鋁合金表面用化學刻蝕和陽極氧化法制備的超疏水膜層的耐蝕性能

李松梅李 彬 劉建華 于 美

(北京航空航天大學材料科學與工程學院，空天材料與服役教育部重點實驗室，北京 100191)

鋁合金表面用化學刻蝕和陽極氧化法制備的超疏水膜層的耐蝕性能

李松梅＊李 彬 劉建華 于 美

(北京航空航天大學材料科學與工程學院，空天材料與服役教育部重點實驗室，北京 100191)

通過化學刻蝕和陽極氧化在AA2024鋁合金表面制備超疏水表面。當化學刻蝕時間超過3 min時，表面在很寬pH值范圍內顯示出水靜態接觸角大于150°。SEM和AFM照片表明化學刻蝕時間決定了試樣表面形貌和粗糙度。FTIR用來研究氟硅烷(G502)與AA2024表面的結合。結果說明FAS(氟硅烷)分子與鋁合金表面的三氧化二鋁發生反應，并在陽極氧化膜層表面展示出優異的結合性能。超疏水表面的耐腐蝕性能通過在質量分數為3.5%的NaCl溶液中進行動電位極化和交流阻抗(EIS)測試。電化學測試結果和等效電路模型顯示出超疏水表面顯著改善抗腐蝕性能。

超疏水；化學刻蝕；陽極氧化；耐腐蝕

Superhydrophobic surface with a water contact angle of more than 150°has drawn a great deal attention because of its potential application in the industrial area and biological process[1],such as selfclearing material[2],anti-icing coating[3],corrosion-free coating[4-5]and so on.In nature,there are many living things with superhydrophobic surfaces,such as lotus leaf,butterfly wing,etc.From the lotus leaf,we know that the superhydrophobicity of a material depends on not only its surface energy but also its surface morphology[1].In the past decade,many methods were developed to fabricate superhydrophobic surfaces andreferences were in the field of nano-printing[6],electrospun[7], sol-gel[8]and so on. Currently,fabrication of superhydrophobic surfaces on metal has attracted considerable research attention.For example,Wu and co-workers[9]obtained superhydrophobic surface assembly of FAS (fluorinated agent silane)molecules on rough morphology created by chemical etching.Femtosecond Laser ablation was used by Kietzig[10]et al. to create roughness on steel.Wettability of the roughness steel transformed from hydrophilicity to hydrophobicity by laying the specimen in the natural environment,and reached superhydrophobicity when the lay time was over 50 days.

Aluminum and its alloy have excellent physical and mechanical properties such as low density,good electromagnetism and high strength/weight ratio.Thus,they are expected to find applications in various industries such as aerospace and automobile.However,the poor corrosion resistance limits their application.Most of the corrosion occurs when the metallic matrix contacts with water and oxygen or other corrosion environment.One of the most effective corrosion protections for aluminum alloy is to treat the metal or alloy with chromium.However,chromium is toxic and harmful to the environment.Superhydrophobic surface treatment is one of the efficient strategies to protect aluminum alloy from corrosion because the surface is water repellent and separates the metallic matrix from water and corrosion environment.In ourprevious study[4],superhydrophobic surface was fabricated on aluminum alloy by anodization and self-assembly,butthe method was time consuming and demanded more energy.

Here we report the preparation of superhydrophobic surface by chemical etching,anodization and self-assembly of FAS molecules.The static water contact angle was measured in wide pH value range.FTIR was employed to investigate the AA2024 surface combination of the fluorinated agent silane (FAS)molecules.Corrosion resistance of the superhydrophobic surface was estimated by electrochemical measurements in 3.5wt% NaCl aqueous solution.

