Effect of Chromium Content on the Oxidation Behavior of Ni-Cr Model Alloys in Superheated Steam
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INTRODUCTION
THE steam oxidation of Ni-based alloys at high temperatures has attracted increasing interest in recent years. These materials are candidates for use in superheated steam generators in future generations of power plants, i.e., advanced ultra-supercritical power plants.[1– 3] Metal loss, internal oxidation, decarburization, and the decrease of heat transfer due to oxidation contribute to the degradation of the mechanical properties and performance of Ni-based alloys.[4,5] Although numerous investigations have been conducted to determine the oxidation behavior of Ni-based alloys in dry atmospheres, relatively few oxidation studies of these materials in steam environments have been conducted to date. Steam can affect the solubility and/or diffusivity of oxygen in the alloy and thus impact its oxidation rate[4,6,7]; therefore, detailed knowledge regarding the oxidation behavior of Ni-based alloys in steam environments is required for the accurate prediction of their oxidation rates. This work has examined surface preparation as a method to uncouple the effects of surface cold work and chromium content on the oxidation behavior of Ni-Cr model alloys. Indeed, rapid diffusion of Cr from the base metal to the oxide film has been shown to occur in the cold work zone, located in the subsurface of chromiaforming alloys. This phenomenon results in significant
FETHI HAMDANI, PhD Student, is with the INSA de Lyon, MATEIS CNRS UMR5510, 69621 Villeurbanne, France, and also with the Graduate School of Engineering, Tohoku University, 6-6-01-2 Aoba, Aramaki, Aoba-Ku, Sendai 980-8579, Japan. HIROSHI ABE, Assistant Professor, and YUTAKA WATANABE, Professor, are with the Graduate School of Engineering, Tohoku University. Contact e-mail: [email protected] BENOIT TER-OVANESSIAN, Assistant Professor, and BERNARD NORMAND, Professor, are with the INSA de Lyon, MATEIS CNRS UMR5510. Manuscript submitted September 14, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Cr enrichment in the oxide film and thus improved oxidation resistance.[8–14] The present work focuses on the effect of increasing the Cr content of alloys in terms of oxidation. A sufficient quantity of chromium in Ni-based alloys is known to promote oxidation resistance due to the establishment and maintenance of a protective oxide scale of Cr2O3.[4,15] Essuman et al.[15] studied the oxidation of NiCr model alloys in water vapor. These authors focused on identifying the critical Cr content required to form a compact Cr oxide layer. Therefore, oxidation characteristic, as Cr content was increased above the identified critical value (i.e., 25 wt pct), was not investigated, hence the properties of the protective oxide were assumed being unchanged. However, it is suggested in the present study that improving the protectiveness of the Cr oxide, which is strongly related to its properties, can be achieved by optimizing chromium content. This method has not been considered in previous studies. The present study was conducted to determine the optimal Cr content for the o
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