The Effect of Environmental Sulfur on the Establishment and Structural Stability of Alumina Scales
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The Effect of Environmental Sulfur on the Establishment and Structural Stability of Alumina Scales Xu Liu • Brian Gleeson
Received: 25 May 2012 / Published online: 6 September 2013 Ó Springer Science+Business Media New York 2013
Abstract Single-phase c-Ni of composition (in at.%) Ni–6.3Al–5.4Cr is borderline between forming Al2O3 internally or externally. Oxidation of this alloy in air and O2 ? 1 %SO2 was carried out at 1,000 °C for 20 h. In air, the alloy oxidized in a mixed mode, with regions forming a non-protective product of internal Al2O3/ NiAl2O4 and external NiO. When oxidized in O2 ? 1 %SO2, the alloy formed a continuous Al2O3 scale. Thus, a small amount of sulfur in the atmosphere promoted the transition from internal to external Al2O3-scale formation. In a parallel study, single-phase c0 -Ni3Al of composition (in at.%) Ni–5Cr–20Al–3Pt–0.1Hf–0.05Y was oxidized at 900 °C for 20 h in air, O2 ? 0.1 %SO2 and in air with an Na2SO4 deposit. For all conditions, external alumina scales were established. Metastable –-Al2O3 formed when oxidation took place in air alone, whereas the stable a-Al2O3 O formed during oxidation in O2 ? 0.1 %SO2 and in air with an Na2SO4 deposit. Thus, sulfur from the salt deposit or gas atmosphere promoted the O –-Al2O3 ? a-Al2O3 transformation. Keywords Ni–Cr–Al alloys Sulfur Alumina scale Scale establishment Phase transformation
Introduction Sulfur is found in a number of applications in which alloys and metallic coatings are exposed to high temperatures. In the case of hot corrosion, sulfur is primarily found X. Liu (&) B. Gleeson Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA e-mail: [email protected] B. Gleeson e-mail: [email protected]
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in the Na2SO4-based deposit. Environmental sources of sulfur may be at elevated levels due to the fuel source being of a low grade or the service environment having a relatively high SO2 and/or sulfate content. A great deal of research has focused on the high-temperature corrosion of metals and alloys in oxidizing-sulfidizing environments [1–3]. Much of this research has been reviewed by Gleeson [4], Gesmundo et al. [5], Stroosnijder and Quadakkers [6], Stringer [7], Grabke et al. [8], and others [9–11]. Most of these studies were done under high Pso2 conditions, in which sulfide usually formed during the reaction process. Very limited work has dealt with oxidation in low Pso2 conditions [12, 13]. Moreover, past research was concerned primarily with the formation and behavior of Cr2O3 in sulfur-containing gases [1]. Only a few studies investigated the formation and growth behavior of Al2O3 in environments containing sulfur. The main focus of those studies was the effect of sulfur on scale adhesion—a detrimental effect that is well-known when the alloy contains sulfur as an impurity [14]. Kubena et al. [15] studied the effect of environmental sulfur on the cyclic oxidation resistance of alumina-scale forming nickel-base superalloys. Their r
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