Study On High Temperature Corrosion Resistance And Mechanism Of Protective Coatings For Water-cooled Wall Of Low-NO_x Combustion Boiler

After the application of low-NO_x combustion technology in coal-fired power plants,the water-cooled wall may form high-temperature corrosion due to oxygen-poor atmosphere combustion,which would seriously jeopardize the safety and reliability of boiler.Therefore,it is of great theoretical and practical value to develop high-temperature corrosion protection methods for low-NO_x combustion coal-fired boilers.A nickel-based alloy coating?Ni-Cr?and a ceramic coating?Al-Si-Cr?were sprayed on the surface of water-cooled wall substrate.The high temperature corrosion resistance of two coatings was systematically investigated by high temperature corrosion tests.At the same time,the two coatings were applied to a300MW unit lean coal boiler,and the high temperature corrosion resistance under boiler operating conditions was further verified and analyzed.In this paper,the high temperature reducing gas corrosion test of the two coatings was carried out first.The corrosive environment is CO/SO₂/H₂S/HCl reducing corrosive atmosphere.The cross-sectional morphology and elemental composition of the coatings before and after test were analyzed by X-Ray Fluorescence?XRF?,X-ray Diffraction?XRD?,Scanning Electron Microscope?SEM?and Energy Dispersive Spectrometer?EDS?.The results show that the Al-Si-Cr coating consists of spinel oxide,and the inner structure is dense with no obvious defect,and the coating is tightly combined with the water-cooled wall substrate.The dense oxide can effectively prevent the penetration of S and Cl corrosion gases.The Ni-Cr coating is mainly composed of corrosion-resistant Cr₂O₃ oxide film and nickel-chromium alloy,but there is a"diffusion channel"formed by defects such as pores,which causes corrosion gas to penetrate into the coating interior and damage the coating.The corrosion rate of Ni-Cr coating is about twice that of Al-Si-Cr coating.Secondly,the two protective coatings were subjected to a high temperature molten sulfate corrosion test in reducing atmosphere.The results show that the high temperature sulfate corrosion resistance of ceramic coating is derived from the dense inert oxide film composed of Al,Si and Cr elements.However,due to defects such as cracks generated by internal thermal stress release,the sulfate penetrates into the interior of the coating to reduce corrosion resistance.The Ni-Cr coating showed good anti-sulfate corrosion effect by Cr₂O₃oxide films.The corrosion rate of Al-Si-Cr coating is about 1.5 times that of Ni-Cr coating.Finally,Ni-Cr coating and Al-Si-Cr coating were applied to the water-cooled wall surface of a 300MW unit low-NO_x combustion coal-fired boiler.The results of nearly one-year practical application showed that high temperature corrosion of water-cooled water of low-NO_x combustion boiler is mainly caused by corrosive reducing gases and sulfides:Corrosive medium such as gas phase H₂S and active sulfur atom in the flue gas can penetrate the sulfur-containing deposition layer on the surface of the coating and react with the coating's oxide film,resulting in the invalidation of Ni-Cr alloy coating.The Al-Si-Cr ceramic coating has better high temperature corrosion resistance and anti-fouling performance than Ni-Cr alloy coating due to its denser inert oxide layer and less structural defects.