Research On Corrosion Of Materials In Power Plant's Superheater In Supercritical Water

Supercritical and ultra supercritical thermal power unit can significantly improve the efficiency of power plant,reduce CO2 emission.It mainly develops supercritical(SC)and ultra-supercritical(USC)thermal power unit with high efficiency and parameters in the future in china.However,with the improvement of steam parameter,high-temperature heating surface of the power plant boiler,especially the superheater materials,will operate in highly corrosive supercritical water.The oxide scale exfoliation from the steam side of boilers tube can cause occurrence of overtemperature and tube burst,which influence ultra supercritical boiler safe and stable operation for a long time.Therefore,research about the influence of different steam parameters on boiler tube corrosion characteristics helps to reveal the growth mechanism of the oxide.Therefore,research on oxide film corrosion mechanism under different parameters is very meaningful.The oxidation experiment of ferritic-martensitic steel P92 and austenitic steel TP347 HFG were carried out by exposure to different temperature,pressure,dissolved oxygen and flow state.Temperature,pressure,dissolved oxygen(DO)and velocity are 550-700 ?,0.1-25 MPa,0-2000 ppb and 0-5 ml/min,respectively.The oxidation kinetics curve of the two kinds of steel under different temperature,pressure,flow rate and dissolved oxygen were obtained,the microstructure and the main oxide phase composition of oxide film were analyzed using scanning electron microscopy(SEM)and X-ray diffraction(XRD).In addition,to assess the role of dissolved oxygen in supercritical water on the growth processes of oxide scales,a marker experiment of ferritic-martensitic steel P92 was performed by exposing to supercritical water(H216O)containing marked 18O2 at 600 ? under 25 MPa.The mechanism of the effect of dissolved oxygen on oxidation is discussed.By analyzing oxidation characteristics of martensitic steel P92 and austenitic steel TP347 HFG at different temperature,it can be found that the diffusion rate of iron along the inner oxide layer is the oxidation rate-controlling step,and the change of temperature will affect the oxidation kinetics of two kinds of steel.By analyzing the influence of pressure,it can be found that the oxidation rate of martensitic steel P92 increased with the increase of the pressure and the oxidation rate increased dramatically especially in the supercritical water.The oxidation experiment of F-M steel P92 was performed at 600 ? under 25 MPa in SCW(H216O)with a dissolved oxygen(18O2)content of 2000 ppb.The SIMS oxygen isotope profile indicates that both O2 and H2 O react to grow oxide scale.Dissolved oxygen mainly changes the oxidation reduction potential of the SCW and the oxygen potential distribution through the oxide scale,leading to the increase in the oxidation rate.Moreover,the the content of dissolved oxygen in supercritical water has a great impact on the oxide phase.The combination of result in this paper,it has been analysed the exfoliation behaviour of oxide scale formed on martensitic steel P92 and austenitic steel TP347 HFG.Furthmore,exfoliating mechanism of the two steels has been given in this paper.