Feng Peng,Dongdong Zhng,Xunyong Liu,d,*,Yu Zhng,*

aDepartment of Orthopedics,Guangdong Provincial People’s Hospital,Guangdong Academy of Medical Sciences,Guangzhou 510080,Guangdong,China

b State Key Laboratory of High Performance Ceramics and Superfin Microstructure,Shanghai Institute of Ceramics,Chinese Academy of Sciences,Shanghai 200050,China

c Center of Materials Science and Optoelectronics Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

d Cixi Center of Biomaterials Surface Engineering,Shanghai Institute of Ceramics,Chinese Academy of Sciences,Ningbo 315300,China

Abstract Magnesium(Mg)is a vital engineering material owing to its light weight and excellent mechanical properties.However,poor corrosion resistance limits its widely applications as well as its economic value.Hence,surface modificatio is essential for Mg and its alloys.Among the various coatings,superhydrophobic coating,which is inspired by nature,has received increasing attentions in the past decade.With a water contact angle larger than 150°,superhydrophobic coating can provide sufficien protection for Mg-based substrates.The model of superhydrophobic states and the protection mechanism of superhydrophobic coating are discussed in this review.Especially,the methods for fabricating superhydrophobic coatings on Mg alloys are reviewed.Meanwhile,some functional superhydrophobic coatings on Mg alloys are summarized.Finally,the challenges and future directions are proposed.We hope that this paper will provide a serviceable review for future research on superhydrophobic coatings on Mg 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:Magnesium alloys;Superhydrophobic coatings;Corrosion resistance.

1.Introduction

Magnesium(Mg)is the lightest structural metal with a density of 1.7g/cm3,which is approximately 60% of that of aluminum(Al,2.7g/cm3)and 20%of that of iron(Fe,7.9g/cm3).In addition,it exhibits high thermal conductivity,low thermal expansion,high damping capacity,excellent electromagnetic shielding,and machinability.Therefore,it is widely utilized in automotive,aerospace,electronics,and other industries[1-3].Notably,in the past decade,researchers have explored the potential of Mg alloys as biomedical implants,and some products have been certifie by Conformite Europeemme(CE)and Korea Food and Drug Administration(KFDA)[4,5].However,with an inherent low standard electrode potential(?2.37V vs NHE),Mg and its alloys easily being corroded in aqueous or humid environments.Furthermore,the potential of the second phase(βphase)in Mg-based substrates is more positive than that of the pure Mg phase(αphase)[6].Thus,when an aqueous solution permeates a substrate,βphase acts as a cathodic area andαphase acts as a anodic area,and resulting in microgalvanic corrosion(Fig.1).Although corrosion products,including MgO,Mg(OH)2,and other Mg salts,are deposited on the surface of Mg alloys forming a corrosion barrier,the layer is porous and is easily attacked by anions,especially Cl?[7].In this case,the corrosion layer cannot stop the proceeding of corrosion.Therefore,the fast corrosion of Mg alloys is highly risky for their largescale applications.

To improve the corrosion performance of Mg-based products,researchers have proposed several methods.The firs is purification Less impurities in the Mg substrate indicate a lower risk of microgalvanic corrosion.Nor et al.compared the corrosion behavior of high-purity Mg,AZ91,ZE41,and Mg2Zn0.2Mn.They found that high-purity Mg showed the lowest hydrogen evolution in 3 wt% NaCl[8].The second is alloying.Alloying elements can react with Mg and form intermetallic phases,which are distributed along the grain boundaries or dissolve in Mg matrix,thus influencin its corrosion resistance[9].The third is the improvement of processing technology.For example,Li et al.used high-press torsion technology to fabricate a ZEK100 Mg alloy[10].It was found that the grain size was refine and a large number of twins were introduced,thus leading to an enhanced corrosion resistance.Nevertheless,Mg-based substrates are still easily corroded despite these strategies.Surface modification which alters the surface chemical composition of Mg-based substrates,can protect the substrates from contact with corrosive aqueous[11-13].Therefore,it is widely studied to enhance the corrosion resistance of Mg-based products.

Fig.1.Microgalvanic corrosion of Mg alloys with multiphase.Reprinted with permission from Ref.[6].Copyright 2012,Wiley.

Numerous methods have been applied to Mg alloys,including alkaline treatment,hydrofluori acid treatment,plasma electrolytic oxidation(PEO),electrophoretic deposition(EPD),and plasma immersion ion implantation(PIII),to fabricate coatings with different chemical constituents,such as metal hydroxides,hydrotalcites,metal oxides,Ca-P compounds,and polymers[14-17].In recent years,superhydrophobic coatings(SHC)with a static water contact angle(WCA)larger than 150° and a water sliding angle(WSA)smaller than 10° on Mg alloys have gained considerable attention owing to their unique advantages in corrosion protection[18-20].The firs SHC on Mg alloy was reported by Liang et al.in 2007[21].They used PEO as the pretreatment,and then immersed the sample in acrylic acid to obtain a superhydrophobic surface.Subsequently,increasing number of researchers have focused on this area.Fig.2 shows the published paper of SHC on Mg alloys retrieved from the Web of Science.A rapidly increasing number of published papers can be observed in the past fi e years.

