Safoora Farshid,Mahshid Kharaziha

Biomaterials Research Group,Department of Materials Engineering,Isfahan University of Technology,Isfahan,8415683111,Iran

Abstract Magnesium(Mg)and its alloys have become a hot research topic in various industries owing to the specifi physical and chemical properties.However,high corrosion rate is considered the key lifetime-limiting.Plasma electrolytic oxidation(PEO)method is a simple strategy to deposit an oxide layer on the surface of light metals such as magnesium alloys,to control corrosion rate and promote some other properties,depending on their performances.Nevertheless,their features including their micropore size,distribution,and interconnectivity,and microcracks have not been improved to an acceptable level to support long-term performances of the magnesium-based substrates.Studies have introduced micro/nano-enabled approaches to enhance various properties of PEO coatings such as corrosion resistance,tribological properties,self-healing ability,bioactivity,biocompatibility,antibacterial properties,or catalytic performances.These strategies consist of incorporating of micro and nanoparticles into the PEO layers to produce multi-functional surfaces or the formation of multi-layered coatings to cover the defects of PEO coatings.In this perspective,the present paper aims to overview various nano/micro-enabled strategies to promote the properties of PEO coatings on magnesium alloys.The main focus is given to the functional changes that occurred in response to the incorporation of various types of nano/micro-structures into the PEO coatings on magnesium alloys.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Plasma electrolytic oxidation;Corrosion resistance;Tribological properties;Biological properties;Micro and multi-layer coatings.

1.Introduction

Over the last centuries,magnesium attracted wide researches due to their specifi features,including high specifi strength,low density(1.35-1.85g/cm3),high damping capacity and biodegradability and,bioactivity.These features led to a bunch of applications,consisting of automobile[1],aerospace[2],electronics[3],and biomedical industries[4].However,it is associated with some challenges such as high chemical activity and low corrosion-resistant,poor plasticity and,some weaknesses over wear resistance and tribological features[5].There are several methods to improve the properties of magnesium,including alloying(e.g.AZ31,AZ91,WE43 and MA8 alloys)[6,7],composite formation[8]and surface modification[9-11].

One of the most common approaches to improve magnesium properties is surface modificatio using various techniques including electroless[12-14],electrochemical plating[15,16],conversion coating[17-19],physical vapor deposition[20,21],plasma spray[22,23],anodizing[24],chemical vapor deposition[25,26],and plasma electrolytic oxidation[27-29].Plasma electrolytic oxidation(PEO)is based on the formation of an oxide layer on the surface of metallic substrates,under applying a high voltage[30,31].During this process,micro arcs result in the fusion and bonding of coating and substrate[32].Besides,high amount of porosities and roughness could make it a proper intermediate layer for other coatings[31].However,a high fraction of porosity and micro-cracks on the PEO surface limit its applications by reducing corrosion resistance[33].Exhaustive researches have been performed to modulate the properties of PEO coated magnesium alloy.Modificatio of the electrolytes via incorporation of various nanoparticles to optimize the microstructure and fil some of the pores and cracks is one of the most common approaches[34-36].In these approaches,the properties of nanoparticles such as size,Zeta potential and,high surface energy could affect the corrosion resistance[37,38],tribological properties[39,40],self-healing properties[41],catalytic activities[42]and biological features[43,44].Another approach is the formation of multi-layer coatings based on PEO coating.Ceramic[45,46]and polymer[47-49]coatings have been applied on the PEO layer using various strategies,including sol-gel,electrophoretic deposition,atomic layer deposition,plasma spray and,hydrothermal.In addition,these multi-layer coatings can cover the porosities completely,leading to promote corrosion resistance.According to our knowledge,there is no complete review on nano/micro-enabled based approaches to improve PEO coating features,over the recent years,while the main goal of this text is covering all aspects of these approaches.

Fig.1.Schematic presentative of PEO coating steps on a light-metal substrate.

2.Plasma electrolytic oxidation process

PEO is a high voltage,anodic deposition process that results in an oxide-based coating on light metals,including titanium,aluminum,zirconium,magnesium,and their alloys[33,50].This approach is not common for other types of metals since higher arc voltage and current density are required than light metal substrates,during the coating process,leading to the consumption of high electrical energy[51].PEO process is started with the preparation of samples,including surface polishing,cleaning,and pretreatments followed by providing a proper electrolyte bath,depending on the expected properties.To provide a suitable electrochemical set-up,a pair of electrodes including a cathode(usually steal electrode)and anode(the workpieces)is required.After providing the set-up,proper potential and current are used to develop PEO coating.PEO process could be performed by different current modes,including unipolar or bipolar.Finally,the samples will be air-dried[52].According to Rakoch et al.[53]report,there are four main steps to deposit an oxide layer on a substrate during the PEO process,including anodizing and,or electrolysis,spark discharges,micro-arc discharges and arc discharges.According to the schematic provided in Fig.1,during the anodizing and/or electrolysis step,in addition to remarkable increase of anode voltage around the working electrode,a highly porous coating is formed on the workpiece substrate.In the next step(park discharges),owing to the employment of a voltage more than the dielectric capacity of the oxide layer,a discharge happens.The last two steps(micro-arc discharge and are discharge)occur simultaneously with different geometrical discharges’sizes and intensity and lead to the growth of deposition.At the last stage of arc discharge,micro arcs are applied on the electrolyte rather than coating because of the high liberated energy of discharges.Hence,local micro-sized defects appeared in the coatings,which are not desirable due to a deteriorative effect on the corrosion resistance[53].

