Behavior of Fe-Ni-Cr Alloys in a Complex Multioxidant Environment under Conditions of Dynamic Straining
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I.
INTRODUCTION
THEincreased demand for alternate energy sources has led to the use of high-temperature materials in increasingly aggressive environments. The production of high or medium btu gas from coal provides a perfect example of the severe environmental conditions which will be encountered. The proposed gasifiers operate at 700 to 1000 ~ with complex, aggressive environments containing oxygen, sulfur, carbon, and hydrogen as the reactive species. For economic reasons, creep or stress-rupture (S-R) properties and corrosion resistance are normally determined independently with the creep properties measured in air or an inert environment and the corrosion resistance determined in the absence of stress. This procedure neglects the interactions which may occur between corrosion and creep, i.e., the potential acceleration of corrosion brought about by dynamic straining and the potential increase or decrease in creep resistance brought about by reactions with the environment. In this paper, the necessary thermodynamic conditions for the various oxidation reactions and the possible effects of these reactions on creep-rupture will be reviewed. The possible interactions between corrosion and creep will also be discussed. The results of S-R tests of three Fe-Ni-Cr alloys in a gaseous environment containing oxygen, sulfur, and carbon will then be presented and discussed.
A. Thermodynamics of Multioxidant Environments Pettit et al. ~have treated the thermodynamics of environmental attack for environments containing two oxidants. A review of their analysis will be presented here since it is essential for an understanding of the environmental interactions considered in this paper. It should be emphasized, however, that the treatment presented below is for surface film formation only; it does not deal with the possible formation of internal oxidation products. Formation of a protective surface scale can lead to reductions in the activity of one or both species across the scale. Equilibrium conditions R.A. PAGE and J.E. HACK, Senior Research Metallurgists, and R. D. BROWN, Senior Research Engineer, are with Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78284. This paper is based upon a presentation delivered at the symposium "Effects of Environment on Elevated Temperature Mechanical Properties" held at the February, 1982 meeting of TMS-AIME in Dallas, Texas, under the sponsorship of the Corrosion and Environmental Effects Joint Committee (TMS/MSD). METALLURGICAL TRANSACTIONS A
at the scale/metal interface establish the activity of one oxidant, the oxidant involved in scale formation, while the activities of any secondary oxidants are established by their activities in the gas phase and their penetration rates through the scale. Hence, it is possible that the ratio ax/ao, where ax and ao are the activities of the oxidizing species X and O, respectively, at the scale/metal interface could differ from that in the gas phase, thus, leading to the internal formation of AX under a surface scale of AO, or alte
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