Preparation And Characterization Of AZ91 Magnesium Alloy Conductive Composite Coating For Vehicle Radar Chassis

With the light weight requirements of military equipment,magnesium alloys are gradually applied in the above fields.Due to the active nature of magnesium alloys,it is prone to corrosion,which limits its application range.Nowadays,micro-arc oxidation（MAO）has been used to modify magnesium alloys to increase the corrosion resistance.However,electrical conductivity is also required to achieve electrostatic shielding,especially for the usage as case material for vehicle radar.Therefore,it is necessary to prepare coatings with electrical conductivity and corrosion resistance on the basis of micro-arc oxidation layer.In this study,dopant-modified polypyrrole powder（PPy）and graphene composite polypyrrole powder（PPy/GN）were prepared by chemical oxidation method.The above two powders were used as conductive fillers,and epoxy resin E44（EP）was used.Two types of conductive anti-corrosion coatings were obtained by mixing,epoxy/polypyrrole（EP/PPy）and epoxy/polypyrrole/graphene（EP/PPy/GN）were prepared on the surface of AZ91 magnesium alloy by spin coating method,exploring the effects of different process parameters on the corrosion resistance and electrical conductivity of composite coatings.The main results of this thesis are as follows:（1）PPy powders were prepared by using hydrochloric acid（HCl）,oxalic acid（H₂C₂O₄）and sodium benzenesulfonate（C₆H₅SO₃Na）as dopants.The powders were successfully prepared and characterized by SEM&EDS,XRD and FT-IR techniques.Four probes powder conductivity test shows that the conductivity of the PPy powder after doping was improved.Among them,the conductivity of PPy modified with sodium benzenesulfonate was the highest,which reached to 4.612 S/cm.The PPy modified by sodium benzenesulfonate was compounded with different mass ratios（1%,5%,10%）of GN.The results of SEM&EDS,XRD and FT-IR observation showed that the addition of GN made the morphology of the composite powder to be lamellar.The four-probe powder conductivity test showed that the conductivity of the PPy/GN composite powder was improved.Among them,the PPy after 10%GN composite had the highest conductivity of 20.2 S/cm.（2）The optimized parameter combination was obtained by comparing the morphology and corrosion resistance of the coatings obtained under different spin coating process parameters,that is,when the spin coating speed was 1000 rpm and the spin coating time was 10 s,the EP/PPy1%coating well covered the substrate with good uniformity.corrosion current density was examined to be 10^-6A·cm^-2,which was one order of magnitude lower than that of the other control groups.Two composite coatings were prepared by mixing C₆H₅SO₃Na modified PPy powder and GN modified PPy-C₆H₅SO₃Na powder with EP uniformly.With the addition of PPy powder,the water contact angle of the coating was between 80-90°,and the roughness R_avalue was between 10-20 nm;and with the addition of PPy/GN powder,the water contact angle of the coating is between 100-120°.The roughness R_avalues were between 20-70 nm,and the coating thicknesses were 58±6μm and 76±7μm,respectively.（3）The corrosion resistance of the coatings was compared and analyzed by electrochemical experiments,accelerated heating immersion experiments and neutral salt spray resistance tests.The results showed that the coatings contained epoxy resin/polypyrrole 10%（EP/PPy10%）and epoxy resin/polypyrrole/graphene 5%（EP/PPy/GN5%）exhibted better corrosion resistance,and the self-corrosion current densities were 10^-8A·cm^-2and 10^-7A·cm^-2,respectively.The EP/PPy10%and EP/PPy/GN5%coatings maintained good surface morphologies after 96 h of salt spray test.The resistivity of the coating decreased with the increase of the powder content,which was 3-4 orders of magnitude lower than that of the EP coating,and all were less than 10⁸Ω·m,which meets the requirements of the national requirements for materials to conduct static electricity.With the increase of PPy and PPy/GN,the distance between powders decreases so that the barrier of electron transport can be broken,and the large specific surface area of GN can increase the connection between PPy and make the resistivity decrease,but PPy/GN exceeds 10%,the corrosion resistance of the coating was reduced to a certain extent,which may be caused by excessive GN,which will cause defects in the coating.These defect will provide corrosion channels,accelerated corrosion.In summary,in this thesis,two kinds of conductive anti-corrosion coatings,EP/PPy and EP/PPy/GN were successfully prepared on the surface of AZ91magnesium alloy by spin coating method.The degree of improvement,and the resistivity all meet the national static conduction requirements,it has a production guiding significance for the preparation of conductive anti-corrosion coatings on the radar chassis using the AZ91 magnesium alloy micro-arc oxidation surface.