| As one of three basic parts of the industry with widest application range,the fastener plays an irreplaceable role in social production.With the gradual development of marine resources and the construction of various marine infrastructure,the demand for fasteners is increasing.In addition,due to the high temperature,high humidity,high salt fog,much mold,strong sunlight and strong wind(three highs,one much and two strong),the corrosion of fasteners is particularly prominent in the southeast coast of China and the nearby islands and reefs all year round.Especially in the case of inconvenient sea transportation,it is difficult to carry out repair and maintenance work on various structures and equipments.Moreover,the fasteners can only be replaced by new fasteners,which causes great inconvenience and waste.Mechanical energy-assisted thermal infiltration technology is the most widely used advanced surface treatment technology in the fastener industry in recent years,which can produce an infiltrated layer with excellent corrosion resistance and wear resistance,in addition,of which the production process is green and pollution-free.In this thesis,the mechanical energy-assisted thermal infiltration technique is adopted,and zinc,aluminum and nickel powder are used as the infiltration agent.By adjusting the ratio of Zn,Al and Ni elements in the infiltration agent and the mechanical energy-assisted infiltration process parameters at low temperature environment of 450?,an alloy infiltrated layer with superior corrosion resistance is prepared on the surface of 45#steel matrix to achieve the purpose of extending the service life of fastener bolts.The main research contents of the thesis are as follows:(1)The surface and cross section morphology of infiltrated layers were observed and analyzed through pre-test and optimization experiments.The surface hardness,porosity and bonding strength of infiltrated layers were measured and calculated.The optimized infiltration agent formulation and the infiltration process parameters were determined as follows:infiltration agent(15%Al,35%Zn),activator(0.02%NH4Cl),catalyzer(0.05%RE)and filler(the balance is Al2O3),the holding temperature is 450?,the holding time is 4h,and the infiltration furnace speed is 7r/min.(2)XRD diffraction analysis and energy spectrum analysis were used to analyze the phase composition and composition distribution of infiltrated layers.The results show that the main phases of the optimized infiltrated layer are Al2O3,?1 phase(Fe11Zn40)and ?1 phases(FeZn6.67,FeZn8.87,FeZn10.98)of Zn-Fe alloys.The Al element is mainly concentrated on the surface of the infiltrated layer,the Zn-Fe alloy near the surface of the infiltrated layer is mainly FeZn10.98,and the Zn-Fe alloy near the interface between the infiltrated layer and the substrate is mainly FeZn6.67.There is a transition zone between the substrate and the infiltrated layer with a thickness of about 5 ?m.A carbon-rich layer exists in the transition zone,and the content of Zn and Fe elements changes sharply in the transition zone.(3)Friction and wear tests were carried out on the infiltrated layers prepared with different formulations and process parameters.The wear resistance of the infiltrated layer was characterized by the three-dimensional morphology,width,depth,wear volume and friction coefficient curves of the wear marks.The results show that the wear resistance of the infiltrated layers is better than that of the substrate.The increase of Al content in the infiltrated layer helps to enhance the wear resistance of the surface of the infiltrated layer,and the change of process parameters has no significant effects on the wear resistance of the infiltrated layers.(4)The electrochemical analysis was carried out on the infiltrated layers prepared with different infiltration agent formulations.The corrosion mechanism of the optimized infiltrated layer in a 3.5wt.%NaCl environment was studied.The main corrosion process is found as follows:1)in the time zone of 0-24h,the impedance modulus of the low frequency region is reduced,and the radius of the capacitive anti-arc becomes smaller,the corrosion solution invades the infiltrated layer and the layer is quickly corroded;1)in the time zone of 72-360h,the corrosion is diffused clearly,and the impedance modulus of the low frequency region and the radius of the capacitive anti-arc gradually increases,and the corrosion rate of the infiltrated layer gradually decreases;3)after soaking for 360h,the infiltrated layer still has excellent antiseptic effect.(5)The corrosion resistance of the infiltrated layer was tested by the full immersion test and the salt spray test,and the composition of corrosion products was analyzed.The results show that after the optimized layer was soaked for 1000the corrosion rate is only 0.88×10-4g·m-2·min-1.And the corrosion products can be firmly filled into the pores on the surface of the infiltrated layer.The corrosion products in the early stage of salt spray corrosion are mainly ZnO and Zn5(OH)8Cl2H2O.The corrosion products in the middle stage of salt spray corrosion are ZnAl2O4 and the unstable corrosion intermediate FeOCl.The ZnAlO4 phase in the corrosion products in the later stage of salt spray corrosion disappears,but Zn(OH)2 and Al(OH)3 phase appear. |
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