In this review,we firs illustrate the model of wettability states and then discuss the protection mechanism of SHC.Furthermore,we propose the general strategy to design such coating and summarize the methods used to fabricate SHC on Mg alloys in the recent years.Moreover,some functional SHC on Mg alloys are also discussed.Finally,we propose future challenges and directions in this research area.

Fig.2.Published papers of SHC on Mg alloys since 2007.

2.Models of wettability

WCA is an important index for assessing the wettability of materials.When the phases of solid-liquid-gas reach equilibrium,the angle between the solid-liquid junction line and the tangent line of the droplet is define as WCA.On a desirable smooth surface(Fig.3a),the WCA(θ)can be calculated according to Young’s equation:

whereσSA,σSL,andσLArepresent the interfacial tension of solid-air,solid-liquid,and liquid-air,respectively.

A surface can be classifie on the basis of WCA:i)superhydrophilic surface(θ≤5°);ii)hydrophilic surface(5°≤θ≤90°);iii)hydrophobic surface(90°≤θ≤150°);and iiii)superhydrophobic surface(θ≥150°).

However,for an actual solid,its surface cannot be perfectly smooth,and there must be rough sites.Supposing the rough surface is completely infiltrate by liquid(Fig.3b),then the actual contact area of solid-liquid is larger than that of the perfectly smooth(Young’s situation).To address this issue,Wenzel introduced a dimensionless surface roughness factor(γ)to amend the Young’s equation:

whereγis the ratio of the actual surface area to the apparent surface area.

Obviously,the value ofγis larger than 1.Hence,the roughness can strengthen the wettability of the surface,that is,a hydrophilic surface will be more hydrophilic,while a hydrophobic surface will be more hydrophobic.

Further,considering the capillary effect,the liquid cannot fully infiltrat rough surfaces.Therefore,some air will be sealed on the rough surface and lead to the existence of solidliquid and air-liquid contact states,rather than only solidliquid contact state for Young’s and Wenzel’s models.The above model is called the Cassie model(Fig.3c),and the WCA can be determined according to:

Fig.3.Wetting states:(a)Young,(b)Wenzel,(c)Cassie.Reprinted with permission from Ref.[25].Copyright 2016,Springer.

wheref1andf2represent the contact area ratio of solid-liquid and air-liquid,respectively,whereasθ1andθ2represent the intrinsic contact angle of solid-liquid interface and air-liquid interface,respectively.

For the contact area ratio,it is understandable that the sum off1andf2values is 1,that is,f1+f2=1.For the air-liquid interface,the contact angle is 180° Therefore,the Cassie’s equation can be represented as:

Thus,for a hydrophobic surface(θ1>90o),the actual contact angleθincreases when the value off1decreases or the intrinsic contact angleθ1increases.Therefore,increasing the roughness and intrinsic contact angle are the key principles employed to construct a superhydrophobic surface.Notably,the SHC is generally inspired from“l(fā)otus effect”,which means droplets can roll off easily from the surface.Therefore,a small sliding angle(<10°)is required for a“l(fā)otus effect”inspired SHC,which endows the surface with a self-cleaning ability.

For a uniform surface,most situations can be ascribed to Wenzel or Cassie model.Also,there is condition between the two states and Jiang et al.proposed a“Cassie impregnating wetting state”to complete the wetting models[22].In this state,grooves on the surface are fully impregnated,while plateaus on the surface are dry(Fig.4a).This surface exhibits a high adhesive force with water owing to the semi-impregnating state(Fig.4b).However,in practice,the problem is that the composition and morphology of a surface is always uneven in microcosmic,so Wenzel,Cassie and“Cassie impregnating wetting state”might all be involved on one surface.

3.Protection mechanism of superhydrophobic coatings on Mg alloys

With a special wettability,SHC demonstrates significan corrosion protection for metals[19,20,23-25].The SHC on Mg alloy can decrease the corrosion current density by several orders of magnitude.The current developed SHC on Mg alloys generally contains two layers:a rough layer and a low surface energy layer.Furthermore,there is trapped air in the superhydrophobic surface with the Cassie state or Cassie impregnating wetting state.Therefore,according to the structure of SHC on Mg alloys,a“three barriers”corrosion protection mechanism is proposed in this review(Fig.5).The firs barrier is the trapped air.The existence of air between the corrosive liquid and the surface can reduce the corrosion area.The second barrier is the low surface energy layer.Long-chain moleculars(such as long C-C and C-F chains)are frequently utilized to decrease the surface energy.These molecules indicate poor conductivity.Hence,a low surface energy layer can effectively inhibit the electro-transfer during the corrosion process,thus suppressing the corrosion of the Mg substrate.The third barrier is the rough surface.A compact rough layer on Mg substrate can significantl prevent the permeation of the corrosion liquid.

