Experimental Study On Corrosion Behaviour Of SiO₂ Reinforced Magnesium Based Nanocomposites

With the development of modern science and technology,the material performance requirements are higher and often hope that the material has a certain special performance,but also has a good overall performance.The limited ductility and low wet corrosivity of metallic magnesium at room temperature limit its use in many applications.In order to enhance the mechanical properties of the metallic magnesium material,it may be an effective method to add a cheap low volume fraction of nanoparticles?NP?resulting in decentralized reinforcement.Nano-silica is a high-tech ultra-fine inorganic new material,its particle size is very small,the specific surface area is large,surface adsorption strong,high chemical purity,good dispersion and other specific performance advantages.So,it has irreplaceable effect in many areas.Because of the series of advantages of magnesium and nano-silica materials,pure magnesium is used as the matrix of Mg-based composites,and the cost of the two materials is low.Therefore,the study of SiO₂ particles reinforced magnesium based nanocomposites by powder metallurgy method will have a certain reference value.Related workers have successfully synthesized SiO₂ nanoparticles?NP?reinforced magnesium based nanocomposites by two-directional microwave assisted rapid sintering technology of powder metallurgy technology,and analyzed the micro-hardness,0.2% CYS,compressive fracture strain,damping capacity and damping loss rate performance.The results show that with the addition of SiO₂ nanoparticles?NP?,these properties have been improved and enhanced in different degrees.The excellent mechanical properties of Mg/SiO₂ new magnesium-based nanocomposites indicate that it will have not alternative application prospects for most of the metal materials.Poor corrosion resistance is common performance for magnesium matrix composite material.even it is also very easy to form a layer of MgO layer at room temperature,but this layer of magnesium oxide layer is loose and porous,magnesium matrix composite matrix can not be provided a good protective effect.There are many researchers have studied the corrosion behavior of similar magnesium-based composites in typical NaCl and Na₂SO₄ solutions.Therefore,it is necessary to study the corrosion behavior of the Mg/SiO₂ new magnesium-based nanocomposites in some typical media?NaCl and Na₂SO₄?.In this paper,the microstructure characteristics of Mg/Si O₂ new composites prepared by powder metallurgy hot extrusion method were analyzed by optical microscope,scanning electron microscope,energy spectrum analysis and XRD diffraction.The corrosion resistance performance was evaluated by immersion experiments and electrochemical experiments in NaCl and Na₂SO₄ solution.Corrosion morphology and corrosion cross-sectional morphology were observed and energy spectrum analysis for a certain time corrosion of the sample by scanning electron microscopy?SEM?.Corrosion behavior was analyzed by electrochemical impedance spectroscopy?EIS?and corrosion mechanism model was proposed.The final conclusions are as follows.?1?The microstructure characteristics of the new Mg/SiO₂ composites were analyzed in this paper.The results show that the black aggregate of the second phase?compound of magnesium,silicon and oxygen?is distributed on the grain boundary by powder metallurgy hot extrusion method,and the grain size of Mg/SiO₂ composites decreases with the increase of SiO₂ content.?2?In the NaCl solution,the SiO₂ content has obvious effect on the corrosion of the composites.The weight loss method,the SEM observation and the electrochemi-cal method show that the corrosion rate of the composites becomes faster with the increase of SiO₂ content.The temperature also has a significant effect on the corrosion of the composite.The temperature was raised from 25 ? to 35 ?,the corrosion rate of composite materials increased nearly doubled.The electrochemical impedance spectroscopy?EIS?analysis showed that the impedance arc radius and charge transfer resistance of the composites decreased with the increase of the immersion time,and corrosion rate is accelerated.The corrosion process basically reached a relatively stable condition about 10 h,and the corrosion process is proceeding smoothly.In the early stage of corrosion,local pitting occurred near the grain boundary.After a period of corrosion,the area of local pitting at the grain boundary gradually increased and was expanded to the Mg/SiO₂ composite matrix.This is characterized by intergranular corrosion.At last,the corrosion of the composite material extends into the interior of the matrix,a large area of corrosion occurred.The addition of SiO₂ accelerates the corrosion of pure Mg matrix,which is due to the formation of Mg2 Si during the in-situ autogenous reaction of the Si O₂ powder and the Mg powder.Mg2 Si as the cathode and Mg as the anode,and the occurrence of microgalvanic corrosion accelerates the corrosion process.?3?In Na₂SO₄ solution,it is the same law the the SiO₂ content has obvious effect on the corrosion of the composites as in NaCl solution.The weight loss method,SEM observation and electrochemical method all show that the corrosion rate of the composites increases with the increase of SiO₂ content.The temperature also has a significant impact for the corrosion of composite material.The temperature was raised from 25 ? to 35 ?,the corrosion rate of Mg/SiO₂ composites is nearly 0.5 times in Na₂SO₄ solution,which is smaller than in NaCl solution.However,the corrosion process in the Na₂SO₄ solution and in NaCl solution is different,electrochemical impedance spectroscopy analysis shows that the corrosion rate is also accelerated with the immersion time increases,the corrosion process basically reached a relatively stable condition about 10 h,and the corrosion process is proceeding smoothly.In the early stage of corrosion,a protective film was formed at the interface of the corrosion.After a period of corrosion,corrosion occurred at the grain boundary,and the intergranular corrosion occurred.Finally,the composite material produced a large area of corrosion.?4?The corrosion rate of the composites in Na₂SO₄ solution is much smaller than in NaCl solution.The reason is as follows.1)the composite material forms a layer of thin corrosion products protective film in Na₂SO₄ solution at the beginning,the thin layer plays a certain blocking role on the corrosion.However,the composite material forms local corrosion products by the local corrosion in Na₂SO₄ solution at t-he beginning,and it has no protective effect for the bare metal;2)Cl-radius is much smaller than the SO42-,and the migration and aggressive of Cl-much higher than SO42-.?5?The addition of SiO₂ accelerates the corrosion of pure Mg matrix.The reason is as follows.1)The formation of Mg2 Si during the in-situ autogenous reaction of the SiO₂ powder and the Mg powder.Mg2 Si as the cathode and Mg as the anode,and the occurrence of microgalvanic corrosion accelerates the corrosion process.2)During the sintering process,SiO₂ makes the grain refinement of the matrix,the grain boundary increases,and the grain boundary dislocation is high.Thus,the grain boundary occurs to corrosion firstly and it accelerats corrosion process.