Preparation And Properties Of High Temperature Oxidation Resistant Coatings For Carbon Steel

Three kinds of glassy carbon-steel-oxidation-resistant coatings were prepared with bauxite,borax,glass powder and titanium dioxide(rutile type)as main raw materials.The phase composition and morphology of the coatings were analyzed by metallographic microscope,X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).The results show that:corundum and quartz are the main phase compositions of the binary coatings prepared from bauxite and borax,and the decarbonization depth at the edge of the matrix is 25%of those unprotected samples,and the weight loss decreases by 88%on average after 5 hours of heating.When the mass ratio of bauxite to borax is between 1:2 and 1:1,Al2O3 reacts with Na2BO4 to form Al3O6.When the mass ratio of bauxite and borax is 1:2,the coating has the best oxidation resistance,and it has a smooth and continuous glassy substance with dense surface morphology,less pores and self-shedding at room temperature,and it’s coefficient of thermal expansion is about 7.36×10-6 m/℃,the weight loss rate is only 0.009%after oxidation at 900℃ for 5 h.The main phases of coatings prepared with bauxite,borax and glass powder are corundum and quartz,and the depth of the decarburization layer on the edge is 21%of those uncoated sample;when the additive amount of glass powder is 3g,the coating has the best oxidation resistance,and it has a smooth and continuous glassy substance with dense surface morphology,less pores and self-shedding at room temperature,and it’s coefficient of thermal expansion is about 6.37×10-6 m/℃,the weight loss rate is only 0.007%after oxidation at 900℃ for 5 h.the main phases of coatings prepared with bauxite,borax,glass powder and titanium dioxide(rutile type)are corundum,quartz,titanium dioxide and a kind of aluminum-titanium oxide TiAl2O5,and the depth of the decarburization layer on the edge of base is 16%of those uncoated sample;when the additive amount of titanium dioxide powder is 3g,the coating has the best oxidation resistance,and it has a smooth and continuous glassy substance with dense surface morphology,less pores and self-shedding at room temperature,and it’s coefficient of thermal expansion is about 6.32X 10-6m/℃,the weight loss rate is only 0.005%after oxidation at 900℃ for 5 h,so the coating can be used as a temporary protective coating for heat treatment.The three kinds of coatings all have the oxidation resistant and will not affect the structure of the matrix,and compared with uncoated sample,all the three kinds of coatings’ mass change rate reduced by 79%after heating for 5 hours,and the depth of decarbonization layer at the edge of three matrixes were reduced by 86%.All of those coatings are with good thermal stability,and its compact surface structure,few pores,no cracks and inclusions.