4.Methods for fabricating superhydrophobic coatings on Mg alloys

The design and fabrication of SHC dates back to 1999[26],while the SHC on Mg alloys emerged in 2007[21].With the development of surface modificatio technologies,increasing number of methods have been proposed to prepare SHC on Mg alloys.As discussed in Section 2,there are two essential conditions for designing SHC.First,a microstructure is required to increase the roughness of the surface.According to the Wenzel and Cassie equations,an increase in roughness results in the decrease inf1value.Second,a low surface energy agent is necessary to decrease the intrinsic surface energy of the surface.A decrease in surface energy will result in an increase in solid-liquid tension,and thus,a higher WCA can be obtained according to Young’s equation.Guided by the two principles,various strategies have been proposed,which can be divided into one-step and two-step methods.

4.1.Two-step method

Two-step method is the most frequently employed strategy to fabricate SHC on Mg alloys.For a typical two-step method,pretreatment is applied to construct a rough surface,followed by grafting long-chain molecules(C-C or C-F)with low surface energy onto the rough surface.Therefore,the following discussion is based on the technology of pretreatment and expanding on the construction of the outer layer.

Fig.4.Schematic illustration the Cassie impregnating wetting state and the Cassie’s state(a),SEM images of a red rose petal,and shape of water on petal’s surface and on surface when it is turned upside down(b).Reprinted with permission from Ref.[22].Copyright 2008,American Chemical Society.

Fig.5.Schematic illustration of the corrosion protection of SHC on Mg alloys via three barriers.

PEO is widely used as a preliminary treatment for its porous structure.Under a high electric field the anode(Mg substrate)is oxidized and a metal oxide layer formed.PEO coating can significantl increase the surface roughness,and it also possesses high adhesion to the substrate and favorable corrosion protection.Based on PEO pretreatment,various methods have been exploited to fabricate SHC on Mg alloys.Dip coating is an effective way to graft long-chain molecules[27-30].For example,Cui et al.placed PEO-treated AZ31 alloy in stearic acid(SA)and myristic acid(MA)solutions,respectively[31,32].Both the prepared surfaces indicated WCA values higher than 150° Furthermore,EPD was applied to deposit SA on PEO-coated Mg-4Li-1Ca by Cui et al.,and the surface demonstrated a WCA of 153.5°[33].In addition,the self-organization of nanoparticles was involved.Boinovich et al.applied a precursor solution containing silica nanoparticle,hydrophobic agent methoxy-{3-([2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]pro yl-silane},and low-volatile dehydrated dispersion medium to fabricate SHC on Mg alloy[34].To further enhance the adhesion of long-chain molecules,they boiled the initial prepared PEO coating to increase the surface density of chemisorption-active sites,namely hydroxyl groups[35].The WCA of the PEO coating before and after boiling was 45.9°±2.9° and 22°±7°,respectively.In another study,ethylene glycol was ultized to increase the surface hydroxyl groups on the PEO coating,and then reacted with octadecylphosphonic acid(ODPA)to obtain a SHC(Fig.6)[36].Moreover,to increase the roughness of the PEO coating,a nanostructure was prepared atop of the PEO coating[37,38].Jiang et al.used phytic acid(PA)and Ce(NO3)3to layer-by-layer assembly construct a micro-nano hierarchical structure on a PEO-treated AZ91 alloy,and then modifie with 1H,1H,2H,2H-Perfluorode yltriethoxysilane(FAS)(Fig.7a)[38].The as-prepared surface indicated a WCA of up to 159°(Fig.7b).

EPD(including electroless plating),which use Mg as cathode,is an alternative method to fabricate nano-micro structures on Mg alloys[39-41].The current research is focused on Ni-based coatings.After EPD treatment,the dip-coating method was frequently used to endow the surface with a low surface energy[42-44].She et al.used H3BO3as a buffer agent,EDA·2HCl as a crystal modifie to prepare a pineconelike Ni coating on AZ91D alloyviaEPD.The sample was immersed in SA(0.01mM)for 5min to obtain a superhydrophobic surface[45].The as-prepared surface indicated a WCA of 163.3°±20.7°and a WSA of 1.2°±0.9°Moreover,the SHC exhibited good mechanical stability and long-term stability.Liu et al.placed Ni-plating treated AZ31D alloy in a perfluorocapryli acid(PFA)solution[46].The as-prepared surface showed superamphiphobic ability,with water and oil contact angle of 160.2°±1° and 152.4°±1°,respectively.In addition,the PA-modifie surface presented a favorable chemical stability over a large pH value range(2-12).In particular,Yuan et al.used a plated Ni-P surface as pretreatment on AZ61 alloy,and treated them with a hydrothermal reaction and finall modifie with SA(Fig.8a)[47].The surface displayed a maximum WCA of 155.6°±0.3° as well as excellent self-cleaning capability(Fig.8b).Besides dip coating,long-chain molecules could be deposited on EPD-treated surfaceviaEPD[48,49].For example,Wang et al.utilized Ni electroless plating as pretreatment of AZ91 alloy,followed by electrodepositing Cu coating,and finall depositing lauric acid(LA)under a direct current supply with LA and sodium acetate as the electrolyte[49].