3.The mechanism of PEO coating formation on magnesium alloys

There are lots of theories to understand how PEO coatings deposit on various substrates[54-59].Mechanism of the PEO coating formation cannot be predicted easily,due to different reactions happened on the PEO coatings.These factors consist of chemical,electrochemical,plasma chemical,and thermal reactions[54].Moreover,substrate composition,type and concentration of electrolytes and electrical parameters could control the deposition process.General reactions that happen during the PEO coating of Mg-based alloys consist of the following steps:

The PEO process begins with the formation of a barrier oxide fil on anode through the firs seconds.Coating grows from weak spots(intermetallic Al-Mn phase)because of differences between the chemical activity of different phases(Step 1).By increasing the applied voltage,dielectric breakdown happens in the oxide fil(Step 2).Then,by increasing the time and voltage,products spread over the total surface.After that,oxide fil grows by enhancing sparks leading to thickening the layer at sparking spots(Step 3).Consequently,porosities are produced between the spots in the oxide layers.In continue,larger pores and some micro-cracks create owing to larger sparks,and lead to porous layers(Step 4).Consequently,the plasma media is produced and chemical reactions enhance due to discharges that promote temperature and pressure.In continue,new phases are produced by the reaction of electrolyte anions and substrate cations and puncture conversion products(Step 5).As sparks continue,oxide layers grow gradually,and defects are covered via layer by layer coating process[56,59].

3.The role of various parameters on the PEO process of magnesium alloys

Various types of electrolytes have been applied to develop PEO coatings,including aluminates,phosphates,and silicates[60].Numerous properties of electrolytes,including composition,concentration,conductivity,and pH of solution can effectively control the morphology,thickness,and porosity of coatings leading to the modulation of their tribological and corrosion protection properties.For example,sodium phosphate-based electrolytes produce Mg2P2O7and NaMg(PO3)2,which provide improved corrosion protection[61].Table 1 shows the role of more common electrolyte compositions on the properties of the coatings.The chemical composition of the electrolytes can indirectly control the electrical conductivity of the solution which is a crucial parameter in the formation of dense and compact PEO coatings.According to Ghanbari et al.study[62],the thickness of coatings reduced and their porosity enhanced with increasing the conductivity of solution.As a result,KOH has been well known as a proper composition to diminish the conductivity of electrolytes.Moreover,it has been investigated that the main phase of PEO coatings on the AZ31 substrate in such electrolyte is MgO.Moreover,polyvalent metal anions such as Tungstate can produce new phases and microstructures on the coatings.

Table 1Role of common electrolyte compositions on PEO properties.

Table 2The effects of different processing factors on corrosion resistance of PEO coated magnesium based alloys.

In addition to electrolyte properties,electrical parameters,including applied voltages,current density,and current mode can modulate the PEO coating properties[68].The current mode of hybrid,bipolar or unipolar can also be used to develop appropriate coatings on different substrates.Results show that bipolar and hybrid currents(unipolar followed by bipolar)decrease defects such as gas pores and micro-cracks,leading to the formation of compact coatings with enhanced corrosion resistance[69].Current density also affects the deposition rate of PEO process.Zhuang et al.[68]studied the role of the current density on PEO coated AZ31 alloy.They applied different current densities and found that,the deposition rate and the thickness of coatings enhanced with increasing current density.However,arc intensity enhanced with increasing current density leading to the formation of more porosities in the coatings.Hence,an optimum current density was reported about 10 A/dm2to provide a more compact coating with the optimum thickness and tiny pores.Moreover,the current frequency effectively modulates the PEO process,as widely reported Elsewhere[70,71].However,the results seem not consistent.For instance,Srinivasan et al.[69]applied the PEO process in a KOH/Na3(PO4)solution at different frequencies(10,100 and 1000Hz)in DC mode.They found that,while higher frequency(1000Hz)led to the formation of MgO coating with tiny pores and smaller micro-cracks,lower frequency(10Hz)produced an anti-corrosion coating with remarkable amounts of Mg3(PO4)2[72].The different roles of applied frequency might be due to its synergic effects on the thickness,phase composition and density of coating[58].Besides,applied voltages also play a crucial role in the thickness of coating and porosity.The rate of deposition enhances with increasing voltages leading to thickening of the coatings.However,the pore size of PEO coatings enhances with increasing applied voltage,which could have destructive effects on their performances[50].Therefore,an optimum voltage should be selected,depending on the coating properties.

4.Properties of PEO coatings on magnesium alloys

According to the effective roles of parameters during the PEO process,various features of PEO coatings have been developed with different corrosion resistivity and tribological and biological properties.These properties are brifl descused,bellow.

4.1.Corrosion resistance of PEO coatings on magnesium alloys

Corrosion resistance is the main goal of PEO coatings on magnesium alloys[73].In this regard,PEO coatings with low amounts of defects and compact structure are desired thanks to their corrosion protective ability[74].The quality of the inner PEO coatings could be influence by different process parameters.Table 2 summarizes the role of various parameters to control the corrosion resistance of PEO coatings.The chemical composition of produced phases can control the corrosion resistivity of the coatings.For instance,incorporation of KOH enhances the corrosion resistance because of less porosity and formation of Mg3PO4[72].Moreover,increasing the time of PEO process often results in the penetration of electrolyte into porosities and cover them,leading to the more compact and protective layer.However,new pores often produce by increasing the process time because of new micro-arcs[75].Other parameters,including electrical factors such as current density,could effectively change the thickness and porosity of PEO coatings that result in the modulation of corrosion resistance[76,77].

4.2.Tribological behavior of PEO coatings on magnesium alloys

Another important property considered in the use of PEO coatings is wear-resistance.Generally,the tribological behavior of magnesium and its alloys is poor owing to week hardness.Various surface modificatio approaches such as physical vapor deposition(PVD)[82,83],thermal spray[84,85],and PEO[40,86]have been developed to overcome the tribological behavior of magnesium alloys.It is worth mentioning that PEO could provide high adhesive strength and its hardness in comparison to other approaches.As a result,it could be used for automotive applications to achieve high tribological properties besides corrosion resistance[55].White et al.[28]investigated the mechanical and corrosion resistance of PEO coating in a silicate-based electrolyte and found that wear resistance and hardness of magnesium alloy enhanced after PEO coating due to the formation of MgO phase with lower wear rate than AZ31 substrate.In another study,Kennedy et al.[87]studied the formation of PEO coating on Mg-7Y-1Zn alloy in a silicate-electrolyte and demonstrated increased hardness and wear behaviour of the substrate due to formation of a thick oxide layer.

