A facile electrodeposition process to fabricate corrosion-resistant superhydrophobic surface on carbon steel

Applied Surface Science 368 (2016) 435–442

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A facile electrodeposition process to fabricate corrosion-resistant superhydrophobic surface on carbon steel Yi Fan a,b , Yi He a,b,∗ , Pingya Luo a,∗∗ , Xi Chen b , Bo Liu b a State Key Lab of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Rd. 8, Xindu District, Chengdu City, Sichuan Province 610500, PR China b School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu City, Sichuan Province 610500, PR China

a r t i c l e

i n f o

Article history: Received 4 November 2015 Received in revised form 10 January 2016 Accepted 27 January 2016 Available online 29 January 2016 Keywords: Superhydrophobicity Electrodeposition Carbon steel Corrosion Self-cleaning

a b s t r a c t Superhydrophobic Fe film with hierarchical micro/nano papillae structures is prepared on C45 steel surface by one-step electrochemical method. The superhydrophobic surface was measured with a water contact angle of 160.5 ± 0.5◦ and a sliding angle of 2 ± 0.5◦ . The morphology of the fabricated surface film was characterized by field emission scanning electron microscopy (FE-SEM), and the surface structure seems like accumulated hierarchical micro-nano scaled particles. Furthermore, according to the results of Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS), the chemical composition of surface film was iron complex with organic acid. Besides, the electrochemical measurements showed that the superhydrophobic surface improved the corrosion resistance of carbon steel in 3.5 wt.% NaCl solution significantly. The superhydrophobic layer can perform as a barrier and provide a stable air–liquid interface which inhibit penetration of corrosive medium. In addition, the as-prepared steel exhibited an excellent self-cleaning ability that was not favor to the accumulation of contaminants. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Carbon steel is almost the largest amount of metal that is used in industry and daily life for its excellent and low-cost properties. However, it is an active metal which tends to corrode easily in most cases. One of the effective methods to prevent steel from corrosion is surface coating technology. Since, it can block the transportation of corrosive medium to the substrate and thus inhibit the formation of corrosion cell. In recent years, a surface wetting behavior which has drew much attention is the effect of Lotus [1,2]. Under the slightest gravitational force, water drops can roll off the Lotus leaves with taking along away pollutants. Such wetting behavior is called superhydrophobicity that water contact angle (CA) of the surface is above 150◦ , and sliding angle (SA) below 10◦ [3]. Therefore, it could consider a similar process in the interface between steel and corrosive matters, corrosion inhibition through repelling matters from the steel surface, and making them lightly roll off under external forces.

∗ Corresponding author at: State Key Lab of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Rd. 8, Xindu District, Chengdu City, Sichuan Province 610500, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. He), [email protected] (P. Luo). http://dx.doi.org/10.1016/j.apsusc.2016.01.252 0169-4332/© 2016 Elsevier B.V. All rights reserved.

Generally, high surface rough structure and low surface energy [4,5] are indispensable for fabrication of superhydrophobic surface. The conventional means for fabricating superhydrophobic surface on metals are trying to create rough microstructure with low surface energy material, such as sol–gel [6,7], anodization [8,9], electrodeposition [10–12], spray-deposition [13], chemical etching [14–19], nanocasting [20] and so on. Whereas, it is hard to accomplish the fabricating high roughness surface with low energy material simultaneously. As a simple method, deposition of assembled hydrophobic molecular layer on rough surface tends to be accepted for superhydrophobic film fabrication [21–23] in practical application. Furthermore, this treatment is easy to control micronanostructure array by adjusting the parameters. Currently, several researches have focused on the application of superhydrophobic coatings as a protective method. According to the theory, it could be supposed that the hydrophobicity can play a role of a barrier against attack by hydrophilicity contact. Results have shown that, in aqueous solution, the corrosion resistance of Cu, Al, Mg, Zn was improved significantly [21,24–31]. In addition, the corrosion resistance of superhydrophobic surfaces in condensing condition has drew much interest. In humid atmosphere, steam condensed between the rough surface structures, and the superhydrophobicity could be damaged. It is required to assess the tendency occurrence of electrolyte film and the surface morphology performs more important role in anti-corrosion performance [25,32]. However, the