Fig.6.Schematic illustration of the fabrication process of SHC on Mg alloy via ethylene glycol and DOPA(a),SEM images and contact angles of PEO coating(b)and SHC(c).Reprinted with permission from Ref.[36].Copyright 2016,Royal Society of Chemistry.

Compared with PEO and EPD pretreatment,hydrothermal treatment(HT)is a more facile and cost-effective method to construct a rough surface on Mg alloys.Mg(OH)2coating with petal-shaped nano-sheet structures could be achieved on Mg alloys using deionized water,NaOH or H2O2solution as a hydrothermal liquid[50-56].Large amount of hydroxyl groups exist in the hydrothermally treated surface which can be utilized to graft-COOH groups;thus,a SHC can be easily obtained by simply immersing in SA(Fig.9)[53],dodecafluoroheptyl-pro yltrimethoxylsilane(Actyflon-G502 or 1H,1H,2H,2H-perfluorooctyltriethoxysilan(PFOTES)solution.In addition to Mg(OH)2,a fibrou szaibelyite coating,mainly composed of MgBO2(OH),was prepared on AZ31 alloy by using Mg(NO3)2and H3BO3as a hydrothermal liquid[57].Layered double hydroxide(LDH)coating,with the ability to anti-corrode of Cl?,was reported to possess better corrosion resistance than Mg(OH)2coating[14,58,59].Therefore,many researchers prepared LDHs coating,which displays similar surface view with Mg(OH)2coating,on Mg alloys as a rough surface[60-62].Zhou et al.in-situfabricated an Zn-Al LDH coating on AZ91 alloyviaa hydrothermal crystallization method,and grafted with SAviaa dip coating method[63].The WCA of the prepared coating was 165.6°When exposed to the air for 100 days,the WCA remained larger than 150°,suggesting a favorable durability(Fig.10).Wu et al.prepared Mg-Al LDH coating on AZ31 alloy,and grafted it with SA,sodium laurate(SL),MA,and 1H,1H,2H,2H-perfluorode yltrimethoxysilane(PFDTMS)viadip coating method,respectively[64].It was found that when immobilized with SA,SL,and MA,the surfaces revealed superhydrophobic property with WCA of 150.6°,153.7°,and 152°,respectively.However,the WCA was only 145.5° for the PFDTMS-modifie surface.EPD was also involved to deposit low surface energy agent on hydrothermally treated surface.For example,EPD was used to deposit MA on Mg-Mn LDH coated Mg[65].Notably,Wu et al.used RF magnetron sputtering technology to fabricate a polytetrafluoroet ylene(PTFE)layer on Mg-Al LDH coated AZ80 alloy[66].The surface exhibited a WCA of 170.1°±1.2°,and desirable corrosion protection in H2SO4.In addition to Mg(OH)2and LDH coatings,TiO2and hydroxyapatite were used to construct micro-structures on Mg alloys by HT[67,68].Zang et al.prepared a TiO2coating on AZ91 alloy.Cu-thiolate layers were then deposited on TiO2by combing electroless plating and self-assembling method.The as-fabricated surface exhibited a lotus seedpod structure and a WCA of 153.9°±2.7°[67].

Fig.7.Schematic illustration of the fabrication process of SHC on AZ91 alloy via cyclic assembly of PA and Ce(NO3)3(a),and surface morphology of PEO coting and SHC,cross section view and WCA of SHC(b).Reprinted with permission from Ref.[38].Copyright 2018,Elsevier.

