Bioinspired Composite Coating with Extreme Underwater

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Bio-inspired composite coating with extreme underwater superoleophobicity and good stability for wax prevention in petroleum industry Weitao Liang, Liqun Zhu, Weiping Li, Xin Yang, Chang Xu, and Huicong Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03234 • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 22, 2015

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Bio-inspired composite coating with extreme underwater superoleophobicity and good stability for wax prevention in petroleum industry Weitao Liang, Liqun Zhu, Weiping Li, Xin Yang, Chang Xu, Huicong Liu*

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China

KEY WORDS: Bio-inspired; Wax prevention; Wetting behavior; Superoleophobicity. ABSTRACT. Wax deposition is a detrimental problem that happens during the crude oil production and transportation, which greatly reduces the transport efficiency and causes huge economy loss. To avoid wax deposition, a bio-inspired composite coating with excellent wax prevention and anti corrosion properties is developed in this study. The prepared coating is composed of three films, including an electrodeposited Zn film for improving corrosion resistance, a phosphating film for constructing fish-scale morphology and a silicon dioxide film modified by simply spin coating method for endowing surface with superhydrophilicity. A good wax prevention performance has been investigated in a wax deposition test. The surface morphology, composition, wetting behaviors and stability are systematic studied and a wax prevention mechanism is proposed, which can be calculated to “water film theory”. This composite coating strategy which shows excellent property in both wax prevention and stability is expected to be widely applied in petroleum industry.

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1 Introduction Crude oil, also known as the blood of the industry, is a very important industrial material for the wide application in energy, synthetic and many other areas1. It’s important to ensure the adequate supply of crude oil. Wax deposition is a detrimental problem that happens during the crude oil production and transportation. Wax deposited in the pipeline would reduce the effective cross area and raises the flow resistance, which leads to short supply and great economy loss2,3. The industry and economic demands emphasize the need of developing new effective method to avoid wax deposition. Up to now, many researchers have developed various different methods to solve the problem, such as paraffin inhibitor4, microbial5, magnetic fields dewaxing6 and anti-wax coatings7-12. However, most of these methods are either inefficient or costly. Among the methods mentioned above, anti-wax coatings have attracted much attention due to the simple fabrication, low cost and wide application. Many materials have been employed to fabricate coatings for wax prevention，such as cement8, glass9, polymer10, amorphous materials11 and metal oxide12. However, the wax prevention performance of those coatings still needs to be improved7. It is necessary and urgent to develop effective self-cleaning surfaces for wax prevention, especially in a facile and low-cost way. Besides, for the application in industry, corrosion resistance, high-temperature resistance and good mechanical property are also required. Our group has committed to develop efficient and low-cost wax prevention coatings for several years13-18. In our previous work, chemical conversion coatings with different surface morphology and containing different elements were fabricated on carbon steels to prevent wax from deposition, and good performance was investigated13-16. Besides, an alternate current etching method was also used to fabricate flower-like conversion coatings on carbon steel to avoid wax deposition17. It’s known


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that the pipeline would experience serious corrosion threatening during the working duration19. However, all the methods mentioned above, both chemical conversion and alternate current etch, would destroy the original surface structure of the carbon steel during the preparation duration and lead to a reduction of the corrosion resistance and mechanical property, which greatly limit their practical applications. In addition, the mechanism of wax prevention was concluded to “water film theory”, which indicates that a stronger water film is expected in anti-wax coatings. In recent years, inspired by the self-cleaning phenomenon in nature, such as seabird, shell and fish scale, bio-inspired materials have incited broad attention20-23. In this work, inspired by the self-cleaning ability of fish scale in oil-polluted water, we first report a facile fabrication of composite coatings with fish-scale morphology on carbon steel used in pipeline, which present both excellent performance in wax prevention test and anti-corrosion test. Fish scales are composed of calcium phosphate, protein, and a thin layer of mucus that leads to their hydrophilic behavior23. Inspired by the special micro structure and surface composition of the fish scale, the prepared coating is composed of three films, including an electrodeposited Zn film for improving corrosion resistance, a phosphating film for constructing fish-scale morphology and a silicon dioxide film modified by simply spin coating method for increasing the surface superhydrophilicity. To further understand the mechanism of wax prevention, bare carbon steel, only Zn coated carbon steel and phosphating coating are also fabricated. The surface morphology and composition of all the specimens are systematic studied and wetting behaviors of the prepared specimens are also investigated. A possible wax prevention mechanism based on “water film theory” is proposed. The “water film” in this work, attracted by hydroxyls of the modified silicon dioxide on the specimen surface, is stronger than that attracted by Van der Waals force and it is prone to better anti-wax property. Besides, the corrosion


