Study On The "Smart" Healing Coatings Of Mg Alloys Based On Micro-Arc Oxidation Coatings

As we all known,application of protective coating is a cost-effective and widespread approach to corrosion protection for metallic substrates.However,in the case of a defect during using,these coatings can often cause severe localized corrosion of metal substrate.To avoid such problems,the concept of a self-healing "smart"coating has been proposed by researchers in recent years that minor damage or defects are actively repaired by such a coating without the need for any external intervention.Embedding corrosion inhibitors directly into coatings is one of the easiest ways to obtain a healing coating.However,the following problems are likely poor dispersion of the corrosion inhibitor throughout the coating,or even lack of functionality because of the interaction between the corrosion inhibitor and coating layer.Therefore,encapsulating the inhibitor in the "container" have been developed by researchers.When the coating is subjected to stimuli from the environment,such as pH,aggressive species,the stored corrosion inhibitor will be triggered release from "containers" and further result in retarding corrosion.The key of this kind of "smart" protection system is the fabrication of encapsulation "containers",and layered double hydroxides(LDHs)have been widely investigated as "nano-containers" because of their excellent ion-exchange capability.In particular,attempts to combine a LDHs coating with a micro-arc oxidation(MAO)coating has become another research hotspot in the field of"smart" protection.On the one hand,preparation of the LDHs on the MAO coating can seal the pores of the MAO coating,which can further improve the corrosion protection provided by the coating for the metallic substrate.To the other hand,the LDHs can store and release inhibitors.If this way is suitable for active protection of magnesium alloy,it will greatly promote the development and application of magnesium alloy.Combining the advantages of MAO coatings and LDHs coatings,the possibility of growing LDHs on MAO coatings was discussed in this paper,the influence of different properties of MAO coatings(formed at different voltages or electrolytes)and different conditions of hydrothermal treatment on the subsequent growth of LDHs were studied.On the basis of above studies,the key "rules" was put forward to optimize the preparation process according to different performance requirements of coatings.In order to further improve performance of coatings,the loading of inhibitors and rare earth modification of MAO/LDHs composite coating were also studied.Effect of MAO coatings formed at different voltages on the in situ growth of LDHs was firstly investigated.MAO coatings were produced at different voltages on the magnesium alloy in this work,leading to the formation of MAO coatings with different properties(e.g.composition,structure and morphology).Then,LDHs grown in-situ on MAO coatings was further studied.The obtained results showed that the dissolution of the MAO coating during hydrothermal treatment determined the properties of final coatings,especially in the structure and thickness of final coatings.When MAO coating formed under lower voltage(150 V or 200 V)was hydrothermal treated,the MAO coating was completely dissolved,leading to formation of final coating with single-layer structure;When MAO coating formed under higher voltage(250 V,300 V or 350 V)was hydrothermal treated,the MAO coating cannot be completely dissolved,leading to formation of final coating with double-layer structure.In addition,all final coatings significantly increased in thickness with the thickest one being 18 ?m.The corrosion resistance tests showed that due to a synergistic effect of LDHs and spinel,MAO coating formed at the 350 V oxidation voltage,modified with LDHs,provided the best corrosion protection.Roles of electrolyte for formation of MAO coatings and hydrothermal treatment solution during the growth of LDHs was further specified.In detail,MAO coatings were produced on magnesium alloy in aluminate,phosphate and silicate-based electrolytes,and followed by hydrothermal treatments in order to synthesis LDHs on MAO coatings.LDHs synthesis was done in three different growth solutions(deionized water,sodium nitrate and aluminum nitrate containing solution).In frame of this work it was shown,that it was difficult to form LDHs on Si-based MAO coating,due to stable Mg2SiO4 phases in comparison with aluminate and phosphate based MAO coatings.The corrosion resistance tests confirmed that,the hydrothermal treatments in Al3+ containing solution played a positive role on overall corrosion resistance for phosphate and silicate-based MAO coatings,but not for Al-based MAO coatings.Based on the characteristics of ion exchange of LDHs gallery,a systematic screening for corrosion inhibitors of magnesium alloys that can be loaded into MAO/LDHs composite coatings was performed at different treatment conditions.In this work,twelve inorganic or organic inhibitors were selected,and different ion-exchange reaction conditions were used,including different pH of inhibitor solution(7,9 or 11),different hydrothermal temperature(50? or 90?),different reaction time(7h,24h or 48h).After ion-exchange reaction,XRD was applied to screen in which conduction inhibitors can be intercalated into LDHs gallery and these inhibitors that could be intercalated successfully into LDHs are ranked by their inhibition efficiency.It showed that MAO/LDHs samples modified with sodium phosphate,2,5-pyridinedicarboxylic acid and potassium oxalate monohydrate at a certain condition had the superior corrosion resistance.These three above coatings with inhibitor were further investigated by SEM,EDS and XRD.At last,a composite coating based on a phosphate loaded LDHs on a cerium-modified MAO coating was prepared.Such composite coating was produced via(i)MAO with Ce salt sealing,on which LDHs were prepared via a hydrothermal treatment,and(ii)then loaded with phosphate intercalation ions via an ion-exchange reaction.The final composite coating(characterized using XRD,XPS,FT-IR,SEM,EDS and GDOES)consisted of LDHs/Mg(OH)2/CeO2/Ce(OH)3 with a non-uniform Ce distribution.The corrosion resistance and localized electrochemical tests showed that,the composite coating after modification had the most superior corrosion protection and self-healing ability,attributed to the synergistic effect between rare earth cation and intercalation anion inhibitors.