| With the continuous increase of the operating parameters of ultra-supercritical power plants,the requirements for the safety performance of their steel materials are also higher and higher.In order to study the high-temperature corrosion behavior of a new type of austenitic heat-resistant steel C-HRA-5 at the flue gas side,500h experiments were carried out together with the control materials S31042 and S30432.Two temperature conditions of 650℃ and 700℃,the SO2 volume fraction conditions of 0.1%and 0.5%were selected to investigate the effect of temperature and SO2 concentration on the corrosion process.Samples covered with no-coal-ash,realcoal-ash and simulative-coal-ash were tested at the above temperature and SO2 concentration in order to study the corrosion characteristics of austenitic heat-resistant steel in the enviroment of flue gas,the enviroment of molten salt and flue gas and the enviroment of coal ash and flue gas.The kinetic curves were drawn,and the microscopic morphology and element distribution of the three metals under different conditions were characterized by scanning electron microscopy and energy dispersive spectroscopy to compare the corrosion resistance of the three austenitic heat resistant steel.The following are the main conclusions of the study:Due to the difference in the severity of environmental conditions and the corrosion resistance of materials,there are also differences in the corrosion stages of the three materials under different conditions after 500 hours of corrosion.In some environments,corrosion products have no obvious layered structure,and in more harsh environments,layered corrosion products appear.On the surface of the S31042 and C-HRA-5,the layered corrosion products show the characteristics of scattered distribution,composed of external iron oxide layer,internal chromium oxide corrosion pit and diffusion area,and the internal corrosion products are mixed with sulfide.On the surface of the S30432,the distribution of layered corrosion products is relatively continuous,which is composed of external iron oxide layer,internal iron-chromium spinel oxide layer and diffusion area,and the internal corrosion products are mixed with sulfide.The corrosion type is oxygen-sulfur vapor phase corrosion in the enviroment of flue gas,and Type Ⅱ hot corrosion in the other two environments.There are similarities but also differences in the corrosion mechanism.Compared with oxygen-sulfur vapor phase corrosion,Type Ⅱ hot corrosion is faster due to the presence of molten salt and stronger sulfuration.In the environment of flue gas and the environment of coal ash and flue gas with low SO2 concentration,oxidation is dominant.According to the weight gain curve and section micromorphology,C-HRA-5 has the strongest resistance to high temperature oxidation,followed by S31042 and finally S30432.In the environment of flue gas with high SO2 concentration,there is both oxidation and sulfide,and the corrosion resistance of C-HRA-5 is comparable to S31042,S30432 is the worst.In the environment of molten salt and flue gas,the difference in corrosion resistance of the three materials is magnified,S31042 has the strongest resistance to Type Ⅱ hot corrosion,followed by C-HRA-5,and finally S30432.According to the analysis,the difference in the element content of Cr,Co and W causes the difference in their resistance to Type Ⅱ hot corrosion.Increasing temperature and increasing SO2 volume fraction in flue gas can accelerate the thermal corrosion rate.Under the simulated-coal-ash condition,the effect of SO2 volume fraction increase from 0.1%to 0.5%is greater than the temperature increase from 650℃ to 700℃.Since Type Ⅱ hot corrosion resistance of C-HRA-5 is weaker than that of S31042,it will have a good effect to extend the service life through coal desulfurization,if its use temperature is increased in practical application. |
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