Chemical etching and deposition(CED),which only contains an immersing process,is a simple and time-saving route to construct a rough surface on Mg alloys.Generally,the solution used in this method is acidic,and thus a portion of the Mg substrate(high energy sites)will be dissolved to leave a rough surface.Moreover,the released Mg2+or other metal ions in the solution would react with OH?or other anions to form a deposited layer on the Mg substrate thereby increasing its roughness[69-73].The most commonly used metal ions for etching and deposition solution are Ni2+,Cu2+,Ce2+,and Fe2+(among which Cu2+was frequently chosen),while anions are SO42?,Cl?,NO3?,and PO43?[74-79].After CED,low surface energy molecules are depositedviaEPD,hydrothermal,and dip coating methods[79-82].Yin et al.used 5 wt% HNO3and 1mM Cu(NO3)2mixed solution etched AZ31 alloy and a lotus-like surface was achieved.After modificatio with KH-832,its WCA attained 157.3°±0.5°(Fig.11a)[83].Wang et al.fabricated a fl wer-like cluster structure on a Mg substrate by etching with H2SO4for 4min and then H2O2for 150s,and the WCA was 154° after immersion in SA(Fig.11b)[84].Interestingly,Gupta et al.found that when AZ31 was treated with H2SO4for 5min and then H2O2for 5min,it exhibited a WCA of 152° although no long-chain molecule was grafted on the surface[85].Ishizaki et al.prepared a vertical CeO2coating on AZ31 alloy by immersing in an etching solution Ce(NO3)3(0.05M,pH=4.5),and modifie it with FAS[86,87].The as-prepared coating showed a WCA of approximately 155°,and its corrosion current density was decreased by 1 order of magnitude compared with untreated AZ31 alloy.NaF was used to fabricate nanostructures on the Ca-P chemical deposition layer by Zhang et al.[88].The as-prepared surface displayed a typical three-level hierarchical structure consisting of micro-protrusions,submicro-lumps and nano-grains.Upon further modifie with SA,the surface exhibited a WCA of 159°

Fig.8.Fabrication process of SHC on AZ61 alloy combing EPD,hydrothermal treatment and dip coating(a),and self-cleaning behavior of the prepared superhydrophobic surface.Reprinted with permission from Ref.[47].Copyright 2017,Royal Society of Chemistry.

Fig.9.Schematic illustration of formation mechanism of SHC on Mg alloys via HT and immersing in SA solution.Reprinted with permission from Ref.[53].Copyright 2019,Elsevier.

The above four technologies(PEO,EPD,HT,and CED)are the frequently studied methods for constructing rough surfaces on Mg alloys[18-20,24,25,89].Nevertheless,many other technologies,including spray coating,laser ablation(LAb)and layer-by-layer self-assembly(LBL),have been applied to increase the surface roughness of Mg alloys.AZ31B alloy was spray coated with 5 wt% NaCl by Wang et al.,and the sample was immersed in FAS to obtain a superhydrophobic surface[90].Shi et al.employed a spray method to deposit a polyphenylene sulfid(PPS)and SiO2mixture on an AZ31 alloy to create a rough surface,and further sprayed with PTFE to lower its surface energy[91].A periodic papillary-like pits surface(Fig.12b)was formed on the AZ31 alloy via the LAb method(Fig.12a)by Li et al.[92].They used AgNO3to etch the surface to increase roughness and modifie it with SA to lower the surface energy.Interestingly,in another study,Wei et al.found that the LAb-treated AZ91 exhibited a WCA of 158.8°±2° after annealing at 160 °C for 60min[93].However,the authors did not provide a reasonable explanation for this phenomenon.In addition,Qu et al.used PA and CeCl3to construct a rough LBL surface on AZ31B alloy and modifie the surface with hexadecyltrimethoxysilane(HDMS)[94].When LBL treated for 4 cycles,the surface showed the best superhydrophobicity with a WCA of 167.3°±2.1° and a WSA of 2.7°±0.8°

Fig.10.Digital photographs of water droplet on SHC exposed in the air:(a)as-prepared,(b)after 10 days,(c)after 20 days,(d)after 40 days,(e)after 70 days,(f)after 100 days.Reprinted with permission from Ref.[63].Copyright 2015,Elsevier.

Fig.11.Morphology and WCA of the surface treated with HNO3/Cu(NO3)2 mixed solution and then modifie with KH-832(a,b),or treated with H2SO4 and H2O2,and then modifie with SA(c,d).Reprinted with permission from Refs.[83,84].Copyright 2010,Elsevier.

In summary,the characteristics of SHC fabricated by the two-step method are displayed in Table 1.It can be observed that after coated with SHC,the corrosion current density of Mg alloys decreased by 2-4 orders of magnitude,suggesting the significan corrosion protection of SHC preparedviaa two-step method.Among the literatures in Table 1,only 6 studies detected the mechanical durability and adhesion force of the superhydrophobic coating[45,47,48,53,88,90].The most used method to evaluate the mechanical durability is sandpaper abrasion test,while cross cutting test is mostly used to evaluate the adhesion force of the coating.Nevertheless,these two methods are only suitable for lab-scale detection,and cannot represent the practical application.However,in a study by Wang et al.,waterfall/jet test,which is more rigorous than sandpaper abrasion test,was also applied to evaluate the mechanical durability of the prepared superhydrophobic coating on AZ31[90].These methods to test mechanical durability and adhesion force were also applied on the superhydrophobic coatings prepared by one-step method.

