Formation of aluminum oxide scales in sulfur-containing high temperature environments
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I.
INTRODUCTION
THE formation of a continuous layer of a-A1203 scale provides the protection for most high temperature alloys and coatings. In simple oxidation environments containing only one oxidant, namely oxygen, the most severe problem associated with the aluminum oxide is spalling of the scale under thermal cycling conditions. Cracking of the oxide scale under these conditions has been subject to extensive studies by many authors.l-5 In particular the role of oxygen active elements (Hf, Y, Ce) in enhancing the adhesion of the oxide scale under thermal cycling has been discussed extensively in a number of excellent review papers. 6'7'8 However, little attention has been given to the study of cracking of the oxide scale under isothermal conditions. There was no pressing need for such studies because most high temperature alloys can easily reform an oxide scale to seal cracks when exposed to simple oxidation environments. However, when the alloy is exposed to mixed oxidant environments (e.g., coal gasification atmospheres), the second oxidant (especially sulfur) can easily penetrate the scale through the cracks and form sulfides at the scale/alloy interface, resulting in accelerated corrosion. 9't° A previous study n has shown that the aluminum oxide scale cracks continuously during oxidation at 900 °C due to the growth stress of the oxide scale. Therefore, it was expected that the presence of a small amount of sulfur in the environment would have a detrimental effect on the oxidation resistance of alloys at high temperatures. The main objective of this investigation was the study of the effect that sulfur exerts on the formation of an a-A1203 scale at high temperature.
T.T. HUANG, Assistant Professor, and Y. L. CHANG, Graduate Student, with the Department of Chemical Engineering and Materials Science, and E. PFENDER, Professor, Mechanical Engineering, are with The University of Minnesota, Minneapolis, MN 55455. R. RICHTER, formerly a Graduate Student of Chemical Engineering and Materials Science Department of The University of Minnesota, is now a Processing Engineer with Texas Instruments Inc., Dallas, TX. Manuscript submitted September 17, 1984. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
A model alloy, Fe-18 wt pct Cr-6 wt pct A1, was prepared by arc melting 99.99 pct pure metals in argon atmosphere. Each specimen was polished and annealed under vacuum at 1000 °C for 5 hours. The composition of the gas mixtures in this study was controlled by calibrated precision rotameters. In order to ensure that the gas mixture reached equilibrium, the mixture passed through a platinum-coated ceramic catalyst. The compositions of various gas mixtures are listed in Table I. Figure 1 presents phase stability diagrams of AI-S-O, Cr-S-O, and Fe-S-O systems at 900 °C. n All gas mixtures used in oxidation tests fall into the AleO3 stable region as shown in Figure 1. All oxidation tests were conducted at 900 °C. The surface analyses of the oxidized specimens were performed with a scanning electron microscope (SEM) and a sca
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