film.This indicates that disruption of passivity is not a sufficient requirement for localized corrosion to begin. EXPERIMENTALIn situ x-ray microprobe measurements of concentration gradients of species in artificial one-dimensional pits were made using foil electrodes about 0.015 mm thick (1,2). Fig. 1 shows a schematic of the electrochemical cells used to study effects of chemistry changes in pit solutions and at pit surfaces during metal dissolution in restricted geometries. The figure represents an artificial pit produced by dissolving back the cross section of a foil mounted between plastic sheets. The dissolving surface was under potentiostatic control. The original edge of...

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

“…This is consistent with the other study which showed higher concentrations near the interface and it was reportedly caused by lower diffusion coefficient of ions near the interface. 7,8) In this study, the deviation was not so clear as reported, which can be attributed to a relatively larger beam size of 0.1 mm in height. …”

“…It was reported that 1.08 M of chromium ion was close to the dissolving interface of an artificial pit for Type304 SS in 0.5 M HCl and 0.5 M NaCl solution, higher than that observed in this study. 7,8) In the reported experiments, stainless steels were dissolving with salt films on the surface, therefore the difference of concentration of chromium ions near the dissolving interface might arise from the difference of solubility of metal chlorides and bromides. Solubility of metal chlorides were estimated as 5.02 M, 7,8) but solubility for metal bromide has not been obtained yet.…”

“…7,8) In the reported experiments, stainless steels were dissolving with salt films on the surface, therefore the difference of concentration of chromium ions near the dissolving interface might arise from the difference of solubility of metal chlorides and bromides. Solubility of metal chlorides were estimated as 5.02 M, 7,8) but solubility for metal bromide has not been obtained yet. Therefore, measurement for concentration of bromide inside the artificial crevice was carried out when the crevice reaches about 2.5 mm depth.…”

“…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 concentration and coordination states of dissolved metal ions and aggressive anions near the solution/steel interface. Concentration of ions, especially chromium and bromide ions, coordination state of these ions inside the artificial crevice for stainless steels have been investigated by the in situ XAFS and XANES measurements.…”

“…7,8) Another source quotes the solubility of FeCl 2 and FeBr 2 at 10°C as 64.4 and 109 g/cm 3 or 5.08 and 5.05 M, respectively. 12) From these experimental results, difference of chromium ion concentration at the dissolving interface between in chloride 7,8) and in bromide shown in the present study should relate to content of chromium in salt films on stainless steels; the content of chromium ion was higher in metal chloride than that in metal bromide.As shown in Figs. 2 and 3, In the present experiment, concentrations of chromium and bromide ions inside the artificial crevice showed slight deviation form the linear dependence.…”