Study On Intrinsic Self-healing Protective Coating For Magnesium Alloys

As having many advantages such as low density,high specific strength and good thermal conductivity and so on,magnesium alloys are widely used in many applications,such as electronics,transportation and energy,etc.However,magnesium alloys are chemically active and easy to be corroded,which greatly limits its application and development.In order to protect magnesium alloy from corrosion,coating protection technology has been widely used.However,it is easy to be subjected to mechanical or chemical damages when the coating is used for a long time or in harsh environment,which leads to the failure of its protective performance.Inspired by the wound healing of natural organism,the concept of self-healing protective coating is proposed,and self-healing protection has gradually become an important development direction of metal corrosion protection.However,there are few reports about the self-healing protective coatings of magnesium alloys.The related reports only involve extrinsic self-healing coatings,that is,the self-healing function of the coating depends on the controlled release process of corrosion inhibitor or repair agent.This kind of extrinsic self-healing protective coating has the disadvantages of slow healing speed,low healing degree and limited healing times,which can not provide long-term protection for magnesium alloys.With the development of polymer technology,self-healing materials based on reversible covalent or noncovalent bond interactions have attracted much attention.These materials are typical intrinsic self-healing materials when used as self-healing materials,and the self-healing coatings have the advantages of fast healing speed,high healing degree and multiple healing times.However,it is a great challenge to use this kind of intrinsic self-healing materials for the protection of active metals such as magnesium alloys.The main difficulties are as follows:（i）Polymer solution will cause unnecessary corrosion to magnesium alloy in the coating-forming process,so it can not be directly used for magnesium alloys;（ii）There is a lack of sufficient adhesion between polymer coating and magnesium alloy substrate.In order to solve the above problems,the strategy of"bottom coating-self-healing polymer outer coating"was designed and constructed on the surface of magnesium alloy.The introduction of the bottom coating cannot only avoid the corrosion of magnesium alloy during the preparation of the polymer coating,but also provide a good adhesion between the polymer coating and the magnesium alloy substrate.By using the above strategy,the self-healing protective coating with the characteristics of fast healing speed,high healing degree and multiple healing times was successfully preparedon magnesium alloy.The surface morphology and composition of the samples were characterized by field emission scanning electron microscopy（FESEM）,atomic force microscopy（AFM）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared（FTIR）and Raman spectroscopy.The formation mechanism of outer polymer coating was studied by confocal laser scanning microscopy（CLSM）.The self-healing ability and the self-healing stabilities of the coatings were characterized by healing various damages.The healing kinetic of the coating was studied by in-situ observing the cutting-healing process using optical microscope.The self-healing mechanism of the coating was researched based on the mechanical properties,water absorption capacity and electrochemical impedance spectroscopy（EIS）.The protective performance of the coating was characterized by immersion in corrosion medium and EIS tests.The results show that the self-healing protective coating has good self-healing and protective properties.The main content is listed as follows:（1）Preparation and characterization of the bottom coating.The bottom coating was prepared on the surface of magnesium alloy by boiling water bath,sodium hydroxide solution bath,hydrothermal treatment in pure water and in sodium hydroxide solution,respectively.The results show that the bottom coating obtained by hydrothermal treatment in sodium hydroxide solution has the best performance.The coating is composed of Mg O and Mg（OH）₂,which contains a large amount of hydroxyl groups.The coating is firmly bonded with the substrate（adhesion strength grade is 5B）.In addition,the surface of the bottom coating is superhydrophilic with a water contact angle of only 8.2°,which is conducive to the full contact between the polymer solution and the surface of the bottom coating.The bottom coating not only improves the corrosion resistance of magnesium alloy in polymer solution,which avoids the corrosion of magnesium alloy substrate in the subsequent polymer outer coating forming process,but also provides a good adhesion between the polymer outer coating and the magnesium alloy substrate.