Research On Preparation And Characterization Of Protective Coating For Welded Joint Of Steel Structure In Seashore Equipment

The welded joint is affected by various factors,such as chemical composition of substrate material,welding filling materials,welding process and service condition of welded structure,etc.Therefore,the chemical composition and metallurgical condition of the welded joints become very different from that of the substrate metal.In the coastal environment with complex corrosion conditions,the welded joints tend to be preferentially corroded,which seriously affects the operation safety of equipment.Based on high energy ball milling technology,titanium nano polymer welding coating for welded joint is prepared in this thesis.The welded joints of both 304 stainless steel and Q235 steel welded joint were used as the research objects,and the titanium nano polymer welding coating is coated on the surface of welded joints.The corrosion resistance of coating on welded joint is investigated by neutral salt spray test,salt immersion test and aging test.The corrosion resistance mechanism of coating is studied by electrochemical analysis.The main research contents of this thesis are as follows:(1)Using A102 and J422 as welding filling materials,304 stainless steel weld samples and Q235 steel welding samples are prepared by manual arc welding(MAW).The microstructure and phase composition of the welded joint are observed by scanning electrical microscope(SEM)and optical microscope(OM).The results show that the austenite grain of fusion zone of 304 stainless steel sample is coarse and accompanied by a small amount of carbide precipitation,which is the main reason for the corrosion of welded joint.The weld zone of Q235 steel sample has typical widmannstatten structure,and contains a large number of intragranular acicular ferrite and a small amount of pseudo pearlite.(2)Using E-44 epoxy resin as protective agent and main film forming material,polyamide as grinding aids,absolute ethanol as dispersing agent,mica iron oxide as filler,stable dispersed titanium nano polymer is obtained by high speed milling process in a high energy ball milling tank with proper amount of titanium powder.The titanium nano polymer,epoxy resin,diluent,leveling agent,light stabilizer,plasticizer,curing agent and others are evenly mixed and matured,for the preparation of titanium nano polymer welding coating.The titanium nano polymer welding coating is prepared on the surface of tinplate by spraying method.And the basic mechanical properties of the coating is characterized according to the national standard.The results show that the titanium nano polymer welding coating has good adhesion strength,good flexibility,good impact resistance and so on.(3)The corrosion resistance of the welding coating is evaluated by neutral salt spray tests,salt immersion tests and aging test.The results of neutral salt spray test show that the coating can effectively reduce the corrosion tendency of welded joint of Q235 steel and 304 stainless steel at the initial stage of immersion.The results of neutral salt spray tests show that the coating can resist 4000h salt fog corrosion,and effectively improve the corrosion resistance of welded joint of Q235 steel and 304 stainless steel.The results of electrochemical impedance spectroscopy(EIS)shows that the titanium nano polymer welding coating can act as a high resistance and low capacitance shielding insulating layer,and has excellent corrosion protection performance.