Recent progress in superhydrophobic and superamphiphobic coatings for magnesium and its alloys

Viswanathan S.Saji

Center of Research Excellence in Corrosion,King Fahd University of Petroleum & Minerals(KFUPM),Dhahran 31261,Saudi Arabia.

Abstract Superhydrophobic coatings are projected as a practical approach to tackle the weak aqueous corrosion resistance of Mg/Mg alloys.The present review provides the most recent updates in this area.The various low surface energy treatments reported are presented first,followed by the methods employed for developing hierarchical surface micro/nanostructuring.Reported works in different application areas,including anti-corrosion,biomedical and anti-icing are systematically discussed.Concise descriptions of self-healing characteristics and long-term durability of the superhydrophobic surfaces provided.Reports on superamphiphobic surfaces also deliberated.? 2021 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,Mg alloys;Superhydrophobic;Superamphiphobic;Coatings;Self-healing;Corrosion.

1.Introduction

Bioinspired superhydrophobic(SHPC)surfaces having apparent contact angle(Cθ)>150°[1–3]have enticed substantial topical research attention due to their imminent industrial applications[4–6].The extreme non-wettability of SHPC surfaces is associated with the effective maintenance of a substrate/water interface air layer[4,7,8].Typically,superhydrophobicity(SHPY)could be fabricated by tailoring a surface with optimum surface roughness(micro/nanostructuring)and low surface energy(SE)[4,9–12].Several approaches were put forward in achieving both the hierarchical surface structuring and the low SE[4].SHPC surfaces with smaller sliding angles(Sθ<5°)and contact angle hysteresis(Hθ<10°)are bestowed with the additional self-cleaning property[1–3].Precise basics of SHPY[4,5,9,10,13–22]and fundamental theories of surface wettability are described elsewhere[7,23,24].

On the other hand,Mg and its alloys have found intense topical research attention due to the exceptional qualities like higher specific strength,excellent vibration/shock absorption,good thermal/electrical conductivity,biodegradability and biocompatibility[25–27];making them suitable for several industrial applications including automotive,electronics,biomedical,aerospace,and defence[28–34].The most crucial shortcoming of Mg alloys that hinder its extensive utilization is the feeble aqueous corrosion resistance[35–39].When compared to Al and Ti alloys having robust passive surface film,Mg forms a less corrosion resistant film made up of inner Mg-O and outer porous Mg(OH)2layer[36–38].Several reviews are available where more details of Mg corrosion and its prevention methods can be found[34–36,38–44].Among the different corrosion prevention strategies,surface coating technologies are the most preferred[29,30,40–46].

Current research trend suggests that one of the brilliant methods to enhance the corrosion resistance of Mg alloys is the SHPC surface modification.Despite the several interesting recent reports,only a few review articles are available in this area[45,46].The first review provides an account of the different fabrication approaches of SHPC surfaces[45],whereas the second one[46]focused on corrosion protection application.Only a few works published in 2020 are discussed in the later[46].Considering the high importance of the field,we made an effort to present the most recent advancements(2015?2020)in this area.In addition to the fabrication approaches,works reported in different application areas,viz.anti-corrosion,biomedical and anti-icing are carefully presented.Long-term durability and self-healing properties of SHPC coatings debated.Analogous works reported on superamphiphobic(SAPC)surfaces also shown.

Fig.1.Pie-charts on(A)Different low SE materials,and(B & C)Different methods employed for the low SE treatment with(B)SA/MA/LA-based and(C)Silane/fluorine-based compounds.(1)to(6)of(B)correspond to(1)Room temperature immersion followed by room temperature drying,(2)High temperature immersion followed by room temperature drying,(3)Room temperature immersion followed by high temperature drying,(4)High temperature immersion followed by high temperature drying,(5)ED,and(6)HT/ST.(1)to(9)of(C)correspond to(1)Room temperature immersion followed by room temperature drying,(2)High temperature immersion followed by room temperature drying,(3)Room temperature immersion followed by high temperature drying,(4)High temperature immersion followed by high temperature drying,(5)ED,(6)HT/ST,(7)Spraying,(8)Blade-coating/drop-coating,and(9)Others.The chart is based on the number of journal publications from 2015 to 2020(Source:SciFinder,CAS).

2.Fabrication approaches

2.1.Low surface energy treatment

The most widely employed low SE treatment for making SHPC surfaces,irrespective of the substrate metal used(Al,Ti,Cu,Mg,etc.)rely on the silane and fluorine chemistry.This is particularly evident when analyzing the reported works on SHPC coatings for metals with robust passive surface oxides such as Al and Ti[22].Despite the toxicity concerns,the F-substituted silanes are typically preferred,due to the high C–F bond strength,low critical surface tension(6 m·Nm?1)of–CF3group,and good chemical and biological inertness[47–49].Analysis of the reported works,however,showed that the scenario is different for Mg and its alloys(Fig.1A),Here,the most widely employed category of compounds are those based on long-chain saturated fatty acids such as stearic acid(SA)[50–92],myristic acid(MA)[56,87,91,93–100]and lauric acid(LA)[56,91,101].The three compounds together constitute～55% of the total works reported.Among them,the most widely employed candidate is SA(～40%).The silane and fluorine-based compounds constitute～33% of the total reports[56,76,94,102–129](Fig.1A).Only a few works used fluoropolymer/NaF without the silane chemistry[130–132].A few works employed SA and silanes together[76]or their performances were compared[56,94].Other polymers such as polypropylene(PP)[102,133,134],polyvinyl chloride(PVC)[135],and octadecyl phosphonic acid[136]were also employed.One work employed candle soot coating[137].A few reports on thiols[138,139]and oleate[140,141]compounds are also available(Fig.1A)(Tables S1and S2,Supporting Information).

Figs.1B & 1C correspond to various treatment methods employed for the low SE modification with SA/MA/LA(Fig.1B)and fluorine/silane(Fig.1C)based systems.More than 65% of the reported works with SA/MA/LA employed direct solution immersion as a means to impart the low SE.～18% of the reports utilized electrodeposition(ED)whereas～16% used hydrothermal(HT)processing.Among the direct immersion methods,～40% of the works used room temperature immersion followed by room temperature drying,whereas,～12%of the works employed high temperature immersion(50?99°C)accompanied by room temperature drying.～10% of the reports used room temperature for immersion,and high temperature(60?120°C)for drying and～4%used high temperature immersion and high temperature/hot air drying(Fig.1B)(Table S1,Supporting Information).Most of the reports on ED and HT methods employed one-step processing to achieve both the micro/nanostructuring and the low SE modification(see§2.3).

Even though the silane and fluorine-based modifications displayed a similar trend where～55% of the works used direct immersion,most of them(～27%)employed room temperature immersion followed by high temperature curing(80?130°C).～15% of the works used a high temperature immersion(45?80°C)accompanied by room temperature drying,whereas only～12% used room temperature immersion and room temperature drying Fig.1C).This variation can be correlated to the chemistry of interaction of these compounds with the hydroxylated Mg surface.Typically,the fatty acids such as SA reacts with the Mg(OH)2by neutralization reaction(Eq.(1))whereas the silanes undergo hydrolysis and condensation polymerization(Eqs.(2),((3))[56].High temperature processing is typically helpful to make robust bonding with the surface hydroxides.Only a few works with the silane/fluorine compounds used ED and HT methods(～3% each).Interestingly,～15% of the works employed spraying,and～9% works utilized blade-coating or drop-coating.Other methods such as RF sputtering,thermal evaporation,electrospinning and magnet-assisted process were also employed(Fig.1C)(Table S2,Supporting Information).A few works used an additional post-high temperature annealing[121,131].

The major advantages of the long-chain saturated fatty acids such as SA when compared to fluorosilanes are indeed the environmental friendliness,and economic feasibility.As Mg/Mg alloys are having a higher fraction of surface hydroxides,a thicker layer formation by SA could be feasible via the carboxylic acid groups and hence a more protective barrier effect.However,several studies on metals with robust surface oxides suggested that monolayer scale fluorosilanes are more durable than the long-chain saturated fatty acids[22].The adsorbed silanols could react with each other allowing cross-linking in the coating.Silanes/fluorosilanes are advantageous over unsaturated fatty acids(such as oleic acid)and thiols because of their thermal stability to significantly higher temperatures[142,143].

Fig.2.Bar diagram showing the different methods employed for fabricating surface micro/nanostructuring.The chart is based on the number of journal publications from 2015 to 2020(Source:SciFinder,CAS).

2.2.Micro/nanostructure development

Achieving Cθ>120°merely by controlling the surface chemistry,even with the lowermost SE compound usually is not feasible.Hierarchical surface structure is obligatory to create SHPC surfaces[11,12].More than 90% of the reported works employed a two-step processing where the first step was used to create the required surface roughness,and the second step for the low SE modification(see§2.4).Instead,a few works attained the SHPY by single-step processing where both the surface structuring and the low SE modification realized simultaneously(see§2.3).Several works employed multi-step processing where an additional coating was usually used to avoid the rapid corrosion of Mg.

Fig.2 displays the typically used methods for the hierarchical surface structuring.The mostly used method is the HT.～25 % of the reports utilized HT or solvothermal (ST) processing [52,56,58–60,72,73,77,81,82,84,85,91,92,94,109,112,119,120,127,133,138,140,144].The second most utilized way(～15 %)is the direct immersion [51,54,74,76,86,87,89,90,95,97,98,102,110,124,130].～9 % of the works used chemical etching[55,64,104,122,139,141] or intentional corrosion[125,137],or electrochemical etching[103].ED(～9 %)[50,57,67,69,96,99–101,132]and electroless deposition(ELD)(～5 %)[67,71,77,101,132]were used in several reports.Anodization/electrochemical anodic oxidation(EAO)(～9%)[56,61,68,83,91,94,114,127,135]and micro-arc oxidation(MAO)(～10%)[63,70,78,79,107,111,118,126,131,136,145]were also employed by many.EAO/MAO methods have the advantage of fabricating a thick conversion coated oxide layer on the surface and hence could provide a better corrosion resistance ascribed to the robust barrier effect.～6 % of the works employed spray-coating[60,75,105,115,121,129].Two reports made use of laser texturing[66,107].Other methods such as electrical discharge machining[53],sputtering[123],scrap-coating[116,128],salt spray[125]and electrospinning[113]were also employed.A few works employed core-shell structures[51,117]or host-guest feedback active coatings in the process[124].Comparative advantages of these different methods such as HT,ED and solution immersion are described elsewhere[46].

Fig.3.(A)Schematic of one-step HT processing.(B & C)Variation of Cθand Sθof one-step HT fabricated surfaces with different solution(B)Temperatures and(C)Composition.Reproduced(A)with permission from[120],(B)from[73]and(C)from[72];? 2017 Elsevier B.V.

2.3.Single-step processing

A few works achieved the SHPY by single-step processing[57,72,73,81,82,96–100,116,120,121,140,144].The mostly employed approach is the one-step HT/ST[72,73,81,82,120,140,144],followed by the one-step ED[57,96,99,100].Other methods reported include one-step immersion[97,98]and one-step blade/spray-coating[116,121].

2.3.1.One-step hydrothermal

HT method is a simple,eco-friendly,and economical approach to fabricate SHPC Mg alloys.The one-step HT method(Fig.3A)could well attain the required surface roughness by Mg(OH)2layer formation,whereas the low SE compound added in the HT bath could covalently graft with the surface–OH groups.Feng et al.has shown that the Cθof the hydrophilic bare substrate(～38.3°)increased to 153.8°after the HT treatment in SA+water+ethanol solution(ethanol:water=1:1.4,50 mM SA,80°C,10 h).The corresponding Sθwas～4°.SEM surface view images revealed microscale bar-like and plate-like structures encompassed of several layers and cavities.XRD studies presented that the HT-treated sample consisted of Mg(OH)2and MgO(magnesium stearate was in amorphous state).The surface grafting of SA was confirmed by FTIR analysis[120].The wettability varied significantly with the HT bath temperature(Fig.3B)and the composition(Fig.3C).Zhang et al.have shown that onestep HT processing in ZnCl2+SA+water+ethanol solution(50?80°C,1?3 h)resulted in Cθas high as 161.7±0.8°and Sθof 4.8±0.9°.The incorporation of ZnCl2was helpful for additional surface etching.SEM images revealed compact porous structures(1–2μm diameter)with random distribution.A processing temperature of 80°C(Fig.3B)and time of 2 h were found to be the optimum where maximum Cθand minimum Sθwere obtained[73].Kang et al.reported onestep HT synthesized SHPC hydroxyapatite(HA)coating.The HT bath has consisted of calcium acetate,NaH2PO4,HCl,SA,water,and ethanol(170°C,2 h).The Cθvaried significantly with the volume ratio of ethanol and water in the bath.The highest Cθ(152.8°)and lowest Sθ(<2°)were obtained when the volume ratio was 6:4(Fig.3C).XRD peaks detected Mg(OH)2,HA and calcium stearate.Here,calcium stearate served the purpose of low SE material.The surface displayed a flaky hierarchical structure typical to HT-treated Mg[72].

Peng et al.made-up a SHPC surface on pure Mg by HT treatment in Al(NO3)3+NaOH+sodium oleate(SO)solution(120°C,3?24 h).The Cθvaried considerably with the SO concentration,but not with the HT time.SO concentration of 400μM and HT time of 12 h were selected as the optimal parameters.XRD and XPS plots confirmed the presence of Mg(OH)2and Mg-Al LDHs(layered double hydroxides)along with the surface anchored oleate groups.The resultant SHPY was sticky(Cassie impregnating wetting state)in nature[140].Li et al.employed Ni(SO4)2+SA+water+ethanol solution for the onestep processing(150°C,1.5 MPa,8 h).The Cθof the bare Mg alloy was 60°whereas the as-fabricated SHPC surface shown Cθof 156.7°.The SHPC surface was enclosed with peony-like microstructures comprising of hexagonal flake-like structures.FTIR confirmed the occurrence of surface anchored long-chain aliphatic groups[144].Hu et al.attained slippery SHPY(Cθ～158.5°,Sθ～2°)by one-step HT in SA+water+ethanol solution[81,82].The surface displaced a few large microclusters and microspheres protrusions with fine embedded nanostructures.FTIR confirmed the successful grafting of SA[81].

Fig.4.Variation of Cθand/or Sθof one-step ED surfaces with different(A)Electrolyte composition,(B & D)Deposition time,and(C)Deposition voltage.Reproduced(A)with permission from[57]? 2019 Elsevier B.V.for Chongqing University;(B)from[96]? 2017 Elsevier B.V.;and(C & D)from[100]?2015 American Chemical Society.