4.3.Biological properties of PEO coatings on magnesium alloys

Magnesium and its alloys are the next generations of degradable metals for biomedical applications.Low weight and density,elastic modulus near bone,and its biocompatibility make magnesium suitable for orthopedic and cardiovascular applications[81,88].However,their high degradation rate and low corrosion resistance limit its application.One of the best strategies to control the degradation rate of magnesium alloys is the PEO coating[89-91].In vivo studies revealed that PEO coated ZK60 could be safe in a short time because of its corrosion protection,while long term corrosion and high hydrogen evolution may prevent bone growth[92].In another study,PEO coating on Mg-Ca alloy resulted in a bioactive calcium-phosphate layer,leading to promoted cell attachment and proliferation.Corrosion and hydrogen evolution tests also revealed higher protection of PEO coated samples.However,it was not adequate,and gas accumulation at the interface of coating and magnesium could damage coatings[93].

Despite significan improvement in the corrosion resistivity and tribological behavior of PEO coating for magnesium and its alloys,their use alone is not recommended.The porous outer layer of PEO coatings may allow to penetrate corrosive medium into the magnesium substrates.Besides,the formation of amorphous phases at outer layers of PEO coatings reduces wear resistance.overall simple PEO coating could not improve the tribological properties,significantl[37,94].Moreover,some biological applications focusing on the bioactivity,biocompatibility and hemocompatibility of PEO coated magnesium-based alloys revealed that single PEO coatings could not be satisfactory.Researchers have proposed different modification strategies on the PEO coatings,including the incorporating nanoparticles,post-heat treatment approaches,and formation of secondary coatings,named duplex coatings.These strategies have often been reviewed elsewhere[95-97].Here,the micro and nano-enabled approaches which have been applied to promote the PEO coating properties will be investigated.

Fig.2.The role of incorporation of nanoparticles to control various properties of PEO coatings on magnesium-based alloys.

5.Nanoparticles incorporated PEO coatings for different approaches

Despite the significan properties of pure PEO,it could not provide an ideal coating in various applications.In this regard,modificatio of PEO coatings using nanoparticles is one of the promising strategies.At this strategy,the modificatio mechanism is modulation of pH value,conductivity,and viscosity of electrolytes,depending on the properties of nanoparticles,which result in PEO coatings with new features.In addition,nanoparticles could fil the porosity of PEO coatings,which improve the corrosion resistivity,tribological and biological properties of coatings(Fig.2)[38,58,79,98-102].Various properties of nanoparticles,including average and distribution of particle size[103],melting point[104],concentration and Zeta potential[105],modulate the PEO coating properties and numerous studies have been conducted on the effective role of nanoparticles in PEO coating properties[106].In the following,the roles of various nanoparticles in controlling the properties of PEO-based coatings are discussed.

5.1.Corrosion resistivity of nanoparticles incorporated PEO coatings

One of the promising strategies to control corrosion resistance of PEO coatings is the incorporation of nanoparticles.Various types of nanoparticles have been applied to improve the corrosion resistance of coatings in various ways.Table 3 summarizes these types of nanoparticles and their roles to improve the corrosion resistance of PEO coatings.Between various types of nanoparticles,graphene and its derivatives noticeably influenc the corrosion protection[102,107,108].Zhao et al.[102]introduced a novel approach to improve the corrosion resistance of PEO coating of Mg alloy by incorporating GO in the electrolyte.According to Fig.3A,the incorporation of GO upon 2g/L reduced the porosity of the PEO coating leading to the optimal corrosion resistance.They proposed that GO embedded into the inner layer(Fig.3A(i))and the outer layer(Fig.3A(ii))of the PEO coating.GO nanosheets acted as a barrier to hinder the diffusion of aggressive electrolyte to the magnesium interface and enhanced the tortuosity for the electrolyte diffusion pathway.

ZrO2and TiO2are also metal oxide nanoparticles that could influenc the corrosion resistance of PEO coatings thanks to their high chemical stability[109,110].Rahman et al.[109]applied a two-step PEO approach to develop ZrO2incorporated PEO coatings(Fig.3B(i)).In this approach,a thick primary PEO layer was processed in a secondary solution.In this step,the plasma and discharge formation happed only on the limited spots such as pores,and cracks leading to the formation of a localized coating layer.Owing to the complicated nature of the PEO process,numerous simultaneous procedures were participated to encompass ZrO2nanoparticles,including electrophoretic forces,gas evolution,physical mixing particles because of the plasma shock waves,and the inherent changing molten magnesium oxide(Fig.3B(ii)).Consequently,ZrO2incorporated PEO coating resulted in the formation of a cluster-type structure covering the PEO defects.The greatest corrosion resistance(Rp～81.17 kΩcm2)of the coatings was reported for the ZrO2concentration of 2g/L.Nevertheless,higher concentrations of ZrO2nanoparticles resulted in a poor crystallinity of the coating owing to unstable and lower intensity discharges,which was unsuccessful in providing high corrosion resistance performance.The effective roles of this two-layer coating on the corrosion resistance of the magnesium alloy are mentioned in Fig.3B(iii).According to this schematic,R1,R2,R3and R4represent the solution resistance,pore resistance,the inner layer resistance and the polarization resistance on the electrolyte/metal interface,respectively.Moreover,the term CPE originated from the constant phase element applied to reduce the imperfect capacitive responses and to balance for porosity and surface roughness.Finally,it could be concluded that nanoparticles could assist in protecting the corrosion resistivity via the formation of a compact PEO inner layer or to increase the thickness of coatings,depending on the nanoparticle concentration.Therefore,the most important point is discovering an optimized amount of nanoparticle,without any agglomeration.

