Superhydrophobic surface of Mg alloys:A review

M.Yeganeh,N.Mohammadi

Department of Materials Science and Engineering,Faculty of Engineering,Shahid Chamran University of Ahvaz,Ahvaz,Iran

1.Introduction

As one of the lightest engineering materials,magnesium alloys have exhibited quite special properties including high specific strength,low density,high thermal conductivity,electromagnetic shielding,excellent machinability,good vibration and shock absorption,high damping capacity, fine electromagnetic interference shielding properties,good weldability under the controlled ambient,excellent castability,and recyclability which lead to the specific applications such as automotive,aerospace,communication,electronics,medical,and other industries[1-6].However,the inherent poor corrosion resistance of Mg alloys due to their low standard electrode potential is the major shortcoming regarding their applications in the aqueous or atmospheric media[7,8].Besides,magnesium endures high chemical activity and the formed oxide film of magnesium alloy under the natural conditions is porous and non-protective[9].Another problem is that the chemical activity of industrial magnesium alloys is typically inhomogeneous.For instance,AZ91D alloy which has been extensively used in the automobile industries is made of two phases includingα(Mg)andβ(Mg17Al12)phase[10].Various surface treatment methods have been applied to reduce the chemical and electrochemical activity of magnesium including physical vapor deposition(PVD)[11-19],electroless plating[16-20],plasma electrolytic oxidation(PEO)[21-27],chemical conversion treatments[28-34],sol-gel coatings[35-39],calcium phosphate coatings[40-44],hydroxyapatite coatings[45-49],and polymer coatings[50-55].One of the recent interesting strategies to improve the corrosion resistance of Mg alloys is the development of superhydrophobicity at the surface of these alloys[10,56-59].Superhydrophobic surfaces are generally defined as the surfaces which water contact angles(θ)show the greater values than 150°and the sliding angles are smaller than 5°[60,61].It could exhibit self-cleaning,anti-icing,anti-bacterial,anti-reflection, fluidic drag reduction,anti-corrosion,and anti-fogging behavior of surfaces if the superhydrophobic properties are created in the textiles,glasses,polymers,and metals[62-68].If the direct contact of surface regarding magnesium alloy with the reactive environment reduces,the chemical activity of magnesium alloy decreases sufficiently.Thus,synthesis of a superhydrophobic layer/surface would be an attractive approach to improve the corrosion resistance of magnesium alloy,and therefore it could expect more potential applications for these interesting alloys[69].

Surface modification of Mg alloys due to their high activity and inhomogeneity is difficult[10].Moreover,the formation of the corrosion products is quite complicated and not well understood yet.For example,when AZ91 Mg alloy is dipped in the aqueous solutions,the formed film on the surface of theαphase consists of three layers including an exterior layer of Mg(OH)2,a middle layer of MgO,and an interior layer of Al2O3.The surface film on theβphase is different from that onαphase which leads to the inhomogeneity of surface chemistry[15,18,19].Consequently,it is a great challenge to control the formation of corrosion products under the laboratory conditions[70-73].

In recent years,the fabrication of superhydrophobic surfaces has become an interesting area of active researching.In general,the main techniques which are applied for manufacturing of micro-and nanostructured layers,such as lithography,etching,chemical and electrochemical deposition,casting,plasma treatment,and replication have been used for the synthesis of superhydrophobic surfaces[74].There are two significant requirements to fabricate a superhydrophobic surface including the roughening of the surface and the development of low surface energy.These two requirements lead to two major routes of producing superhydrophobic surfaces.The first route is to make a rough surface from an intrinsic hydrophobic material,and the second one is to modify an initially rough surface by changing the surface chemistry or applying a hydrophobic component layer[63,74,75].

In this review,the procedures and the routes of superhydrophobic surfaces(SHPSs)which were fabricated on the Mg alloys were summarized.It must be mentioned that most of these procedures require a distinct method of surface treatments(immersion,coating and roughening processes)with the various modification processes to decrease the surface energy.The main idea of categorization in this review relies on the synthesis processes before the modification stages.In this review,all processes categorize on the basis of chemical,electrochemical or physical surface treatment including hydrothermal technique,chemical and electrochemical deposition,conversion and polymer coating,and etching processes.Besides,it must be noted that before the processes,the Mg surfaces were polished,cleaned by alkaline or acidic solutions,and deoxidized by mixing of acidic and/or basic solutions[76].