Table 1Summarization of two-step methods to fabricate SHC on Mg alloys and the characterizations of SHC.

Fig.12.Setup of laser ablation(a)and surface morphology of the obtained SHC on AZ31 alloy(b).Reprinted with permission from Ref.[92].Copyright 2018,Elsevier.

4.2.One-step method

One-step method means that a rough surface and low surface energy on Mg alloys is achieved simultaneously in one process.The methods applied in the two-step method can be used in the one-step method to fabricate a SHC on Mg alloys,except the PEO method.This is because PEO technology is based on the oxidation of Mg substrate under a high voltage,and long-chain molecules can hardly be grafted onto PEO surface during the oxidation process.The other commonly used technologies,including EPD,HT,and CEP,are involved in the construction of SHC on Mg alloys.

EPD is an effective and simple method to fabricate SHC in one step,because the negatively charged long-chain molecules with a low surface energy can easily be electrostatically attracted to the Mg surface by metal cations.As discussed earlier,all the electrolytes of EPD used in the two-step method contained Ni cations.In contrast,the electrolytes of EPD used in the one-step method mainly were Ce3+,Mg2+,andCa2+,which are more environmentally friendly than Ni2+[95-99].For example,Liu et al.used Ce(NO3)3,MA,and ethanol as the electrolyte,Mg-Mn-Ce alloy as the cathode,and platinum plate as the anode[100,101].During the EPD process,Ce3+would introduce myristic molecules(with one carboxyl group)to deposit on the Mg-Mn-Ce surface(Fig.13),leading to a superhydrophobic property.

Fig.13.Schematic illustration of EPD process to one-step fabricate SHC on Mg alloy.Reprinted with permission from Ref.[101].Copyright 2015,American Chemical Society.

HT is extensively studied as a one-step method to fabricate SHC on Mg alloys.In a typical HT process,Mg plates and hydrothermal solution are added to a stainless-steel autoclave[102-104].The hydrothermal solution contains a long-chain molecule and the dissolvent is a mixture of ethanol and deionized water.Ethanol is favorable for the dissolution of inorganic long-chain molecules,and deionized water is favorable for the diffusion of cations and anions.Therefore,in this system,Mg2+would dissolve from the substrate and be deposited on the surface in the form of Mg(OH)2,and the negatively charged long-chain molecule would bond with Mg2+simultaneously,lowering its surface energy.In addition,many other soluble salts can be added to the hydrothermal solution to alter the composition of the rough surface layer[105-108].For examples,Qian et al.used Cu(NO3)2and SA as hydrothermal solutions,and Cu2O phase was detected on SHC[106],while Kang et al.used Ca(CH3COO)2and NaH2PO4as hydrothermal solutions,and hydroxyapatite phase was detected[108].

CED can be applied to fabricate SHC on Mg alloys in a single step.Through a simple immersion process,metal oxide or metal hydroxide is deposited on the Mg surface,accompanied by a long-chain molecule bonded on the Mg surface.Typically,alkaline metal ions are dissolved in deionized water and long-chain molecules are dissolved in ethanol.The two solutions are uniformly mixed and used as reaction solutions.Zhao et al.used FeCl3and tetradecanoic acid(TA)as treatment solutions,and AZ31 plates were immersed in the solution at 60 °C for 2h[109].The asprepared surface displayed a WCA of 165°±2° In addition,Ce(NO3)3/MA,ZnCl2/SA were used as reaction solutions to fabricate SHC on Mg alloys,respectively[110-112].Similar to CED,Wu et al.developed a novel strategy to fabricate SHC on an etched Mg surface by an immersing process[113].They modifie rhombic-dodecahedral zeolitic imidazolate framework(ZIF-8@SiO2,a kind of nanoparticle)with hexadecyltrimethoxysilan(HDTMS),and then the modifie nanoparticles were self-assembly immobilized on etched AZ31 surfaceviaan immersing process.

In addition to the above mentioned three one-step methods,researchers have developed other one-step strategies to prepare SHC on Mg alloys.Ishizaki et al.used plasma-enhanced chemical vapor deposition(PECVD)with a gas mixture of trimethylmethoxysilane(TMMOS)and Ar as the raw materials to deposit a SHC on AZ31 alloy[114].Qian et al.sprayed PFOTES modifie SiO2on an AZ31B alloy to construct a rough and low surface energy surface[115].Similarly,Xie et al.employed polydimethylsiloxane(PDMS)to modify SiO2.The nanoparticles were painted on the AZ31surface(Fig.14a)[116].Polat et al.used polystyrene(PS)to modify fluorosilane-functionalize SiO2,and then the particles were electrospun coated on AZ31 alloy(Fig.14b)[117].

