Steam Effects on the Oxidation Behaviour of Al 2 O 3 -Scale Forming Ni-Based Alloys
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Steam Effects on the Oxidation Behaviour of Al2O3-Scale Forming Ni-Based Alloys Wei Zhao • Brian Gleeson
Received: 16 April 2012 / Published online: 19 January 2013 Ó Springer Science+Business Media New York 2013
Abstract The effects of steam on the 1,000 °C oxidation behavior of Al2O3-scale forming Ni-based alloys were studied by conducting tests in dry air and air ? 30 % steam (wet air) gas environments. It was found that the critical concentration of Al (N*Al) to form a continuous alumina scale is increased when the environment is wet air. From the morphology of and the depth at which the internal oxides formed in the two oxidizing conditions, it was determined that the enrichment factor a remained the same in dry and wet air, though the internal-oxide precipitates were coarser and their average spacing greater in the wet air. Based on an assessment of the parameters contained in Wagner’s criterion for transition from internal oxidation to external scale formation, it was deduced that the only factor that can change the critical concentration N*Al to the extent measured is the critical volume fraction f*v . Keywords of Al
Steam effect Oxidation map Ni-based alloy Critical concentration
Introduction Nickel-based super alloys are used extensively at elevated temperatures due to their good oxidation and corrosion resistance [1]. The protection of an alloy typically relies on its ability to form a continuous Al2O3 or Cr2O3 scale during thermal exposure. The design of Ni-based superalloy compositions is usually based on the Ni–Cr–Al system with other elements added primarily for strength[2, 3]. Therefore, W. Zhao (&) B. Gleeson Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 3700 O’Hara Street, 15260 Pittsburgh, PA, US e-mail: [email protected] B. Gleeson e-mail: [email protected]
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Oxid Met (2013) 79:613–625
a good understanding of the oxidation behavior of the Ni–Cr–Al system is very helpful from the standpoint of alloy design. The Ni–Cr–Al system has been widely studied under a variety of test conditions [2–7]. Among those studies, Giggins and Pettit [2] investigated the oxidation behavior of Ni–Cr–Al alloys in 0.1 atm oxygen at 1,000–1,200 °C. From their seminal study, three trends in oxidation behavior were identified and qualitatively summarized on an oxidation map. Figure 1 shows a typical oxidation map at 1,000 °C for Ni–Cr–Al alloys exposed to a dry oxidizing condition. The three trends or regions on the map correspond to NiO-scale forming, Al2O3-scale forming and Cr2O3-scale forming. The boundary between the NiO- and Al2O3-scale forming regions is important, since it defines the lowest Al concentration (N*Al) in an Ni–Cr– Al alloy required to form a continuous Al2O3 scale. This critical concentration is dictated by a variety of thermodynamic and kinetic factors associated with the alloy and the environment [8–13]. Values of N*Al can be estimated on the basis of two criteria proposed by Wagner [14, 15]. In one criterion [14], N*Al represents the minimum
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