Study On Corrosion Behaviour Of Binderless WC-Al₂O₃ Hard Composites

Cemented carbide is widely used in cutting tools,drilling tools,drawing dies and wear-resistant coatings due to its many advantages such as high hardness and high wear resistance.With the continuous expansion of the application range of cemented carbide,the service environment is becoming more and more complex,such as special sealing materials,high-pressure medium nozzle materials,sliding materials and precision mold materials and other applications.In addition to its excellent mechanical properties,high requirements are also placed on its corrosion resistance.However,the Co is prone to corrosion due to its active chemical properties,and Co resources in China are relatively scarce and expensive.Therefore,the development of binderless WC-based cemented carbide has become an inevitable demand.Although scholars at home and abroad have carried out a lot of research on the corrosion resistance of traditional WC-Co cemented carbide,there are few reports on the corrosion resistance of binderless WC-Al₂O₃ hard composites.Therefore,the corrosion behavior and corrosion mechanism of WC-Al₂O₃ composites in three typical corrosion media were studied.This research has great academic value and practical application significance.The main research work and results of this paper include the following aspects:1.The corrosion behavior of WC-Al₂O₃ composite in three typical corrosion media with different concentrations was studied.The results show that WC-Al₂O₃composite has the best corrosion resistance in H₂SO₄ solution,followed by NaCl solution,and the worst corrosion resistance in Na OH solution.With the increase of NaCl and Na OH solution concentration,the corrosion resistance of WC-Al₂O₃composite is gradually improved,and the oxygen absorption reaction becomes the dominant factor in the electrochemical reaction.This is due to the increase of solution concentration,the dissolved oxygen content is significantly reduced,which can inhibit the electrochemical reaction.With the increase of H₂SO₄ solution concentration,the corrosion resistance of WC-Al₂O₃ composite is gradually reduced,and the hydrogen evolution reaction becomes the dominant factor in the electrochemical reaction.This is due to the increase of H⁺ion after the increase of solution concentration,which can accelerate the electrochemical reaction.2.The corrosion mechanism of WC-Al₂O₃ composite in three typical corrosive media were investigated,and it was found that the different corrosion mechanisms in three typical corrosive media resulted in significant different corrosion resistance.In H₂SO₄ solution,the relatively stable WO₃ oxide film isolates the contact between the material surface and the corrosive medium and inhibits the electron conduction,thus improving the corrosion resistance of the composite.In NaCl solution,in addition to the formation of WO₃ oxide film,the electrochemical corrosion of WC phase and chemical corrosion of Al₂O₃ phase can both generate OH^-ion,which leads to the increase in the local p H value of the solution,and the internal WC phase will be dissolved in local alkaline environment.Therefore,the corrosion resistance in NaCl solution is slightly worse than that in H₂SO₄ solution.However,in Na OH solution,the surface of WC-Al₂O₃ composite fails to form an effective oxide film,and the WC matrix is significantly dissolved,which leads to the peeling of Al₂O₃ phase and reduces the corrosion resistance of the material.3.The influence of TiC additive on the corrosion resistance of WC-Al₂O₃composite in three typical corrosive media were studied.The influence mechanism of grain size,chemical composition and density on the corrosion resistance of WC-Al₂O₃composite was revealed in depth.The results show that the chemical composition becomes the main controlling factor affecting the electrochemical corrosion reaction of WC-Al₂O₃ composite when the TiC content is 0-10wt.%.However,when the TiC content reaches 20wt.%,the corrosion resistance is significantly reduced,which is due to the decrease of the density of the composite.At this time,the density of composite becomes the main controlling factor affecting the electrochemical corrosion reaction of WC-Al₂O₃ composite.4.Two kinds of WC-Al₂O₃ composites with different grain sizes were prepared,and their corrosion resistance in three typical corrosive media were compared.The results show that the coarse-grained WC-Al₂O₃ composite has better corrosion resistance than the fine-grained WC-Al₂O₃ composite.This is mainly due to the large grain size resulting in less contact between grain boundaries and corrosive solution,compared with the fine-grained WC-Al₂O₃ composite,the coarse-grained WC-Al₂O₃composite has higer corrosion potential and lower corrosion current density.5.The corrosion behavior of binderless WC-Al₂O₃ hard composite and traditional WC-Co cemented carbide were compared.The results show that:(1)The corrosion resistance of WC-Al₂O₃ composite in H₂SO₄ and NaCl solution is better than that of traditional WC-Co cemented carbide.Especially in H₂SO₄ solution,the corrosion resistance of WC-Al₂O₃ composite is much higher than that of traditional WC-Co cemented carbide.In Na OH solution,the corrosion resistance of WC-Al₂O₃ composite is lower than that of traditional WC-Co cemented carbide.(2)When TiC additive is introduced into WC-Al₂O₃ composite,the corrosion resistance of WC-Al₂O₃-TiC composite is better than that of traditional WC-Co cemented carbide in three solutions.(3)Compared with WC-Co cemented carbide,temperature has little effect on the corrosion resistance of WC-Al₂O₃ composite in three solutions.Therefore,when the temperature changes greatly,the application of WC-Al₂O₃ composite has more advantages.(4)Compared with WC-Co cemented carbide,in NaCl and H₂SO₄solutions,the surface roughness has less effect on the corrosion resistance of WC-Al₂O₃composite.In Na OH solution,the surface roughness has greater fluence on the corrosion resistance of WC-Al₂O₃ composite.6.There are many corrosion pits and obvious corrosion cracks in WC-Al₂O₃composite after being corroded under the ultrasonic vibration in corrosive solution.This may be due to the oscillating effect caused by ultrasonic,which makes the grains or corrosion products on the surface easier to fall off,thus accelerating the initiation and growth of cracks.However,WC-Al₂O₃ composite containing TiC additive still have good stability of corrosion products under the ultrasonic vibration.