Abstract
Supercritical water (SCW) has attracted increasing attention since SCW boiler power plants were implemented to increase the efficiency of fossil-base power plants. The SCW reactor (SCWR) design has been selected as one of the Generation IV reactor concepts because of its higher thermal efficiency and plant simplification as compared to current light water reactors (LWRs). To support SCWR development, a study of the corrosion behavior of austenitic steels 316, D9, NF709 and Alloy 800H in supercritical water (SCW) at 500°C and 600°C was conducted. The measured weight changes indicate that all these austenitic steels exhibit limited oxidation in SCW in these test temperatures. For most steels tested, a mainly magnetite/spinel structured scale with an outer Fe-oxide layer and inner Fe-Cr-Ni oxide layer formed on the surface. At 500°C, the interfaces between the scale and metal and especially between the scale and the environment were irregular. Grain boundaries influenced the growth rate of the oxides, with a thinner oxide layer developing close to grain boundaries. At 600°C, a continuous chromia layer developed in D9 between the outer oxide scale and the base steel, resulting in improved corrosion resistance. The formation of this chromia layer appears to occur in alloys with smaller grain sizes. Regardless of test temperature, alloy 800H experienced severe spallation during the exposure, however alterations of grain boundary structure have been shown to eliminate this spallation.