5.2.Tribological properties of nanoparticles incorporated PEO coatings

Wear-resistant coatings have been achieved by incorporating nano-additives in PEO coatings.Table 4 summarizes various types of nanoparticles showing the effective role in the tribological features of PEO coatings on magnesium alloys.Carbon-based nanoparticles(e.g.,graphene,graphite nanoparticles,and GO)have been promising for improvement of tribological properties of various types of coatings[111].However,a proper coating method should be applied to provide adequate adhesive strength.In this regard,PEO coating with high adhesive strength can be considered to be incorporated with carbon-based nanoparticles and control tribological properties.The significan role of GO on the tribological properties of PEO coatings designed on magnesium alloy has been recently examined by Zhang et al.[101].GO nanosheets with exceptional hydrophilicity and great specifi surface area easily attract free electrons or negative ions,leading to convert a negatively charged colloid in the electrolyte.Consequently,the conductivity and viscosity of these electrolytes increased during PEO process leading to improvement of the micro-arc discharges created on the anode surface,increased electron avalanche,and reduced breakdown.According to Fig.4A(i),the negative ions generated in the presence of GO resulted in the formation of several crater-like pores and a reduction in the average size of the pan-like structures.Consequently,the coating density was meaningfully improved due to the formation of minor-sized and blocked pores.According to Fig.4A(ii),this compact nanocomposite coating revealed a low,stable friction coefficien and high wear persistence[101].In addition to the formation of compact structure,other studies mention the effective role of intrinsic hardness and self-lubricant properties of graphene-based components to promote the tribological properties of PEO coatings[80,98].

Table 3List of different nanoparticles incorporated electrolyte and their effects on the corrosion resistivity of PEO coatings.

Table 4Effects of various nanoparticles on the tribological performance of PEO coatings.

Table 5The list of nanoparticles applied to promote catalytic activity of PEO coatings on magnesium alloys.

Other types of hard nanoparticles such as WC,SiC and SiO2have also been incorporated in PEO coatings to enhance the tribological performance of magnesium alloys,besides succeeding an appropriate adhesive coating[112,114].For instance,it was found that WC nanoparticles distributed over surface uniformly and significantl reduced the wear rate from 28.65mg/mN to 6.67mg/mN.Nasiri Vatan et al.[112]demonstrated that a 20 min PEO process in a phosphate-electrolyte consisting of WC nanoparticles(5g/L)resulted in the formation of compact coating with strong cohesion and adhesion,besides significan tribological behavior[112].SiO2is another hard nanoparticle applied to promote the tribological performance of various PEO coated samples.In a study,the effect of nano and micro-sized SiO2on the PEO coatings formed on AM50 magnesium alloy was considered[113].Nano-and micro-sized SiO2particles were insitu synthesized into phosphate-based electrolytes.According to the schematic provided in Fig.4B(i),while the uptake of the nanoparticles happened mostly via discharge channels and open pores,micro-sized SiO2particles were engaged via the coating surface.Consequently,P and O were uniformly dispersed in the coating,while Si signal from the nanoparticles was less uniformly distributed with strong signals along with the pore band and discharge channels(Fig.4B(ii)).Results revealed that compared to micro-sized SiO2,these particles diminished the wear rate more effi ciently from 7.3×10?4mm3N?1m?1(micro-sized SiO2)to 4×10?4mm3N?1m?1(nano-sized SiO2)(Fig.4B(iii)),while corrosion resistance decreased[113].

Fig.3.Evaluation of nanoparticles on the corrosion resistance of PEO coatings:A)Effect of GO additive on the corrosion resistance of the PEO coatings on the AZ31:i)The schematic illustrating the corrosion resistance of PEO coating.ii)The surface morphology of the GO incorporated PEO coating.iii)Potentiodynamic polarization curves of the uncoated AZ31 and the GO incorporated PEO-coated samples consisting of various concentrations of GO nanosheets.Reproduced with permission from Ref.[102],License Number:4,938,291,382,708,2020,Elsevier.B)Effect of ZrO2 nanoparticles on the localized PEO coatings formed on AZ91 alloy:i)Schematic illustration of localized PEO process.ii)Different moving mechanisms participated in the incorporation of ZrO2 nanoparticles into the coatings.iii)Equivalent circuit model of the coatings illustrating different parameters[109].

5.3.Catalytic activity of nanoparticles incorporated PEO coatings

Catalysts are materials that participate in reactions without consumption[115].PEO coatings have also been applied as catalysts due to their porous microstructure and ability to incorporate catalytic compounds[116,117].Table 5 shows the catalytic applications of nanoparticles loaded PEO coatings formed on magnesium alloy.Al Zoubi et al.[80]employed the photocatalytic activity of TiO2nanoparticles to promote catalytic activity of PEO coating.They developed fl wer-like hybrid MgO-TiO2nanoparticles mixed with organic compounds(HQ(8-hydroxyquinoline),APY(2-aminopyridine),and HQ-APH(2-aminophenol))using twostep PEO and dip chemical coating process(Fig.3C(i)).Photocatalyst property was activated by UV-visible absorption and transferring an electron from TiO2into C atom of the organic part(Fig.4C(ii)).Results signifie that synergistic effects of aromatic heterocycles on TiO2and MgO containing inorganic layer revealed main roles in the improvement of photocatalytic activity on the photodecomposition of methylene blue(MB)(Fig.4C(ii,iii)).In another research,the photocatalytic performance of ZnO/PEO coating was investigated.ZnO nanoparticles were incorporated into the PEO electrolyte in different amounts between 2 and 8g/L.It was indicated that while MgO revealed weak photocatalytic activity,ZnO nanoparticles improved it owing to more unsaturated coordination on the surfaces.It could be worth mentioning that proper adhesion of coatings led to the stability of the catalyst.Thus,catalytic activities endured for a long-time,and life of workpiece increased[42].