2.3.2.One-step electrodepostion

One-step ED was successfully employed in making SHPC surfaces on Mg alloys[57,96,99,100].Zheng et al.performed ED in SA+Mg(NO3)2+anhydrous alcohol at 30 V for 30 min.The Cθof the fabricated surface varied significantly with the SA/Mg2+ratio in the electrolyte(Fig.4A).The highest Cθof 156.2±4.9°was obtained for a sample with SA/Mg2+ratio of 10.By the Cassie-Baxter equation,the proportion of the four surfaces(Fig.4A)in actual contact with the solution was calculated to be 40,16.5,12.6,and 13.7%,respectively.SEM images revealed that the SHPC surface consisted of microspheres with embedded nanosheets.The main coating component responsible for the SHPY was magnesium stearate,as shown by XPS and FTIR studies.During the ED process,SA ionizes to form stearate and H+ions,accompanied by the H+reduction,and the simultaneous cathodic deposition of magnesium stearate[57].Zhong et al.reported a calcium myristic SHPC coating from an electrolyte containing CaCl2,MA and ethanol(50 V,1?90 min).After 90 min of deposition,～33μm thickness coating was formed.The wettability varied considerably with the ED time(Fig.4B).The maximum Cθ(156.6°)was recorded for a 30 min ED sample[96].Liu et al.have reported one-step ED SHPC coating from Ce(NO3)3+MA+ethanol solution.The Cθvaried significantly with the ED voltage(Fig.4C)and ED time(Fig.4D).The highest Cθof 159.6±0.5°was observed for a sample deposited at 30 V,and that was attributed to the uniform hierarchical micro/nanopapillae structured surface.Except the one deposited at 10 V,the Sθof all SHPC samples were less than 10°(Fig.4C).SHPY could be achieved even at the shortest ED time of 1 min(Fig.4D).During the cathodic deposition,Ce3+and MA reacted and formed cerium myristate and H+ions[100].Zhao and Kang employed one-step ED to fabricate SHPC surface on both the cathode and the anode in the ED cell.Here,both the electrodes were made up of the same Mg alloy.The electrolyte was Mg(NO3)2+MA+ethanol.At 20?30 V,the Cθand Sθof the deposited coatings on both the electrodes were greater than 150°and less than 4°,respectively.The hydrophobicity on the anodic surface was determined by the anodic oxidation,Mg2+release and magnesium myristate formation,whereas,on the cathodic surface,the formation of magnesium myristate and H+played the important role[99].

2.3.3.Others

One-step immersion was effectively used for fabricating SHPC surfaces[97,98].Ishizaki et al.reported an easy and ultrafast single-step immersion in a solution comprised of MA,Ce(NO3)3,ethanol and water(1?30 min,pH 2 adjusted by HNO3).The method was advantageous as no perfluoro molecules or heat treatment was used.The Cθreached above 150°after 1 min of immersion and almost kept constant at the range of 150?156°,irrespective of the immersion time.Hθvalues were less than 10°.SEM images revealed microclusters(diameters～2–3μm)of needle-like assemblies with embedded nanoscale structures.The surface analysis detected MA and CeOx,where the outermost surface was rich in CH3(CH2)14COO–[97].

These studies depict that the SHPY achieved by one-step methods more or less make use of the highly active nature of the Mg alloys.Irrespective of the technique used,the Mg surface when in contact with an aqueous solution undergoes instantaneous dissolution and Mg(OH)2formation.This process could result in the development of nano/microstructures on the surface,and that along with the simultaneous incorporation of the low SE material could lead to SHPY.

SHPC coating was also fabricated by single-step spraycoating or blade-coating[116,121].Xie et al.fabricated hierarchical polydimethylsiloxane/silicon dioxide(PDMS/SiO2)coating with two different sized(S and L with 40 nm and 50–250 nm)SiO2nanoparticles(NPs).The paint was prepared by adding PDMS and Sylgard ?184(curing agent)to a mixed homogenous solution of 1H,1H,2H,2H-tridecafluoron-octyltriethoxysilane and SiO2NPs;and the sample after the blade-coating was cured at 160°C.The micro/nanostructure fabricated by the appropriate blending of the S and L NPs were found to be decisive.Samples fabricated from 0.2 and 0.3 g of S SiO2along with 0.7 g of L SiO2displayed SHPY.As the S particles increased from 0.1 to 0.3 g,the root means square(RMS)roughness of the coated surface enhanced(153.42 to 188.18 nm),and the Cθaugmented(147.2°to 153.6°)[116].Shi et al.fabricated composite coating comprising of polytetrafluoroethylene(PTFE),polyphenylene sulfide(PPS),and SiO2NPs(40 nm,0–4 g·L?1)(PPSPTFE/SiO2)by spray-coating.The Cθof the coated surfaces were at the range of 152–145.5°±0.3°and the Sθwere<5°.A PPS-PTFE coating also demonstrated high Cθ(152±0.2°),and that was attributed to the cross-linked structure between PPS and PTFE and the low SE of PTFE.The study also showed that Cθdeclined as the SiO2content in the composition increased,and that was associated with the increased SE and the morphology variation caused by the additional SiO2NPs[121].

2.4.Two-step/multi-step processing

This section is classified primarily based on the first method used for the processing.

2.4.1.Hydrothermal/Solvothermal

Several works used HT/ST as the first step followed by the low SE treatment[52,58,59,77,80,84,85,88,109,119,133,138].Jin et al.treated Mg alloy hydrothermally in deionized water(160°C,2–5 h),and subsequently,immersed in hexane solution of SA.The micro/nanostructured intersected lamellar Mg(OH)2obtained after the HT processing(Fig.5A)combined with the low SE created the SHPC surface.By obeying the Wenzel’s theory,the enhanced roughness of the HT-treated samples resulted in hydrophilicity(Fig.5C).The SHPY achieved upon modification varied significantly with the concentration of SA solution(Fig.5D)and the time of immersion(Fig.5E).The maximum Cθ(～159°)was obtained for a 3 h immersed sample in 7 mM SA solution.The corresponding Sθwas～7°.The SA modification resulted in considerable variation in the surface morphology where the original flake-like structures(Fig.5A)were modified to petals-like structures(Fig.5B).As discussed above,the presence of surface anchored SA was determined by FTIR(Fig.5f),where two additional peaks related to the linear C-C stretching vibration of the aliphatic groups(1110 cm?1)and the–COO group of SA(1626 cm?1)were appeared[59].Li et al.also realized SHPC surface by water-only HT processing(120°C,6 h)followed by modification with ethanol solution of dodecafluoroheptyl-propyl-trimethoxylsilane.The Cθof the bare,HT-treated and SHPC surfaces were 83.1°,4.8°and 152.1°,respectively.When the desired roughness of the HT-treated surface and the low SE combined,the superhydrophilic(SHLC)surface easily turns out to be SHPC[119].Unlike Jin et al.[59],the HT-treated sample displayed petallike nanoslices,and the morphological variation after the low SE treatment was not significant.XPS results concluded the formation of C10F12H9Si(O-surface)3,as silanol groups reacted with the surface–OH groups[119].

Wu et al.employed 0.01 M NaOH for the HT processing(120°C,10 h)of AZ80 alloy,followed by PTFE RF magnetron sputtering.The HT-treated film(20–50 nm thickness)displayed cross-linked microsheets(0.5–1μm)morphology.Their grazing incidence XRD depth profile analysis indicated the formation of a double-layered film with Mg–Al LDHs(MgxAl1?x(OH)2·nH2O)rich top layer and Mg(OH)2rich bottom layer.The typical dissolution-precipitation that happened with theα+βphases of the alloy helped in the formation of the LDHs film.The triple-layered structure resulted after the PTFE modification displayed SHPY with Cθof 170.1±1.2°.The corresponding Cθof a PTFE sputter-coated bare surface(without HT processing)was only 110.6±0.5°,signifying the importance of the HT process and the resulting surface roughness.The Cθof the bare surface was 35.8±0.8°,whereas the HT-treated sample without the PTFE modification displayed Cθof 16.5±1.8°[109].Zhang et al.prepared Mg(OH)2/Mg-Al LDHs film on AZ31 via a joint coprecipitation-HT process and subsequent SA modification.To an aqueous solution of Al(NO3)3and Mg(NO3)2(Al3+:Mg2+=3:1 molar ratio),a mixture solution of NaOH and Na2CO3was added,and the resulted slurry stirred vigorously(48 h,65°C)and 12 h aged,and used for the HT processing(120°C,36 h).A 0.01 M aqueous DMF solution of SA was used for the low SE modification(99°C,0.5 h)[80].Zhou et al.reported SHPC Zn-Al LDHs film on AZ91D alloy via HT and SA treatment.An aqueous solution consisting of 0.05 M Al(OH)3and 0.1 M Zn(CH3COO)2(pH adjusted to 10–11 by NH3)was used for the HT process(60°C,5 h).At pH 10,porous nest-like microstructure was formed,and as the pH increased to 11,micro-rods began to appear.XRD results confirmed the presence of ZnO and Zn6Al2(OH)16CO3·4H2O.The Cθof the modified surface was～165.6°.By Cassie-Baxter equation,the air fraction was calculated to be 98.3%[88].

Fig.5.SEM surface view images of(A)3 h HT-treated sample and(B)SHPC sample after SA modification.(C)Variation of Cθof HT-treated sample with HT treatment time.(D & E)Cθof SHPC sample as a function of(D)SA concentration and(E)SA modification time.(F)FT-IR spectra of(a)3 h HT-treated sample and(b)SHPC sample after SA modification.Reproduced with permission from[59]? 2019 Elsevier B.V.

Zhang et al.reported Mg(OH)2coating via HT process in NaOH solution(5.66 wt.%,140°C,4 h)followed by dip-coating in PP+maleic anhydride grafted-PP(PPMAH)+xylene solution(0.2:1.8:100).The thickness of the PP and Mg(OH)2layers formed were～31.5 and 23.5μm,respectively.The hybrid Mg(OH)2/PP coating demonstrated SHPY(Cθof 165.5±3.6°and Sθof 4±0.6°)and that was attributed to the micron-scale spherical structures and the low SE PP layer.The SE of the SHPC coating as calculated by Van Oss approach was 6.7 mJ·m?2[133].The authors in an earlier work employed HT process(120°C,8 h,5.66 wt.% NaOH)and SA modification and the fabricated surface displayed Cθof～157.6°[85].Wang et al.also achieved SHPY by HT(160°C,4 h,NaOH)and SA immersion(50°C,2 h).The HT-treated surface with a nanoplate morphology was SHLC(Cθ～0°).There was no apparent disparity in the surface morphology after the SA treatment.Cθas high as 155±1°and Sθof～2°were obtained[92].

Ding et al.fabricated a SHPC and self-healing coating by in-situ growth of WO42?-LDHs by co-precipitation(80°C,48 h)and HT(130°C,36 h)methods.The HT solution was comprised of Al(NO3)3,Mg(NO3)2,Na2WO4,NaOH,and water(pH 10).The HT-treated sample was surface modified and sealed by ureido cross-linked PDMS(U-PDMS)and low SE laurate-modified LDHs powder(La-LDHs).The spray-coated La-LDHs powder was distributed uniformly in the U-PDMS matrix,and that was also helpful to prevent the easy wash off of the intercalated WO42?inhibitors.The modified surface displayed Cθof～163°[112](see§4).

Several works fabricated an additional surface coating before the HT process,intending to enhance the barrier effect and to assist the hierarchical surface structuring.Yuan et al.made up a SHPC surface on Ni–P ELD-coated sample via HT treatment(Ni(NO3)2+ethanol,100–160°C,9–24 h)and SA modification(0.01 M,5 h).The Ni–P coating with nodular microstructure displayed Cθof～67°.The HT process created nanostructures on the Ni–P coating and the sample fabricated at 120°C for 15 h revealed a maximum Cθof～155.6°and Sθof～2°.The Cθvaried significantly with the HT temperature and SHPY was not obtained at 100°C or 160°C[77].A few studies employed HT fabricated TiO2coatings[52,138].Zang et al.employed HT process(100°C,overnight,Ti(SO4)2)to fabricate TiO2coating and the coated sample was subjected to ultrasound-assisted Cu ELD(formation of copper microspheres),and subsequent modification with 0.1 M n-dodecanethiol ethanol solution.The densely packed Cuthiolate layer displayed microspheres with embedded nanotextured topography with cavities and islands.The SHPY could be effortlessly interchanged with superhydrophilicity(SHLY)by alternating thiol modification and heat treatment(350°C)steps[138].Zhang et al.prepared thin film TiO2layer by HT method(Ti(SO4)2,120°C,10 h)and then loaded wrinkled SiO2(immersed in 1 wt.% poly-dimethyl diallyl ammonium chloride,and plunged into a SiO2dispersion)to construct a pod-like structure.After a subsequent SA treatment(1 wt.%ethanol solution,1 h),the surface(at 10 mg·mL?1SiO2)becomes SHPC with Cθof～158°and Sθ<10°.The embedded SiO2particles were helpful to provide robust mechanical stability[52].

Zhu et al.reported HA-based SHPC composite coating by HT processing in CaCl2+KH2PO4+Na-EDTA solution(95°C,8 h)and successive soaking in SA(0.1 M ethanol solution,4 h).SEM images showed that the surface was covered entirely with a dense HA layer composed of long flakes and flat blocks.Cθs of the bare,HT-treated,and SA-modified surfaces were 41.9°,18.8°and 154.5°,respectively[58].Gu et al.fabricated ChCl-urea deep eutectic solvent(DES)-based conversion coating(composed of MgCO3and MgH2phases)and subsequently surface modified with SA(10 mM ethanol solution,5 h).The film obtained after the ionothermal process(160°C,2h)and SA modification displayed Cθof 151.5±1°[84].

2.4.2.Electrodeposition/electroless deposition

Several works used ED in a two-step process[50,67,69,101,106,132].Liu et al.reported SA/CeO2bilayer coating by CeO2ED(Ce(NO3)3+NH3NO3+ethanol+water electrolyte,0.65–3.25 mA·cm?2,5–90 min)and SA immersion(0.05 M,1 h).The optimum current density for the deposition was 0.65 mA·cm?2.SEM images presented petallike nanosheets morphology(Fig.6).It was also noticed that as the ED time augmented to 60 min,the surface displayed SHPY even without SA modification and that was credited to the particular crack-free nanosheet structure and the intrinsically hydrophobic property of CeO2.SA modification,however,easily converted all the coatings(from 5 to 90 min of ED)to SHPC(Fig.6)[50].

Wang et al.used a multi-step approach where chemical etching(CuCl2solution),Ni ED and SA modification were sequentially used.The CuCl2etching caused the formation of MgCl2,and also contributed to the surface structuring.Ni ED resulted in evenly disseminated micro-sized protrusions with embedded nanostructures.The SA-modified/etched surface displayed Cθand Sθof 151°and 7.6°,respectively,whereas the corresponding values of the SA-modified/etched/Ni ED alloy were 153°and 7.2°.Even though the samples with and without Ni ED achieved SHPY,the combined approach involving Ni ED was attractive as it provided an additional barrier effect[69].

