High Cr ODS steels performance under supercritical water environment

This paper summarizes the results of supercritical water corrosion studies of two ferritic oxide dispersion strengthen (ODS) steels MA956 and PM2000 at the temperature of the upper limit of potential peak cladding temperature under normal operation, according to the conceptual design being developed in the EU. As the high temperature and pressure above the thermodynamic critical point of water result in higher oxidation rate for conventional austenitic alloys than observed in sub-critical light water reactor (LWR) conditions, ensuring adequate corrosion resistance is critical for thin-wall components like fuel cladding. This study concentrated on the investigation of two effects, surface finish and orientation of the cuts. Two different surface treated coupons were prepared in order to study the effect of cold work in sample surface on corrosion resistance. Samples were exposed in supercritical water at 650 °C and 25 MPa, for up to 1800 h. The corrosion rate was evaluated by measuring the weight change of the samples and by cross-section examinations. The microstructure of the oxide layers was analyzed using a scanning electron microscope (SEM) in conjunction with energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). Weight gain results of both ODS steels proved a good resistance to general corrosion. Nevertheless the cross-sectional SEM study showed signs of nodular corrosion, observed mostly on the ground specimens after long exposure times.