Preparation Of 5005 Aluminum Alloy Red Mud Ceramic Coating By Plasma Electrolytic Oxidation Technology

5005 aluminum alloy can naturally form an amorphous oxide film in the atmosphere,but it cannot make up for the low surface hardness,poor corrosion resistance,poor wear resistance and other disadvantages in harsh environment,and it is easy to be damaged by corrosion,wearing,scratching and other surface damages during use.Plasma electrolytic oxidation(PEO),also known as the micro-arc oxidation(MAO),is a kind of surface modification technology that uses high voltage and current to directly oxidize and sinter the substrate metal into a high-performance ceramic oxide coating with certain thickness,high density and crystal oxide structure under the coaction of thermochemistry,plasma chemistry and electrochemistry.Adding additives to the electrolyte is an effective way to improve the properties of PEO ceramic coating.Bayer red mud(RM)is a strong alkaline solid waste residue after extracting alumina from bauxite,it proves to be a promising ceramic coating material due to the high content of Al₂O₃,Fe₂O₃,SiO₂,CaTiO₃and TiO₂.In this work,firstly,the mineral composition,morphology,particle size and pore properties of Bayer RM at various heat-treated temperatures were studied by means of differential thermal-thermogravimetric analyzer(DTA-TG),X-ray photoelectron spectroscopy(XPS),X-ray diffractometer(XRD),scanning electron microscope(SEM),laser particle size analyzer,electrochemical workstation,specific surface area and pore analyzer.Then a RM PEO ceramic coating was prepared on the surface of 5005 aluminum alloy by using PEO technology with RM heat-treated at various temperatures as electrolyte additive,respectively.The effects of PEO parameters on the morphology,growth mechanism,phase composition,microstructure and properties of the ceramic coating were studied.In order to improve the compactness of the RM PEO ceramic coating,a layered double hydroxides(LDHs)was grown in situ on the surface of the coating by hydrothermal method,and the coating was sealed and repaired,and by using the tunability of laminate elements and the exchangeability of interlayer anions of LDHs,Cl^–can be effectively isolated and captured,and actively release the corrosion inhibitor to repair the corrosion area,so as to achieve the purpose of dual protection.Microhardness tester,coating adhesion automatic scratch tester,abrasive wear tester,electrochemical workstation and material test system were used to characterize the hardness,binding force,wear resistance,corrosion resistance and creep performance of the coating.The main results are as follows:1.The composition of RM is complex,which is mainly composed of hematite,katoite,cancrinite,calcite,fayalite,disapore,goethite,gibbsite,and perovskite at room temperature.RM has small particle size and belongs to mesoporous material.After heat treatment,the color,phase composition,morphology,particle size,specific surface area and pore volume of RM are changed significantly.2.The pure PEO ceramic coating consists of?-Al₂O₃and?-Al₂O₃,while the RM PEO ceramic coating is composed of?-Al₂O₃,?-Al₂O₃,CaTiO₃,CaCO₃,SiO₂,Fe₂O₃and compounds formed at high temperature.The surface properties of 5005 aluminum alloy substrate are significantly improved by the presence of PEO ceramic coating.At the same time,compared with the pure PEO ceramic coating,the RM PEO ceramic coating has larger thickness,denser structure,stronger binding force,better wear resistance and corrosion resistance.Under the same PEO parameters,RM PEO ceramic coatings with different properties can be obtained by selecting different heat-treated temperatures,among them,the coating prepared by RM heat-treated at 300?has better comprehensive properties.3.A dense LDHs nanosheet was grown in situ on the surface of RM PEO ceramic coating,which can effectively seal and repair the coating and improve its compactness and corrosion resistance.Solution pH is the most critical factor in the synthesis of LDHs.The results show that the weak acid solution with the synthesis temperature of about 70?is better for the nucleation and growth of LDHs.The longer the synthesis time,the fuller the nucleation and growth of LDHs,and the better the corrosion resistance.The corrosion resistance of LDHs can be further improved by inserting anionic corrosion inhibitor or cationic corrosion inhibitor alone,especially the LDHs with anionic and cationic corrosion inhibitors has the best corrosion resistance.4.The results of indentation creep test show that the RM PEO ceramic coating after sealing treatment has high hardness,elastic modulus and elastic recovery ability.The properties of the coating can be adjusted according to the requirements of different application environments.Through the optimization of the preparation process and the later sealing treatment of the coating,not only the hardness,binding force,wear resistance and corrosion resistance of the coating are improved,but also the indentation creep performance is improved.