A SHPC ZIF-8/PVDF/LDHs coating was fabricated by ED and dip-coating.Zn?Al LDHs film was first fabricated by ED in Al(NO3)3+Zn(NO3)2at?1.0 V for 400 s.The low SE top layer was then fabricated by dipping the ED sample in ZIF-8 powder-dispersed DMF comprising various amounts of 1H,1H,2H,2H-perfluorodecyltriethoxysilane(PFTS)and polyvinylidene fluoride(PVDF).The LDHs film was helpful to strengthen the binding force between the top layer and the substrate.Attributed to the formation of the hydrophilic oxide with increased surface roughness,Cθof the LDHs surface was considerably lesser(45.2±2°)than that of the bare substrate(80.7±2°).Upon coating the LDHs surface with PVDF,Cθaugmented to 104.8±2°.After incorporating ZIF-8 in the coating,Cθbecomes 117±2°.On further incorporation of the low SE PFTS,Cθ>150°and Sθ<3°were obtained[106].

A few works utilized ELD along with HT[77,138],ED[67,101,132]and immersion[71]methods.Typically,ELD was applied to reduce the substrate aggressiveness to facilitate a subsequent coating.The deposited coating could also provide the desired surface roughness.Yuan et al.employed Ni-P ELD,and the coated alloy was further subjected to HT processing[77]whereas Zang et al.deposited TiO2coating first and subsequently deposited Cu by ELD[138].The typical microsphere morphology of Cu deposit could provide the required microscale surface roughness[138].Song et al.fabricated multi-layer coating with Ni-P ELD(inner layer),Cu ED(intermediate layer)and Ni ED(top layer),and the SHPY was achieved by a subsequent SA modification[67].Tan et al.subjected an acid/alkaline pre-treated sample to Ni ELD and subsequent Cu ED.The ELD/ED coated sample was then subjected to a DC voltage-assisted deposition of LA and sebacic acid in HOOC(CH2)8COOH+CH3(CH2)10COOH+sodium acetate+ethanol solution.The modified surface was comprised of flower-like microstructures with embedded nanoslices.The Cθvaried considerably as the mole fraction of HOOC(CH2)8COOH in the solution varied.The surface wettability changed from SHPY(Cθ～154°)to SHLY(Cθ～0°),asXCOOHincreased[101].Zhu et al.attained a SHPC surface via Ni-P ELD,nano-Ag deposition(immersion in AgNO3solution)and SA modification.The deposition of nano-Ag particles created the hierarchical structure,which endowed excellent SHPY[71].

2.4.3.Immersion

Several studies employed immersion as the first step,followed by the low SE treatment[54,62,74,86,87,89,90,93,95,102,110,130].Xun et al.achieved self-cleaning SHPY by a simple and eco-friendly two-step immersion process with ensuing SA treatment.The two-step process involved consecutive immersion in 0.01 M and 0.1 M concentrations of MnSO4.The process was helpful to enhance the coating/substrate interfacial bonding and the durability of the SHPC surface.The resulted MnO2-coated surface displayed numerous randomly distributed vertical plates having porous wrinkles(20–40μm diameter)and embedded MnO2NPs(50–100 nm).AFM studies showed that the MnO2-coated surface exhibited a higher RMS roughness(146.3 nm),and that increased significantly after the SA modification(298.0 nm).The fabricated SHPC surface displayed a Cθof 156.9±3.7°[54].Xie et al.made-up a SHPC coating by simple immersion in ZnCl2solution(34 g·L?1)followed by SA modification(14.2 g·L?1,4h,50°C,drying at 120°C).The first step resulted in the formation of micron mastoids,along with numerous vertical nano-sheets.The Cθof the bare,ZnCl2only-modified,SA only-modified,and ZnCl2/SA-modified surfaces were 49.20°,16.76°,109.32°,and 162.04°,respectively[62].Kuang et al.fabricated Mg–Mn LDHs coating and the surface was modified by MA.The Mg samples were first immersed in CO2bubbled 0.025 M MnCl2solution and subsequently in 0.05 M Na2CO3solution,and further in 0.1 M MA solution.XRD results indicated the formation of Mg6Mn2(OH)16CO3·4H2O,MnO(OH),and MnCO3in the LDHs film.The LDHs displayed rough hillocky surface structure with embedded sheet-like crystallites.After MA modification,the surface presented a micro/nanostructure resembling rose petals.The measured Cθwas～152.2°[95].

Fig.6.(Top)SEM images of 60 min ED/SA-modified coating.(Bottom)Cθas a function of ED time,before and after SA modification[50].Reproduced with permission from[50]? 2020 Elsevier B.V.

Zhang et al.described a self-healing and SHPC chemical conversion coating made from a Cr(III)-based DES(0.3 M CrCl3+0.05 M NH4H2PO2+choline chloride)with subsequent SA modification.The hierarchical porous microstructure with island-like platforms having Cr2O3nodules combined with the low SE modification has resulted a sticky SHPC surface with Cθof～157°[90].Yang et al.reported SHPC stannate coating with self-healing functionality.The alkali/acid pre-treated Mg sample was immersed in a solution comprised of 50 Na2SnO3,10 NaOH,and 10 g·L?1CH3COONa(pH 13.6,90°C,1 h).Subsequently,the sample was surface modified in an alcohol solution of 0.01 M SA at different temperatures(23–50°C)and immersion times(0–60 min)[86].A zinc phosphate conversion coating was fabricated by Yuan et al.via immersion in 10 Na2HPO4+6 Zn(NO3)2+4 NaNO2+2 g·L?1NaF solution(pH 3 adjusted by H3PO4,55°C);and subsequently subjected to HT processing in a solution comprised of 0.01–0.045 Zn(NO3)2,0.0075 Na3C6H5O7,0.01 M ethanol solution of SA and water(pH 9 adjusted by NH3,140°C,24 h).The as-formed zinc phosphate coating showed block structure with irregular micro-cracks which were disappeared after the SA modification;evenly distributed small particles were also seen on the modified surface.XRD and FTIR studies confirmed the presence of Zn(CH3(CH2)16COO)2.The Cθvaried significantly with the concentration of Zn(NO3)2in the HT solution.At 0.035 M of Zn(NO3)2,Cθ>150°and Sθ<10°were achieved[74].

Ou et al.fabricated an inorganic-organic conversion coating by repeated alternative immersion(3 min each)in CeCl3(20 mM)and phytic acid(PA,5 mM);and subsequently dipped in an ethanol solution of hexadecyltrimethoxysilane(HDMS,10μL in 10 mL ethanol,6 h).The surface anchoring of PA was attributed to the phosphate–Mg2+chelating effect and the Ce3+assimilation to the PA layer was suggested to have occurred via P–O–Ce linkages.The surface?OH groups on the PA deposit can favour the robust linkage of HDMS molecules.An optimized sample with 4 cycles of PA/Ce deposition presented Cθof 167.3±2.1°and Sθof 2.7±0.8°[110].Zhang et al.fabricated hierarchical structured calcium phosphate coating by chemical conversion coating in 40 M H3PO4+40 g·L?1Ca(NO3)2and subsequent low SE modification in 5 g·L?1NaF at 80°C.The Cθbefore and after the NaF modification were 29°and 150°,respectively[130].Ou et al.reported a SHPC coating via consecutive immersion(30 s)in 10 mM AgNO3+0.1 M NaF aqueous solution and 10 mM ethanolic solution of SA.Presence of NaF was found to be necessary to initiate Ag galvanic deposition through barrier oxides dissolution[89].Wu et al.fabricated a double-layer SHPC surface by a two-step immersion where the sample was first immersed in octadecyltrichlorosilane solution,and dried at 110°C;and subsequently immersed in FeCl3+SA solution at 60°C[76].

A few studies explored immersion as the first step,followed by ED for the low SE treatment.Zhao et al.subjected phosphate conversion coated(immersion in 30 KMnO4+70 K2HPO4+34 KH2PO4+20 g·L?1H3PO4,7 min)sample for ED in an ethanolic solution of SA+MA+Ce(NO3)3(0.8 mA·cm?2,5 min).The coated surface revealed large numbers of microspheres(～4μm diameter)that were composed of copious nanostrips(～100 nm diameter).The Cθand the Sθwere measured to be 160.19°and 1.5°,respectively.The SHPC surface was mainly composed of Ce(CH3(CH2)12COO)3,Ce(CH3(CH2)16COO)3and phosphate[87].Kuang et al.fabricated LDHs coating by two-step immersion at 50°C.The sample firstly immersed in an acidic solution of 0.025 M MnCl2and HCO3?/CO32?saturated carbonate for 2 h,and after that in an alkaline solution of 0.5 M Na2CO3for 1 h.An auxiliary electric field was also used for directional movement of ions.The LDHs-coated sample was then subjected to ED in an ethanolic solution of CaCl2and MA(30 V,50°C,5–20 min).The surface morphology of LDHs coating revealed dense nanoplates having 300–600 nm of length and 15–30 nm of width(Fig.7a).3D profiler image displayed flat surface with 0.418μm roughness(Fig.7f).The SHPC surface obtained after the ED presented a flowerlike microstructure wrapped by calcium myristate particles(Fig.7e)with a surface roughness of 5.85μm(Fig.7j).XRD plots revealed the formation of Mg6Mn2(OH)16CO3·4H2O and calcium myristate phases.The Cθand Sθof the prepared surfaces varied considerably with the ED time(Fig.7).Cθs of bare(46.7±5°)and LDHs-coated samples(9.5±4°)were also shown in the figure[93].

Zhang et al.prepared a SHPC polymer coating on alkaline pre-treated Mg alloy via successive treatments with(3-aminopropyl)trimethoxysilane(APTMS)and PP.The sample was first dipped in a 3:22:75 vol.% solution of APTMS,ethanol and water for 15 min,and subsequently soaked in 0.2:1.6:100 vol.% solution of PP,PP-g-MAH and xylene for 5 min.The thickness of the first and the second layers of the coating were 10.65±1.25μm and 49.65±1.38μm,respectively.The double-layered coating displayed porous spherical microstructure and low SE(10.38mJ·m?2)with measured Cθof 162±3.4°and Sθof 5±0.6°[102].

Zang et al.reported a SHPC coating based on MnO2microspheres enclosed in SA shells.The sample was first coated with MnO2spheres with embedded nanostructures through one-step dipping in MnSO4precursor solution.The sample was further dipped in 0.01 M ethanolic solution of SA,and dried at 60°C.The Cθof MnO2-coated and MnO2/SA-coated samples were 40.2±5.7°and 160.5±5.4°,respectively[51].Wu et al.reported a core-shell SHPC coating made up of ZIF-SiO2.The fabrication steps include synthesis of ZIF nanocrystals via co-precipitation,preparation of ZIF-SiO2by sol-gel process,surface modification of ZIF-SiO2particles by HDMS,and SHPC coating fabrication by dipping in n-hexane solution of HDMS-modified ZIF-SiO2.The coating displayed Cθof～153°[117].

2.4.4.Chemical/electrochemical etching

Several works utilised chemical etching(other than the acid/alkali pre-treatments,see§2.5)to fabricate the hierarchical surface structure[55,104,122,139,141].This is particularly relevant for Mg alloys having bothαandβphases,where chemical etching could leads to preferential dissolution(preferential corrosion of anodicαmatrix)due to the different electrochemical potentials of the phases.Safarpour et al.explored two-step chemical etching process on as-cast as well as solution annealed and aged AZ91 alloy where the sample was first immersed in 1 vol.% H2SO4(240 s)and afterward in 20 vol.% H2O2(120 s);followed by surface modification in 0.05 M ethanolic solution of SA.The etched surface was composed of numerous microfeatures enclosed by needleand lint-like nanofeatures.The as-cast sample presented petaltype SHPY.However,the annealed and aged sample displayed lotus-type SHPY,and that was attributed to the redistribution ofβ-precipitates in theα-matrix[55].Tang et al.prepared a SHPC surface by acid etching(1 ml HCl/50 mL H2O)and ultrasonic treatment,followed by surface modification in 3 wt.% PFTS at 45°C.The modified surface showed Cθ～158°and Sθ～2°[104].Wang et al.employed aqueous CuCl2(34 g·L?1,80°C)for surface etching and ethanolic solution of oleic acid(14.2 g·L?1,4 h)for the surface modification.The dried(at 120°C)sample was SHPC with Cθof～155°[141].

A SHPC Mg alloy was fabricated by H2SO4etching(provides rough structure),AgNO3treatment(nanoscale protrusions and grooves),and dodecyl mercaptan modification(low SE).The Cθof the bare alloy was～38°whereas both the acid-etched(Cθ～5)and AgNO3-treated(Cθ～8)surfaces were SHLC.Upon mercaptan modification,Cθincreased to 154°,and Sθreached 5°.The wettability varied significantly with the acid concentration,etching time and the AgNO3concentration.The optimum parameters were found to be 1.0 wt.% H2SO4,4 min,0.001 M AgNO3,0.10 M mercaptan and 5 h[139].In a report by Gray-Munro and Campbell,the Mg sample was first immersed in 2 vol.%H2SO4(4 min,80°C)followed by immersion in 20 vol.%H2O2(150 s,80°C)along with ultrasonication.The etched sample was surface modified in a solution comprising of 1 mL of 3-mercaptopropyltrimethoxysilane(MPTS),31 mL of PDMS,484 mL of water and 98.5 mL of methanol[122].

Li et al.conducted electrochemical etching in an aqueous solution of NaCl and NaNO3(0.2 M each)at 0.5 A·cm?2(0–15 min,200 r·min?1,anode-Mg sample,cathode-a Cu plate).For the low SE modification,the etched sample was soaked in 1 wt.% ethanolic 1H,1H,2H,2Hperfluorooctyltriethoxysilane(PFOTS)and dried at 120°C.The Cθand Sθof the SHPC surface varied considerably with the etching time(Fig.8A&B).With the upsurge of etching time,preferential dissolution increased and microstructures with puffy pores formed.A schematic of SAM formation by the fluorosilane is also provided(Fig.8C)[103].

Fig.7.SEM and 3D profiler images of(a,f)As-prepared LDHs film and(e,j)SHPC coating obtained at ED time of 20 min.Cθand Sθof pure Mg,LDHs-coated surface(0 min),and SHPC surfaces obtained at different ED times were also shown.Reproduced with permission from[93]? 2019 Elsevier B.V.

