An Exploration of the Effect of Temperature on Different Alloys in a Supercritical Carbon Dioxide Environment

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 130-131).In the constant effort to increase efficiency, safety margins, and lower cost, a new breed of nuclear reactors, Generation IV, is being developed in which supercritical carbon dioxide (SCO₂) is a prime coolant candidate. SCO₂ allows for higher efficiencies, reduced pumping power, lower plant temperature, and more compactness compared to the other gas coolants currently being examined. However, limited corrosion data currently exists for materials that are potential pressure boundary candidates under high pressures and temperatures in SCO₂ environments. The goal of this investigation was to understand the effect of temperature on corrosion on potential structural materials in a SCO₂ environment A total of 7 different alloys, 6 nickel based and 1 austenitic stainless steel (AUSS), were examined in three sets of experiments. The experiments exposed the specimens to SCO₂ at temperatures ranging from 650 "C to 750 °C, pressures from 12.5 to 20 MPa, and for durations of up to 1000 hours. The nickel based alloys demonstrated very promising results as the weight gain rates were almost an order of magnitude lower than the stainless steel. The average nickel based sample exposed to SCO₂ at a temperature of 750°C and a pressure of 12.5 MPa showed a weight gain rate of 0.0063 mg/cm 2*day, while the stainless steel sample had a weight gain rate of 0.096 mg/cm 2*day after a duration of 1000 hours. This was expected as the combination of nickel and chromium forms a higher integrity and more stable passive film than iron and chromium. Additionally, nickel has a lower oxygen affinity than iron and therefore the migration of cations into the scale is lower.(cont.) The chromium content for the AUSS 316L was also the lowest, which most likely contributed to the high oxidation rates. The tests conducted at 750 °C and 12.5 MPa showed the highest weight gain rates for the nickel based alloys, which was expected as the corrosion rate should follow an Arrhenius trend. The effect of pressure was small compared to the effect of temperature as a 43% reduction in pressure and a 5% increase in temperature produced significantly higher corrosion rates in the nickel based alloys. The AUSS 316L, behaved counter-intuitively as the highest temperature experiment, 750 °C and 12.5 MPa, had the lowest weight gain rate. This behavior may be explained by the increase in temperature, which caused an increase in the diffusion rate within the alloy. This facilitated a faster growth rate of an inner "healing" layer composed of chromium rich oxide, which may have restricted the outward diffusion of cations and inward diffusion of anions.by Michael William Dunlevy.S.M