The Corrosion Behavior of Nickel-Base Austenitic Alloys for Nuclear Hydrogen Generation
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0929-II05-06
The Corrosion Behavior of Nickel-Base Austenitic Alloys for Nuclear Hydrogen Generation Rama S Koripelli, Joydeep Pal, Ajit K Roy Mechenical Engineering, UNLV, Las Vegas, NV, 89119
ABSTRACT Three nickel-base austenitic alloys, namely Alloy C-22, Alloy C-276 and Waspaloy have been tested for evaluation of their corrosion resistance in an acidic solution at ambient and elevated temperatures. The results of stress corrosion cracking studies indicate that none of these materials did exhibit any failure at constant load. The cracking susceptibility determined by the slow strain rate technique was gradually enhanced at higher temperatures showing reduced ductility and true failure stress. The critical potentials determined by the polarization technique, became more active (negative) with increasing temperature. The fractograpic evaluations by scanning electron microscopy (SEM) revealed ductile failure in Alloys C-22 and C-276. However, Waspaloy showed brittle failure at elevated temperature. INTRODUCTION The United States Department of Energy (USDOE) is currently exploring hydrogen generation, based on a thermochemical process known as sulfur-iodine (S-I) cycle. This process involves the utilization of heat from nuclear power plant to generate hydrogen by the formation and decomposition of sulfuric acid (H2SO4) and hydrogen iodide (HI), as illustrated in Figures 1 and 2 respectively. During this process the maximum temperature associated with decomposition of H2SO4 will be approximately 850°C. Thus, the structural materials to be used in the hydrogen generation plant must possess superior corrosion resistance due to the presence of H2SO4 at elevated temperatures. Based on a literature review, three nickel-base alloys namely Alloy C-22, Alloy C-276 and Waspaloy have been identified as candidate structural material for heat exchanger applications in nuclear hydrogen generation. The susceptibility of all three alloys to stress corrosion cracking (SCC) and localized corrosion has been determined using different state-of-the-art techniques. Further, fractographic evaluations of the broken specimens have been determined by SEM. This paper presents the results of SCC, localized corrosion and fractogrphic evaluations of all three alloys.
Figure 1. Hydrogen Generation Plant
Figure 2. S-I Cycle
EXPERIMENTAL PROCEDURES Alloy C-22, Alloy C-276, and Waspaloy were procured from a vendor in a solutionannealed condition. The chemical composition and the metallurgical microstructures of all three alloys are given in Table I and Figure 3, respectively. Smooth and notched cylindrical specimens were machined from round bars of these alloys in such a way that the gage section was parallel to the longitudinal rolling direction. The stress concentration factor due to the presence of a notch in these cylindrical specimens was approximately 1.45 [1]. The susceptibility of these alloys to SCC was determined under both constant load and slow-strain-rate (SSR) conditions. A strain rate of 3.3 X 10-6 sec-1 was used in the SSR testi
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