2.4.5.Electrochemical anodic oxidation

Several studies used EAO to fabricate a robust oxide layer as a preliminary step[56,61,68,83,91,94,114,135].A subsequent HT processing would facilitate the formation of a thick LDHs film[56,91,94].Wang et al.reported a selfhealing coating via in-situ growth of corrosion inhibitorintercalated LDHs.The sample after EAO(20 V,30 min,0.18 M NaOH+0.05 M NaAlO2electrolyte)was subjected to HT(125°C,12 h)in corrosion inhibitor(0.1 M Na3VO4or Na2MoO4)containing solution;and afterwards,the surface was modified with sodium stearate(SS),LA or MA by another HT process(15 mL 0.01 M ethanolic solution+15 mL water,140°C,12 h).Cθs of SS,LA,and MA-modified surfaces were 139.4°,148.6°and 145.2°,respectively[91].

A few studies compared SA/MA with silanes.Wu et al.fabricated SHPC surface by modifying in-situ grown LDHs film with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane(PFTMS),SA,MA or sodium laurate(SL).Anodized(NaAlO2+NaOH,30 min,20 V)and HT-treated(0.1 M NaNO3,125°C,9 h)samples were used.The as-prepared LDHs film showed dense hexagonal nanoplatelets,with nestlike nanostructures.The SL,MA,SA and PFTMS-modified surfaces displayed RMS roughness of 550,260,170 and 160 nm,respectively.The corresponding Cθs were 153.7°,152°,150.6°,and 145.5°[56].The authors also reported a similar study where the anodized and HT-treated sample was modified with MA and PFTMS.The results revealed that LDHs with largest flakes were the preferred sites for MA adsorption,but unfavourable for PFTMS adsorption[94].

Liu et al.employed EAO in 0.2 M NaCl(20–120 mA·cm?2,3–60 min)and SA modification in 0.02 M ethanol solution to achieve the SHPY.The as-prepared EAO surface had numerous pits and cracks(Cθ～0°),and the SHLC surface becomes SHPC after SA treatment(Cθ～163°).The wettability was closely associated with the EAO current density and time.Within 80 mA·cm?2,Cθpresented an increasing trend with the upsurge of current density whereas above this value,a reverse trend was noticed,and the variation was correlated with the associated surface structure.Cθof the EAO/SA-modified sample(fabricated at 80 mA·cm?2)augmented from 143.3°to 163.8°as the EAO time increased from 3 to 30 min.With further rise of time,Cθwas reduced as the uniform surface structure damaged[83].Khalifeh and Burleigh subjected highly purified Mg to EAO(30–70°C,10–240 s,120 V AC or 4–6 V DC,borate benzoate+NaOH+KOH)and SA modification(0.05–0.15 M ethanol solution).The surface after EAO displayed flaky structure and tubular morphology.The samples anodized at 120 V AC(EAO time of 10 s)and 5 V DC(180 and 240 s)effortlessly achieved SHPY[68].Zhang and Lin prepared SHPC calcium stearate coating by both DC and pulsed ED on anodized Mg alloy.Here,the first step was EAO in Ca(NO3)2+Mg(NO3)2+ethanol(100 V,10 min),and that was followed by ED of low SE calcium stearate in calcium nitrate+SA+ethanol.SEM images revealed flower-shaped protrusions with both DC and pulse ED samples;however,the pulse deposited coating appeared more homogeneous(Fig.9).The Cθvaried significantly with the deposition condition,owing to the associated variation of morphology and thickness of the coating.Fig.9 also presents the Cθvariation with the duty cycle employed for the pulse ED[61].

Fig.8.(A & B)Variation of Cθand Sθwith etching time.(inset)Cθvariation with time of droplet on the surface.(C)Schematic showing SAM formation.Reproduced with permission from[103]? 2020 Elsevier Ltd.

Fig.9.(Top)SEM images of(a)Anodized Mg alloy,and(b,c)ED surfaces by(b)DC(50 V,60 min)and(c)Pulse(50% duty cycle)methods.(inset)High magnification images.(Bottom)Variation of Cθbefore(anodized Mg alloy),and after DC ED,and pulse ED with different duty cycles[61].Reproduced(B)from[61]? 2019 Elsevier B.V.

Yang et al.reported a SHPC coating by EAO(15 min,10 V)followed by soaking in PVC–THF ethanolic solution(1-3 layers,～1.8 mm/s).The SHPY was not obtained when the ethanol content in the PVC–THF solution exceeded 46.4 vol.%[135].Zhang et al.reported a SHPC surface via anodic treatment(30 min,25 and 50 mA·cm?2)in choline chlorideethylene glycol-based DES(800 rpm,85±3°C)and subsequent surface modification in 0.5 wt.% ethanolic PFOTS(Cθ～158±1°and Sθ～1°)[114].

2.4.6.Micro-arc oxidation

MAO(typically performed at a higher applied potential than EAO)was employed in several studies[63,70,78,79,111,118,131,136].The method is advantageous in producing a robust thick oxide layer with good abrasion and corrosion resistance.MAO-coated surfaces normally consisted of a thick porous exterior layer and a thin barrier layer.The subsequent low SE modification could act as a sealing coating to the porous outer layer[22,79].

Zhang et al.reported a SHPC MAO-coated(170–180 V,600 s,100 Hz,50% duty cycle,8 g·L?1PA+10 g·L?1NaOH)alloy by simple SA modification(0.01 M SA in DMF/H2O solution,0.5?7 h).The typical morphology of the MAO surface with micropores/cracks was altered to petal-like clusters after the low SE treatment where all the pores/cracks were covered with the thicker SA layer.XRD and FTIR studies confirmed the formation of magnesium stearate.Cθs of the unmodified and 7 h SA-modified MAO samples were～33°and 155.5°,respectively[79].Liu et al.also achieved SHPY by MAO(250–450 V,50–3000 Hz,2–30 min,45% duty cycle,15 g·L?1Na2SiO3+5 g·L?1NaOH)and SA modification(10 g·L?1ethanolic solution).Cθvaried considerably with the MAO voltage and reached maximum(～154.7°)at 350 V and that was correlated to the resultant surface morphology.The effect of frequency was much less.Cθincreased first with the frequency,and then decreased,reaching the maximum at 1000 Hz.A similar variation was observed with the MAO time,and the maximum Cθwas reached at 5 min[70].An MAO-coated alloy modified by tetrafluoroethylene(TFE,solution immersion)and heat-treated at 250°C displayed Cθas high as 171°[131].

Wang et al.subjected MAO-coated(360 V,600 Hz,480 s,10% duty cycle,Na2SiO3+NaOH+KF)sample for HT treatment(Al(NO3)3,pH 10.5,125°C,6–24 h),and SA modification(0.05 M,60°C,5 h).The as-prepared porous morphology of MAO surface transformed to nest/flake-like clusters after 24 h of HT process.The LDHs completely enclosed the micro-pores/cracks of the MAO layer.The sample after SA modification displayed Cθof～151.21°[63].

Jiang et al.fabricated a SHPC composite coating by combining MAO(300 V,600 Hz,5% duty cycle,10 min,15 g·L?1Na2SiO3+10 g·L?1NaOH),chemical conversion coating by alternate immersion in PA and Ce(NO3)3(0.01 M,60 s),and PFTS modification(1 wt.% ethanol solution,1 h)(Fig.10).The PA-Ce conversion coating was helpful to develop a hierarchical surface structure as well as to seal the defects in the MAO coating.XRD studies showed that the peak intensities of the MgO and Mg2SiO4phases detected in MAO-coated sample were reduced significantly after the cyclic immersion step,and that was ascribed to the formation of PA-Ce(III)complex precipitates(Fig.10).The arithmetic average roughness of the MAO layer was 71.16 nm,and that increased to 169.63 nm and 205.02 nm,after 2 and 3 steps of cyclic assembly,respectively.Cθof the bare MAO,PFTS-modified MAO,PFTS-modified MAO/PA-Ce(2 cycles),and PFTS-modified MAO/PA-Ce(3 cycles)samples were～70°,121°,151°,and 159°,in that order.The enhancement of Cθwas credited to the graded surface structure and the higher surface roughness[118].The authors also reported a SHPC and self-healing coating by combining MAO with electro-assisted sol-gel deposition where an one-step ED was utilized to assemble SHPC silica film encapsulated with 8-hydroxyquinoline(8HQ)corrosion inhibitor[111](see§4).

Li and Yuan reported SHPC(Cθ～163±2°)organophosphonate self-assembled MAO-coated Mg alloy.The alloy after MAO coating(4 A·dm?2,8 min,2000 Hz,15% duty cycle,Na2SiO3+NaOH+triethanolamine electrolyte)was immersed in ethylene glycol first and then in octadecylphosphonic acid(0.3 mM,THF solvent,48 h)[136].Zhang et al.described an ED/MAO SHPC coating with Cθof 150.9°.After MAO(400 V,10 min,10 sodium phosphate+1 KOH+5 g·L?1clay particles,10±2°C)process,the sample was etched in diluted H3PO4for 15 s,and calcium stearate coating was electrodeposited(0.05 M Ca(NO3)2+0.05 M SA in ethanol,50 V,60 min).SEM images revealed a hierarchical structure composed of protrusions(～30μm diameter)with embedded nanostructures.The thickness of the MAO and ED layers were～12μm and 23μm,respectively[78].

Qiu et al.fabricated MAO layer(Na2SiO3+KOH+NaF,1000 Hz,10%,6 A·dm?2,360 s),and subsequently applied a coating of dopamine-anchored iron oxide NPs(comprised of magnetic Fe3O4core and HDMS surface layer).Fe3O4NPscoated with HDMS was incubated in an aqueous dopamine solution to produce the dopamine-anchored NPs.The MAO sample was dipped in the NPs dispersion,and a magnetic field was applied.The resulting SHPC surface displayed Cθof～157°[126].

2.4.7.Spraying

Several works employed spraying/painting[60,75,105,115,121].Liu et al.fabricated a robust rare-earth-containing SHPC coating by spraying La(OH)3sol,followed by SA modification.The La(OH)3sol was prepared by adding excessive NH4OH into La(NO3)3aqueous solution(0.1 M)under vigorous stirring,filtered,washed,and dispersed in pure water.The bare,spray-coated only,SA-modified only(0.028 M ethanol solution,20 h),and SA-modified/spray-coated surfaces displayed Cθs of 83.1°,43.0°,117.8°and 156.8°,respectively.The SHPC surface was composed of cross-linked networks of fiber-like La(OH)3nanowires[75].Qian et al.prepared a spray-coated SHPC coating based on nanometer and micrometer sized SiO2NPs.The spray-coating solution was prepared by vigorous mixing of ethanolic solutions of tetraethoxysilane(TEOS),NH3and PFOTS(1 wt.%)with fumed silica NPs.The coated surface displayed Cθof 164.5°and Sθof 1.2°[115].

Fig.10.Scheme showing the fabrication steps.Reproduced with permission from[118]? 2018 Elsevier B.V.

Zhou et al.fabricated SHPC cluster-like ZnO coating(Fig.11A)on acid-etched Mg alloy by spraying epoxy resin and ZnO seeds solution,followed by HT in ZnO growth solution(Zn(NO3)2+hexamethylenetetramine+water,100°C,5 h),and subsequent SA modification.The hydrolysis of Zn2+in the alkaline environment favoured ZnO rod’s growth on the ZnO seeds.The typical surface morphology of the coating is presented in Fig.11B.Optimizing the amount of epoxy resin in the process was found to be crucial.If present in excess,the resin totally covered the ZnO NPs,whereas,at insufficient quantity,the coating adhesion and its bonding with ZnO NPs were reduced.A 2:10 mass ratio of epoxy resin to ZnO was found to be the optimum(Fig.11C).The concentration of the growth solution was also decisive in determining Cθ[60].Yu et al.designed an organic coating reinforced with SHPC Al2O3NPs by a simple mechanical stirring method[105](see§3).

2.4.8.Others

A few studies employed laser ablation as the first step[66,107].Laser-assisted surface texturing is an emerging procedure for altering chemistry and roughness of functional surfaces.Zhang et al.reported a SHPC coating by multistep processing involving laser etching(λ-1064 nm,spot diameter-20 m),MAO(15 Na2SiO3+13 KF+2 g·L?1NaOH,200 V,700 Hz),modified-epoxy coating,SiO2-NPs-dispersion dripping,and PFTS modification.The laseretched and MAO-treated surface was first spin-coated with epoxy resin,and then SiO2/PFTS-modified.Cθas high as 161.7°was measured[107].Li et al.fabricated periodic microstructures of papillary-like pits(～80μm)via laser ablation and subsequently modified with AgNO3(immersion,0.1 M,3 min)and SA(0.15 M ethanolic solution,1 h).The modified surface displayed sticky SHPY with Cθof 158.2°.AgNO3immersion was helpful to achieve nano-scale silver dendritic surface structure.The average surface roughness of the bare,laser-etched,AgNO3-treated,and SHPC surfaces were～0.2,6.6,5.8 and 5.6μm,respectively[66].

La et al.reported DC sputtered Mg-Y thin film modified with thermal vacuum-deposited PTFE for energy-saving windows application.The porous PTFE-coated surface exhibited Cθof～152°[123].Qiu et al.employed high speed wire electrical discharge machining(HS-WEDM)and SA modification[53].Polat et al.utilized electrospinning.Here,a mixture solution of polystyrene(PS,10 wt.%),fluorosilanemodified SiO2(2–8 wt.%)and DMF was used.The assimilation of modified SiO2NPs endowed PS fibers with high surface roughness,and a 4 wt.% NPs-incorporated coating yielded the maximum Cθof～165°[113].Wang et al.employed a salt spray method(35°C,5 wt.% NaCl,100% humidity)to generate a hierarchical surface structure.Subsequent surface modification with 1 wt.% PFTMS(immersion,30 min)yielded SHPY with Cθof 152.65°and Sθof 5°[125].

Most of the above reported works achieved slippery SHPY with excellent self-cleaning properties[50–52,55,59,60,69,72-74,76,77,81,82,86,87,89,99–104,106,108–110,112,114–116,120,121,125,128,133,138,139,141].

Fig.11.(A)Scheme showing the preparation process.(B)SEM surface view image of the sample obtained from 2:10 mass ratios of epoxy resin:ZnO.(C)Cθof coatings obtained from various ZnO:epoxy resin mass ratios.Reproduced with permission from[60]? 2019 Elsevier B.V.

2.5.Pre-treatments

Several works employed acid[50,52,60,86,102,106,121,127,141],alkali[88,102,115],or acid and alkali[64,71,86,92,99,101,132]etching as a pre-treatment step before the single/multi-step processing.

Different acid solutions such as 1.5 M HCl+0.5 M CH3COOH(etching time～420 s)[52],12 M HCl+CH3COOH+water[60],0.5 M HCl[106],36 vol.%HNO3:85 vol.% H3PO4:anhydrous ethanol:water=1:3:10:186(120 s)[50],1 wt.% H2SO4+1 wt.% HF(2 min)[121],1.5 M H2SO4(10 s)[127],and 20 g·L?1aqueous C6H8O7(35 s)[141]were used.

