Corrosion Behaviors And Mechanisms Of Candidate Materials For Supercritical Water-Cooled Reactor

The great advantages of supercritical water-cooled reactor (SCWR) such as economy, continuity and sustainability, may make it the main reactor type for large scale electricity production. However, the SCW environment is extremely corrosive to the metal materials which can result in the failure in the applications of present core components and fuel cladding materials on SCWR. Evaluation of several candidate materials for SCWR has been conducted on the major fire tube materials used in supercritical fossil fired plant and fuel cladding materials used in current pressurized water reactors (PWRs), covering zirconium alloys, ferritic/martensitic steels (F/M), austenitic stainless steels, nickel-based alloys and oxide dispersion strengthened (ODS) steels. The research on the corrosion behavior of candidate materials in SCW environment keep great importance for the selection and development of materials for core components and fuel cladding used in SCWR.The corrosion behaviors of Zr-2.5Nb, P92, 12Cr, 304NG, AL-6XN, Hastelloy-C276 and MA956, etc were investigated in SCW including static and dynamic states at 500, 550, 600 and 650℃and at pressure of 25MPa for 1000-1400h in this paper. The surface morphology, microstructure and element distribution of each candidate material were analyzed by Optics Microscope, Scanning Electron Microscope (SEM), Energy-dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD).The results showed that Cr and Al may be the key elements for alloys to form a stable oxide film with great corrosion resistance in SCW. The oxidation of alloys exposed in SCW always follows a parabolic growth kinetic. After long time corrosion in SCW, zirconium alloy, F/M steel and low-Cr steels would have high corrosion rate while those high-Cr, Ni steels such as austenitic steels and nickel-based alloys had low corrosion rate. MA956 with high content of Cr had the lowest corrosion rate.In SCW, F/M and austenitic steels always formed a dual-layer oxide film with Fe-rich magnetite of the outer layer and Cr-rich spinel of the inner layer and cracks could often found on the surface of oxide film on F/M steels while nodular corrosion or exfoliation occurred on austenitic steels. Nickel-based alloys and MA956 always formed an extremely thin oxide film having the structure of Fe-Cr-Ni spinel and might have some fields of pitting or spallation. In addition, temperature and dissolved oxygen also had significant influence on the corrosion behavior of alloys exposed in SCW.According to the present experimental results, high-Cr level ODS steels (MA956, 18Cr-ODS), nickel-based alloys (C276, 625) and super austenitic steels (6XN, HR3C) have pretty excellent corrosion resistance in SCW among all candidate materials. Therefore, the three kinds of candidate materials may be used as core components and fuel cladding materials in SCWR or as reference materials for research and development of new alloys used in SCWR.