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Fig. 1. Wettability of the superhydrophobic steel surface.

feasibility of employing a superhydrophobic film on carbon steel to resist corrosion is still scarce. In this article, a facile and fast electrochemical method is applied to fabricate superhydrophobic surface with hierarchical structures on C45 steel. The influences of the process were investigated, and the properties of the surface were characterized with technologies, such as FT-IR, XPS, CA test, FE-SEM and electrochemical measurements. Based on these, the building process and anti-corrosion mechanism of the superhydrophobic surface on C45 steel were discussed. It is believed that this research would guide the corrosion protection of steel.

adventitious carbon. The water CAs of 5 ␮L droplet on samples were determined by using contact angle meter (KRUSS DSA30S) at room temperature. The average value of three tests at different locations was adopted as the CA. 2.4. Electrochemical experiments

Carbon steel (C45) sheets were cut into 30 mm × 15 mm × 2 mm. A series of chemicals, containing lauric acid, myristic acid, palmitic acid, stearic acid, ferric chloride, ethanol and sodium chloride, were purchased from Kelong Chemical Reagent Co., Ltd. and used as obtained. Water applied in all experiments and tests was acquired from a UPC-III water purification system.

Polarization curves and electrochemical impedance spectra (EIS) were measured with a system (CorrTest CS350) in 3.5 wt.% NaCl solution at ambient temperature. The conventional threeelectrode cell was employed with a platinum plate as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The steel samples of 1 cm2 exposed area was applied as the working electrode. EIS was conducted in the frequency range of 10−2 to 105 Hz with AC amplitude of 5 mV. The potentiodynamic polarization measurements were taken with scanning from 200 mVSCE below OCP (open circuit potential) to 250 mVSCE , at a rate of 0.6 mV s−1 . The parameters of corrosion current density (Icorr ) and corrosion potential (Ecorr ) were calculated from the intersection of the anodic and cathodic Tafel curves employing the Tafel extrapolation way. Before all the tests, the working electrodes were immersed in the corrosive media for 30 min to get a steady OCP.

2.2. Fabrication of superhydrophobic film

3. Results and discussion

Modification of steel surface was made on C45 using onestep electrodeposition method. In a typical step, steel sheets were abraded using emery paper up to 800 and 1200 grade, and then washed with ethanol and blow-drying. After that, the electrodeposition of superhydrophobic film was conducted in a two-electrode cell, in which a platinum wire acted as an anode, steel sheet as cathode. The film was deposited by employing a constant potential ranging from 2 V to 30 V for various duration in ferric chloride/palmitic acid/ethanol solution at room temperature. Afterward, the steel sample was taken out, cleaned with ethanol, and dried with nitrogen flow. The potential was provided by a DC power supply system (ZHAOXIN RXN-605D). If there is no specific explanation, the specimen tested and characterized is prepared in ethanol solution of 0.05 M ferric chloride and 0.1 M palmitic acid at a constant potential of 30 V for 10 min.

3.1. Morphology and wettability

2. Experimental 2.1. Reagents and materials

2.3. Surface characterization Morphology of superhydrophobic surfaces of the C45 steel samples was characterized with field-emission scanning electron microscope (FE-SEM, JSM-7500F). FT-IR spectra of the specimens were measured by using a WQF520 spectrometer. Chemical composition on the sample surface was obtained by X-ray photoelectron spectroscopy (XPS, KRATOS XSAM 800 type multi-function X-ray photoelectron spectrometer, Al K␣ ray), and the binding energies were referenced to the C 1s line at 284.6 eV from