2.Superhydrophobic processes

As mentioned earlier,SHPSs could be developed by chemical,electrochemical,and physical surface treatment.These methods could facilitate the formation of coatings or surface roughening on Mg alloys.Chemical routes are useful methods to fabricate SHPSs.In this regard,chemical processes are one of the most widespread methods for development of superhydrophobic structures.The most conventional methods to create superhydrophobicity on the surface of Mg alloy by chemical routes include hydrothermal treatment,conversion coating,chemical deposition treatments,and etching.In these processes,the surface is mainly treated by chemical substances which could react with the surface or deposit on it.One of the other reliable routes to obtain this goal is electrochemical techniques.These techniques are powerful methods to manipulate the surface of the electrodes in the electrolytic media by applying suitable electrical power.Changing the surface morphology or composition by electrochemical techniques including electrodeposition,anodizing,plasma electrolytic oxidation(PEO),and etc.could help to form superhydrophobic structures.The other processes including polymer coating could belong to the chemical or physical routes according to the technical procedures of surface treatment.

2.1.Hydrothermal process

SHPSs which are fabricated by hydrothermal process,obtained by the application of heating during the immersion of Mg plates into a Teflon-lined autoclave in the appropriate solutions including H2O,H2O2,urea,and NaOH followed by cooling,drying,and surface modification using fluoroalkyl silane(FAS),stearic acid(SA)or silanes[77-85].FAS molecules are extensively used as a hydrophobic modifying agent since those are well-known organic substances with an extremely low free energy surface due to C-F groups[63].However,it is widely accepted that such fluorinated chemical substances are expensive and could result in potential risks to human health and environment[69].Generally,the modifying substances could react with the surface of Mg to achieve an organic tail with hydrophobic behavior.For instance,CH3(CH2)16COO-bonding on the magnesium alloy surface was observed in the case of using stearic acid[81].

Gao et al.[80]fabricated superhydrophobic AZ31 Mg alloy through a hydrothermal process in the solution of water and hydrogen peroxide at 180°C for 3h followed by the immersion in the FAS(CF3(CF2)7CH2CH2Si(OCH3)3)containing solution for 1h and subsequently heating at 180°C for 1h.They reported the formation of magnesium hydroxide after the hydrothermal process.The surface of the Mg alloy was relatively smooth and partially coated with the flakelike structure.Water contact angle of the as-prepared superhydrophobic surface was measured 164±2°which was larger than that of untreated magnesium alloy surfaces with a value of 34±2°.FAS could decrease the surface free energy of the magnesium alloy[86].

The measured water contact angle varied from about 150-167°,while the sliding angle changed from 1 to more than 10°(Table 1).Most authors attributed to the mechanism of superhydrophobicity to the Cassie-Baxter relations.Superhydrophobicity could be explained by the Cassie-Baxter equation[87]:

whereθandθ0are the water contact angles on the rough and plane surfaces,respectively.fslis the fractional interface area of solid/water on the surface.Theθ0is the intrinsic contact angle of the solid surface.When the fractional area of solid/water decreases,the value ofθwould increase,and therefore the air trapped by the rough structures could generate a large contact angle and a small sliding angle.The large contact area between the water and air leads to the prevention of the penetration of water or aggressive species on the surface[87].

Fig.1a-d shows the surface morphology regarding Mg alloys obtained by the hydrothermal treatments[70,77,81,82].As can be observed in these figures,the most frequent morphologies related to the hydrothermal process are plate-like,needle-like or flower-like morphology,which could act as a barrier to the invasion of water droplets.

Some authors performed FTIR technique to evaluate the presence of the modifying agent on the surface of Mg alloy after the treatment[81,82].FTIR spectra of the superhydrophobic surface of Mg alloy and pure stearic acid(SA)as a modifying agent are shown in Fig.2a and b[82].The asymmetric and symmetric stretching vibration of C-H peaks were located at 2848 cm-1and 2918 cm-1[83],which could be attributed the presence of the aliphatic groups on the superhydrophobic surface arising from stearic acid[82].In some studies,the presence of Mg(OH)2was evaluated by the O-H stretching vibration in the region of 3500-3600 cm-1[81].The observed peaks at about 850 cm-1were assigned to the Mg-O vibration mode.Besides,the band of-COO from SA appeared at 1560 cm-1[81].