Fig.4.Effect of nanoparticles to control the tribological behavior and catalytic performance of PEO coated magnesium alloys:A)Influenc of GO additive on the tribological properties of PEO coated magnesium alloy;i)Schematic representing the GO modifie PEO coating growth.ii)Friction coefficien evolution of the GO modifie PEO coating compared to pure PEO coating.Reproduced with permission from Ref.[101],License Number:4,938,310,428,973,2020,Elsevier.B)The role of SiO2 nanoparticles and microparticles on the tribological properties of PEO coated magnesium alloy;i)Schematic showing the incorporation mechanism of SiO2 into PEO coating.ii)EDS-mapping analysis of SiO2 incorporated PEO coatings.iii)Wear depth profil of the SiO2 incorporated PEO coatings.Reproduced with permission from Ref.[113],License Number:4,938,320,030,746,2020,Elsevier.C)Effect of TiO2?MgO-organic coating on the catalytic performance of PEO coating:i)SEM image of TiO2 based PEO coating.ii)Schematic illustrating the electron transfer and charge separation in the MgO-TiO2 based coating under visible light irradiation.iii)The photocatalytic degradation of MB using TiO2-HQ-APH under UV-vis light.Reproduced with permission from Ref.[80],License Number:4,938,320,487,409,2020,Elsevier.

5.4.Biological properties of nanoparticles incorporated PEO coatings

Recently,PEO coated magnesium alloys have been widely studied for biomedical applications[119-122].However,to promote the biological properties of PEO coatings,including biocompatibility,hemocompatibility,antibacterial properties,and bioactivity,various types of nanoparticles were incorporated in them.Table 6 summarizes different nano-additives used for biomedical applications.Antibacterial surfaces have been widely applied in different biomedical implants to prevent bacterial adhesion and reduce infection risks[123].

Table 6Nanoparticles incorporated PEO coating for improvement of biological applications.

Table 7Nanoadditives applied for self-healing coatings on magnesium alloy.

Various types of nanoparticles,including Ag[43],and Cu[128]have been incorporated in PEO coatings to promote its antibacterials performance.Sukuroglu et al.[43]in-situ synthesized Ag nanoparticles-PEO coatings via decomposition of silver nitrate during the PEO process.Results revealed the release of Ag ions from PEO coatings resulted in the introduction of antibacterial performances.Bordbar-Khiabani et al.[35]improved the bioactivity of PEO coatings via incorporation ZnO nanoparticles in size range of 25 and 65nm.According to Fig.5A(i),ZnO nanoparticles migrated into a magnesium-based substrate owing to its negative charge and trapped into PEO layers.In vitro bioactivity evaluation(Fig.5A(ii))revealed higher concentrations of ZnO resulted in more release of Zn2+ions leading to deposition of calcium phosphate layer and improvement of bioactivity.

Wen et al.[126]incorporated GO/HA nanopowder within PEO coating in a one-step process.Negatively charged nanoparticles were obtained by addition of 10ml/l ethylene glycol into the electrolyte to suspend nanoparticles monotonically.GO/HA nanoparticles covered the pores leading to reduced corrosion current density of just PEO from 122.1(μA/cm2)to 36.43(μA/cm2).It’s proper corrosion behavior in SBF followed by stability in body,and GO microstructure and HA increased possibility of bone regeneration.In another interesting research,halloysite nanotubes(HNTs)were incorporated into PEO coating to form a multi-functional coating on AM50 magnesium alloy[127].It is proposed that HNTs can be used as a nanocontainer for controlled release of drugs during implantation of stents or bone implants.Strong corrosion resistance was achieved by refinemen of morphology and covering the porosities.

5.5.Self-healing properties of nanoparticles incorporated PEO coatings

Motivated by biological systems,recently,efforts have focused to develop smart self-healing constituents which can fi themselves once damages occur.Thanks to this superior ability,self-healing materials show a longer life and better ability to reduce the full life-cycle cost than conventional materials[129-131].Various strategies have been applied to induce self-healing ability,including the incorporation of various types of nanoparticles and organic compounds and the formation of hybrid coatings[132-134]Table 7 presents PEO coated-magnesium alloys with self-healing properties induced by nano-additives.Some types of nanoparticles,including cerium oxide(ceria),show the intrinsic self-healing ability,and some types of nanoparticles have been applied as nanocontainers for corrosion inhibitors to develop anti-corrosive coatings.For instance,ceria nanoparticles are among the most common nano-additives to achieve self-healing properties[99,100,135].Toorani et al.[100]incorporated ceria nanoparticles within the PEO coating in various concentrations and investigated the self-healing ability of nanocomposite coatings.According to Fig.5B(i),self-healing ability happened at the optimal concentration of ceria nanoparticle(3 g/L)during soaking in a corrosive environment via changing the pH value and release of cerium ions.The interaction between Ce3+and OH?resulted in forming a compact layer of corrosion product in the internal layers of PEO coating.Results revealed that this self-healing coating could fi the prolonged damage and recover their corrosion resistance.Fig.5B(ii)shows the Bode diagrams of the PEO coatings,in the absence and presence of 3wt.% CeO2(PEO-3Ce),after potentiodynamic polarization(PDS)test immersed for up to 10h.Results revealed that the impedance modulus of PEO-3Ce in the proposed area enhanced after one hour,and then kept almost constant.According to Fig.5B(iii),the cavities formed on the surface of PEO-3Ce were small and were blocked more severely than the other one.

