High Temperature Sulfidation Resistance of DO3 - Ordered FeAl Tested in H2S-rich Gases
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0980-II05-09
High Temperature Sulfidation Resistance of DO3 - Ordered FeAl Tested in H2S-Rich Gases Pierre-Yvan Thery1, Anna Fraczkiewicz1, Cecile Genevois1, Xavier Longaygue2, and Francois Ropital2 1 Centre SMS UMR CNRS 5146, Ecole Nationale Superieure des Mines de St-Etienne, 158 Cours Fauriel, St-Etienne, 42100, France, Metropolitan 2 Institut Francais du Petrole, 1-4 rue de Bois-Préau, Rueil-Malmaison, 92852, France
ABSTRACT The sulfidation behaviour at 973 K in a 30 vol.% H2S-H2-H2O-N2 gas mixture of Fe-25 % at. Al, DO3-ordered alloys, binary or alloyed with niobium, was investigated. Binary alloys presented a good sulfidation resistance in comparison with 321 stainless steel, even if a large amount of iron sulfides FeS grew over the alumina layer on corroded samples. The sulfidation resistance of DO3 alloys could be significantly improved by an oxidation treatment prior to corrosion test. The Nb in iron aluminides was supposed to preferably react with sulphur and therefore, to enhance sulfidation resistance through preventing the nucleation and growth of harmful FeS. Unfortunately, the scales of Nb-allloyed FeAl contained only a small amount of Nb3S4 which did not prevent efficiently the formation of FeS. Adding Nb to Fe-25Al even resulted in a severe deterioration of the microstructure during sulfidation due to intense diffusion of S inward the specimen.
INTRODUCTION One of main characteristics of Fe-Al ordered alloys is their excellent chemical resistance, even at high temperatures, and especially in oxygen- or sulphur-rich atmospheres. The presence of an impermeable, homogeneous, alumina-rich oxide layer at the surfaces seems to be at the origin of these beneficial properties. Yet, the structure of the oxide layer depends on aluminium content in the alloy: a minimum content of about 18 % of Al [1] seems to be necessary to obtain an adequate resistance of the material, at least under oxidising conditions. Sulfidation of Fe-Al alloys was investigated in several studies [1- 4], in different conditions. Typically, some deterioration of sample surfaces was observed, and was related to the growth of FeS on the sample surface, due to an inefficient, permeable surface layer. Kai et all. [3] suggested that further improvement of FeAl behaviour can be obtained if some niobium is added to the alloy [5]. Actually, in Nb-alloyed alloys, niobium sulfides that are more stable than iron sulfides, should grow preferentially at the corroded surfaces and prevent the presence of harmful FeS. In this work, the sulfidation behaviour of D03-ordered Fe-Al alloys, containing 25 at. % Al, was studied in conditions similar to industrial ammonia cracking. Especially, the effect of an addition of niobium on alloy behaviour was analysed.
EXPERIMENTAL DETAILS Two alloys, Fe-25Al-100ppm B and Fe-25Al-2Nb-100ppm B (at. %) were prepared in laboratory by the cold crucible method from high purity source materials. The alloys were studied in their as-cast state; this procedure results in a large grain size (about 500 µm). The niobium cont
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