X-ray photoelectron spectroscopy(XPS)was applied to detect the presence of surface modifying agents including FAS or SA[79,80,82,84].Fig.3a and b illustrates XPS spectrarelated to the Mg surface after immersion in the urea and its modification with FAS containing solution.It showed the existence of C,O,F,and Si elements,which indicated the coverage of the surface by FAS components[84].The observed peaks could prove the presence of C-F bond and the interaction between FAS and the surface.The reaction between FAS and the surface film was described as a condensation reaction between-Si(OCH3)3from FAS and the-OH on the surfaces which resulted in a new chemical bond at the interface[84].

Table 1 The measured water contact angle and while the sliding angle.

Fig.1.Surface morphology of Mg alloys treated by hydrothermal processes:(a)[70],(b)[77],(c)[81],and(d)[82].

XRD patterns in some cases confirmed the formation of the complex[84,85]or simple compounds such as Mg(OH)2[78,80,81].For example,Fig.4a-c shows the peaks regarding hydromagnesite(Mg5(CO3)4(OH)2·4H2O(JPCD-25-0513))after the immersion of Mg alloy in the urea containing solution[84].As can be observed,the created compounds(Fig.4a and b)showed the distinct peaks compared to Mg alloy(Fig.4c).It must be noted that the modifying compounds such as FAS or SA did not influence the surface composition due to the formation of a thin film on the surface,which could not be easily detected by the typical XRD techniques.

Fig.2.FTIR spectra regarding the superhydrophobic surface and SA.(a)superhydrophobic surface and(b)stearic acid[82].

The hydrothermal process could help to form dense or plate-like morphology which is favorable to provide SHPSs.However,this technique needs specific instruments such as autoclaves and numerous steps with different surface treatments,which limits its applications to provide the reasonable surfaces.

2.2.Chemical deposition

Chemical deposition is one the most significant methods to create SHPSs.In this category,chemical processes involve chemical salts deposition,conversion coatings,sol-gel process,and chemical vapor deposition.In these chemical methods,by using different inorganic salts including sulfates,nitrates,chloride,and etc.different coatings form which could decrease the diffusion of the aggressive species to the Mg alloys surface[56,59,88-98].

Fig.3.XPS spectra of(a)the superhydrophobic surface of Mg alloy and(b)F1s peak regarding FAS[84].

Fig.4.XRD patterns of(a)hydromagnesite powder,(b)hydromagnesite film,and(c)Mg alloy substrate[84].

In these routes,Mg plates are put in the solution or gas phase which possesses mainly metal substances.As the surface of Mg is a reactive metal,it could be reacted with the solution or gas and therefore an appropriate conversion coating or film forms on the surface.The obtained surface is the groundwork for the following treatments.

Zhang and co-workers[88]immersed AZ31 plates in the iron sulfate containing solution for 30min,followed by hydrothermal modification with the stearic acid.The immersion process led to the formation of hierarchically micro/nano-Fe(OH)3film.The modified Mg alloy with the stearic acid showed superhydrophobicity and the measured contact angle was in the range of 163.7±2.9°[88].In another work,Zhou et al.[93]immersed Mg plates in an autoclave containing Zn(CH3COO)2·2H2O and Al(OH)3at 60°C for 5h.The treated plates were immersed in an ethanol solution of stearic acid for 5h at room temperature to achieve superhydrophobic behavior.The formed composition of ZnO and Zn-Al layered double hydroxide(LDH)was responsible for superhydrophobicity.Therefore,the water droplet cannot penetrate into the rough structure and therefore slide simply.The measured air fraction for the formed superhydrophobic surface was estimated 98.3%,which meant that 1.7%of solution could approach the surface[93].

The measured water contact angle regarding the chemical deposition varied from about 152-167°,while the sliding angle changed from 4 to 5°(Table 1).Fig.5a-d shows the obtained SHPSs of Mg alloys regarding the chemical deposition technique.As can be observed in Fig.5,the morphology of SHPSs such as the previous ones possessed the needle or flower-like morphology.

Fig.5.Surface morphology of Mg treated by different chemical deposition process:(a)[90],(b)[93],(c)[94],and(d)[95].