Alkali solutions of 1.5 M NaOH[88],1 M NaOH(70°C,30 min)[102],and 0.25 NaOH+0.25 Na2SO3+20 g·L?1Na2CO3(5 min)[115]were used in different reports.

Alkaline degreasing followed by immersion in 10 vol.% HNO3(10 s)[92],alkali washing in NaOH+Na3PO4+H3PO4(10 min,65°C)followed by cleaning in H3PO4+KF aqueous solution(10 s)[71],degreasing in 80 g·L?1NaOH+30 g·L?1Na3PO4(10 min,80°C)accompanied by pickling in 30 mL·L?1HNO3+605 mL·L?1H3PO4(30 s)[99],degreasing in NaOH+Na2CO3+C12H25NaSO4(5 min,70°C)and immersion in HNO3+H3PO4+CH3CH2OH+Mn(H2PO4)2(2 min)[86,101],degreasing in 45 g·L?1NaOH+10 g·L?1Na3PO4(10 min,65°C)followed by immersion in 125 g·L?1CrO3+100 mL·L?1HNO3(30–40 s)[132],and etching in 1.7 M HCl(12 min)followed by immersion in 0.027 M NH3(2 h)[64]were also employed in different reported works.A table showing the pre-treatment steps and the corresponding methods used for the nano/microstructring and the low SE treatment is provided in the Supporting Information(Table S3).

The acid etching step is typically conducted to get rid of the surface oxide layer and to increase the activity of the metallic substrate.The method is also helpful to make the surface rougher.Alkali etching is normally beneficial to degrease the surface as well as to fabricate a hydroxylated rich surface.A few reports employed a boiling water pre-treatment intending to develop a surface morphology similar to that of HT-treated samples[110].

3.Application domains

3.1.Corrosion protection

Corrosion is a critical industrial challenge demanding right preventive action[146–153].Surface SHPY is considered as one of the most acceptable solutions for enhancing the anticorrosion properties.The existence of the robust air layer at the surface/solution interface and the associated water repellency could provide superior aqueous corrosion resistance for the underlying metallic substrate[4,11,41,46].The air layer could act as a dielectric limiting the electron movement and could serve as an excellent passivation layer.

All the reported works on SHPC Mg/Mg alloys revealed improved anti-corrosion performance when compared to their bare counterpart.No work so far has reported a reverse trend(see below).This section mostly discusses the electrochemical corrosion parameters reported in various studies on SHPC samples.Results of long-term immersion studies are also provided.It needs to be reiterated that the durability of SHPC surfaces(see§5)is critical in determining their long-term corrosion resistance.

3.1.1.One-step processed superhydrophobic coatings

One-step HT[73,81,120,140,144],ED[57,96,99,100],immersed-coated[97,98]or painted[116,121]SHPC samples revealed superior corrosion resistance during electrochemical studies.By potentiodynamic polarization(PDP)studies,Fang et al.have displayed that the corrosion current densities(icorr)of the bare and SHPC(one-step HT-treated)samples(in 3.5 wt.% NaCl)were 1.48×10?4and 3.32×10?7A·cm?2,respectively.The equivalent corrosion potentials(Ecorr)were?1.50 and?1.34 V(vsSCE).A more positiveEcorrand lowericorrvalues reflect better corrosion protection.However,their long-time immersion studies revealed considerably decreased Cθafter 60 h[120].The continuous immersion in very aggressive solutions could deteriorate the packed hydrophobic molecules resulting in coating defects leading to corrosion.Zhang et al.in a similar work showed only one order of reduction oficorrfor the SHPC sample.Theicorr(Ecorr)of the bare and SHPC samples were 7.94×10?5(?1.69 V)and 7.94×10?6A·cm?2(?1.57 V),respectively[73].Despite the similar method used,the considerable difference in the corrosion resistance of the above two studies possibly be ascribed to the difference in the processing conditions/materials.The ZnCl2used in this work could result in an additional surface etching.Their electrochemical impedance spectroscopy(EIS)studies,however,revealed～16-fold higher impedance to the SHPC sample than the bare.The Cθmeasured after 36 h of continuous immersion was 151.6°[73].Studies on HT-treated and oleate-modified SHPC sample in phosphate buffer saline(PBS)showed that the polarization resistance(Rp)was～20 times greater to the bare;whereas theRpof the unmodified HT-treated sample(LDHs-coated)was only slightly higher.The average mass loss of the SHPC sample after 30 days of immersion was 21.4 mg,which was substantially lower than the unmodified HT-treated(26 mg)and the bare(123.3 mg)samples[140].Similarly,other studies also revealed excellent corrosion resistance for one-step HT-treated SHPC samples when compared to their bare counterparts[81,144].

ED is a conventionally used method for fabricating corrosion-resistant coatings[154,155].Zheng et al.have shown thaticorrof an one-step ED SHPC sample(5.80×10?8A·cm?2)was 3-orders inferior to that of the bare(7.02×10?5A·cm?2).The SHPC sample maintained high Cθvalues during one week of continuous immersion in 3.5 wt.% NaCl.XRD studies indicated that the adsorbed low SE calcium myristate underwent hydration with rise of dipping time[57].Zhong et al.also showed better long-term performance for an one-step ED calcium myristic SHPC coating during one weak of continuous immersion study in 3.5% NaCl.High low-frequency impedance modulus(～109Ω·cm2)was noted after 1 day of immersion.The two time constants appeared at high and medium frequencies were correlated to the coating and the electrical double layer,and the associated coating resistance(Rcoat)and charge transfer resistance(Rct)were determined.TheRcoatof the optimum(60 min ED)sample,however,reduced significantly(5.139×102Ω·cm2)after 8 days of immersion t.The corresponding Cθwas～102.6°[96].Zhao and Kang[99],and Liu et al.[100]also showed that one-step ED SHPC Mg alloys had better corrosion resistance when compared to their bare counterparts during both short-and long-term corrosion studies.

One-step solution immersion derived SHPC samples also displayed higher corrosion protection[97,98].PDP studies in 5 wt.%NaCl revealed that theicorrof SHPC samples obtained after 1,3,10,and 30 min of immersion were 8.95×10?5,1.21×10?5,1.08×10?6,and 4.41×10?7A·cm?2,in that order.The equivalentEcorrwere?1.48,?1.19,?1.45 and?0.43 V(vsAg/AgCl).An immersion period of 30 min has resulted in the coating with the best corrosion resistance[97].

Electrochemical studies on one-step blade-coated PDMS/SiO2SHPC samples in 3.5% NaCl revealed that a sample coated with 0.3 g of 40 nm SiO2NPs had a better protective effect than a corresponding 0.2 g added coating.Rctof bare,and 0.2 and 0.3 g of SiO2NPs added coatings were 11.57,4076 and 6.67×105Ω·cm2,respectively[116].The superior corrosion resistance obtained for a SHPC PPSPTFE/SiO2composite coating in 3.5% NaCl was attributed to the SPHY as well as to the barrier effect of the cross-linked polymer coating.The best corrosion resistance was obtained for 2 g·L?1SiO2-incorporated coating[121].

3.1.2.Two-step/multi-step processed superhydrophobic coatings

This section discusses the electrochemical and long-term corrosion studies of the works discussed in§2.4.Jin et al.with their continuous soaking studies in 3.5% NaCl displayed that aHT-treated and low SE-modified SHPCsample displayed localized corrosion only after 10 days.On the other hand,the bare sample underwent severe corrosion within one day,and the unmodified HT-treated sample revealed localized corrosion after 3 days of immersion.The SHPC coating significantly reduced theicorrby more than 2-orders of magnitude[59].Li et al.have shown that theicorrof the HT and silanetreated SHPC sample decreased by～1/8thof the bare alloy and～1/4thof the unmodified HT-treated alloy.TheRcoatof the HT-treated and HT/silane-treated samples were 4435 and 35149Ω·cm2,respectively,validating the significant role of the SHPC surface in enhancing the corrosion resistance[119].A HT-treated and PTEF-sputtered SHPC sample having threelayered Mg(OH)2/Mg-Al LDHs/fluropolymer coating provided superior corrosion protection both in 3.5 wt.% NaCl and 5 mM H2SO4.Theicorrof the SHPC sample was 33.4 and 15.8 times higher than the corresponding HT-treated samples tested in H2SO4and NaCl solutions,respectively[109].Zhang et al.showed that an LDHs-coated and SA-modified SHPC sample substantially enhanced the anti-corrosion property,where theicorrof the SHPC,LDHs-coated,and bare samples were 3.4×10?10,3.9×10?7and 4.7×10?5A·cm?2,in that order.EIS,hydrogen evolution(HE)studies and corrosion morphology analysis provided supportive evidences[80].TheRpvalue reported for an SA-modified and LDHs-coated SHPC sample in 3.5 wt.% NaCl was 4680Ω·cm2and that was significantly higher than the corresponding values of unmodified LDHs-coated(406.2)and bare(151.9)samples.EIS plots revealed that the SHPC sample had the highest impedance modulus and the largest high and medium frequency capacitance loops[88].Zhang et al.demonstrated that a HT/PP-dip-coated SHPC alloy had significantly higherRp(1.06×107Ω·cm2)in 3.5 wt.% NaCl,considerably greater than that of the unmodified HT-treated(4.31×105Ω·cm2)and the bare(2.12×102Ω·cm2)samples.TheRctcalculated from the EIS plots for the SHPC,HT-treated and bare samples were 935,526 and 0.327 KΩ·cm2,respectively.The hydrogen evolution volume(HEV)for the bare and SHPC samples after 250 h of immersion in 3.5 wt.% NaCl was measured to be～7±0.03,and 0.05±0.03 mL·cm?2,correspondingly[133].Continuous immersion studies(120 h,3.5 wt.%NaCl)by Zhang et al.have shown that the bare sample underwent severe corrosion,whereas HT-treated sample suffered a few localized attacks only.The corresponding SHPC(HTtreated/SA-modified)sample,however,displayed a smooth surface free of localized corrosion[85].Similar results were reported with several other studies such as ST-treated/SAmodified[84],HT-treated/Ni-P-plated/SA-modified[77],HTtreated/TiO2-coated/Cu-plated/thiol-modified[138],and HTtreated/TiO2-coated/SiO2/SA-modified[52]samples,confirming that SHPC surfaces might effortlessly capture air at the coating/solution interface and that could prevent the diffusion of the aggressive ions.

PDP and EIS studies on SHPC coatings fabricated byED as the first-steprevealed superior corrosion protection.Liu et al.have shown thaticorrof the CeO2/SA-coated SHPC sample was more than 2-orders of magnitude lower than the bare.Their time-dependent PDP studies revealed a slight increase oficorrafter 48 h of immersion in 3.5 wt.% NaCl.After 100 h,theicorrincreased considerably,ascribed to the localized corrosion and the deterioration of the coating.The inductive loop of the Nyquist plot becomes much evident indicative of the increased corrosion[50].PDP studies in 3.5 wt.%NaCl on chemical-etched/Ni ED/SA-modified SHPC sample also shown matching results.TheEcorrof the SHPC sample was 0.77 V more positive to the etched/SA-modified sample and 1.1 V more positive to the bare sample.The correspondingicorrwas～1/6 lower to the etched/SA sample and～1/42 lower to the bare sample.Both the Ni coating and the SHPC surface contributed to the excellent corrosion resistance.Their continuous immersion studies in 3.5% NaCl also displayed better corrosion resistance to the SHPC sample[69].Theicorrof SHPC PFTS/ZIF-8/PVDF/LDHs-coated sample in 3.5% NaCl was 3-orders of magnitude inferior to the bare sample.Rctof the bare,LDHs-coated(by ED),PVDF/LDHscoated,and ZIF-8/PVDF/LDHs-coated,and the SHPC samples were 61.58,8.1×102,5.9×103,2.2×104,and 6.6×104Ω·cm2,respectively.The SHPC sample maintained the highRct(>104Ω·cm2)during 7 days of continuous immersion.The much-improved protection was credited to the combined effect of the LDHs film and the top SHPC layer[106].Similarly,SHPC Ni-ELD/Cu-ED/LA-modified[101],and Ni-P-ELD/nano-Ag-deposited/SA-modified[71]samples also displayed excellent corrosion resistance where theicorrwas～2-3 magnitude nobler to the bare sample.

Several studies onimmersion-coated SHPC samplessuch as MnO2/SA[54],Mg(OH)2/Zn/SA[62],phosphate/SA[87]and silane/SA[76]also revealed excellent corrosion resistance.Ding et al.have fabricated a SHPC multi-layer coating(Fig.12A)with U-PDMS and WO42?(corrosion inhibitor)and laurate(low SE)-modified LDHs film(see§5).PDP results depict(Fig.12B)the enhanced corrosion resistance where the SHPC coating presented more positiveEcorrand much reducedicorrvalues.The EIS Bode plots of SHPC sample revealed larger low-frequency impedance moduli(Fig.12C)and high-frequency phase angles of 90°(Fig.12D)indicating better barrier effect.The analogous impedance moduli of the U-PDMS/WO42?-LDHs coating and the SHPC coating was attributed to the similar thickness.However,their long-period immersion experiments revealed that the impedance plateau of U-PDMS/WO42?-LDHs coating decreased within 72 h of immersion,indicating diffusionassisted coating degradation,whereas the SHPC sample displayed relatively high low frequency impedance(～105Ω·cm2),even after 15 days[112].EIS studies on a Mg–Mn LDHs/MA SHPC coating demonstrated low-frequency impedance of～20000Ω·cm2and that was more than 2-times of the LDHs-coated sample and 10-times of the bare sample.However,their long-time immersion in simulated body fluid(SBF)revealed that the Cθof the SHPC coating decreased to～60°after 48 h of immersion,and that was ascribed primarily to the desorption of the low SE film[95].Kaung et al.showed thatRctvalues of bare,LDHs-coated,and SHPC LDHs/ED-SA samples were 171.1,605.8 and 6936Ω·cm2,respectively.The measured HEV was also in the same order[93].

The low SE modification could seal the cracked surface morphology of chemical conversion coatings[74].A SHPC coating fabricated by organic(PA)–inorganic(Ce)conversion coating,followed by HDMS modification,has shown excellent corrosion resistance.SEM analysis after 120 h of salt spray studies disclosed that～73%of the area of PA/Ce/HDMS coating was not corroded.On the other hand,a corresponding PA/HDMS coating was completely corroded[110].HE studies on a dipcoated SHPC PAPTMS/PP sample revealed the following order for the measured HEVs:bare(5.35±0.45)>PAPTMS coated(3.80±0.22)>SHPC PAPTMS/PP coated(0.65±0.31 mL·cm?2)[102].

