Study On The Corrosion Resistance In Moltenzinc Of Plasma-Sprayed Coating Of Internal Heating Tube

This experiment adopt the design idea of composite coating.The composite coating whichwas composed of Fe-Al alloy coating, metal ceramic FeAl₂O₄-Fe-Al₂O₃ composite coating andceramic ZrO₂ coating (sealing hole)was prepared on the surface of low-carbon steel tube byplasma spraying.The internal heating tube of plasma-sprayed had high strength,toughness,strong thermal conductivity and excellent resistance to liquid zinc corrosion.It couldsave energy and increase efficiency.First, Al-Fe₂O₃ composite powders were synthesized by spray granulation method. Theaverage property of composite powders was: 60% collection efficiency, 0.97kg/cm apparentdensity, 7.5s/50g flowability.The best parameter of spray granulation method was: X bindercontent, Y solid powder, Z atomization pressure, W1 drying temperature.Secondly, Al-Fe₂O₃ composite powders of different ratio were sprayed on the low-carbonsteel.The main phase of composite coating was Fe,Al₂O₃,FeAl₂O₄ and FeAl alloy. The more ofthe Al content of original powders, the more of the Fe content of the coating, Iron oxidechanged from Fe3O₄ to FeO. When the weight ratio of Al powder and Fe₂O₃ powder reached to1:2, iron oxide disappeared completely, the content of Fe was the most in the coating. When theweight ratio of Al powder and Fe₂O₃ powder reached to 1:1, the main phase of compositecoating changed from FeAl₂O₄ to Fe-Al alloy. Thereinto, 1:1 was FeAl,2:1 was FeAl andFeAl2,3:1 was FeAl and FeAl3 (Fe24Al76.8).Especially 3:1, the layer structure of coating wasdisappeared, the unreacted Al powder appeared.Again, Fe-Al alloy powders, metal ceramic FeAl₂O₄-Fe-Al₂O₃ composite powders andceramic ZrO₂ powders were prepared on the surface of the internal heating tube. The more ofthe Al content of original powders, the stronger of the thermal conductivity, the worse of thethermal shock stability and the corrosion resistance of the internal heating tube.Comprehensive consideration,the best internal heating tube interlayer was the composite coating whose weightratio of Al powder and Fe₂O₃ powder was 1:2. The composite coating will ensure not only theresistance to liquid zinc corrosion but also the thermal conductivityand the thermal shockstability.At last, the corrosion process of FeAl₂O₄ composite coating and ZrO₂ coating was studiedin liquid zinc corrosion test.The corrosion of FeAl₂O₄ composite coating: Zn atomic permeatedFeAl₂O₄ phase of composite coating firstly. It would form FeAl₂O₄ solidsolution which had Zn-Zn (Fe_xAl_2-x) O₄ with Al atomic replacement. Along with the corrosion, Zn(Fe_xAl_2-x)O₄ wasresolved into (Zn_1-xFe_x)(ZnxFe2-x)O₄ and Zn1+xO. Then (Zn_1-xFe_x)(ZnxFe2-x)O₄ would resolveinto Fe-Zn alloy of dendrite and irregular shape.The composite coating was damaged finally.The corrosion of ZrO₂ coating was the resolution of c-ZrO₂ and Zr0.92Y0.08O1.96 intoZr0.82Y0.18O1.91. After corrosion 1440h in the molten zinc, the main phase of ZrO₂ coating wouldchange.It would cause the whole coating turn into an unstable state which would broke thecoating. By measuring, the corrosion life of internal heating tube coating was around 1200h.Going forward, the nm-ZrO₂ coating had low elastic modulus and porosity so that it hadmore superior ability to resistance corrosion of liquid zinc than um-ZrO₂. By measuring, thecorrosion life of nm-ZrO₂ coating was around 1200h.