Fig.6.FTIR spectra corresponding(a)FAS,(b)Mg alloy surfaces treated via a chemical corrosion without FAS modification,and(c)Mg alloy surfaces treated via a chemical corrosion and FAS modification[92].

A few authors studied different chemical bonding resulted from different modification using FTIR technique[59,89,92,94,97].Although,XPS results demonstrated the appearance of C 1s,O 1s,and F 1s related to the presence of modifying compounds on the surface of Mg alloys[88,89,91,95,96].Fig.6a-c shows FTIR spectra corresponding FAS(Fig.6a),Mg alloy before the treatment by FAS(Fig.6b),and Mg alloy after treatment with FAS(Fig.6c).The absorption bands about 1406,1325,1239,and 1144 cm-1were related to the C-F stretching vibration of the-CF2-and CF3in the FAS molecules,which was due to the reaction of Mg alloy with FAS as substance[92].

Another chemical conversion coating based on Cr(III)was developed by Zhang et al.[99].Superhydrophobicity was achieved for the coating by the modification with stearic acid.A water contact angle value of≈157°was obtained for the Cr(III)chemical conversion coating by stearic acid modification due to the formation of hierarchically porous structure[99].

Chemical deposition techniques are powerful ones to supply unique surfaces in the low time and steps.SHPSs could obtain without the heat treatment and can be completed within a few hours[96].This is a good potential for industrial demands.However,chemical substances could have toxicity effect on the human or environment.Besides,the disposal of chemical agents at the industrial level would be a critical issue.

2.3.Electroless and/or electrodeposition techniques

Generally,electroless and/or electrodeposition induced superhydrophobicity carried out in two common methods by researchers.In the first method,the main process is a combination of Ni electroless plating and the electrodeposition of metals or alloys such as Ni,Cu,Ni-Co,and Cu-Zn followed by modification with the organic molecules such as stearic acid,lauric acid,etc.[58,100-109].In the second one,an electrolytic salt of a metal including Ce(NO3)3,NiCl2,and CeCl2was applied to deposit on the Mg surface.Usually,in this process,a modifier molecule such as myristic acid,perfluorocaprylic acid,lauric acid,and stearic acid was used[58,100,101].

Liu et al.[102]synthesized SHPS on the AZ91D Mg alloy by means of Ni electroless plating,Ni electrodeposition,and finally immersion in the stearic acid.The Mg plates were immersed in the plating solution containing NiSO4·6H2O,C6H8O7·H2O,NaH2PO2·H2O,NH3·H2O,HF,NH4NF2,CN2H4S,and the pH value of the solution was adjusted 5.5-6.5.The electroless process was performed within 60min at 85°C.The obtained samples were electroplated in the bath containing NiCl2·6H2O and H3BO3at 55°C,and the pH value of the solution was set in the range of 4.5-5.0.In this bath,a constant current power with current densities of 15mA cm-2was applied to the electroless plated Mg.After electrodeposition,Mg plates were immersed in the stearic acid containing ethanol solution at room temperature for 60min[102].The prepared surface showed a contact angle as high as 160.8±1°and a sliding angle as low as 1.8±1°.Besides,the measured contact angle after two weeks of exposure in the air illustrated that the sample remained superhydrophobe.FTIR results showed the presence of stearic acid bonds on the surface.-CH3and-CH2-groups related to the stearic acid could reduce the energy of the Mg surface and provide superhydrophobicity[102].

Wang et al.[105]applied Ni coating on the Mg surface by electroless plating.The electroless process was needed due to reducing the activity of Mg-base alloys.In the next step,Cu metal was electrodeposited from a bath containing CuSO4·5H2O,KNaC4H4O6·4H2O,NaOH,and H3BO3by using a direct current density of 500mA dm-2.Finally,Mg coated sample was modified in an ethanolic lauric acid solution by applying the anodic current density with the value of 0.6mA cm-2.The measured contact angle before and after modification showed that electroplated Mg possessed a contact angle of about 0°,while the modified surface with lauric acid presented a superhydrophobic surface with a contact angle of 153.9°and a sliding angle of 2°.The appearance of copper laurate(CH3(CH2)10COO)2Cu)on the surface which was verified by FITR spectra could result in a superhydrophobic surface with a high roughness and low energy[105].

Fig.7.SEM morphologies of AZ91D after Ni electroless/electrodeposition and modification by SA with different magnifications[106].