In contrary to ceria,HNTs have been widely investigated as nano-carriers for organic corrosion inhibitors[136-138].HNTs are two-layered aluminosilicate structures which can provide appropriate vessels for corrosion inhibitors.The small particle size of HNTs with their great thermal stability makes them promising candidates to create self-healing PEO coatings via encompassing in-situ inhibitor-loaded HNTs in the surface,via a simple treatment approach.Sun et al.[41]inorporated benzotriazole(BTA,a corrosion inhibitor)within HNT carriers to provide a self-healing functionality in PEO coatings on AM50 Mg alloy.Results revealed that in addition to the significan role of HNTs to promote the mechanical properties of PEO coating,pH increasing at the corrosion sites encouraged BTA-mediated nucleation of Mg(OH)2.Consequently,the formation of a compact Mg(OH)2layer on the corrosion site hindered pitting corrosion.

Fig.5.Role of nanoparticles to improve biological properties and self-healing ability of PEO coating:A)Role of ZnO nanoparticles on the bioactivity of PEO coating:i)The mechanism of coating growth,including(1)attraction of negatively charged ZnO nanoparticles to magnesium-based anode,(2)deposition of ZnO nanoparticles on the anode,(3)formation of a thin passive fil and trapping of ZnO nanoparticles by this layer,(4)micro-discharging,local melting and molten oxide eruption on the substrate,(5)sintering and incorporation of ZnO nanoparticles in the oxide coating and(6)the fina cross-section of PEO coatings prepared in(1)low concentration and(2)high concentration of ZnO nanoparticles.Schematic illustration of the Ca3(PO4)2 layer formation after immersion in SBF solution,for(ii)simple and(ii)ZnO nanoparticles incorporated PEO coatings.Reproduced with permission from Ref.[35],License Number:4,938,340,466,576,2020,Elsevier.B)Self-healing PEO coatings containing CeO2 nanoparticles:i)The schematic showing the role of CeO2 nanoparticles to modulate the corrosion mechanism of PEO coatings,ii)Bode diagrams of the PEO-coated sample,before and after incorporation of 3wt.% CeO2,after PDS test.iii)SEM images of the coatings after 12h immersion in the 3.5wt% NaCl solution,after the PDS test.Reproduced with permission from Ref.[100],License Number:4,938,360,061,713,2020,Elsevier.

6.Micro/Nano-enabled multi-layer PEO coatings for different approaches

PEO coatings include defects such as pores and cracks because of hydrogen gas formation and sever sparks leading to the penetration of corrosive medium into the substrate and weakening corrosion resistivity and tribological performances[139-141].Despite the significan successful results of nanoparticle addition to PEO layers,some issues,including their agglomeration and non-complete covering of the surface,may lead to reduce in corrosion resistance and mechanical properties.In this regard,various post-treatment strategies have been developed[142-145].These nano/micoenabled based approaches have been applied to improve the properties of PEO coatings.These approaches are often based on the hybrid coatings formed via the formation of the secondary ceramic,polymer or nanocomposite coatings on the middle PEO layer aiming to reduce the porosity and incorporation of biological components.Here,the nano-enabled approaches performed via various strategies are discussed.

6.1.Nano-ceramic/PEO double-layer coating on magnesium alloys

The most common approaches to control corrosion resistance,bioactivity and biocompatibility of PEO coatings on magnesium-based alloys are the formation of secondary nanoceramic based coatings developed using various chemical and electrochemical properties including sol-gel,electrophoretic deposition,plasma spray and etc.Table 8 presents various types of double-layer coatings based on nano-ceramic/PEO using different approaches.

Table 8Nano-ceramic/PEO coatings developed on magnesium-based substrates.

Table 9Various polymer/PEO hybrid coatings and their effects on the properties of magnesium alloys.

Between various strategies,the sol-gel is a promising approach commonly applied to prepare ceramics and glasses[164].The sol-gel process consists of a condensation reaction over the molecular precursors in a liquid medium to create an oxide network[60].The sol-gel method has some advantages such as simple facilities,easy to produce,apply on different substrates,and low energy consumption[165].However,the thickness of sol-gel based coatings is not often controllable and adhesive strength is not adequate to improve the corrosion resistance of the magnesium alloys[166-168].To overcome these disadvantages,sol-gel coatings have been combined with other strategies including PEO leading to decrease in the pore size and formation of the strong interaction between two layers[167,169].For instance,nanocrystal TiO2layer was deposited on the PEO coating over sol-gel process by a dip-coating method and subsequent heat-treatment at 150 °C and 350 °C.The TiO2layer covered the pores which resulted in decreased corrosion resistance from 2.35μA/cm2(uncoated AZ91)and 1.61μA/cm2(PEO)to 0.08μA/cm2(PEO-TiO2)[146].TiO2was formed in the pores and bonded to hydroxyl-group of PEO surface.However,the formation of these types of hybrid coatings often faces with thermal crack formation,specificall during the post-treatment process,due to the different thermal expansion coefficien values of various layers.This strategy could also be useful as a carrier for various molecules to greatly enhance their properties.For instance,Ivanou et al.[147]developed nanohybrid coatings on ZE41 alloy to induce self-healing property.In this research,porous PEO coating incorporated with poly(ethylene oxide)was primarily fabricated and was subsequently impregnated with corrosion inhibitor 1,2,4-triazole.Consequently,it was coated with a SiO2-based sol-gel fil modifie with TiO2.Nanocomposite coatings enhanced impedance from 3×108Ωcm2(for PEO/sol-gel)to 4×108Ωcm2(for PEO/triazole/sol-gel).The proper thickness and self-healing effect of PEO/triazole/sol-gel coating were also the most important properties of this nanohybrid coating.