1 Experimental

1.1 Preparation of superhydrophobic surface on Aluminium alloy AA2024

Aluminum alloy AA2024(composition:4.5%Cu,1.5%Mg,0.5%Fe,0.6%Mn,0.5%Si,0.5%others and Al is the rest)with a size of 60mm×40mm×3mm was used as the substrate.The substrates were ground by emery paper (No.100,500,1000,grit sizes were 165,25,13 μm,respectively)gradually,and then ultrasonically cleaned in acetone and distilled water for 10 min,respectively.Diluted hydrochloric acid(VHCl∶VH2O=1∶1)was used as chemical etching solution at 15 ℃ .Chemical etching time was 2～4 min.Anodization process was conducted in the solution with 45 g·L-1sulfuric acid and 10 g·L-1boracic acid.The anodizing parameters were 0.6 A·cm-2,25 ℃,20 min.After anodization,the samples were immersed in 100 mL FAS solution containing 0.6 g FAS(G502,C13F12H18SiO3),40 mL methanol and 60 mL H2O(prepared by stirring for 4 h at 30℃)for 2 h at 40℃.

1.2 Characterization of surfaces

Water contact angles for as-prepared surfaces were estimated with opticscontactangle meter(Dataphysics OCA20)based on a sessile drop measuring method.The volume of the test water droplet was 6 μL.The contact angle of samples was obtained by averaging fivedifferentpoints.The surface morphologies of the prepared samples were estimated by scanning electron microscope(FE-SEM,Apllo 300,Japan)and atomic force microscope(AFM,Veeco,MutiModeNanoscope Ⅲ a,USA).SEM accelerating voltage was 15 kV.The AFM test was Tapping Mode and test area was 15×15μm.Chemical bonds were characterized by Fourier transform infrared spectroscopy(FTIR;NEXUS-470,Nicolet).

1.3 Electrochemical measurements

Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)measurements were used to estimate the corrosion resistant of superhydrophobic surfaces.Electrochemical workstation (Princeton Applied Research 2273)was employed to test electrochemical measurements based on three electrode system. All electrochemical measurements were performed in 3.5wt% NaCl aqueous solutions at room temperature.Before electrochemical measurements,the specimens were immersed in the aqueous solution for 10 min to obtain a stable surface.The prepared surface was used as the working electrode with test area of 3.14 cm2.A saturated calomel electrode was used as the reference electrode and a platinum sheet was used as the counter electrode.Potentiodynamic polarization curves were subsequently measured with respect to the open circuit potential(OCP)at a scanning rate of 2 mV·s-1from -0.5 V to 1 V.Electrochemical impedance spectroscopic measurements were conducted in the frequency ranges between 10 mHz and 100 kHz with a sinusoidal perturbation of 10 mV.The program Zsimpwin 3.2 was used to obtain fitting parameters based on equivalent circuit.

2 Results and discussion

2.1 Fabrication of Superhydrophobic surfaces

Schemes for the sample fabrication are shown in Fig.1.All of the samples were assembled by FAS molecules.The sample only treated by chemical etching for 2,3 and 4 min is denoted as CE2,CE3,CE4,respectively,and the sample only anodized for 20 min is denoted as A,and the samples treated by chemical etching for 2,3 and 4 min and anodization for 20 min is named as CE2A,CE3A and CE4A,respectively.Fig.2 shows the water contact angle measurement on samples obtained by different treatments under wide pH value range.It can be seen clearly that the surfaces anodized for 20 min (A)has the lowest water contact angle from 90°to 100°in all pH value ranges.The surface chemical etching for 2 min(CE2)has water contact angle values from 132°to 139°,and exhibits hydrophobic property,so is the surface treated by chemical etching for 2 min and by anodization for 20 min (CE2A).The contact angle is greater than 150°if chemical etching time for the sample is over 3 min (CE3,CE4,CE3A and CE4A).The water contact angle has a little decrease when the pH value of water is above 9.

Thus we can conclude that in the process of superhydrophobic surfaces preparation, the determining factor is chemical etching.The wettability of the surfaces changes with chemical etching time.The wettability ofthe surfaces becomes more hydrophobic by extending chemical etching time.Watercontactangledoesnotshow anyfurther variations when chemical time is over 3 min.As shown in Fig.1,the morphology of the surfaces is altered by chemical etching.These can be observed by FE-SEM photographs and AFM images in the next section.The surface chemical property is changed by anodization and self-assembly.This will be discussedin the chemical characterization section.