Fig.12.(A)Layered structure of the SHPC coating.(B)PDP and(C & D)EIS Bode plots in 3.5 wt.% NaCl.Reproduced with permission from[112]?2018 Elsevier B.V.

All the studies reported withEAO or MAO as the firststeprevealed better corrosion resistance.EAO/MAO coating is advantageous in providing a thick conversion coated oxide layer.The subsequent low SE modification is also helpful to reduce the typical coating defects.Liu et al.through PDP studies(in SBF)presented that theEcorrof EAO/SAcoated(SHPC),EAO-coated(SHLC),and bare samples were–1.351,–1.671 and–1.819 V(vsSCE),respectively.The equivalenticorrvalues were 0.127,0.647,and 1.248 mA·cm?2[83].Zhang et al.on their study on SHPC EAO/ED-calcium stearate coated alloy showed that the corrosion resistance varied with the duty cycle employed for the pulse ED.A sample fabricated with a duty cycle of 50% had the maximum pitting potential(Epit)of 1.17 V,compared to?1.62 V(vsSCE)recorded for the bare alloy.The SHPC sample also displayed highest coating thickness and better coating morphology[61].EIS studies on SHPC surface fabricated by EAO,followed by HT and various low SE modifications(SA,SL,MA and PFTMS)disclosed that the corrosion resistance depends on the type of the low SE material used.TheRctof the PFTMS,MA,SA and SL-modified samples were,respectively,27793,11021,9311 and 5139Ω·cm2(3.5 wt.%NaCl,after 1 h).The correspondingRctvalues after 14 days of immersion were 2126,1960,1767 and 1387Ω·cm2.The results indicated that PFTMS modification provided comparatively better protection[56].A SHPC EAO/PVC–THF coating reported by Yang et al.displayed 3-orders higher corrosion resistance than the bare sample[135].PDP studies by Zhang et al.disclosed that the time of the low SE treatment(immersion,99°C)significantly affected the protection efficency.TheEcorrof the bare,MAO-coated,MAO/0.5 h SA,MAO/1 h SA,MAO/3 h SA,and MAO/7 h SA coated surfaces were–1.55,–1.75,–1.61,–1.74,–1.76,and–1.73 V(vsSCE,3.5 wt.% NaCl),respectively,while the correspondingicorrvalues were 4.23×10?5,6.12×10?6,1.89×10?6,9.28×10?7,1.55×10?7,and 5.36×10?8A·cm?2.The low frequency impedance values of the MAO/SA coatings in the above order were～104,105,105,and 107Ω·cm2.The inherent pores of MA coatings were filled by the SA and the Mg(OH)2layers,and that was supportive to enhance the corrosion resistance.The long-term immersion,however,could results in the dissolution of the adsorbed Mg stearates and Mg(OH)2,resulting in the formation of soluble MgCl2[79].Liu et al.have shown that the HEVs of bare,MAO-coated,and SHPC samples after immersed in SBF for 300 h was 58.42,8.48 and 3.05 mL·cm?2,respectively[70].SEM and EDS analysis of SHPC MAO/LDHs/SA-coated sample after 288 h of soaking in 3.5 wt.% NaCl revealed that the morphology and composition of the surface remained the same without any significant variation.However,the sample edges were more distinct,and there was desorption of SA,as indicated by the decline of Cθto 126.76°.Corresponding unmodified MAO-coated sample,however,underwent severe corrosion damage whereas the unmodified MAO/LDHs-coated sample retained the original flake-like morphology in most areas,and only a few corrosion pits were observed.EDS results suggested that ion-exchange and Cl?capturing happened in the LDHs film[63].Corrosion studies of SHPC MAO/organophosphonate-coated sample disclosed 2-orders of magnitude inferioricorr(1.12×10?8A·cm?2)when compared to the MAO-coated counterpart(3.12×10?6A·cm?2)[136].A SHPC MAO/PA/Ce/silane coating revealed excellent corrosion resistance.Here,the synergistic effect of chemical(organic PA and inorganic Ce)and electrochemical(MAO)conversion coatings was utilized along with the low SE silane modification.Theicorrof the bare,MAO-coated,and SHPC samples were 2.1×10?5,8.2×10?7,and 3.5×10?8A·cm?2(immersion,3.5 wt.%NaCl,1 h).After 72 h,the corresponding values were found to be 2.8×10?4,5.1×10?6and 4.8×10?8A·cm?2[118].The authors in a more recent work reported a SHPC coating with self-healing capability[111](see§5).A SHPC MAO sample made by magnet-induced assembly technique by using dopamine binders and iron oxide NPs(magnetic Fe3O4core and HDMS surface)also displayed excellent corrosion resistance[126].Cui et al.studied MAO/ED(zinc stearate)composite anti-corrosion coating.The low SE ED zinc stearate layer provided the surface SHPY and also acted as an effective sealing layer to the porous MAO layer[145].In recent work,Zhang et al.have shown that a SHPC MAO/PP coated Mg alloy(Cθ-167.2±0.8°and Sθ-2.7±0.5°)significantly reduced theicorr(8.76×10?9A·cm?2).The coating integrity was well maintained even after 248 h of immersion in 3.5 wt.% NaCl.The MAO layer was helpful to develop a well-adhered coating[156].

Spray-coated SHPC ZnO/epoxy resin provided superior corrosion resistance,and that was credited to both the robust barrier protection and the non-wettability[60].Longterm immersion studies of a LaOH sol spray-coated,and SAmodified SHPC sample showed that after 48 h,Cθs of the bare,only spray-coated,and only SA-modified samples were decreased to～15°,20.3°,and 19.8°,respectively,while the SHPC sample retained the Cθat～150°.Theicorrvalues for the four alloys were in the order:9.23×10?6,8.805×10?7,2.22×10?6,and 1.10×10?8A·cm?2[75].A few studies reported SHPC NPs-incorporated spray-coated samples[105,115].Yu et al.designed an epoxy coating reinforced by silane functionalized SHPC Al2O3particles(SF-Al2O3,26 vol.% loading)using a simple mechanical stirring method(Fig.13).Their long-term open circuit potential(OCP)decay studies showed superior performance for the SF-Al2O3-incorporated coating when compared to pure epoxy and unfunctionalized(UF)Al2O3-added coatings(Fig.13).The EIS impedance values obtained at 0.01 Hz data also support this(Fig.13)where the SF-Al2O3epoxy showed much higher impedance values even after 144 days(6.5×109Ω·cm2).The impedance moduli plots also suggested that incorporation of UF-Al2O3had an adverse influence on the barrier effect of the epoxy coating[105].PFOTS surface-modified SiO2particles spray-coated Mg alloy enhanced theRpby more than 4-orders of magnitude[115].

SHPC coatings fabricated by chemical etching[139,141],laser etching[66,107],HS-WEDM[53],and electrospinning[113]also displayed better corrosion protection than their bare counterparts.Fig.14 shows the PDP plots of a SHPC sample produced by HS-WEDM followed by SA modification.The enhanced corrosion resistance is evident.The SHPC sample displayed the lowesticorr(269.5μA·cm?2)when compared to the bare(27.8μA·cm?2)and the unmodified HS-WEDM(193.8μA·cm?2)samples.The extended passivation region seen with the SHPC sample supports the interface air layer formation(Fig.14)[53].

All these reports undoubtedly proved that irrespective of the type and methods used for the hierarchical surface structuring and the low SE modification,SHPC samples displayed excellent corrosion resistance,far superior to their bare counterparts.However,in most cases,long-term immersion studies failed to prove their suitability in aggressive aqueous environments such as 3.5 wt.% NaCl.Most of the studies attributed the failure to the desorption of the low SE material with time.Typically,the higher activity of Mg contributes to this as localized corrosion could trigger at coating defects and that ultimately ends up with severe uniform corrosion.A firmly adhered SHPC coating without significant coating defects could further improve the long-term aqueous corrosion resistance.More details on the durability of SHPC coatings and their excellent atmospheric corrosion resistance are discussed in§5.

3.2.Biomedical

Mg alloys are researched extensively as biomedical implants.The desirable mechanical properties of Mg alloys such as matching elastic modulus and strength to that of human bone are advantageous;however,the miserable corrosion resistance restricts their clinical application[83].This section discusses works reported on SPHC Mg/Mg alloys specifically for biomedical applications.

Kang et al.reported an one-step HT-deposited SHPC HA coating with improved corrosion protection and biocompatibility.Theicorrof the modified alloy in Hanks’solution was～4-orders of magnitude lower.The SHPC surface also demonstrated excellent blood compatibility with<0.1% of hemolysis when compared to over 30%of hemolysis recorded for the bare alloy(whole blood,60 min).The significantly reduced hemolysis was attributed to the excellent biocompatibility of HA as well as to the reduced actual contact area due to the SHPY.The enhanced biocompatibility and stability could contribute to rapid post-implantation tissue healing[72].One-step HT fabricated SHPC surface in oleate presented improved cytocompatibility as evidenced by the fastest endothelial cell migration rate.When compared to the bare(19.6%)and LDHs-coated(64.3%)samples,the SHPC sample showed significantly higher(84.7%)cell viability.The SHPY,along with the lowered corrosion resistance,was suggested to be the reason for the increased cell adhesion and proliferation.Their platelet adhesion and hemolysis assays suggested improved hemocompatibility for the SHPC surface[140].

Fig.13.(Top)Schematic of the coating process.(Bottom)OCP measurement(left)and impedance moduli at 0.01 Hz(Right)with immersion in 3.5 wt.%NaCl.Reproduced with permission from[105];? 2020 Elsevier B.V.

Fig.14.(A)PDP plots in 3.5 wt.% NaCl.A schematic of SHPC surface is also shown.Reproduced with permission from[53];? 2020 WILEY-VCH.

Xun et al.on their studies withS.aureusandE.colidisclosed that the bacteria attachment and biofilm formation was significantly reduced on MnO2/SA SHPC surface,and that was primarily attributed to the strong water repellency.The fluorescence microscopy images during the cell adhesion test using NIH3T3 mouse fibroblasts cells demonstrated virtually no cell adhesion on the SHPC surface,revealing excellent opposition to the fibroblasts cells[54].Cell adhesion and differentiation studies(followed over 21 days)with calcium stearate/MAO-coated sample revealed that the SHPC surface reduced human osteoblast proliferation,but enhanced osteoblast differentiation,as observed by the formation of HA,suggesting that the modified sample could be suitable for bone applications,where fast bone-bonding is essential.On the other hand,the corresponding unmodified MAO sample displayed an enhanced osteoblast proliferation[78].Cytotoxicity test with HEK293 human embryonic kidney cells revealed that a SHPC HT/SA-coated surface efficiently reduced the toxicity of the base substrate.The study also showed that the SHPC surface decreasedE.coliadhesion considerably and hence could reduce the risk of infection during/after implantation[92].

Fig.15.(Left)Hemolytic rate of the differently modified samples.(Right)Cell viability after cultured for 1,3 and 5 days.Reproduced with permission from[62];? 2019 Elsevier B.V.

Several research efforts were dedicated to Mg alloys as bioabsorbable implants.Achieving SHPY is an excellent approach to extend their corrosion resistance.Corrosion studies on EAO/SA-coated SHPC sample in SBF showed that EAO is a preferable method for making SHPC Mg samples with improved corrosion resistance in this application[68].Degradation studies(by determination of pH and osmolality)in fetal bovine serum-added modified eagle medium revealed lower pH(～7.6)and osmolality values(after 10 days)for a SHPC calcium stearate/MAO-coated sample when compared to the bare one[78].The long-term immersion studies of HT/SAcoated SHPC alloy in Hank’s solution showed that pitting corrosion could only be found after 80 days and the corrosion area was much smaller when compared to the corresponding HT-treated SHLC alloy(pitting corrosion appeared after 60 days).The results were supported by the ICP-AES analysis of the Mg content in Hank’s solution[92].Several studies such as HT-treated HA/SA[58],EAO/SA[83],LDHs/MA[95]and laser-etched[107]SHPC samples also displayed superior biocompatibility and corrosion resistance.

Xie et al.reported improved corrosion protection and biocompatibility of ZnCl2/SA-modified SHPC Mg alloy.Their platelets adhesion tests presented uniform coverage with several spherical platelets on the bare alloy,whereas,only a few platelets adhered on the SHPC sample.The hemolysis rate of the SHPC sample was the lowest(Fig.15A).The low toxicity and the better biocompatibility of the deposited Zn also played a role in the improved performance.Their cell viability(smooth muscle cells)studies showed that after 24 h of culture,the viability was highest on the SHPC sample(Fig.15B).The cell viability of all the samples decreased with increase of immersion time.After 5 days,both the SA-modified sample and the SHPC sample displayed lower cytotoxicity,and that was attributed mainly to their surface hydrophobicity[62].

3.3.Anti-icing

A SHPC sample could be a promising icephobic material.Surfaces with low free energy and heat transfer efficiency could diminish the water freezing and the ice adhesion[104,144,157].Tang et al.investigated ice adhesion behaviour of a SHPC sample prepared by chemical etching and ultrasonic treatment.Experiments were performed by using a home-built apparatus in the humidity chamber(30°C,～80 RH).The cooling stage was attuned to?20°C at 4°C·min?1,and kept for over 4 h.Finally,the probe of the force transductor was moved horizontally(5 mm·s?1)and the force upon the detaching was noted.The SHPC surface demonstrated excellent icephobic property with significantly reduced ice adhesion force(160–260 kPa).The corresponding value recorded for the bare surface was～950 kPa.The enhanced performance was primarily attributed to the significantly decreased water/substrate contact area[104].

Li et al.reported decent anti-icing performance for an onestep HT synthesized SHPC surface.Here,the sample was positioned flat on a watch glass in a freezer at?15°C,and then methyl orange-coloured deionized water(0.04 mL)was poured onto the surface.The water changes the colour from dark orange to light orange upon freezing.The results revealed that the freezing process starts at～120 s on the bare sample,whereas the process begins at～600 s on the SHPC sample.The enhanced icing delay was attributed to the low SE and the SHPY[144].Wu et al.reported significant icing retardation by a HT-treated and PTEF-sputtered SHPC surface[109].A reported study on core-shell structured ZIF-SiO2SHPC coating also displayed much delayed ice adhesion.Ice removal from the SHPC surface was easier,and the surface remained clean after deicing[117].

3.4.Others

Zhang et al.investigated photocatalytic performance of SHPC TiO2-SiO2/SA-coated Mg alloy.On exposure to UV light for 12 h,the SHPC sample placed in a methyl orange solution(10 mL·L?1)degraded～64.1% of methyl orange.On the other hand,a sample without SA modification revealed a degradation rate of 76.9%[52].La et al.reported DC sputtered Mg-Y thin films modified with thermal vacuum-deposited PTFE for switchable mirror applications.When compared with the unmodified sample,the luminous transmittance and the solar transmittance of the SHPC surface was enhanced by～7%,and～5%,respectively.The enhanced property was also attributed to the lower refractive index of the PTFE layer[123].A few reports on SHPC Mg alloys for potential applications in oil separation[99],and micro-fluidic devices[101]are also available.

