The Corrosion Oxidation Of Ferritic-martensitic Steel In Supercritical Water

Under the demand for energy saving and environmental protection, the nuclear energy will be rising proportional in the future. For the Second and third generation PWR provides mature technical support, supercritical water reactor (SCWR) have attracted increasing attention in the world for one of the six Generation IV nuclear reactor. Thermal eciency of SCWR can be 45%( PWR is 33% approximately), and it has the advantage of simple core structure, lower operation cost etc. For the SCWR operateing on the ultra high pressure, temperature and strong neutron irradiation condition, the mechanical and chemical properties of materials using in fuel cladding or structure cannot meet the requirements. Therefore, the selection of fuel cladding and structure materials for using in SCWR has become one of the key technology. At present, the hotspot is to research the materials corrosion behavior in SCWR.Ferritic-martensitic(F/M)steels exhibit low swelling and activation under irradiation, low coefficient of thermal expansion, high thermal conductivity, good oxidation and corrosion resistance at elevated temperatures. It also be used in supercritical coal-fired power plant commonly. So it becomes the probable candidate using in supercritical water reactor (SCWR).The corrosion behavior of two ferritic-martensitic steel K1 and K2 exposed to supercritical water at 500℃and a pressure of 25MPa for exposure times of 200h, 400h, 600h, 800h, 1000h was investigated. The results from Scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) analyses indicated that the oxide scale exhibits typical duplex oxide structure, in which the scale is composed of an outer layer of more porous magnetite(Fe3O4), and an inner layer of iron chromium spinel. The results of corrosion dynamic manifest that two materials follow the Parabolic rule, and the corrosion oxidation rate of K2 steel is faster than K1. The formation of pores is related to the defect types present in the magnetite structure, there are two major defect types in magnetite, one is interstitial, and another is vacancy which may collapse into pores when vacancy concentration is high enough in supercritical water. The K1 steel have higher alloying element in chromium and tungsten, but lower carbon than K2 steel. The matrix of K1 steel also have more ferritic phase. So it exhibited lower corrosion rate and higher anti-oxidant properties.