Investigation of surface interactions between volatile chromium species and ceramics

Abstract

Stainless steels are often used in high temperature (≥500°C) applications such as solid oxide fuel cells (SOFCs), combustion engine exhaust systems, and in power/chemical plant process equipment. At high temperature and in oxidizing conditions, chromium containing oxides, such as chromia, may form protective surface layers on the underlying alloy. Reactive evaporation of chromium, however, may occur from the protective surface layers given these conditions, resulting in the formation of volatile chromium species such as CrO 2 (OH) 2 . These volatile chromium species may then interact with surrounding materials, potentially resulting in hazardous compound formation, or having detrimental effects on system performance, as in the case of SOFCs. To better understand the interaction of volatile chromium condensation/ deposition on material substrates, volatile chromium species were generated from chromia powder at 500°C to 900°C and flowed past coupons of alumina and mica and quartz wool at temperatures ranging from 150°C to 900°C for 24-and 100-hour exposures. The ceramic surfaces were characterized as a function of these exposures using X-ray photoelectron spectroscopy (XPS). Analysis of Cr 2p 3/2 peak positions revealed the influence of temperature, material, and exposure time on the oxidation states of surface chromium compounds and extent of chromium deposition.Potential mechanisms are proposed to help explain the observed trends.

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

“…Chemical composition is more similar amongst the quartz and manufacturer insulations, which is mostly silica. These observations are reinforced by the results from the studies of Tatar et al [14][15][16] that found alumina to collect more Cr than silica.…”

“…3-5 as well as the measured total Cr/Cr(VI) collected on the fibers as presented in Fig. 6 can be explained by a series of events proposed by Tatar et al [14][15][16] The aluminosilicate fibers are exposed to volatile Cr species in an environment of air or air and water vapor at temperatures between 100 °C-400 °C. This temperature range encompasses the boiling point of chromic acid (∼200 °C), and the vapors may begin to become physiosorbed to the fiber surfaces at this point.…”

“…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.…”

The Influence of Water Concentration on High-Temperature Reactive Evaporation and Condensation of Cr in 800°C Air

“…These volatile species proceed downstream and deposit on the cathode through chemical and electrochemical deposition and impede SOFC operation, an occurrence known as "chromium poisoning". 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., (12)(13)(14). Condensed species can be initially identified by color of stains appearing on light colored ceramic substrate materials (e.g., alumina).…”

Reactive Condensation of Cr Vapor on Aluminosilicates Containing Alkaline Oxides

“…While the reactive evaporation of Cr well-documented, there are relatively few publications focusing on the chemical and physical processes that occur during its downstream condensation, e.g., (10)(11)(12). Reactive condensation of Cr onto various surfaces under different conditions such as refractory ceramic materials like alumina and silica, and on SOFC components, such as those made from manganese or strontium, is a well-known occurrence.…”