4.Self-healing superhydrophobic coatings

SHPC coatings with an additional ability for self-healing the service damages have attracted significant research curiosity.Self-healing property can be assigned to a coating via different approaches[158,159].

Conventionally,chromate corrosion inhibitor pigments are known for their self-healing property,attributed to the ability of the hexavalent chromium for orientated migration to the damaged regions.Zhang et al.demonstrated a self-healing SHPC Cr2O3/SA coating fabricated by a simple immersion process.The incorporation of the Cr-oxide was helpful for self-healing the intentional scratches in 3.5 wt.% NaCl(1 h of immersion).Their continuous immersion studies,however,showed that the sample completely lost the hydrophobicity after 7 h of exposure[90].Absence of more studies on hexavalent Cr is indeed associated with the toxicity and the environmental guidelines[124,158].Yang et al.studied selfhealing stannate conversion coating.The stannate coated sample was submerged in 3.5 wt.% NaCl for 4 h for self-healing the defects and then modified with SA.An ionic strengthbased solving-reprecipitation theory was used to explain the self-healing mechanism[86].

Smart on-demand release coatings with embedded nanocontainers are projected as the most suitable option to fabricate self-healing coatings[159].Only a few works are reported on SHPC and self-healing coatings for Mg alloys in this line.Ding et al.described a host-guest feedback active coating by assimilation of guest(mechanized silica NPs,SNPs)nanoreservoirs into host(self-assembled nanophase particle,SANP)coating.The solution for the dip-coating was made by ultrasonically disseminating SNPs(2 mg·mL?1)in SANP sol,and then adding triethylenetetramine.The SNPs was comprised of mesoporous silica NPs and supramolecular pseudorotaxanes,and 2-hydroxy-4-methoxy-acetophenone(inhibitor)was trapped in the mesopores.The inhibitor release will happen only in the presence of alkali/Mg2+stimuli due to the corrosion initiation at micro-regions.To make the surface SHPC,the hierarchical micro/nanostructured surface was subsequently modified by PFTS(immersion,10 wt.% ethanol solution).The SHPC coating displayed excellent anti-corrosion performance,and the Mg alloy remained free from corrosion during 15 days of soaking in 0.05 M NaCl.The capability to heal the scratched surface was evident.EIS Bode plots of SANP,SNPs-SANP and PFTS/SNPs-SANP coatings recorded after 5 h of immersion not showed any noteworthy variation.However,after 3 days of immersion,the second time constant(at 10?1-100Hz)attributed to the charge-transfer process was distinct in the phase angle plots of the SANP coating.After 7 days,the second time constant has appeared in the plot of SNPs-SANP coating.Interestingly,PFTS/SNPs-SANP coating retained high coating resistance,and an incomplete second-time constant appeared only after 15 days[124].

LDHs films with 2D layered structure and inherent ionexchange property could be conveniently used for loading corrosion inhibitors.LDHs loaded with anionic corrosion inhibitors could conditionally release them and simultaneously entrap the aggressive anions in the solution[101].A SHPC,self-healing anti-corrosion coating with WO42?(corrosion inhibitor)-intercalated LDHs(fabricated by coprecipitation and HT)and post-sealed by U-PDMS(by spincoating)and modified by La-LDHs(low SE laurate-modified LDHs,powder,spray-coating)(see Fig.12A)displayed appreciable self-healing.To study the self-healing property,intentional scratches(width～200μm)were made,and the samples were immersed in 3.5 wt.% NaCl.With the WO42?-LDHs coating,many lightly filled corrosion products heaped up in the spoiled areas;however,such corrosion product’s accumulation was much less in U-PDMS/WO42?-LDHs and La-LDHs/U-PDMS/WO42?-LDHs(SHPC)coatings,and that was attributed to the released corrosion inhibitor.The UPDMS layer was beneficial in avoiding unnecessary inhibitor wastage.The self-healing performance was also studied via scanning vibrating electrode technique(SVET)(Fig.16).Here,a carbonate-intercalated LDHs film(CO32?-LDHs)was used as a control.Both the optical images and the current density maps revealed a significant extent of corrosion in this case(Fig.16A).The corresponding images recorded for the WO42?-LDHs coating suggested self-healing activity where the corrosion areas not spread considerably(Fig.16B).The SHPC coating displayed the slowest corrosion rate(lowest peak currents),and the maximum and prompt self-healing effect(Fig.16D).The duration for the maximum peak current density to reach the noise level was taken as the selfhealing period.The reason for the better performance for the SHPC composite coating was mainly ascribed to the postsealing of the top layer that prevented the rapid leakage of the entrapped inhibitor[112].Wang et al.investigated NO32?,VO42?or MoO42?-intercalated SHPC EAO/HT-treated LDHs coatings.SS,MA or LA was used for the low SE modification.Among the different samples,the MoO42?-intercalated and LA-modified sample revealed the best corrosion protection,and that was credited to the synergistic effect of physical protection by the EAO/LDHs layer,corrosion inhibition by MoO42?and the water repellency due to SHPY.Theicorrof NO32?,VO42?and MoO42?-intercalated LDHs(modified by LA)samples were 0.0057,0.0040 and 0.0017μA·cm?2,respectively.However,the authors have not evaluated the selfhealing properties in detail[91].

Fig.16.SVET 3D current density maps of(A)CO32?-LDHs,(B)WO42?-LDHs,(C)U-PDMS/WO42?-LDHs and(D)La-LDHs/U-PDMS/WO42?-LDHs(SHPC)coatings in 0.05 M NaCl for different periods.Maximum anodic/cathodic current densities over the fault area during the testing period is also shown where the self-healing zone is green highlighted.Reproduced with permission from[112]? 2018 Elsevier B.V.

Jiang et al.described a self-healing coating by combining MAO(in 15 g·L?1Na2SiO3·9H2O+5 g·L?1KOH)and one-step ED(in 90 mL ethanol solution of NaNO3(0.2 M,10 mL)+TEOS(5 mL)+PFTS(2.5 mL)+8HQ inhibitor(0.2 g)).The typical porous morphology of MAO surface and the fabricated porous silica network,both were helpful for the inhibitor loading.The inhibitor loading density,as determined by UV?vis spectrophotometry was～1.15 mg·cm?2.Their PDP studies in 3.5 wt.% NaCl(30 min immersion),however not showed a significant difference of corrosion protection performance for the SHPC coating with(MAO?SHS?8HQ)and without(MAO?SHS)inhibitor loading.Theicorrof the two cases were 9.91×10?9and 5.46×10?9A·cm?2,respectively.Time-dependent EIS studies,however,showed that with prolonged time,the low-frequency inductive loop was evident in both MAO and MAO?SHS samples,but not pronounced in MAO?SHS?8HQ sample.Localized electrochemical studies by SVET also revealed interesting results(Fig.17).Here,mock pit was prepared to simulate coating defect,and the current density at the defective area was monitored after different duration of immersion.For the MAO?SHS sample,maximum anodic current density observed after 0.5 h was 42.8μA·cm?2(Fig.17a)and that further increased and reached at 177.4μA·cm?2after 10 h(Fig.17c)demonstrating that the coating provided only passive protection and the self-healing effect was absent.On the other hand,the MAO?SHS?8HQ coating successfully diminished the initial anodic peak current(47.8μA·cm?2)(Fig.17d)to 7.9μA·cm?2after 10 h of immersion(Fig.17f),revealing notable self-healing effect by the released inhibitors[111].A self-healing SAPC coating reported by Zhao et al.[128]is discussed in§6.

In a recent work,Liu et al.[160]employed HT-synthesized magnesium silicate nanotubes(MSNTs)as nanocontainers to load 2-mercaptabenzimidazole(MBI)corrosion inhibitor to fabricate an epoxy resin-based SHPC and self-healing dual-functional coating.The polished Mg alloy substrate was first brush coated with an optimized mixture of epoxy and MSNTs-MBI,and then dodecyltrimethoxysilane-modified MSNTs-MBI powder was applied on the coated surface with a 200-grit sieve until full coverage,and the surface glasspressed,and cured at 60°C.The fabricated coating displayed Cθ～155°and Sθ～5°.The coating exhibited superior mechanical and chemical durability,and significantly improved anti-corrosion property.Their long-term immersion studies(3.5 wt.% NaCl,366 h)with cross-scratched samples evidenced excellent self-healing attributes.

Fig.17.SVET 3D current density maps in 0.05 M NaCl for(a?c)MAO?SHS and(d?f)MAO?SHS?8HQ coatings.Reproduced with permission from[111]? 2018 American Chemical Society.

A few works are available on liquid-infused porous surfaces corresponding to their SHPC counterparts[46,161].

5.Durability of superhydrophobic surfaces

Typically SHPC surfaces suffer from long-term durability issues that hamper their widespread industrial applications.Several research efforts are dedicated to improving the mechanical,tribological,thermal,and chemical durability of SHPC Mg/Mg alloys.This section discusses results on various durability studies reported in the above-discussed works.It needs to be remembered that the durability results of different studies are difficult to be compared as different materials and methods were used.

5.1.Mechanical and tribological

Studies conducted with abrasive papers on SHPC samples revealed interesting results.Zhang et al.reported that when an one-step HT processed SHPC sample was dragged with a pressure of 1.2 kPa(800 # sandpaper,5 mm·s?1),the surface maintained high Cθ(152.1±0.6°)upto 800 mm of abrasion.After an abrasion distance of 1000 mm,the Cθwas reduced to 146.6±1.2°[73].Liu et al.showed that an one-step ED SHPC surface retained the SHPY upto 400 mm of abrasion(1000 # paper,1.3 kPa).After 500 mm,the measured Cθwas 147.9±1°[100].Zhao and Kang also reported similar results in the abrasion test(1.5 kPa,1 cm·s?1)on one-step ED sample,where Cθ>150°was maintained for 600 mm.After 700 mm,the Cθwas reduced to 149°[99].Abrasion test(5 kPa,1000 # paper)on HT-treated/SA-modified sample showed that the SHPY was lost with a travel distance of 500 mm(Cθdecreased from 159°to 139°)[59].Studies(1000# paper,2.45 kPa)on oleate-modified one-step HT processed sample presented Cθ>140°after 500 mm of abrasion[140].

A Ni–P ED/HT-treated and SA-modified SHPC coating(400 # paper,9.8 kPa)withstand abrasion for 1200 mm with Cθ>150°[77].An ED CeO2/SA coating also maintained SHPY for 1200 mm(see Fig.19 D)(600 # paper,moved to and fro under 50 g)[50].Abrasion test(2000 # paper,100 g,100 cm each in two perpendicular dimensions)on modified EAO/HT-treated samples revealed that the Cθs of SA,SL,MA and PFTMS-modified samples were 96.4°,72.7°,136.9°and 133.3°respectively[56].By scratch test,the critical load for Mg(OH)2and Mg(OH)2/PP SHPC samples were determined to be 2886 and 1323 mN,respectively.The study also showed significantly improved stability for the SHPC surface during a jet impact test[133].All these studies revealed that SHPC surfaces unveiled excellent mechanical durability to some extent.

Wear studies on one-step blade-coated PDMS/SiO2SHPC coating(240 # paper,moved 100 mm to and fro under a weight of 100 g)demonstrated that the abrasion resistance and hence the SHPY could be significantly improved by proper optimization of the size and amount of the incorporated SiO2NPs.As the content of 40 nm sized particles increased from 0 to 0.3 g,the abrasion resistance increased from 12 to 50 cycles.The SHPY was well sustained in the finger touch test also[116].Similarly,one-step spray-coated SHPC PPSPTFE/SiO2also showed superior wear resistance(800 # paper,moved 500 mm to and fro under 100 g)where Cθafter 10 cycles of abrasion was 142.5°[121].The high hardness(918 GPa)of SiO2particles was beneficial in improving the wear resistance.The measured weight loss studies of a spraycoated SiO2-based SHPC sample after abrasion test(1200# paper,100 g)was only～0.0062 g.The Cθremained>150°after 10 cycles[115].A SHPC ZIF-8/PVDF/LDHs coating displayed Cθ>150°even after 50 abrasion cycles(1000 # paper,moved 100 mm to and fro under 100 g)[106].The critical load of a dip-coated PAPTMS/PP SHPC film was found to be～1300 mN lesser to the corresponding PAPTMS coating[102].The friction coefficient and abrasion loss of a SHPC ZIF-8-SiO2coating was significantly lower to the bare sample[117].All these studies confirm that the incorporation of hard NPs such as SiO2is a viable strategy to augment the wear resistance of SHPC coatings.

Fig.18.Wear losses under(A)Dry wear and(B)Wet wear.Reproduced with permission from[53];? 2020 WILEY-VCH.

A 4-fold increase of wear resistance observed for a SHPC MAO/TFE-coated sample when compared to the unmodified MAO sample was attributed to the polymer-containing outer layer,which acted as a dry lubricant[131].The wear amount of a SHPC spray-coated epoxy resin/ZnO/SA sample was considerably lesser due to the strong integration of ZnO and the good anchorage due to the epoxy resin.The surface also displayed better results in tape peeling experiment.After 10 tape peeling tests,the Cθwas still high(163°),suggesting that the cluster-like ZnO had good substrate adhesion.The coating continued to be SHPC even after 100 peeling tape cycles.Abrasion test(2000 # paper)showed that the Cθwas reduced to 132°after～2000 mm[60].The Cθof a LaOH sol spray-coated and SA-modified surface displayed only a marginal reduction(150.2°)after 10 abrasion cycles(800 #paper,5 mm·s?1,0.16 kPa,60 cm).The improved durability was attributed to the strong bonding of La2O3and SA molecules[75].A HS-WEDM/SA SHPC surface significantly reduced the wear loss(Fig.18).Under both the wet and dry conditions(tribometer,silicon nitride ball),the friction coefficient of the SHPC surface was inferior to the bare,with a much reduced volumetric wear loss(Fig.18)[53].

5.2.Chemical

Irrespective of the methods used,most of the SHPC surfaces fabricated on Mg alloys displayed excellent chemical durability over a wider pH range(based on Cθvariation for droplets with different pH).Different studies reported stability in different pH ranges such as pH 1?13(or 14)[50,51,52,60,75,86,101,115],pH 2?12[76,77,117,133,144],pH 3?13(or 14)[58,82,100],or pH 4?13(or 14)[66,140].