In another work which was conducted by She et al.[106],AZ91 plates were coated by Ni electroless and electrodeposition following by chemical modification in the stearic acid.The contact and sliding angle regarding the modified surface was obtained 163.3±0.7°and 1.2±0.9°,respectively.Energy-dispersive X-ray spectroscopy(EDS)analysis showed the presence of carbon which indicated that stearic acid was adsorbed on the Ni coating.Fig.7 shows the SEM morphology of the AZ91D Mg alloy after surface modification by stearic acid.As can be observed in this figure,there are large amounts of small microparticles located on the surface(Fig.7a).Higher magnification of the surface showed that these microparticles consisted of the pinecone-like microclusters with an average diameter of 1-2μm(Fig.7b and c).This cone-like structure(Fig.7d)with the diameters in the nanometer ranges developed the hierarchical structure related to the formed morphology.

Electroless/electrodeposition technique is an efficient route to make different surfaces with various morphologies.However,these techniques need different types of chemical substances which could have an environmental concern,as well as the impurity in the coating could be observed due to competing reactions.Besides,they need several steps of experimental procedures as the hydrothermal methods.

2.4.Electrochemical.conversion coatings

Chemical or electrochemical treatment of the metal or alloy surface in order to change its composition which provides an insulating barrier with low solubility is denoted as the conversion coating.This kind of coating is a combination of surface and solution composition.Conversion coatings are commonly applied to magnesium alloys in order to increase their corrosion resistance.Plasma electrolytic oxidation(PEO)as a conversion coating method is one of the most prevalent methods to form a protective oxide layer on the Mg alloys.One of the major drawbacks of PEO coatings is the presence of micropores or microcracks in the coatings which leads to the penetration of electrolyte and degradation of corrosion resistance[24,110-115].Some researchers enhanced protective properties of PEO coatings by surface modification processes.These processes could facilitate the preparation of SHPSs.It is of great importance to diminish the surface defects of PEO coating in order to enhance its protective properties.In the case of Mg alloys,PEO treatment could enhance corrosion resistance.However,it was reported that PEO coatings on the Mg alloys showed hydrophilic behavior[69,111,116-118].Some researchers modified PEO coatings after its deposition.The most conventional method to obtain this aim is sol-gel process or modification with organic materials.In fact,in this processes,a PEO layer is modified with another coating[119-125].

In the study carried out by Gnedenkov et al.[116],PEO coating was deposited in a silicate- fluoride electrolyte containing Na2SiO3·5H2O and NaF on the surface of MA8 Mg alloy.These PEO coatings exhibited hydrophilic behavior and the obtained contact angles were in the range of 45.9°±2.9°.Superhydrophobic coatings with the contact angles of 166±3°and the sliding angles of 5±3°were fabricated by deposition of aerosil nanoparticle suspension in decane on the PEO coating[116].The hydrophobic agent reduced the free surface energy related to the surface.Besides it possessed three reactive chains of-Si(OCH3)3which supplied a chemical bond between the species in the solution and the surface of PEO coating[116].

Li and Yuan applied a constant current density of 4 A dm-2in an alkaline silicate electrolyte Na2SiO3,NaOH,and triethanolamine to obtain an oxide coating.In the next stage,the PEO coatings were modified with octadecylphosphonic acid(ODPA)followed by rinsing in the ethanol and heating.Pretreatment of PEO coating with ethylene glycol resulted in the increase of hydrophilic behavior of surface which was promoted to the adsorption process of organophosphorus acid.Annealing could maximize the number of covalent P-O bonds at the metal oxide surface,thus creating a more robust layer compared to the chemisorbed carboxylic acids[118].PEO coating showed a hydrophilic behavior with a contact angle of 38±3°,which was promoted after treatment up to 163±2°.Prolonging the immersion time could lead to the diffusion of water or corrosive species,and therefore,the tendency to the hydrophilicity increased.

In another work published by Boinovich et al.[122],a superhydrophobic coating on MA8 Mg alloy was obtained by bipolar PEO in a silicate- fluoride electrolyte followed by immersing in the decane and silane agents.Authors explained that Si-O-Mg bond energy was stronger than Si-O-Si one,which led to the formation of chemical bonding between the hydrophobic agent and the Mg substrate.Besides,the free silanol terminal group was arranged at the other side of bonding[122].The maximum superhydrophobicity obtained by the boiling of PEO coatings in the Na2O×SiO2×H2O solution before the surface treatment for 15min.The contact and sliding angle were obtained 166.0±3.0°and 5.2±3.3°,respectively[122].