2.2 Morphology of the surfaces

FE-SEM photographs and 3-D top AFM images of several samples are shown in Fig.3.(a),(b)and(c)are the FE-SEM photographs for sample CE2,CE3 and CE4,respectively.It can be seen from Fig.3(a)that the surface of the sample by chemical etching for 2 min is not destroyed totally.There exist platforms from the pretreatment in preparation and a few grooves from etching by dilute chlorhydric acid on the surfaces.As a contrast,Fig.3 (b)and (c)are the images of CE3 and CE4.The surfaces are completely destroyed by dilute chlorhydric acid and become rough.There are irregularly shaped particles on thesurfaces.Fig.3(d),(e)and (f)are the 3-D top AFM images of the sample CE2,CE3 and CE4,respectively.From the AFM images we know the RMS(Roughness Measurement of the Surface)of CE2,CE3 and CE4 is292 nm,646 nm and 761 nm,respectively.When chemical etching time is just 2 min,the RMS is only 292 nm.The RMS value of CE3 is 646 nm,which is double of the sample CE2.The RMS value has continued to increase with chemical etching time.There is no difference between CE2(Fig.3(a))and CE2A(Fig.3(g)),and there is also no distinction between CE3 (Fig.3(b))and CE3A(Fig.3(h)).It demonstrates that the self-assembly does not change the morphology of aluminum alloy surfaces when compared the Fig.3(h)and(i).In summary,chemical etching plays an essential role in changing morphologies and RMS.In contrast to chemical etching,anodization does not have any effects on the morphology.When chemical etching time is 3 min,the surface of aluminum alloy is destroyed by dilute chlorhydric acid,and the water contact angle of the surfaces reaches 150°no matter the specimen is treated by anodization or not.

2.3 Chemical characterization of superhydrophobic

Fig.4 shows the FTIR spectra of several samples.From the whole spectrum of the sample anodized without self-assembly of FAS molecules(Fig.4(a)),one can see that there is only one peak at 1 138 cm-1due to Al-O-Al stretching modes.Fig.4(b)is the spectrum of the sample self-assembled by FAS molecules on anodization AA2024,there are two peaks at 1 127 cm-1and 1 160 cm-1,assigned to Si-O and Al-O-Si,respectively.There is one more peak at 1 245 cm-1assigned to-CF2and-CF3.These peaks demonstrate thatFAS moleculesareassembled on anodized AA2024.There are no any peaks in the spectrum for the sample self-assembled by FAS molecules on chemical etched AA2024 (CE3)as shown in Fig.4(c).It demonstrates that there are little FAS molecules on chemical etched AA2024 (CE3).There are three peaks for the sample self-assembled by FAS molecules on chemical etched and anodized AA2024 (CE3A)at 1 116 cm-1,1 141 cm-1and 1 241 cm-1,respectively,almost the same as that of the self-assembled FAS molecules on anodized AA2024,which is obvious in Fig.4(b)and(d).The peak at 1 241 cm-1is assigned to-CF2and-CF3,and the peaks at 1 116 cm-1and 1 141 cm-1are assigned to Si-O and Al-O-Si,respectively,the same as that of the self-assembled FAS molecules on anodized AA2024.These peaks demonstrate that FAS molecules are assembled on the sample treated with chemical etching and anodization.

It can be seen from the FTIR spectra that the FAS molecules are assembled on the specimen treated by anodizing.Anodization film on aluminum alloy is a must for self-assembly. Fadeev et al[11]have demonstrated that the FAS molecules reacted with the hydroxyl group on the solid surface have several modes.Hydroxyl group is the pre-requirement for FAS moleculestoreactwith solid surfaces.Takahiro Ishizaki and his co-workers[5,12]have assembled fluoroalkylsilane molecules on magnesium alloy coated with nano-structured cerium oxide lm.The hydroxyl group on the cerium oxide is bonded with fluoroalkylsilane.Liu et al[13]used n-tetradecanoic acid(CH3(CH2)12COOH)to assemble on the copper sheet treated with 7 mol·L-1HNO3for 30 seconds to activate surfaces.The above examples demonstrate that the hydroxyl group is the most important factor inself-assembly.In our study,the hydroxyl group for assembly is provided by anodization,and there are little FAS molecules assembled on the samples treated by chemical etching only.