Fig.14.The process to prepare the PDMS/SiO2 SHC on AZ31 via spray method(a)and the schematic of preparation of PS/SiO2 SHC on AZ31 via electrospun method.Reprinted with permission from Ref.[116,117].Copyright 2018,Elsevier and Tubitak.

Compared with two-step method,one-step method is more facile and time-saving,but the low surface agent in the reaction solution would influen the formation process of rough layer,which might damage the adhesion force of the coating,as well as its corrosion resistance.For two-step method,because a rough surface is prepared in advance,so the morphology and adhesion force of the superhydrophobic surface can be fully optimized.Table 2 presents an overview of the currently established one-step methods for fabricating SHC on Mg alloys,and the coatings’WCA,WSA,and corrosion current density are summarized.The superhydrophobic surfaces of Mg alloys constructed by one-step method exhibit significan protection for Mg substrate,as verifie by the decrease in corrosion current density decreased 2-4 orders of magnitude.

Table 2Summarization of one-step methods to fabricate SHC on Mg alloys and the characterizations of SHC.

Table 3Explanation of acronyms.

Fig.15.Snapshots of a scratched Cr-based superhydrophobic coating on Mg alloy in 3.5 wt% NaCl aqueous solution at room temperature for various times.a)initial state,b)5min,c)30min,and d)60min.Insets are the corresponding side views.Reprinted with permission from Ref.[119].Copyright 2016,Wiley.

5.Functional superhydrophobic coatings on Mg alloys

The previous section mainly focuses on methods for fabricating SHC on Mg alloys.Based on these methods,many researchers have developed functional superhydrophobic surfaces on Mg alloys with the aim of enhancing their corrosion performance and broadening their applications.In this review,SHC with self-healing ability,biologic effect,and other features are discussed.

Physical damage to SHC is sometimes inevitable in the practical applications.SHC with self-healing ability can form a deposited layer at the damaged sites,thus significantl inhibiting fast localized degradation.Self-healing SHC is always prepared by a two-step method,and the inner rough surface is responsible for its self-healing capability.Some self-healing properties are based on the phase transformation of the coating composition.Yang et al.prepared a MgSnO3coating on an AZ91D alloy and modifie it with SA[118].They proposed that the self-healing process of the asprepared SHC can be ascribed to the dissolve-reprecipitation of MgSnO3.Zhang et al.reported a novel Cr-based chemical conversion coating on AZ31B and modifie it with SA[119].When a scratch appeared in the coating,Cr and Cr2O3were oxidized subsequently,and precipitated as phases of Cr-based oxide or other compounds at the scratch site to heal the coating(Fig.15).In addition to the utilization of phase transformation,corrosion inhibitors have been investigated to construct self-healing SHC on Mg alloys.Ding et al.used mesoporous SiO2to load 2-hydroxy-4-methoxyacetophenone(HMAP),while Jiang et al.employed EPD to deposit 8-hydroxyquinoline(8-HQ).Both inhibitors endow the SHC with self-healing properties[120,121].Moreover,Jiang et al.prepared a Mg-Al-MoO42?LDH coating on PEO-treated AZ91D,and modifie it with 1H,1H,2H,2Hperfluorode yltriethoxysilane(PFDS).In addition they further modifie it with perfluoropolyethe(PFPE)[122].The asprepared sample demonstrated dual self-healing effects(Fig.16).First,the PFPE layer could self-replenish into the damaged region.Secondly,once Cl?permeates into the LDH layer,MoO42?would be released and act as an anodic corrosion inhibitor to suppress the corrosion process.It should be noted that after modifie with PFDS,the WCA value was 153°However,after modifie with PFPE,it decreased to 121°

Fig.16.Schematic protection mechanism of the dual self-healing effects.Reprinted with permission from Ref.[122].Copyright 2019,Elsevier.

Researchers have explored the biological applications of SHC on Mg alloys.The researched studied SHC was developed using the methods discussed in Section 4.Studies have reported that SHC exhibits low cytotoxicity and hemolysis ratio as well as inhibits the adhesion of bacterial and blood platelet,which are ascribed to its favorable corrosion resistance and high contact angle,respectively[123-127].Zhang et al.prepared SHC coating on pure MgviaPEO and EPD methods[124].They reported that the as-constructed surface was not suitable for the adhesion and proliferation of osteoblasts,but enhanced its differentiation.However,the data presented in the paper cannot sufficientl support this conclusion because only the observation of the formed hydroxyapatite was given.In addition,considering the results of the authors that few osteoblasts adhered to the surface after culturing for 7 days,the material is not suitable for application in orthopedic implants.In summary,there are few explorations of the clinical applications of SHC-modifie Mg alloys,and thus,more methods and application scenarios should be explored.

