Biomass power plant is an important, green renewable energy utilization facilities, environmental protection and high efficiency. However, due to the high content of alkaline substances in its fuel, alkali metal chloride formed in the process of burning biomass fuel for safe power generation adheres to the wall of the boiler water wall, superheater and other heat exchange tube, and is easy to form composite salt with low melting point under high temperature environment, which causes serious thermal corrosion of the tube wall.
The researchers used electrochemical methods to study the molten salt thermal corrosion of T91 wear-resistant plate, a common pipe used in boiler superheater of biomass power plant, analyzed the mechanism and process of molten salt thermal corrosion of materials by polarization curve and AC impedance spectrum, explored the influence of temperature in the molten salt system on the corrosion resistance of materials and the rule, and obtained the thermodynamic parameters of molten salt thermal corrosion of T91 wear-resistant plate. In order to provide theoretical basis and technical support for corrosion protection of superheater pipe in biomass power plant.
The test material is T91 wear-resistant plate, its chemical composition (mass fraction, %) : C0.08 ~ 0.12, Si0.020 ~ 0.50, Mn0.30 ~ 0.60, S≤0.02, Cr8.00 ~ 9.50, Ni≤0.40, Mo0.85 ~ 1.05, V0.18 ~ 0.25, P≤0.02, residual Fe. First, the test material is processed into 50mm×25mm×2mm, polished with 60-5000 mesh metallography sandpaper in turn until the mirror is smooth, then cleaned with ethanol and acetone solution, dried with filter paper and placed in a drying oven at 150℃ for 2h.
The corroded molten salt is KCl.NaCl (the mass ratio of the two is 1:1) eutectic salt. After soaking the T91 wear-resistant plate in the corroded molten salt at 800℃ for different time, the polarization curve and AC impedance spectrum of T91 steel in the corroded molten salt at 800℃KCl.NaCl were measured respectively, and the self-corrosion potential Ecorr and linear polarization resistance Rp were calculated according to the polarization curve. The test results show that:
(1) In the high-temperature KCl.NaCl molten salt corrosion of T91 wear-resistant plate, self-corrosion potential Ecorr and linear polarization resistance Rp drift to different degrees with the increase of corrosion time, and Ecorr and Rp gradually increase at the later stage of corrosion and reach a stable value. It shows that the surface of T91 wear-resistant plate gradually formed a more stable, strong protective oxide film, so that its corrosion rate gradually reduced.
(2) At the initial corrosion stage, T91 wear-resistant plate corroded quickly, and its corrosion electrochemical impedance spectrum showed typical Warburg impedance characteristics, that is, the electrochemical reaction rate was controlled by the diffusion of oxidizer particles at the molten salt/sample interface; With the progress of corrosion, FeCr2O4 intergranular oxide film is gradually formed on the surface of T91 wear-resistant plate, and the corrosion rate is significantly reduced. The electrochemical impedance spectrum at the later stage of corrosion shows a significant double-capacitance arc resistance characteristic, that is, the reaction rate is mainly controlled by the migration rate of charged particles in the oxide film.
(3) With the increase of reaction temperature, the corrosion current density of T91 wear-resistant plate gradually increases, and the corrosion resistance of the material decreases. The relationship between the electrode reaction rate constant and temperature meets the Arrhenius formula. The activation energy of the thermal corrosion electrochemical reaction of T91 wear-resistant plate in KCl.NaCl molten salt is 33.235kJ/mol.
