Effect Mechanism Of Tensile Stress On The Oxide Scale Formation Of Steel For Power Plant In Supercritical Water

Ultra-supercritical power plant can significantly improve the efficiency of power generation and reduce CO2 emissions.With the increase of temperature and pressure,the inner wall of the metal pipe on the high temperature heating surface of the boiler is served in highly corrosive supercritical water.The oxidation resistance of metal material in supercritical water greatly restricts the service life of equipment and components in high temperature,environment.The material of high temperature heating surface under the real service condition in power plant is not affected by the corrosion environment,but by the coupling effect of corrosion environment and tensile stress.Therefore,the oxidation resistance experiment under tensile stress,in high temperature steam and supercritical water,will be beneficial to reveal the formation mechanism of oxide scale of metals in real service environment,and provide theoretical guidance for the life assessment and prediction of steel for power plant.The corrosion experiments of the ferritic-martensitic T92 and austenitic Super304H steel were investigated in steam at 6MPa and in supercritical water at 25MPa under a temperature of 600℃.The exposure time is 100h,and the tensile stresses were 0、0.2σs and 0.3σsMPa,respectively.The morphology and phase composition of the oxide scales were observed using SEM,EDS and XRD technologies.At the same time,cross structures,element distribution and the thickness of the scale were tested.The influence of tensile stress on the formation mechanism of the oxide scale were also investigated.The results indicated tensile stress can have an obvious effect on the oxidation rate and the crack formation.Compared with the high temperature steam,the thickness of the oxide scale in supercritical water has increased significantly,and the thickness ratio of the outer to the inner oxide scale also varies.The tensile stress can also have an influence on the thickness of the diffusion scale.The tensile stress generates short-term diffusion path in the oxide scale,and more oxidant would transport to the interface of inner scale and the alloy.