Electrophoretic deposition(EPD)is another strategy to develop nanoparticles based coatings[170].During this process,charged nanoparticles in the electrolyte are moved to the workpiece electrode by applying a proper voltage range.The EPD could be applied to coat different substrates by complex shapes[171].The EPD coatings can seal the porosities and micro-cracks of the PEO layer leading to the formation of more protective surfaces.Bakhsheshi-rad et al.[158]developed nanostructured monticellite(CaMgSiO4)coating on the PEO coated magnesium alloy,using EPD technique.Monticellite is a silicate-magnesium based ceramic showing great osteogenic behavior and appropriate mechanical properties(55.2±0.4GPa[172])near to bone(13GPa[173]).Results revealed the Monticellite coating significantl reduced the corrosion rate of Mg-1.2wt%Ca-6.1wt%Zn substrate(4.21mm/year)and PEO coated sample(0.17mm/year)to less than 0.02mm/year owing to the compact structure.Moreover,the monticellite/PEO coated sample revealed significan in vitro bioactivity and reduced hydrogen evaluation and degradation rate.According to Fig.6A,deposition of nanostructured monticellite over the PEO layer resulted in the sealing of micro-defects,which postponed the aggressive ions transportation to the substrate.In other words,during immersion of double-layered coated magnesium samples into the SBF solution,Mg2+and Ca2+ions released from the sample and resulted in the formation of Si(OH)4,following by the formation of silanol groups on the surface.The condensation of Si-OH groups consequently resulted in the formation of negatively charged SiO2gel,which reacted with calcium and phosphate ions in the SBF solution leading to the nucleation and growth of apatite.In another study,to promote corrosion resistance and bioactivity of AZ91 alloy,doublelayer ZnO/PEO layer was developed(Fig.6B(i,ii))[159].ZnO nanoparticles were deposited on the rough microstructure of the PEO layer using the EPD process,leading to cover the micro-cracks and pores(Fig.6B(iii)).The double-layered coating noticeably prevented from corrosive medium penetration,reduced hydrogen evolution and improved corrosion resistance(Fig.6B(iv)).Consequently,ZnO/PEO was proposed as a proper candidate for biomedical degradable implants.

Fig.6.Double-layer nano-ceramic/PEO coatings:A)Schematic showing the corrosion mechanism of uncoated and monticellite/PEO coated Mg alloy.Reproduced with permission from Ref.[158],License Number:4,938,371,214,852,2020,Elsevier.B)Nanostructured ZnO/PEO coatings on magnesium alloy:i)Schematic representing the EPD process for the formation of nanostructured ZnO coating on PEO coated magnesium.ii)Cross-section images of double layer ZnO/PEO coating on magnesium alloy.iii)SEM images of PEO coated and double-layer ZnO/PEO coatings.Iv)Polarization Tafel curves of samples,before and after coating.Reproduced with permission from Ref.[159],License Number:4,938,380,699,897,2020,Elsevier.

6.2.Submicron polymer/PEO coating on magnesium alloys

Thin polymeric layer is one of the most attractive coatings to alter the tribological,biological and chemical features of the PEO surfaces.These thin layers are commonly used to cover the micro-cracks and porosities of PEO layer to fortify corrosion protection of PEO coating.Various types of natural and synthetic polymers have been applied in these hybrid coatings on PEO coatings on magnesium alloys which are summarized in Table 9.

These submicron top polymeric layers have been applied for various approaches,including improvement of corrosion resistance,tribological performance and biological properties.For instance,Toorani et al.[175]developed a three-layer PEO/silane/epoxy coating to promote corrosion resistance and adhesion strength of AZ31B Mg alloy.Following the formation of PEO coating in a phosphate-based electrolyte containing Ce(NO3)3?5H2O,silane layer containing different volume ratios of tetraethoxysilane(TEOS)andγ-amino propyltriethoxysilane(APTES)was coated on the PEO sample by dipping in silane solution for 30s.Finally,the epoxy resin was deposited as the fina layer(Fig.7A(i)).The hybrid coating revealed the significan corrosion resistance(Fig.7A(ii))due to the fillin the porosities and strong adhesion strength between silane and PEO coating(Fig.7A(iii)).PEO/methyltrimetoxysilane(MTMS)doped nano-antimony tin oxide(ATO)was also fabricated on AZ31 magnesium alloy by Li et al.[180].The sandwiched structure coating consisted of PEO layer,agglomerated ATO nanoparticles and MTMS top coat.Distribution of ATO nanoparticles at the PEO and MTMS layers prevented from the fully conducting path Fig.7B(i),beside the formation of a complex path over corrosive ions diffusion into substrate leading to decrease in corrosion potential from?1.51V(PEO coating)to?1.41V(sandwich coating)Fig.7B(ii).In a recent study,the effects of nano-layer polyethylenimine/Kappa Carrageenan on PEO coated AZ91 were investigated for biomedical approaches[183].PEO coating resulted in the formation of porous inter-layer consisting of forsterite(Mg2SiO4),MgO and MgF2.Covering the pores and strong adhesive strength followed by significan corrosion resistance(reduction of corrosion current from 8.01 to 0.37μA/cm2)were the main finding of this study,obtained after the formation of the secondary nano-layered polymeric coating.Nanocrystal hyaluronic acid(HA)/Carboxymethylcellulose was also deposited on the PEO layer on magnesium screws over the hydrothermal treatment by Kim et al.(Fig.7C(i))[182].Highly brittle and porous PEO layer was covered by HA and formed nanocrystals,leading to the improvement in corrosion resistance,by reducing in corrosion potential from?1.55V(PEO coating)to?1.034(PEO/HA).Fig.7C(ii)indicated that HA could bind Ca and P ions and the PEO coating leading to the fillin of pores and cracks and more corrosion-resistant layer.Moreover,the self-healing ability of this bilayer coating revealed,it could lead to delay erosion over implantation.Another study on a submicron polymer coating on PEO coating was conducted by Feng et al.[184]for biological applications.They deposited polydopamine on PEO coated Mg-Zn-Ca alloy.It was evident that some pores and micro-cracks were covered by polydopamine and reduced the pore size followed by reduction in corrosion current density from 2.09×10?5(PEO)to 1.46×10-6A/cm2(hybrid layer).Also,hydroxyl and amine functional groups of polydopamine may affect the cell adhesion and growth and provide more proper pore size,beside hydrophilic coating.