（2）Preparation and characterization of intrinsic self-healing protective coating.With the help of electrostatic interaction,a multilayer polymer outer coating containing polyethyleneimine（PEI）and polyacrylic acid（PAA）was prepared on the surface of the hydrothermal bottom film by repeatedly alternately dipping coating method,and the intrinsic self-healing protective coating was successfully prepared on magnesium alloy.The outer coating thickness increases exponentially and then linearly with the number of coating layers.CLSM characterization results show that the growth mode of the coating is mainly due to the―in-and-out‖diffusion.The thickness,surface roughness and mechanical properties of the coatings are affected by polymer molecular weight and solution ionic strength.Larger polymer molecular weight and higher ionic strength leads to higher coating thickness,surface roughness and mechanical strength.The coating deposited alternately for 30 times by PEI（MW=70000）and PAA（MW=3000）is uniform and dense with the root-mean-square roughness（R_q）of only 1.99 nm,the thickness of 39.4±3.7μm and Young’s modulus of 1.09±0.12 GPa,which is suitable to be used as the self-healing protective coating.Also,the self-healing protective coating has good adhesion to the substrate（adhesion strength grade is 5B）.（3）Study on the performance and self-healing mechanism of intrinsic self-healing protective coating.The self-healing performance and self-healing stabilities of the self-healing protective coatings were studied by multiple scratchting,sandpaper wearing,high temperature and organic solvent immersion tests.The results show that the cuts of different scales（25μm-120μm）on the coatings can be completely healed in the environment of 100%relative humidity（RH）within 25 minutes,and the coating can be completely healed after repeated damages,which proves that the intrinsic self-healing protective coating has excellent self-healing ability with the characteristics of fast healing speed,high healing degree and multiple healing times.In addition,the self-healing protective coating has excellent mechanical stability,thermal stability and chemical stability.The result of self-healing kinetic shows that the cutting-healing process of the coating is mainly affected by environmental humidity,p H and ion strength.The higher humidity,stronger acidity/alkalinity and higher ionic strength result in faster healing speed.Based on the results of mechanical properties,water absorption capacity and EIS tests in different humidity environment,the self-healing mechanism of the coating is proposed as follows:The adsorbed water molecules in high humidity environment can interact with the amine and carboxyl groups in the polymer coating,which weakens the interaction between the polymers to a certain extent and enhances the migration of the polymer molecules.As a result,the polymer molecules migrate and fill in the cuts or other damages and the coating is healed under the action of the electrostatic interaction.The result of corrosion media immersion shows that the self-healing protective coating remains intact after 72 h immersion in artificial sweat.EIS tests show that the coating resistance（R_c）of the self-healing protective coating is as high as 676.75 kΩ·cm²,which is much higher than the R_c value of the hydrothermal-treated sample（33.53 kΩ·cm²）and the charge transfer resistance（R_t）of magnesium alloy substrate（0.95 kΩ·cm²）,indicating that the self-healing protective coating has excellent protective performance for magnesium alloys.（4）Modification of self-healing protective coating.In order to further improve the mechanical and protective properties of the self-healing protective coating,the modified self-healing protective coating was prepared by dispersing Si O₂ nanoparticles into PAA solution,and then prepared by the same method.The results of morphology and composition characterizations show that Si O₂ nanoparticles are successfully introduced into the coating.The modified coating has good adhesion to magnesium alloy substrate（adhesion strength grade is 5B）,excellent self-healing ability（cuts of 30μm-110μm scale can be 100%healed in 100%RH environment,and can be healed multiple times）,good mechanical stability,thermal stability and chemical stability,and the mechanical strength increases to 1.6 times of the original value（Young’s modulus is 1.78±0.22GPa）.The results of corrosion media immersion and EIS tests show that the surface morphology of the modified coating remains intact after 72 h immersion in artificial sweat,and the coating resistance（R_c）is as high as 4.12 MΩ·cm²,which is nearly one order of magnitude higher than that of the self-healing protective coating,suggesting that the protective performance of the modified self-healing protective coating is greatly improved.In order to further investigate the practical application of the self-healing protective coating,a composite modified self-healing protective coating was prepared by combining with the epoxy resin commonly used in production and life.The composite modified protective coating not only maintains excellent self-healing ability,but also greatly improves the protective performance of magnesium alloy（the coating remains intact after soaking in artificial sweat for 72 h,and the R_c value is as high as157.80 MΩ·cm²）.The above results have guiding significance for the practical application of intrinsic self-healing protective coating,and have positive significance to expand the application of magnesium alloys.