Preparation And Mechanism Research Of Duplex Self-Healing Coating Of Mg Alloy Based On The Porosity Of Micro-Arc Oxidation Coating

Magnesium alloys,as the lightest metallic structure materials,have great promising prospect.Their poor corrosion resistance makes the protection coating as a necessity in actual applications.However,once coatings are aged and damaged during service,corrosion is easy to initiate from these damaged areas,accelerating the failures of Mg parts.Self-healing coatings,which can release corrosion inhibitors,is efficient to solve this problem.Although many self-healing coatings are developed,their healing ability are highly limited and many healing mechanisms are unclear.Therefore,a duplex self-healing coating is designed in this research,which loads interphase inhibitor into the pores of micro-arc oxidation(MAO)coating and dopes an organic inhibitor which shows synergism effect with the above-mentioned interphase inhibitor into the top paint to further enhance the self-healing property.The research includes the selection of proper inhibitors,the preparation of self-healing coating,the performance of self-healing property and the study of inhibitor release/migration/synergism mechanism.To select proper inhibitor to prepare self-healing coating,the inhibition effect of 6 inhibitors on AM60 Mg alloy are studied.Among these inhibitors,SDBS(sodium dodecylbenzene sulfonate),SC(sodium citrate)and MBT(2-mercaptobenzothiazole)are organic interface inhibitor,Na3PO4,Ce(NO3)3 and NaVO3 are inorganic interphase inhibitor.The inhibition efficiency of SDBS is up to 97%.MBT,which owns similar structure with SDBS,can inhibit the cathodic reaction.The inhibition efficiency of Na3PO4 is 94%.A type of self-healing coating for AM60 Mg alloy was prepared by loading interphase inhibitor into micro-arc oxidation.Among 3 highly soluble interphase inhibitors Ce(NO3)3,Na3PO4 and NaVO3,the self-healing coating which loads Na3PO4 into MAO pores shows the best property.The exposed Mg substrate in damaged area remains un-corroded after immersion in 3.5 wt.%NaCl solution for more than one week.Its excellent self-healing property is owing to the increase of pH near the release sites of Na3PO4 and the formation of stable protection film Mg3(PO4)2.In comparison with atmospheric impregnation,vacuum impregnation can efficiently load inhibitors into MAO pores both in quantity and depth.However,more impregnation frequency cannot improve the self-healing property.Although the large size of MAO pores is harmful to the corrosion resistance of MAO coating,it is beneficial for the self-healing properties owing to the easy loading of more inhibitors.The synergism effect between organic inhibitor MBT and loaded Na3PO4 is found,and their synergism effect is attributed to the effect of Na3PO4 on the solubility of MBT and the absorption of MBT on the defects of product film.The duplex self-healing coating,which loads Na3PO4 into the pores of MAO coating and dopes MBT into the top paint,shows better healing property than the above self-healing coating.When the duplex self-healing coating is damaged,abundant highly soluble Na3PO4 is quickly released from the cross-section of scratch,and takes the main rule in self-healing process.Besides,small amount of MBT which is released from the top organic coating is absorbed in the defects of healing film,further enhancing the self-healing property.Especially,the release life of MBT is long,which prolongs the healing life of coating.The early corrosion behavior of AM60 Mg alloy,MAO coatings and self-healing coatings with different width of scratches during outdoor exposure is studied.The main corrosion morphology of AM60 is cracked corrosion point.With increasing exposure time,the corrosion points become more and larger.The main corrosion products are MgO,Mg(OH)2,MgCO3 and MgSO4.During the outdoor exposure,MAO coating is partly dissolved,leading to the local detachment of MAO coating.For the blank composite coating which mixed none inhibitor,due to the aggregation of corrosion media and the acceleration effect of scratch,the corrosion in scratch area is worse than the uncoated AM60.Self-healing coating can release Na3PO4 and MBT,inhibiting the corrosion in scratched area.When the scratch is wider,the healing property is still strong.