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resistance, high-temperature resistance and mechanical property of the composite coating were also tested and good performance was investigated. This composite coating is expected to widely application in petroleum industry. 2 Experimental 2.1 Materials All the chemicals used in this work were of analytical grade without further treatment. Tetraethoxysilane (TEOS), ammonium hydroxide, absolute ethanol and Tween-20 were obtained commercially and used as received. A3 carbon steels with the size of 20 mm×40 mm×2 mm were used as substrates. The A3 carbon steels were polished by sandpaper and rinsed by deionized water, degreased by acetone, and then etched in the aqueous solution of 30% HCl for 30 s.

2.2 Preparation of the composite coatings


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Figrue.1 (A) Digital image of the real fish scales; (B) SEM image of prepared fish-scale like composite coating (insets are the digital images of water contact angle and oil contact angle); (C) Schematic diagram of the prepared composite coating.

The Zn coating was prepared by electrodepodition method in the bath containing 2 M ZnSO4·7H2O, 0.04-0.05 M Al2(SO4)3·18H2O, 0.08-0.1 M KAl(SO4)2·12H2O and 0.02 M surfactant. The phosphating process was carried out in a phosphating agent containing Mn(H2PO4)2·2H2O, Fe(H2PO4)2, MnPO4 and Zn(NO3)2. The silicon dioxide film was prepared by spinning method using a self-prepared silicon dioxide sol, which has been reported to be hydrophilic24. At last, the prepared specimens were dried naturally in air while waiting for further research of the properties.

2.3 Wax deposition test The crude oil for wax deposition test was from Daqing Oilfield (in Hei Longjiang province, China), and the main characteristics are shown in Table 1, indicating a typical of waxy crude oil. The crude oil contains about 50 wt% water, which is consistent with most of the actual situations. The wax deposition procedure was simulated in the laboratory using a self-designed apparatus based on the cold-finger method25,

26

. The deposition apparatus included four basic parts: temperature

controller system (thermostat water bath and circulating watercooling), stirring system, waxy crude oil and washing system, as shown in Supporting Information Figure S1. In the wax deposition test, crude oil was heated to 80 °C in a water bath and stirred thoroughly (ω = 20 rad·S-1) to ensure complete dissolution and well-distribution of wax. Then the specimens were fixed onto the inner wall of the container. After the oil was kept at 80 °C for 0.5 h, it was cooled by circulating water cooling (water temperature is 25 °C). It takes about 2 hours until the oil temperature decreased to


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30 °C. After that the specimens were taken out. The quantitative determination of the wax prevention deposition weight, DW in abbreviation, was calculated according to Eq. (1):

DW =

(Wt − W0 ) × 100% S0 (1)

Where Wt is the weight of the wax deposited specimen; W0 is the weight of the original specimen; S0 is the surface area of specimen. For anti-wax treatment specimens, a less DW value means a better wax prevention property. Table 1 Characteristics of the waxy crude oil. parameter

value

Wax (wt%)

≈26

Resins (wt%)

≈12

WAT (oC)

≈43

Pour point (oC)

≈32

2.4 Corrosion resistant test The electrochemical measurements were carried out in 3.5 wt.% NaCl solution at room temperature using a conventional three-electrode cell using a saturated calomel electrode (SCE) and a platinum wire as reference and counter electrodes, respectively. The specimen worked as the working electrode area was 1 cm2. The pH was 7.2±0.2 before the corrosion tests. First the open circuit potential (OCP) was measured in order to reach a stable potential (all potentials are given versus SCE). Then, polarization curves were recorded by linear sweep voltammetry (LSV) at a rate of 1 mV/s. All electrochemical tests were conducted in triplicate in order to ensure the reproducibility of results27.