Several studies showed that the Cθquickly reduced and the SHPY destroyed at pH≤2[58,73,82,100]and that was attributed to the desorption of the low SE component.However,several other studies showed that the fabricated SHPC surface displayed Cθ>150°even at pH 1[50,51,52,60,75,86,101,115].For example,an ED CeO2/SA SHPC surface displayed excellent stability over a wider pH,including pH 1(Fig.19B)[50].A HT made TiO2-SiO2/SA SHPC coating displayed Cθ～151.8°at pH 1[52].A Ni ELD/Cu ED SHPC sample also showed excellent durability over a wider pH ranging from 1 to 14[101].A few studies,however,showed that the SHPC surface was not stable even at less acidic pH values(pH 3?4)[58,66,82,100,140].The lesser durability of an oleate-modified one-step HT synthesized sample at pH<4 was attributed to the collapse of LDHs phases in the acidic media[140].

Most of the studies,however,not investigated pHdependent long-term durability through continuous immersion.It is expected that such direct immersion will indeed result in significant reduction of chemical durability at very acidic pH ranges.Ishizaki et al.showed that one-step dipcoated Ce/MA SHPC surface after 10 h of dipping in aqueous solutions of pH 4,7,and 10 displayed Cθof 90°,119°and 138°,respectively,demonstrating that the chemical durability gets compromised with time[98].

Fig.19.Durability as evaluated by Cθvariations during(A)Atmospheric air exposure,(B)Contact with droplets with diverse pH,(C)3.5 wt.% NaCl soaking,and(D)Abrasion testing(50-balance weight,600 # paper).Reproduced with permission from[50];? 2020 Elsevier B.V.

Fig.20.(a)PDP,(b)Nyquist,(c)Bode impedance and(d)Bode phase angle plots in 3.5 wt.% NaCl.Photographs of EP/PF-PS-SiO2 coated samples(e)with liquid drops,(f)with liquid jets bouncing off,and(g)in bulk liquids.Reproduced with permission from[129];? 2020 Elsevier B.V.

The long-term durability of SHPC surfaces is typically determined by immersion in aggressive chloride solutions.Most of the studies revealed that the Cθremained>150°after soaked in 3.5 wt.% NaCl for shorter durations such as 24 h[82,122]or 48 h[99,111].Zhu et al.showed that Cθwas only slightly decreased(from 158.5°to 155.8°)after immersion for 24 h[82].Cθof a SHPC MAO/ED/silane coating after 48 h was only～123°[111].The ED CeO2/SA coating displayed significant Cθreduction after～15 h(Fig.19C),and the loss of SHPY was attributed to the SA desorption and the porous surface.The coating,however,remained hydrophobic even after 100 h of immersion[50].However,a few studies,such as SHPC La-LDHs/U-PDMS/WO42?-LDHs coating showed Cθ>150°even after immersion for more than 2 weeks[112].A silica-based spray-coated SHPC coating also showed no significant shift of Cθwithin one week of test time[115].The long-term immersion in chloride/acidic solutions typically affects the coating stability primarily due to the desorption of the coated materials and the instigation of corrosion through localized defects and sample edges.

5.3.Thermal

Only a few studies investigated the thermal durability of SHPC Mg alloys[66,76,103].The thermal stability typically depends on the decomposition temperature of the adsorbed low SE material.

A two-step immersion-coated silane/SA modified SHPC sample retained high Cθvalues after 1 h of exposure at 120°C,suggesting the suitably of such surfaces in daily life[76].Electrochemical-etched and silane-modified SHPC surface displayed Cθat the range of 157–162°at 100–260°C(exposed for 2 h).However,as the temperature augmented to 280°C,the Cθreduced to 141°,and that was attributed to the high temperature decomposition of the adsorbed fluorosilane molecules[103].Laser/chemical-etched/SA-modified surface displayed Cθ>150°after heating at 180°C for 30 min[66].

5.4.Air exposure

All the reported works supported the long-term durability of SHPC surfaces during air exposure studies.Several works employed exposure periods of 1 to 4 months that include laser/chemical-etched/SA-coated(air exposure,1 month)[66],HT-treated and HA/SA-coated(indoor air exposure,2 months)[58],one-step HT-treated(air exposure,2 months)[82],ED CeO2/SA-coated(air exposure,2 months)[50],spray-coated SiO2-based(air exposure,2 months)[115],stannate/SAcoated(outdoor air exposure through rainy,windy,sunny weather,16 weeks)[86],and MAO-based(air exposure,4 months)[136]SHPC samples.All these studies showed that the surface retained the SHPY during the exposure periods,with Cθ>150°.Fig.19A shows the representative variation of Cθrecorded for an ED CeO2/SA-coated SHPC sample during 2 months of air exposure[50].

Several studies employed more longer durations that includes Ni-P ELD/HT/SA-coated(air exposure,180 days)[77],La-LDHs/U-PDMS/WO42—LDHs-coated(ambient air,6 months)[112],chemical-etched and oleic acid-modified(air exposure,6 months)[141],one-step ED/MA-coated(ambient air,7 months)[99],one-step immersion-coated Ce/MA(air exposure,1 year)[98],and one-step HT Ni/SA-coated(air exposure,one year)[144]SHPC samples.Irrespective of the methods and materials used,all the samples retained Cθ>150°during the exposure period,as high as 1 year,indicating that SHPC Mg alloys are suitable for several air exposure applications.

6.Superamphiphobic surfaces

A surface having both SHPC and superoleophobic(SOPC)properties,typically known as superamphiphobic(SAPC),has attracted considerable topical research attention.Only a few reports are available on SAPC Mg/Mg alloys.Fundamental details of SOPC and SAPC surfaces are described elsewhere[3–5,17].

Tang et al.has shown that oil Cθas high as 130°was achieved for a chemical-etched and silane-modified SHPC surface[104].Yu et al.reported candle-soot-modified surface with water Cθof～160°and glycerol Cθof～130°.Here,the Mg sample was first corroded in 1M HCl+0.1 M C2H2O4solution and then soaked in 0.1 M K2MnO4to activate the corroded surface,and subsequently flame-soot coated[137].Jeong and Hwang created a SOPC surface by a two-step EAO(first EAO in 0.01 M KHCO3and second EAO in 1 M NaOH),followed by Mg(OH)2formation by HT(180°C,120 min),and SAM formation in 1H,1H,2H,2Hperfluorodecyltrichlorosilane(0.1 vol.% hexane solution).Cθ>150°was measured for glycerol,rapeseed oil,ethylene glycol,and water[127].

Zhao et al.reported SAPC coating by the combination of a compact self-healing epoxy resin(EP)(scarp coating,thickness of 130±5μm)and a porous perfluorodecyl polysiloxane(PF-PS)-modified silica(PF-PS-SiO2)(spraycoating,thickness～13.2±1.3μm).The EP coating has a dense surface,while the silica coating was rough with numerous nanopores.Such an architecture could provide a robust barrier coating along with the required surface roughness for attaining the SHPY.The EP/PF-PS-SiO2coating was SAPC with Cθof 165.8±2.1°and Sθof 1±0.6°for 3.5 wt.% NaCl and Cθof 160±1.8°and Sθof 3.4±0.7°for n-hexadecane[128].The authors in a more recent work compared SHPC and SAPC coatings fabricated by consecutive spray-coating of EP,and hexadecyl polysiloxane-modified SiO2NPs(HD-PS-SiO2)or PF-PS-SiO2.The bilayer coating with EP and HD-PS-SiO2were SHPC(Cθ～165°and Sθ～1°),but superoleophilic(Cθof n-hexadecane～0),whereas the EP/PF-PS-SiO2bilayer coating was SAPC.Fig.20(eg)demonstrate the superamphiphobic of the EP/PF-PS-SiO2coating.Both the SHPC and SAPC coatings displayed superior corrosion resistance as evidenced by the PDP and EIS plots(Fig.20).Theicorrof the EP,EP/HD-PS-SiO2,and EP/PF-PS-SiO2coatings were 6.29×10?7,9.65×10?12and 7.15×10?12A·cm?2,respectively.The inductive loop corresponding to Mg dissolution that observed with the Nyquist plots of the bare sample was not visible with the coated alloys.The semicircular arc of the EP/PF-PS-SiO2coating was most prevalent,higher than that of the EP/HD-PS-SiO2indicative of the excellent corrosion resistance.The highest impedance modulus at low frequencies(～1010Ω·cm2)and higher phase angles at mid-frequencies further confirms the improved protection.The coating maintained the protection efficiency even after 70 days of immersion and 35 days of salt spray[129].

Liu et al.reported a SAPC surface fabricated by combining Ni ELD(85°C,60 min),Ni ED(0.5 M H3BO3+0.4–1.2 M NiCl2·6H2O,15 mA·cm?1,55°C)and perfluorocaprylic acid modification(0.01 M aqueous solution,5 h).The highest Cθs recorded for water and oil were 160.2±1°and 152.4±1°,in that order.The SAPC surface displayed improved corrosion resistance,superior stability over a wider pH range(2–12)and excellent durability during 3 months of air exposure[132].

7.Conclusions and future perspectives

SHPC coatings are projected as the best suitable approach to tackle the weak aqueous corrosion resistance of Mg and its alloys.Despite the significant research outputs,this area demands further R & D to develop industrially relevant coatings with long-term corrosion resistance and excellent chemical and mechanical durability.Here,we systematically analyzed the reported works(2015–2020)and presented in the most accessible way.

The water Cθof the bare Mg/Mg alloys in the reported works varied at the range of 35°to 95°.This significant variation may be associated with the different alloys and the various pre-treatments used.Smooth metal surfaces typically exhibit hydrophilic behaviour due to the high SE.The methods employed for the hierarchical microstructure development such as HT,EAO,MAO etc.often make the surface more hydrophilic due to the enhanced surface roughness/porous structure and the allied capillary effect.Soon after the anchoring of the low SE material,the surface turns out to be SHPC credited to the synergism of surface roughness and the low SE.

A few works achieved SHPY by one-step processing,whereas most of the works employed a two-step processing;the first step for hierarchical surface structuring and the second step for the low SE modification.For one-step processing,the most used method is HT,followed by ED and solution immersion.For the two-step processing,the most used method for the surface structuring is HT,followed by immersion,EAO/MAO and ED/ELD.Other techniques such as chemical/electrochemical etching,laser ablation,spraycoating,scrap-coating,sputtering,electrospinning and electrical discharge machining are also employed.Several works used three-step/multi-step processing,for example,MAO/HTtreatment/low SE modification.

The most used low SE material is the stearic acid.More than 50 % of the works used stearic acid,myristic acid or lauric acid.The second most used are silane and fluorine-based compounds.Other materials such as thiols and oleate and different polymers(polypropylene,polyvinyl chloride)are also used.The most widely used method for imparting the low SE modification is the direct immersion,followed by ED and HT.Methods such as spraying,sputtering,blade-coating,electrospinning and magnet-assisted techniques were also employed.The low SE modification could also act as a sealant coating for the cracked/porous morphology of chemical/electrochemical conversion coatings.

The perfluorinated and organosilane-based chemistries,however,are unattractive due to the toxicity issues.Further studies need to be focused in finding their eco-friendly and efficient alternatives.Naturally,abundant low SE NPs functionalized with naturally extracted low SE fatty acids could be investigated.

The higher activity of Mg alloys could be made use in fabricating the hierarchical surface structure.A few works are reported in this direction where intentional corrosion or salt spray was used to create an adherent thick Mg(OH)2layer.The HT process also results in the development of a thick Mg(OH)2or LDHs film.Corrosion inhibitors can be intercalated with the LDHs film to provide self-healing properties.The EAO or MAO method is advantageous as they can offer a robust thick surface oxide layer.The optimized porous morphology could provide the required surface roughness for the SHPY.Several studies employed a subsequent HT step before the low SE modification to fabricate a desired LDHs coating on the EAO/MAO surface.The higher number of surface–OH groups could aid in the surface functionalization of the low SE compounds having–COOH groups.Several works employed ED/ELD of Cu,Ni-P etc.primarily to inhibit the initial dissolution of Mg samples during the processing steps.

More than 90% of the reported works targeted for enhancing the corrosion resistance.Electrochemical studies in all these reports revealed significantly enhanced corrosion protection for the SHPC sample when compared to the bare.Typically,theicorrwas decreased by 1 to 4 orders of magnitude.However,continuous immersion studies in 3.5 wt.%NaCl in most of the reports revealed that the SHPC sample gets corroded after a few hours or days.A few reports also suggested that more than the desorption of the low SE material,the exposure of the underlying Mg substrate to the aggressive solution through sample edges and defects caused severe corrosion.More works need to be directed in this line to increase the long-term aqueous corrosion resistance of SHPC samples.Spray-coated polymer coatings such as epoxy-incorporated SHPC coatings are particularly attractive.

Self-healing SHPC coating is a striking approach to extend the aqueous corrosion resistance.This area needs to be further investigated.There exists ample scope for developing smart SHPC coatings through the incorporation of novel supramolecules.

Cθvariation as a function of the droplet pH revealed excellent stability for the SHPC surfaces in a wider pH range(1–14).Several studies indicated that SHPY gets compromised at acidic pH below 2.The highly acidic condition could result in rapid desorption of the low SE material.However,only a few studies conducted long-term immersion studies in acidic pH where the Cθwas found compromised after a few hours of immersion.The direct immersion favours localized corrosion through the sample edges and defects.Incorporation of acidresistant materials in the coating could enhance the stability at acidic pH.

SHPC surfaces displayed excellent mechanical durability to some extent.A few studies incorporated functionalized SiO2NPs to enhance the mechanical durability.More studies in this direction are required via incorporation of silane functionalized hard ceramic particles so that the abrasion resistance can be further improved without compromising the SHPY.

On the other hand,all the reports revealed superb durability during air exposure studies.SHPC samples maintained Cθ>150°for more than one year where the Cθremained almost constant throughout the exposure period.

We have also discussed a few works reported in different application areas such as biomedical and anti-icing.The SHPC Mg samples are excellent candidates for bioabsorbable implant applications.More studies in this direction are required to optimize the materials and processing conditions and to develop standard protocols.The electrophoretic deposition could be employed in fabricating biomedical grade SHPC coatings on Mg alloys.

Only a few reports are available on SAPC coatings.Analogous reported works on Al and Ti could be extended for Mg alloys.

Further R & D in this area is expected to result in economic,eco-friendly and simple approaches for developing SHPC coatings with extended corrosion resistance and durability.We also suggest the following areas for further evaluation:(i)highly durable electrochemical/chemical(organic+inorganic)conversion coatings-based SHPC surfaces,(ii)acid-resistant materials-incorporated ELD/EDbased SHPC coatings,(iii)hard ceramic NPs-incorporated epoxy-based SHPC spray-coatings,and(iv)supramoleculesincorporated on-demand release SHPC polymer coatings.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary materials

Supplementary data associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.01.005.