The wettability of the superhydrophobic surface covered sample are presented in Fig. 1. As can be evidenced from Fig. 1, the sample shows superhydrophobic behavior with a water CA of 160.5◦ , and SA is about 2◦ . While, hydrophilic surface of the carbon steel is about 50◦ , and the droplet is stuck even when it turned upside down. From the morphology of superhydrophobic surface, as shown in Fig. 2, it can be concluded that superhydrophobic sample is coated with complicated and accumulated hierarchical micro/nano papillae particles. A accumulated particle presents spherical structure with diameter of 1–2 ␮m, and composed of nano mastoids with size about dozens of nanometers. Obviously, there is much space where air could be trapped between these particles. Consequently, the real touching area among solid surface and water droplet would be decreased, and hydrophobicity of the steel surface is increased significantly. 3.2. Composition of film The FT-IR spectra of superhydrophobic film on the surface and palmitic acid in wavelength range 4500–500 cm−1 is displayed in Fig. 3. In two cases, the corresponding alkyl chain peaks can be found at same wavelength, such as asymmetric and symmetric vibrations of CH2 (2955, 2917, 2848 cm−1 ) [25,28]. However, compared with palmitic acid, the coordinated COO moieties peak

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437

(a)

survey spectra

100000

Intensity (cps)

80000

60000

C1s O1s

Fe2p

40000

20000

0

1000

800

600

400

200

0

Binding energy (eV) 22000

284.6 eV

(b)

C

20000 18000

Intensity (cps)

16000 14000 12000 10000 8000

288.4 eV

6000 4000 2000 296

294

292

290

288

286

284

282

280

278

276

Binding Energy (eV)

Fig. 2. SEM image of superhydrophobic surface.

4000

(c)

Fe

2p3/2

3500

710.6 eV

3000

Intensity (cps)

2p1/2 724.3 eV

2500 2000 1500 1000 500

13.7 eV

0 735

730

725

720

715

710

705

700

Binding Energy (eV) Fig. 3. FT-IR of (a) palmitic acid and (b) superhydrophobic film.

appears at 1588 cm−1 [28]. Moreover, the bonds related to free carboxyl group of palmitic acid disappear in spectrum of superhydrophobic surface, such as the stretching vibration of C O (1098 cm−1 ), stretching vibration of C O (1701 cm−1 ) and out of plane (939 cm−1 ) vibration of OH. Therefore, it is inferred that the as-prepared surface layer formed with low surface energy is iron palmitate. The XPS survey spectra of the superhydrophobic surface on C45 steel is illustrated in Fig. 4. As Fig. 4a shows, it can be concluded that

Fig. 4. (a) The survey spectra of the superhydrophobic surface, (b) C 1s and (c) Fe 2p spectra of superhydrophobic surface.

Fe, O and C are the main composition in the film. Furthermore, C1s spectra of superhydrophobic film presented in Fig. 4b shows two obvious peaks relating to the coordinated COO group (288.4 eV) and alkyl (284.6 eV), which indicate that iron complex with palmitic acid is the origin of carbon. From the Fe 2p spectra (Fig. 4c), the Fe 2p1/2 and Fe 2p3/2 can be found in the binding energy 724.3 eV and 710.6 eV, respectively. The spin–orbit coupling energy gap is 13.7 eV. It is indicated that Fe3+ is the main valence state of iron

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Fig. 5. EDS analysis of the superhydrophobic surface.

Table 1 Contents determined by EDS for the superhydrophobic surface. Element

wt%

Atom%

C O Fe

75.43 17.07 7.50

83.96 14.25 1.79

Fig. 6. Schematic illustration of the formation process.

