Corrosion of 310 stainless steel in H 2 - H 2 O- H 2 S gas mixtures: Studies at constant temperature and fixed oxygen po
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INTRODUCTION
THEinternal components of coal gasifiers are exposed to hostile' environments and elevated temperatures. The complex gaseous environments comprise molecular species that include oxygen, hydrogen, carbon, nitrogen, and sulfur. Various chemical species of the gas phase react collectively with the construction materials and degrade the structural integrity of the coal conversion systems. Of all the gaseous species present, hydrogen sulfide is probably the most corrosive component. A systematic study on the sulfidation of 310 stainless steel,l'2'3 one of the candidate materials widely used in the pilot plant operations, indicated that the high reaction rates are a result of the sulfide scales being multilayered and nonprotective. Commercial steels are developed primarily for operation at high oxygen potentials. Therefore, characterization of their corrosion behavior in multicomponent gaseous environments at elevated temperatures is lacking. Such systematic studies are necessary for optimum selection of corrosion resistant alloys for coal gasification operations. As a part of a continuing study of corrosion problems associated with the internal components of coal gasifiers, the mechanism of corrosion of 310 stainless steel in gas mixtures of hydrogen, water vapor, and hydrogen sulfide is being investigated. This paper deals with the specific influence of sulfur potential (partial pressure) at a constant temperature of 1150 K and a fixed oxygen potential of approximately 6 x 10 -13 Nm -2 on the kinetics of scale formation and on the composition and morphology of D. BHOGESWARA RAO, formerly a Senior Scientist with Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, is now Project Leader, Research and Development Section, Hewlett-Packard Company, Cupertino Integrated Circuits Operation, 10900 Wolfe Road, Cupertino, CA 95014. K. T. JACOB is Associate Professor, Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S 1A4. HOWARD G. NELSON is Chief, Materials Science and Applications Office, NASA-Ames Research Center, Moffett Field, CA 94035. Manuscript submitted April 8, 1981. METALLURGICALTRANSACTIONS A
scale layers. Thermochemical diagrams have been constructed, and the experimental results are compared with those predicted. Based on the experimental results and the thermochemical diagrams, a tentative mechanism for the corrosion of 310 stainless steel has been proposed.
II.
EXPERIMENTAL
A. Materials
Test coupons 2.5 x 1.25 x 0.33 cm were prepared from a commercially produced cold-rolled, SAE 310 stainless steel. The composition of this alloy and the sample preparation procedures were given in an earlier publication. 1 The test coupons were polished through 5-/zm-diamond abrasive and degreased thoroughly by washing in acetone. The surface area of each coupon was calculated to an accuracy of 1 • 10-5 cm 2 by using dimensions obtained with a micrometer. Commercially available high-purity (Matheson) hydrogen, hydrogen sulfide, and a
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