XPS Characterization of Aluminosilicate Fibers Post Interaction with Chromium Oxyhydroxide at 100–230°C

Abstract

Chromium containing materials, e.g. stainless steels, are commonly used in high temperature (>500 • C) applications such as solid oxide fuel cell (SOFC) stacks, combustion exhaust systems, and in various chemical process equipment. At these temperatures and in oxidizing atmospheres, chromium oxide (chromia) surface layers form and grow, effectively protecting the underlying alloy. Also known to form under these conditions, however, are volatile chromium species such as CrO 2 (OH) 2 and CrO 3 . Formation of these volatile species may have detrimental effects not only on the source material, but also on surrounding materials in the system which may interact with these species. To better understand how volatile chromium species interact with materials, volatile chromium species were generated from ferritic stainless steel (FSS) T409 at 700 • C and directed into aluminosilicate fibers at 100-230 • C for 150 hours. Post exposure fibers were noted to possess yellow, brown, and/or green stained regions which were isolated and characterized using X-ray photoelectron spectroscopy (XPS). Examination of Cr 2p 3/2 peaks revealed varied Cr(VI) content and Cr(III) multiplet-split components for different discolored regions. Possible explanations for differences in chromium content by discoloration color are discussed. The volatilization of chromium species from chromium containing materials is a well-documented phenomenon which holds consequences for SOFCs, human health, and the environment. Using chromia as an example for a source, in the presence of oxygen and absence of water vapor, the dominant volatilization pathway proceeds according to Reaction 1.If, however, water vapor is present in addition to oxygen, then the dominant volatilization pathway proceeds according to Reaction 2. 1-7The generation of these gas/vapor species is well understood, but the same cannot be said of how they interact with (e.g. condense onto) other materials. This is of great importance for human health and the environment considering that chromium species may take toxic forms (hexavalent) or non-toxic forms (trivalent). This relative dearth of understanding also holds negative implications for SOFCs, and so many investigative efforts have been undertaken to understand how CrO 2 (OH) 2 interacts with ceramic components in SOFCs. Electrochemical reduction of CrO 2 (OH) 2 has often been observed to occur at the triple phase boundary (TPB), where cathode, electrolyte, and gas phase meet. [8][9][10] This electrochemical reaction is not limited to the TPB, however, and can occur away from the TPB given: the formation of a continuous chromia layer for hole transport, a mixed conducting electrolyte, or a mixed conducting cathode. 8 While there is evidence for preferential electrochemical reduction of CrO 2 (OH) 2 , 11,12 open circuit chemical reactions have also been observed with cathodes containing Mn or Sr. 13 It should be noted that Cr transport to lanthanum strontium manganite (LSM) was observed via solid state diffusion, whereas vapor deposition o...

Investigation of surface interactions between volatile chromium species and ceramics

“…A FWHM of 2 to 2.5 eV, and 70% Gauss (70% Gaussian, 30% Lorentzian) was used for hexavalent GL peaks based on fit with spectra. The lowest binding energy peak displayed is associated with chromia whereas the middle peaks are inferred to consist of mixed valence Cr 6+ and 3+ species . A range of FWHM values are noted to exist in literature for trivalent multiplet splitting components as well, from 0.9 up to 3 eV .…”

“…Variable discoloration, or stain colors, may be understood with reference to a proposed mechanism in a previous study . Volatile chromium species approaching a ceramic surface may physisorb to the surface, given the temperature is near/below the condensation point of the vapor species.…”

“…Interpretation of XPS Cr 2p 3/2 spectra on aluminosilicate surfaces has been discussed in detail in a previous study . A brief review of the interpretation will be given here with reference to Figure .…”

“…At the final stage, the material would appear to be green . It is important to note that there is some disagreement as to the valence states of Cr 3 O 8 and Cr 2 O 5 , but based on previous work and the available literature, the authors attribute these species to mixed valence Cr 3+ and 6+ species …”

Influence of Water Concentration on High-Temperature Reactive Condensation of Chromium Vapors Generated in 800 °C Air

“…However, there are far fewer publications that focus on the chemical and physical processes that occur during condensation reactions of Cr on other materials and surface conditions, e.g. [14][15][16] Condensed species can be initially identified by the color of stains appearing on light colored ceramic substrate materials (e.g., alumina). Green is indicative of trivalent Cr species whereas brown and yellow are indicative of hexavalent Cr species, or mixtures thereof.…”