Corrosion Behavior And Mechanism Of Four Corrosion-resistant Alloys In Flue Gas Condensate From Wet Desulfurization In Power Plant

In recent years,the boiler industry of thermal power plant has developed rapidly and gradually changed to the ultra-supercritical boiler.The flue gas emitted by the boiler contains components that have corrosive effect on metal materials.The reason for the formation of these components is that the flue gas in the boiler cannot completely remove the acid gas(SO₂,etc.)after desulfurization,and it reaches the chimney mouth through the flue gas passage.Due to the low temperature and high humidity of the flue gas itself,the condensation phenomenon occurs at the chimney mouth when it mixes with the outside cold air,forming small droplets to return to the flue gas passage.The main components of these acid droplets are sulfuric acid,hydrochloric acid and nitric acid,which will cause serious damage to the inner wall of the chimney.In this paper,316L,254SMo,C276 and Inconel740H are selected as experimental objects to simulate the corrosion behavior and mechanism of the inner wall of chimney materials in the flue gas condensate of supercritical boiler.The passivation behavior of four corrosion-resistant alloys in natural state and after anodic polarization was studied by open circuit potential,potentiodynamic polarization curve,electrochemical impedance spectroscopy,Mott-Schottky and other electrochemical testing methods.The influence of element content in different corrosion-resistant alloys on the performance of passive film was analyzed by SEM,XPS and other characterization methods.At the same time,the high temperature flue gas environment of the superheater in the boiler is set up,and the corrosion resistance of the four materials is compared.The results are as follows,under the open circuit potential,254SMo stainless steel and Inconel740H nickel base alloy show better corrosion resistance,and their impedance values are significantly higher than 316L stainless steel and C276 nickel base alloy.In the later period of long-term immersion test,the dissolution rate of passive film is higher than its formation rate,which leads to the decrease of the protective performance of passive film to a certain extent.The carrier density of the four kinds of passivation film semiconductors are all n-type semiconductors.With the prolongation of immersion time,the carrier density decreases rapidly and then becomes stable,which further indicates that the corrosion resistance of the passivation film increases.XPS characterization after immersion for 24 h at open circuit potential showed that the flue gas condensate at 60?would lead to Cr enrichment,Fe depletion and Mo oxidation in the passive film.The main composition of the passive film is Cr(III)-Fe(III)-Mo(VI/IV)oxide/hydroxide mixed layer,and the thickness of the passive film formed by 254SMo stainless steel is the best.The results show that the steady-state current density of the four materials increases with the application of different anode potentials,which indicates that the dissolution rate of the passive film increases with the increase of anode potential.At the same time,the results of impedance analysis show that different anode potentials in turn will make the impedance value first increase and then decrease,and reach the maximum impedance value at 0.2 V?0.4 V,indicating that the corrosion resistance of the passive film formed at this potential is the best.The point defect diffusion coefficient calculated by PDM(point defect model)shows that Inconel740H nickel base alloy has better resistance to pitting corrosion initiation in simulated flue gas condensate.In the high temperature flue gas environment,Inconel740H nickel base alloy has better corrosion resistance.In this environment,corrosion products will deposit on the surface of the material,and the forms of corrosion products of the four materials are also different,which is related to the element content of the material itself.The film produced by Inconel740H nickel base alloy has a three-layer structure,and these three-layer structures protect the matrix together.