Preparation Of Thermal Sprayed Metal Coatings And Study On Corrosion Behavior In Simulated Marine Environment

Corrosion is one of the important forms of metal failure,especially in the Marine environment,where the Cl^-existing in seawater or atmosphere is absorbed on the metal surface,causing local metal damage.Therefore,it is of great significance to prepare a layer of corrosion resistance coating by thermal spraying on the surface of metal structure to increase the service life of structural parts and reduce the later maintenance cost.In this thesis,Zn,Al and KM99 coatings were prepared by arc spraying,Cr₂O₃-25Ti O₂coatings were prepared by plasma spraying,and WC-10Co4Cr coatings were prepared by HVOF.Scanning electron microscope was used to observe the surface morphology of powder,wire and coating,and energy spectrum analyzer was used to analyze the surface element composition of coating.The phase composition of filament,powder and coating was analyzed by X-ray diffractometer,and the hardness of coating was tested by microhardness tester.Based on relevant theses and production experience,Zn and Al coatings with different parameters were prepared,and the coatings with optimal parameters were selected by roughness,surface three-dimensional morphology,bonding strength and so on.The coatings were immersed in 3.5%Na Cl solution for 30days.The samples were taken out at different times.Electrochemical workstations were used to detect and analyze the corrosion resistance of the coatings at different stages,and an equivalent circuit model was established according to electrochemical impedance spectroscopy（EIS）.After soaking for 30 days,the surface morphology and phase composition of the coating were observed to determine whether the coating failed.The Zn,Al and KM99 coatings all formed good mechanical bonding with the substrate,and some unmelted particles were contained in the coatings.Cr₂O₃-25Ti O₂coating had a layered structure,and formed Cr₂O₃rich zone and Ti O₂rich zone.XRD results showed that the coating contains Cr₂O₃and Ti O₂phase,and the average hardness of the coating was 1068.86HV_0.2.The average hardness of WC-10Co4Cr coating was1157.22HV_0.2.Electrochemical experiments were divided into polarization curve test and electrochemical AC impedance test.The Zn coating was more prone to corrosion than the substrate at the initial stage of corrosion,while the Al coating had better corrosion resistance at the initial stage of immersion.After 2 days of immersion,the Al coating was more prone to corrosion than the substrate.Zn and Al coatings,as corrosion resistant coatings,mainly had two functions.One was the corrosion resistance of the coating itself.For example,Al coating would form passivation film.The other was the preferential corrosion over the substrate,thereby sacrificing the coating itself to protect the substrate.Ac impedance test could prove that in the process of continuous corrosion of coating,surface corrosion products were absorbed to form corrosion product capacitance,and the corrosive medium would reach the matrix interface and corrosion the matrix.After 30 days of immersion,the main phase on the surface of the Zn coating was Zn,and corrosion had occurred at the interface between the coating and the substrate.Na Cl phase had existed in the pores of Al coating,the overall coating presented honeycomb shape,part of the coating dissolves,and precipitates were formed in the corrosive medium.KM99 coating as Fe-based amorphous anti-corrosion and wear-resistant coating was gradually popular.The main phase of the coating was Fe,and there were amorphous phases.The coating was composed of good melting zone and semi-melting zone.It was laid on the surface of the substrate,forming a layered structure.The average hardness of the coating was 705 HV_0.2.The self-corrosion potential of KM99was higher than that of the matrix,and it had better corrosion resistance than that of the matrix.It could be seen from the equivalent circuit model that the coating had a good bonding ability with the substrate,and the corrosive medium did not enter the coating pores at the initial stage of corrosion.The corrosion products of the coating accumulated at the later stage of soaking and the corrosion rate decreased.Due to the presence of Cr,the plasma sprayed Cr₂O₃-25Ti O₂coating and supersonic flame sprayed WC-10Co4Cr coating showed passivation tendency in polarization curve test,and the corrosive medium reached the matrix interface after 30days of immersion.Optimizing spraying process and post-treatment of coating pores was an important process to increase corrosion resistance of coating.