Fig.7.Multi-layer submicron polymer/PEO coatings:A)PEO/Silane treatment of the epoxy coating on AZ31B alloy:i)Schematic of double-layered Epoxy/PEO coating as well as bonding of silane layers with epoxy layer.ii)Changing of the impedance modulus at the frequency of 10kHz for PEO/Epoxy dual-layer coating and PEO/Silane/Epoxy coatings with various volume ratios of TEOS and APTES during 28 days of immersion in 3.5wt.% NaCl solution,obtained from Bode-Bode diagram.iii)Adhesion strength of PEO/Epoxy and PEO/Silane/Epoxy coatings,at dry and wet condition(Reproduced with permission from Ref.[175],License Number:493,859,037,248,2020,Elsevier).B)PEO/methyltrimethoxysilane(MTMS)/nano ATO-doped coating:schematic of i)multi-layers of coating and corrosion mechanism of ii)PEO and iii)multi-layer coatings in body flui(Reproduced with permission from Ref.[180],License Number:4,938,591,334,047,2020,Elsevier).C)HA/CMC/PEO coating on magnesium alloy:i,ii)schematic of corrosion mechanism of coating in SBF solution(Reproduced with permission from Ref.[182],License Number:4,938,591,031,822,2020,Elsevier).D)A superhydrophobic multi-layered coating based on PEO and the cyclic assembly:i)The schematic showing the preparation of superhydrophobic coating on PEO treated AZ91 alloy via cyclic assembly of phytic acid and cerium ions.ii)The water droplet photos on AZ91 surface,before and after coating process(MAO-(PA@Ce)3-FAS samples).iii)Polarization curves of the samples after 72h immersion in 3.5 wt% NaCl solution(Reproduced with permission from Ref.[186],License Number:4,938,600,137,527,2020,Elsevier).

Layer by layer(LBL)process is another promising approach to self-assemble different polymers by electrostatic forces between oppositely charged blocks[185].In a recent study,LBL coating based on chitosan-poly(styrene sulfonate)was applied on a PEO-coated magnesium to improve corrosion resistance.It was found that,PEO/LbL layers decreased corrosion current density from 5.741×10?4to 2.796×10?6A/cm2by neutralization of pH and covering defects led to lessening the localized corrosion and compact structure[181].In an interesting research,a superhydrophobic coating was developed on AZ91 alloy to promote corrosion resistance via combining the PEO and cyclic assembly approaches[186].According to Fig.7D(i),after PEO process,a cyclic assembly in phytic acid and Ce(NO3)3solutions were performed to develop a micro-nano hierarchical structure,leading to a superhydrophobic surface with a contact angle of 159°(Fig.7D(ii)).It was found that,while the prior PEO process notably decreased the cracks produced by hydrogen evolution during the assembling process,the following assembling process repaired the porous defects of the middle layer.Consequently,this multi-layer coating resulted in enhanced corrosion resistance by three orders of magnitude in 3.5 wt% NaCl solution,compared to the substrate(Fig.7D(iii)).Based on the results,it could be stated,multilayer coatings could provide the best types of coverage for defects and useful features for various magnesium performances.

Conclusions and outlooks

Considering the varying demands of different industrial applications of magnesium substrates,more and more researches joined in the improvement of magnesium alloys features.The most important conclusions are:

?The PEO process has been demonstrated as a promising strategy to promote the corrosion resistance and tribological properties of magnesium alloys.

?PEO coatings are certainly not free from defects such as microcracks and pores,leading to limitations on the tribological properties and corrosion resistance and long-term performances.

?Several strategies to improve PEO defects focused on the optimization of electrolytes and process parameters,including concentration and composition of the electrolytes,the voltage and current density as well as frequency,have been researched to improve the features of PEO coatings.

?Formation of denser and thicker PEO coatings have been achieved at the optimized working parameters,but these coatings still need to be improved.

?The next promising strategies are micro/nano-enabled approaches,which look to be rational in terms of reaching acceptable properties to overcome the harmful influenc of micro-pores and micro-cracks.

?Incorporation of nanoparticles into PEO coating lead to significan improving tribological properties and corrosion resistance,biological,catalytic and self-healing properties,depending on the nanoparticle properties.

?Incorporation of bioactive and/or antibacterial nanoparticles could exhibit specifi biological properties to provide a suitable surface for biomedical applications.

?Some of disadvantages of incorporation of nanoparticles that limit their industrial applications are long-term performances,keeping the nanoparticle to disperse uniformly in the electrolyte in case of high concentrations of nanoparticles.

?Second micro/nano-enabled approaches are bilayer or multi-layer coatings based on PEO which can overcome the issues by sealing the defects properly.

?Multi-layer coatings could provide a lower wear rate,longer lifetime,higher corrosion resistance,and additional biological properties.

?Multi-layer coatings can also be applied as carriers for multi-types of drugs and macromolecules to provide controlled and sustained release,making them promising for various types of biomedical applications.

?Despite numerous attempts to fin multi-functional coatings on magnesium alloys,the concern to disregard the localized corrosion attack and preserving adequate mechanical integrity of magnesium alloys for a reasonable period have been continued unresolved,and the missions for the perfect strategy have remained.

?A collection of strategies,including increasing the PEO layer thickness and imparting multi-functionality via multilayer coatings,would offer better properties for the longterm application of PEO coatings on Mg alloys.Thus,many more attempts are needed to fortify magnesium alloys into various applications via these combined strategies.