in the superhydrophobic surface. According to the results of IR and XPS, it can be agreed that iron palmitate was deposited onto the steel surface. Fig. 5 presents the EDS analysis of the powders scraped from the deposit, and the content of elements contained in superhydrophobic layer is listed in Table 1. It was found that the atomic ratio of Fe/C/O was about 1:46.9:7.9. According to the chemical valences of CH3 (CH2 )14 COO− and Fe3+ in the solution, we could infer that the superhydrophobic surface is composed of Fe(CH3 (CH2 )14 COO)3 (Fe/C/O = 1:48:6) [33]. Fig. 6 illustrates the formation process of the superhydrophobic layer which is proposed based on the discussion mentioned above. When the DC voltage was loaded in the system, the Fe3+ ions close to the cathode reacted with palmitic acid to form iron palmitate and hydrogen ions (H+ ). Simultaneously, free H+ ions in the solution increased, and several of them obtained electrons to form H2. Furthermore, the observed H2 which released around the cathode plate would promote the fabrication of the loose micro/nano morphology with superhydrophobicity. The reaction equations can be described as follows: Fe3+ + 3CH3 (CH2 )14 COOH → Fe(CH3 (CH2 )14 COO)3 + 3H+

(1)

2H+ + 2e− → H2

(2)

Fig. 7. The influences of treating methods to the wetting condition. (a) The CA of the treated steel varies as a function of time with different anodic potentials in 0.1 M palmitic acid and 0.05 M ferric chloride ethanol solution. (b) The relationship between wetting condition and treating time in different concentrations of palmitic acid with 0.05 M ferric chloride and applying anodic potential 30 V. (c) Effect of the concentration of Fe3+ on CA with 0.1 M palmitic acid and applying potential 30 V.

3.3. Influences of treating method on the wetting condition of the coated steel So as to optimize the experimental conditions, the effects of different treating methods to the wetting condition of the coated steel have been studied, including anodic potential, electrodeposition time and acid/Fe3+ concentration. The wettability of the treated steel varies as a function of time with different anodic potentials in 0.1 M palmitic acid and 0.05 M ferric chloride ethanol solution is displayed in Fig. 7a. With the applying anodic potential 5 V, the

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Fig. 8. Digital graph of superhydrophobic specimen immersed into 3.5% NaCl solution obtained with oblique angle.

CA of the steel surface held about 120◦ after 10 min treatment, and no obvious changes were observed on the steel surface. While the anodic potential increased to 10 V, the CA maintained around 120◦ at initial 10 min. But after a long time treatment, the CA reached to 150◦ . Obvious roughness have been found on the steel surface. In the case of 30 V, fewer time (10 min) was needed to obtain a superhydrophobic surface, and the CA and SA turned to 160.5◦ , 2◦ , respectively. In conclusion, the formation of superhydrophobic layer is accelerated under more positive anodic potential. In addition, the relationship between wettability and treating time in different concentrations of palmitic acid with 0.05 M ferric chloride, applying anodic potential 30 V, was investigated. As presented in Fig. 7b, it is hard to obtain a superhydrophobicity on the C45 steel surface with low concentration (0.02 M). While, as palmitic acid increased to 0.06 M, the superhydrophobic surface could be obtained within 30 min. Besides, when the concentration of palmitic acid in ethanol solution increased more than 0.1 M, the formation of superhydrophobicity is accelerated significantly. Higher concentration is favorable to the reaction between palmitic acid and Fe3+ . Next, within 0.1 M palmitic acid and applying potential 30 V, effect of the concentration of Fe3+ was studied. As shown in Fig. 7c, when the content of ferric chloride in solution increased more than 0.05 M, the superhydrophobicity could be obtained within 10 min. 3.4. Corrosion inhibition performance In general, as shown in Fig. 8, when the superhydrophobic surface is immersed into the water solution, an exceptionally bright surface is viewed from the side [23–25]. According to the mechanism of total reflection in physics, this phenomenon demonstrates that the air can be trapped in the rough surface. Furthermore, air is a thinner medium between water and solid surface, and light will be reflected totally when it transfers from water to the interface of air with the incidence angle larger than critical angle. Next, the possibility of employing a superhydrophobic surface to protect C45 steel from corrosion in NaCl solution was investigated by electrochemical experiments. As Fig. 9a shows, the difference observed in the polarization curves between superhydrophobic specimen and blank sample is clear. It can be seen that the corrosion potential (Ecorr ) is shifted to the positive region, and the corrosion current density is decreased in the presence of superhydrophobicity. The Ecorr of blank steel is −0.67 V vs (SCE). In contrast, the Ecorr of superhydrophobic steel shifts positively about 0.3 V to the −0.36 V, which suggest a reduced corrosion tendency. The displayed anticorrosion behavior could be almost ascribed to the air pockets trapped in the micro-nano hierarchical structures [34,35]. When the superhydrophobic steel was immersed in 3.5 wt%