2.4 Corrosion resistant performance of the superhydrophobic surface

The corrosion resistant performance of the superhydrophobic surfaces was investigated in NaCl aqueous solution from the electrochemical point of view.Fig.5 shows potentiodynamic polarization curves of(a)bare Al,(b)anodization,(c)CE2A and(d)CE3A immersed in 3.5wt%NaCl aqueous solution.As compared to the corrosion current density (jcorr)of the bare aluminum alloy (1.386×10-7A·cm-2),that of the specimen treated by anodization (1.15×10-10A·cm-2)decreases by more than three orders of magnitude.The jcorrvalues of the surfaces CE2A and CE3A are estimated to be 2.89×10-10A·cm-2and 8.509×10-12A·cm-2,respectively.It should be noted that the jcorrvalue of CE3A decreases by five orders of magnitude compared with that of bare aluminum alloy.This supports the conclusion that the superhydrophobic treatment is effective for improving the corrosion resistance ofaluminum alloy.In addition,the corrosion potential (Ecorr)of the bare aluminum,anodization samples,CE2A and CE3A are-637 mV,-498 mV,-420 mV and-577 mV,respectively.As compared to the Ecorrvalues of the bare aluminum specimen,thatofthe superhydrophobic surfaces(CE3A)are shifted to the positive direction.The significant shift of the Ecorrto the positive direction could be attributed to an improvementin the protective properties of the superhydrophobic surfaces on aluminum alloy.We can draw the same conclusion from the jcorrand Ecorrnumerical data that the corrosion resistant performance of aluminum alloy is greatly improved by superhydrophobic surface treatment.

Fig.6 presents the EIS bode plots of the samples of (a)bare Al,(b)anodization,(c)CE2A and(d)CE3A immersed in 3.5wt%NaCl aqueous solution.Generally,we consider the|Z|values at low frequency as some point of anticorrosion performance.The|Z|values at 10 mHz of(a)bare Al,(b)anodization,(c)CE2A and (d)CE3A immersed in 3.5wt%NaCl aqueous solution are 4.94 kΩ·cm2,3.20 MΩ·cm2,15.4 MΩ·cm2and 33.2 MΩ·cm2,respectively.The|Z|value of superhydrophobic surfaces(CE3A)at 10 mHz decreases by 4 orders of magnitude when compared with that of the bare aluminum.It should be noted that the |Z|value of superhydrophobic surfaces(CE3A)in 10 mHz is just two times that of CE2A,however,it is decuple that of anodization.These results indicate that superhydrophobic treatment tremendously improves anticorrosion performance.As compared to the|Z|value of the specimen treated with anodization,that of the specimen treated with both chemical etching and anodizing (CE2A,CE3A)raises one order of magnitude.It indicates that changing wettability of aluminum alloy surfaces could improve their anticorrosion performance.

It can be seen from Fig.6 that the impedance spectra of bare aluminum and the specimen treated with anodizing have two capacitive loops at medium and low frequency.For the specimen treated with anodizing,the medium loop shifts to higher frequency,and the low loop shifts to lower one.This is because that the structure of aluminum surfaces has been changed by anodization and self-assembly of FAS molecules.The medium loop can be attributed to the natural oxide (anodization and self-assembly of FAS molecules)films on the electrode surface,while the other loop can be attributed to the double layer capacitance.The impedance spectra of CE2A and CE3A have a similar plot with three capacitive loops at high,medium and low frequency,respectively.The loop at high frequency can be attributed to air layer between solid surfacesand solution created by roughness structure on superhydrophobic surfaces,the medium one can be attributed to anodization and selfassembly,and the last one can be due to double layer.This conclusion is well in agreement with the FE-SEM image results.

