Pore-blocking Mechanism And Chlorine Corrosive Resistance Of Precursor Ceramic Coatings

High-temperature chlorine corrosion is a common problem in waste incineration boilers.The precursor ceramic coating technology has a significant application prospect in solving high-temperature chlorine corrosion.This technology mainly uses polysilazane as a precursor,adds ceramic aggregate and solvent,and forms a coating film on the metal surface by spraying or dipping.After sintering at high temperature,it forms a Si O₂-based composite ceramic coating.It has simple process and Convenient operation,strong resistance to high temperature chlorine corrosion and other advantages.However,the coating is prone to pores and cracks during the preparation process,which seriously affects the corrosion resistance of the coating.Therefore,this article first use a self-developed high-temperature microscope observation device to visually observe the pore formation process during sintering of coatings with different proportions,and revealed the pore blocking mechanism,and then obtained an optimized coating formulation with the lowest porosity.Micro-morphology analysis,element analysis,thermal shock performance test and abrasion resistance test of the optimized coating subsequently.Finally,the experimental test and thermal analysis of the anti-chlorine corrosion performance of the coating were performed.The main conclusions are as follows:(1)TG-DSC results show that the polysilazane pyrolysis process can be divided into the deamination reaction stage at room temperature to 500?and the demethylation reaction stage at 500 to 700?.The gas generated by pyrolysis is the main reason for the formation of pores in the coating sintering process.(2)A visualization experiment was performed to the coating sintering process of spraying pure polysilazane,polysilazane with glass powder,polysilazane with zirconia,and polysilazane with zirconia and glass powder.The result shows that the addition of glass powder can solve the problem of cracks after sintering of polysilazane,but there are many bubbles in the coating that cannot escape.In response to this problem,zirconia particles were added to the coating,and the pores were significantly reduced after sintering.Based on this,combined with micro-morphology test,the optimized coating distribution ratio is obtained,which the ratio of polysilazane,zirconia and glass powder is 1:1:2.Analysis of the mechanism of pore blocking by added fillers includes:the healing effect of glass powder;zirconia as an inert filler adjusts the thermal expansion coefficient of the coating,which can effectively prevent cracks;In addition,the coating contains zirconia particles,which can form a skeletal structure and effectively block the aggregation of micro-bubbles forming large bubbles.(3)Ceramic coatings were prepared using the optimal coating composition ratio,and physical properties and performance tests were performed on the coatings.SEM analysis showed that the thickness of the coating was 40?60?m,and the surface was mainly smooth without large bubbles and obvious cracks.EDS analysis showed that the main components in the coating were zirconia and silica.The results of the thermal shock test of the coating showed that the coating had obvious peeling off after 25 times of thermal shock,indicating the coating had good thermal shock resistance.The abrasion resistance test shows that the wear rate of the coating is 1.56 times that of steel,but it can alleviate the wear of metal.(4)The corrosion experiments shows that the coating samples show strong corrosion resistance in the high-temperature corrosion test of HCl and KCl at 400?,450?and 500?.The corrosion resistance of the coated sample is 4.5,8.1 and 8 times higher than that of the steel sample at HCl at 400?,450?and 500?,and the effect is more obvious at KCl,which is 60.5,78.6 and 26.1 times,respectively.After SEM/EDS analysis,it's found that the coating is closely connected with the steel and have good adhesion.There is no crack in the coating and no chlorine element is detected,which indicates that the coating forms a dense and stable protective layer to prevent the diffusion of chlorine element,thus improving the corrosion resistance of the substrate.The results of thermal analysis kinetics show that the reaction mechanism of the corrosion sample conforms to the type of second-order phase interface reaction.In the HCl corrosion experiment,the activation energy of steel sheet is 4632.06 J·mol^-1,and that of coating sample is 6385.15 J·mol^-1.In the KCl corrosion experiment,the activation energy of the steel sample is 3111.60 J·mol^-1,and that of the coating sample is 10441.47 J·mol^-1.The activation energy of the coating samples is higher than that of the steel sheet,indicating that the coating has better corrosion resistance.