Fig. 9. Polarization of EIS results of bare steel and superhydrophobic specimens in 3.5% NaCl solution. (a) Polarization curves, (b) Bode-phase angle vs frequency, and (c) Bode |Z| vs frequency.

NaCl solution, owing to the surface tension between water and the hydrophobic hierarchical structure, it is difficult to penetrate the surface superhydrophobic layer. Therefore, the corrosive media is separated from the steel surface and an air protection shield function is achieved. Moreover, the superhydrophobicity of steel surface could be obtained with kinds of organic carboxylic acids. The steel sheets treated using acids with various lengths of carbon chain were also studied. As shown in Fig. 9a, with increasing number of carbon atoms, the corrosion inhibition performance increased, respectively. The Tafel curves of cathodic and anodic curves were

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Table 2 Corrosion potentials and corrosion current densities of different superhydrophobic surface. Sample

E (V)

I (uA/cm2 )

Steel S12 S14 S16 S18

−0.67 −0.48 −0.38 −0.36 −0.34

16.70 6.70 4.36 2.58 0.85

Fig. 10. Equivalent circuits for EIS results of (a) bare and (b) superhydrophobic steel. Table 3 Electrochemical parameters of equivalent circuit with experimental plots. Sample Steel 12 14 16 18

Rf ( cm2 ) 4924 6305 8206 10,480

Fig. 11. (a) Effect of pH values of water droplet and (b) immersion time in water on the CA and SA of the superhydrophobic steel.

Qf (uF cm−2 ) (n)

Rct ( cm2 )

Qdl (uF cm−2 ) (n)

 (%)

70 (0.9) 40 (0.9) 36 (0.9) 37 (0.9)

691 4133 7811 9588 13,600

1441 (0.8) 1222 (0.8) 595 (0.8) 554 (0.8) 442 (0.8)

92.37 95.10 96.12 97.13

extrapolated to the intersection to calculate the corrosion potential (Ecorr ) and corrosion current density (Icorr ). The slopes of the linear segments were employed to obtain Tafel slopes. Table 2 presents the Ecorr and Icorr of different superhydrophobic surface. The bode plots of the blank steel and superhydrophobic specimens in the NaCl (3.5 wt.%) aqueous solution for 1 h are displayed in Fig. 9b, c. The equivalent circuits are presented in Fig. 10, and furthermore the fitted parameters are listed in Table 3, respectively. The electrochemical behavior of the blank steel of Fig. 9b demonstrates just one time constant. Under such condition, Qdl and Rct is the double layer capacitance and charge transfer resistance, respectively. Rs the solution resistance. In the case of the specimen coated with superhydrophobic iron palmitate film, the equivalent circuit (Fig. 10b) shows two time constant. Qdl implies the solid conduction in the barrier layer, and Rct suggests the impedance corresponded to the interface reactions. As Table 3 shows, the values of Rct in low frequency increase sharply when the superhydrophobic surface built on the steel. What’s more, Rf reflects the electrolyte resistance of the porous surface layer with different sizes of pores, and the surface film’s capacitance which is affected by the factors such as thickness and structure assigns to Qf . Rf of the superhydrophobic specimen increases considerably in comparison with the blank sheet (Table 3). Due to the electrochemically inert areas of

the surface covered with films, the current passed through active micro-porous on the electrode [36,37]. The coverage () of film on C45 steel surfaces is calculated by Eq. (3) [21]. Where Rt0 is the resistance of the blank steel sheet, and Rt (Rt = Rct + Rf ) is the charge transfer resistance of the superhydrophobic specimen.  is the proportion of organic film coverage, and 1 −  represents the active area fraction. The results of coverage efficient of these superhydrophobic specimens are listed in Table 3, respectively. It is reasonable to conclude that the defects of the modified steel surface are decreased significantly. Hence, once the steel surface is superhydrophobic modified, it will has an excellent anticorrosion performance. (1 − ) =