Anodizing is often used for the surface treatment of magnesium alloys to improve their surface properties[126-130].However,there is a little information on the formation of SHPSs by the anodizing of Mg[126].Li et al.[126]fabricated anodic coating on AZ91D under the constant current density with the value of 20mA cm-2in the Na2SiO3electrolyte by the addition of a different volume of silica sol.They reported that the addition of silica sol to the anodizing bath could increase the final roughness of the coating.It was observed that the water contact angle on the anodic film was larger than the blank anodizing film by the addition of 10vol.%silica sol.However,they did not report any contact angle value.The hydrophobicity of the final coating was attributed to the crater-like structure of the anodized film and the hydrophobic behavior of sol-gel film[126].

Conversion coating process could provide adhesive films on the surfaces of Mg alloys,which is a typical route for the improvement of corrosion resistance.However,in some cases,the equipment could be expensive and enough skill is needed.Besides,the modification treatments need an extra process like sol-gel technique,which is more intricate than the simple dipping compared to the other techniques.

2.5.Polymer coatings

Some researchers applied polymer coatings on the surface of Mg alloys to obtain superhydrophobicity.In some cases,a further modification was used,while in the others cases,the composite coatings could form SHPSs.Polymer coating is one the most interesting method to fabricate superhydrophobic surfaces[131-134].Some researchers applied polymer coatings on the surface of Mg alloy to obtain superhydrophobicity.Polyvinyl chloride(PVC),polystyrene(PS),and polypropylene(PP)are usually used to prepare superhydrophobic films due to their intrinsic hydrophobicity and low-cost agents.Generally,the superhydrophobic polymer films were fabricated by non-solvent inducing phase separation method without further modification[131].Yang et al.[131]obtained superhydrophobic surfaces by anodizing and PVC dip coating of AZ91 Mg alloy plates.Superhydrophobic behavior obtained due to an increase in the surface roughness by the formation of non-uniform aggregates of hydrophobic PVC particles and the development of large pores.The reported contact angle related to the coated surface was 159°[131].

In another study which was conducted by Kang et al.[134],they etched Mg-Mn-Ce alloys in the HCl solution and subsequently they electroplated polymer substances from ATP (C3N3S2HNa-N(CH2CH=CH2)C2H4(CF2)7CF3)containing solution by the constant current density of 0.1mA cm-2at 25°C for 20min.The fabricated film showed a con-tact angle of 158.3°which was due to the complete covering of the surface with uniformly distributed superhydrophobic polymeric films[134].

Fig.8.SEM morphology of superhydrophobic surface on the AZ31[142].

Polymer coating could supply economic processes as well as high contact angles.These coating also could possess the intrinsic superhydrophobicity.However,the adhesions of coating and environmental issue related to the polymeric substances are the main concerns to use this technique.

2.6.Etching

Etching is a simple and effective route to form a rough and hierarchical structure on the surface of metals and alloys[135-140].The substrates are etched by an etchant which dissolves preferentially the dislocation or high energy sites in the metal grains to give a rough surface[135].Different etching method such as chemical etching has been applied to produce superhydrophobic structures on the surfaces of Mg alloy.The most preferred etching method for Mg alloys is the chemical etching by an acidic solution.This process leads to surface roughening of Mg alloys.In order to achieve SHPSs,the roughened surface was modified by organic molecules including stearic acid,oleic acid,FAS,and ethanol[10,76,141-144].Chemical etching has been interested by researchers due to low cost with the ability to control parameters,shape,and orientation of substrate[145,146].

Yin et al.[142],reported the production of superhydrophobic structures on the surface of AZ31 Mg alloys by etching in the aqueous solution of HNO3and Cu(NO3)2and immersion in KH-832(C14H32O3Si).The magnesium alloy surface was covered by a uniform micro-and nanoporous lotus-like structure with the average diameter of 5μm(Fig.8).This hierarchical structure could trap a large volume of air,which was the main reason to exhibit superhydrophobicity of the magnesium alloy.The measured contact angle could approach in the range of 157.3±0.5°with the sliding angle smaller than 10°.It was also reported that magnesium alloy after the chemical etching and without any modification by silane substances exhibited superhydrophilic behavior.These results demonstrated that both porous structures as well as the low surface free energy related to the silane groups were associated with the superhydrophobicity of the magnesium alloys[142].