To further determine accurate analysis of the impedance data,the equivalence circuit models are proposed.As shown in Fig.7,the equivalent circuit model(a)is for bare aluminum and anodization(A),and (b)is for CE2A and CE3A immersed in 3.5wt%NaCl aqueous solution.In these circuit models,Rct||Cdlis assigned to the impedance of the interface reaction between the films and substrate,Rc||Ccis assigned to the impedance of the interface

Cair||Rairis assigned to the air layer from reaction between theelectrolytic solution and films,and trapped by rough structures of the superhydrophobic surfaces.There are no pores on bare aluminum and anodization (A)specimen,so the equivalent circuit model (a)can be used to fitting them.Equivalent circuit model(b)can be used for fitting hydrophobic surfaces.The Rctvalues obtained from the fitting results as a corrosion resistant emblem are shown in Table 1.The Rctvalues of bare aluminum,anodization(A),CE2A and CE3A are 4.44 kΩ·cm2,1.85 MΩ·cm2,12.3 MΩ·cm2and 34.2 MΩ·cm2,respectively,almost with the same order of magnitude as|Z|values of electrochemical impedance spectroscopy at 10 mHz.It indicates that our equivalent circuit models are well suited for the electrochemical impedance spectroscopy.Compared the specimen treated with anodization to the bare aluminum,Rchas increased by 3 ordersofmagnitude,and Rctvalue hasalso increased by 3 orders of magnitude.These results indicate that anodization and self-assembly of FAS molecules improve anticorrosion resistance of aluminum,the same conclusion as the|Z|values for electrochemical impedance spectroscopy at 10 mHz and the potentiodynamic polarization curves.Compared the Rctvalues of CE2A and CE3A with theRctof anodization(A),it can see that the anticorrosion performance is improved by wettability ofthe aluminum alloy surfaces,which is the same as|Z|values for electrochemical impedance spectroscopy at 10 mHz.When compared the Rairvalues of CE3A with that of CE2A,it is also found that the anticorrosion performance is influenced by wettability of aluminum surfaces.

Table 1 Electrochemical model impedance parameters from Nyquist plots of different samples

3 Conclusions

This paper has demonstrated a convenient and effective method to prepare superhydrophobic surfaces by means of chemical etching and anodization.A static water contact angle of more than 154 at a wide pH value range can be obtained when the as-prepared surfaces are self-assembled by FAS molecules.Chemical etching time is the critical factor for the surface morphology and the water contact angle.Anodization is a necessary process for fabrication of superhydrophobic surfaces. Moreover, the superhydrophobic surfaces exhibit excellent anticorrosion performance compared with the anodization samples and the bare samples when just applying immersion in the corrosion environment for 30 min.

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Corrosion Resistance of Superhydrophobic Film on Aluminum Alloy Surface Fabricated by Chemical Etching and Anodization

LI Song-Mei＊LI Bin LIU Jian-Hua YU Mei
(Key Laboratory of Aerospace Materials and Performance,Ministry of Education,School of Materials Science and Engineering,Beihang University,Beijing 100191,China)

A superhydrophobic surface was fabricated by chemical etching and anodization on AA2024 aluminum alloy.A static water contact angle of more than 150°was achieved at a wide pH value range when the surface chemical etching time was more than 3 min.The SEM and AFM images showed that the surface morphology and roughness were dependent on chemical etching time.The FTIR results indicated that the FAS (fluorinated agent silane)molecules reacted with alundum on the aluminum alloy surface and the surface exhibited excellent adhesion performance on the anodization specimen.The corrosion resistance of the superhydrophobic surfaces was estimated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)measurements in 3.5wt%NaCl aqueous solution.The electrochemical measurements and appropriate equivalent circuit models revealed that the anticorrosion performance was greatly improved by the superhydrophobic surface.

superhydrophobic;chemical etching;anodization;anticorrosion

O647.5；O614.3+1

A

1001-4861(2012)08-1755-08

2011-11-17。收修改稿日期：2012-03-23。航空科學基金(No.20110251003)資助項目。

＊通訊聯系人。E-mail：songmei_li@buaa.edu.cn