In addition,SHC on Mg alloys is found to possess antiicing or anti-oil properties[46,105,128].Li et al.fabricated a superhydrophobic surface on AZ31 alloy using a one-step HT method with NiSO4and SA as a hydrothermal solution[105].They found that after the water was frozen on SHC at?15°C for 300s(Fig.17a),the frozen water could fl w off from the surface,indicating a favorable anti-icing ability.Liu et al.used EPD and dip coating methods to construct a SHC on AZ91D[46].Immersing the prepared sample in oil,and then removed from the oil,the sample surface was completely dry and exhibited a WCA higher than 150°,suggesting an excellent anti-oil property.Interestingly,Ishizaki et al.found that a color-tuned Mg surface can be achieved by varying the treatment time of HT.They were then modifie with noctadecyltrimethoxysilane(ODS)to obtain superhydrophobic surfaces(Fig.17b)[129].Nevertheless,only a few studies focused on the SHC modifie Mg alloys with anti-icing,antioil or other practical properties.

6.Conclusions and future challenges

Inspired from nature,such as lotus leaves,cicada’s wing,and rose petals,superhydrophobic surfaces have attracted widely attention in recent years.In this review,the wettability state of a solid surface and the corrosion protection mechanism of SHC are discussed.A“three barriers”corrosion protection mechanism is proposed based on the special structure of SHC.The strategy to fabricate SHC on Mg alloys in recent years is presented.These methods are based on the two gold principles:rough surface and low surface energy.The strategies to fabricate SHC on Mg alloys can be divided into one-step and two-step methods.The developed methods and technologies in recent years are summarized in Fig.18,and the applied low surface energy agents are displayed.In the two-step method,a rough surface is firs constructed on Mg alloys with PEO,EPD,HT,CED,or other technologies,followed by grafting the long-chain molecule on a rough surface by dip coating,spray coating,immersion treatment,spin coating,EPD and HT.For the one-step method,a rough surface and low surface energy are achieved in one process,and are mainly applied with EPD,HT or CED technologies.Based on these methods,many functional SHC on Mg alloys have been developed,such as self-healing SHC,anti-icing and anti-oil SHCs,and SHC for biological applications.

Fig.17.Photographs for comparison of icing of AZ31 surface(the left)with SHC surface(the right)at?15 °C at different time from the start of the test,with horizontal and incline sample placement.Horizontal samples:(a)5s,(b)120s,(c)150s and(d)600s,incline samples:(e)5s and(f)300s.Reprinted with permission from Ref.[105].Copyright 2016,Electrochemical Society,Inc.Fabrication of color-tuned SHC on AZ31 surface(g).Reprinted with permission from Ref.[129].Copyright 2011,American Chemical Society.

Fig.18.A summary of one-step and two-step methods to fabricate SHC on Mg alloys,as well as the involved low surface energy agents.

At present,although many efforts have been devoted in fabricating SHC on Mg alloys,there is still a long way to achieve practical applications.Before obtaining successful industrial applications,the following challenges should be solved:

(1)The long-term stability of SHC.Many researchers have demonstrated SHC presents good mechanical and chemical stabilities.However,these stabilities are only tested in the laboratory.For example,mechanical stability is tested by rubbing for a certain distance in abrasive paper,chemical stability is tested in water with different pH values,and long-term stability is tested by placing it in air for several months.These conditions are not sufficien to represent situations for actual applications.Thus,the long-term stability performance of SHC on Mg alloys should be detected in more rigorous environments.

(2)Anti-oil ability.Once the oil spread in the superhydrophobic surface,the low surface energy agents,which are organic molecules,might be dissolved.When the oil is rinsed,the low surface energy agent might be rinsed concurrently,leading to the loss of superhydrophobicity.Considering this,anti-oil SHC or non-organic longchain SHC are preferable for Mg alloys.

(3)Development of multifunctional superhydrophobic surfaces for different application scenarios.The present studies mainly focus on the corrosion resistance of SHC on Mg alloys,and ignore other possibilities.To achieve a practical application,the design and fabrication of SHC should be more purposeful.For examples,for bone implant applications,the SHC should be antibacterial and pro-osteogenesis;for application in engine and transmission systems,the SHC should be anti-oil and heat-resistant;and for application in car body parts,the SHC should be impact-resistance and antifriction.

(4)Putting more attention on the applications of SHC on Mg for marine environment.Although SHC on other substrates have been widely studied for marine environment,but few studies tried to explore the potential of SHC on Mg for marine environment.To broaden the application field of Mg,the corrosion behavior of SHC on Mg should be investigated in high salt,high humidity,high dissolved oxygen level,microorganisms and impact-corrosion,which are the typical environment for marine.

Declaration of Competing Interest

None.

Acknowledgment

This work is financiall supported by the China Postdoctoral Science Foundation(2019M662830),the National Natural Science Foundation of China(31771044),the National Key Research and Development Program of China(2016YFC1100604),Shanghai Committee of Science and Technology,China(184107606000)and International Partnership Program of Chinese Academy of Sciences(GJHZ1850).