Wang et al.[147]fabricated superhydrophobic surfaces on the AZ31 Mg alloy by etching in the CuCl2followed by immersion in the oleic acid(C18H34O2).The contact angle obtained was 155°,which was due to the formation of threedimensional porous structure as well as modifying the effect of oleic acid.Fig.9 shows the interaction of oleic acid with the Mg surface.As seen in this figure,the terminal configurations of organic molecules led to the formation of superhydrophobic tails on the surface of Mg alloy[147].

Besides,Xu et al.created SHPSs on the AZ61 Mg alloy by a preliminary Mg electrochemical machining followed by surface modification by fluoroalkylsilane(FAS)[148].The results showed the presence of hierarchical rough structures and a FAS film with a low surface energy on the Mg alloy surfaces,which created superhydrophobicity with a water contact angle of 165.2°and a sliding angle of about 2°.

Etching is a fast technique which could supply the surfaces at the lowest time and steps with the controlled parameters.However,chemical substances could have toxicity effect on the human or environment.Besides,the choice of the proper etchant with suitable characteristics is another critical issue.

3.Prospects

Different techniques regarding the development of superhydrophobic surfaces and coatings on the surface of Mg alloys have been reported in this review.In the mentioned techniques,the surface could be prepared by the chemical,physical,and electrochemical reactions.Most of the aforementioned approaches to develop SHPSs require particular instruments(hydrothermal and electroless/electrodeposition routes),severe conditions(hydrothermal,conversion coating,and chemical deposition routes),long preparation period(hydrothermal,electroless/electrodeposition,and conversion coatings),expensive(conversion coating,hydrothermal)and can lead to the potential harm to the environment(chemical deposition and electroless/electrodeposition)[149,150].The desired routes should be fast,feasible,low-cost,and eco-friendly[149,150].Future works to fabricate SHPSs could focus on the development of the economic treatment with the lowest assumption of hazardous substances including toxic metals,organic,volatile,and fluoride containing molecules,which is a lack of research studies and the main demand for efficient and eco-friendly products.On the other hand,the development of SHPSs to reach long-term stability with self-healing ability is a new demand in this field[151,152].Besides,new trends could depend on the basis of desired contact angle according to the favorable applications.Table 2 summarizes the main advantages and disadvantages of mentioned techniques.

Fig.9.The schematic illustration of oleic acid bond modes on the surface of Mg alloy[147].

Table 2 Advantages and disadvantages of superhydrophobic treatment related to the Mg surface.

The choice of one route could depend on the cost of the technique and the unique characteristics of the obtained surface.

4.Conclusion

In this paper,the formation of SHPSs by different routes on the surface of Mg alloys was reviewed.Various methods including hydrothermal processes,chemical deposition,electroless/electrodeposition methods,conversion coatings,polymer coatings,and etching were conducted to obtain superhydrophobicity on the surface of Mg alloys.In addition,some different molecules such as FAS,SA,oleic acid,and etc.were applied to gain low surface energy.Water contact angle and sliding angle after the aforementioned techniques approached 170°and 1°,respectively.The main outcomes which could be mentioned by the different techniques are listed below:

1-The flake-like and dense morphology could be obtained by chemical methods including hydrothermal processes.However,the application of chemical substances could be a potential risk to human health and environment.

2-Electroless/electrodeposition technique could offer multilayer coating on the surface to obtain good adhesion and functionality.These methods need further modification with low energy components.However,heavy metal contamination could be a major concern regarding the application of metallic electrolytes.

3-Conversion coatings due to the formation of combined composition possess good adhesion.However,some of these processes suffer an extra coating process such as sol-gel methods.

4-Etching is a simple,fast,low cost,and feasible method to form SHPSs.The main concern in this route is related to the selection of etchant and dipping time.

5-The main challenge for the wide application of the SHPSs regarding Mg alloys is to explore simple and feasible preparation conditions,economic and environment-friendly technologies,and develop SHPSs with high stability and self-healing characteristics.

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