Rt0 Rt

(3)

The chemical stability of the superhydrophobic surface is also investigated. Fig. 11a shows variations of the CA and SA for the superhydrophobic surface as functions of pH value of droplet. The pH value of the water droplet was adjusted by sodium hydroxide and sulfuric acid. It indicates that the sample surface retains superhydrophobicity in pH ranging from 2.0 to 13.0 due to their CA larger than 150◦ and SA less than 10◦ . The as-prepared superhydrophobic surface has good chemical stability in both alkaline and acidic states. While, at pH 1, the CA was less than 150◦ , which reveals that this surface could not support an intense acidity. This may be resulted from the damage of micro/nano structures which was caused by the dissolution of the iron palmitate on the substrate surface. Fig. 11b exhibits the change of the CA and SA as a function of the immersion time in water. The CA is slightly decreased from

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Fig. 12. Process of a 4 ␮L water droplet rolling down the superhydrophobic surface within 0.2 s (the interval of each image is 0.04 s).

Fig. 13. Evolution process of self-cleaning behavior of the superhydrophobic surface.

161◦ to 156◦ and the SA is increased from 2 to 6 as the specimen was immersed in water for 7 days. These results imply that the water immersion does not much affect the surface condition from the wettability. Consequently, the as-prepared steel surface could hold superhydrophobicity as it was immersed in water for several days.

organic acid perform as a frame to provide a stable air–liquid interface and a trapped air barrier is built. On the other hand, the as-prepared steel surface has a small SA which suggests excellent self-cleaning ability. It is believed that the facile fabrication process provides a strategy for carbon steel anti-corrosion in actual industrial applications.

3.5. Self-cleaning effect

Acknowledgements

In general, superhydrophobic surface hold different water adhesion, and the adhesive property is also investigated. As shown in Fig. 12, a 4 ␮L water droplet rolled down from the surface, and it is clear to find that the modified steel surface exhibits an extremely slippery behavior with the SA as low as 2◦ . When water droplets contact this kind of superhydrophobic surface, they slide across the surface straightway within 0.20 s and the interval of each image is 0.04 s. This process evidently demonstrates that the modified steel surface has an excellent water-repellent characteristic. Therefore, such a superhydrophobic C45 steel surface will not favor to the accumulation of contaminants. The self-cleaning effect of superhydrophobic steel surface is studied by employing the chalk dust [24] as contaminants on the surface. The test process of self-cleaning effect is displayed in Fig. 13. Obviously, due to weak adhesion of the particle to the superhydrophobic surface and the joint action of capillary forces caused by water droplet, the water droplet started to roll down immediately with removing the chalk dust. It can be noted that the contaminated surface turn into clean after droplets washing. It infers that this kind of superhydrophobic steel surface has capacity to prevent substrates from aggression in practical application.

YH gratefully acknowledges Prof. H. Wang Analysis and Testing Center of Sichuan University for useful discussions and supports. The authors thank H. Wang for assistance with SEM measurements. The Science and Technology Innovation junior Project in Sichuan Province (2014-072) supported this work.

4. Conclusions In summary, a one-step electrochemical deposition means has been proposed to create superhydrophobic film on C45 steel surface. It is facile to control, and both high concentration of organic carboxylic acid and positive anodic potential can promote the formation of superhydrophobic layer. The surface with high water CA is caused by the hierarchical micro-nano scaled structure of iron palmitate. The chemical composition and specific morphology were characterized by FT-IR, XPS, SEM. The evaluation of potentiodynamic polarization and electrochemical impedance spectra indicate that the corrosion process is effectively decreased by creating a stable superhydrophobic surface. The papillae particles with

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