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2.5 Characterization Digital images of the specimens before and after the wax deposition test were obtained by a digital camera (Olympus, E-PL1). A scanning electron microscope (SEM, JSM-7500F, JEOL Ltd., Japan) was used to observe the surface morphology of the obtained specimens. Before the observation, the specimens were sputter-coated with Au under the vacuum conditions for electric conduction. The crystallographic characterization was investigated by X-ray diffractometer (XRD, ARL XTRA, Themo Electron CO., Switzerland) with Cu K radiation at a scan speed of 6o/min in the 2 theta range from 15o to 90o. The contact angles of bare water on the specimens were measured using a contact angle meter (DSA 20, Krüss Instruments GmbH) on five different positions for each sample surface. The volume of an individual droplet in all measurements was 5 µL. The contact angles of oil on the specimens in water were also measured by the underwater−oil CA (OCA) test. The specimens were upside down in a water-filled glass container because the density of oil was lower than water. First, specimens were immersed in hot water (T ≈ 80 oC to keep crude oil at the liquid state), and then a crude oil droplet was gently injected by a microsyringe on the surfaces, as shown in Supporting Information Figure 2. The OCA can be calculated by Eq. (2)28:

tan

θ 2

=

2h D (2)

Where θ is the OCA, h is the height of the oil droplet and D is the length of the oil droplet contact the specimen surface, which can all be measured directly in the digital image of the OCA.

3 Results and Discussion 3.1 Chemical and structural characterization


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Figure 2 SEM images of (A) bare carbon steel; (B) Zn coated carbon steel; (C) phosphating film coated specimen and (D) Silicon dioxide sol modified specimen; (E) Schematic diagram of the preparation of the composite coating.

Figure 3 XRD patterns of (a) bare A3 carbon steel; (b) Zn coated carbon steel; (c) phosphating film


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coated specimen and (d) Silicon dioxide sol modified specimen. Figure 2 demonstrates the SEM images of the surface morphology in different preparation periods, and the corresponding XRD patterns were shown in Figure 3. The bare A3 carbon steel was relatively smooth with a few significant scratches, which can be attributed to mechanical polishing, as shown in Figure 2 (A). The XRD pattern of Figure 3 (a) shows obvious peaks at 45.07o, 65.19o and 82.35o, indicates it is mainly composed of Fe (PDF=01-1267), which is corresponded to A3 bare steel. With a 15 min electro-deposition, uniform particles with hexagon morphology were obtained, as shown in Figure 2 (B) (high-resolution SEM images were shown in supporting information Figure S3). The generated hexagon particles were evenly distributed on the surface with diameter of about 2 micrometers. The hexagon morphology is corresponded to the typical Zn particles, and the result of XRD pattern in Figure 3 (b) also indicates that the new generated film is Zn (PDF=65-3358) for the well matched peaks at 36.29o, 38.99o, 43.22o, 54.32o, 70.63o and 86.54o. After immersing in the phosphating solution for 15 min under 50 oC, the specimen surface was covered by phosphating film with fish scale like morphology, which was reported to present excellent property in self-cleaning20, 23

. The peaks at 20.17o, 25.88o, 31.64o and 34.44o in Figure 3 (c) match well with the characteristic

peaks of Zn2(FeMn)(PO4)2•4H2O (PDF=17-0747), which indicates that the fish scale like phosphating film is mainly composed of Zn2(FeMn)(PO4)2•4H2O. The surface morphology modified by silicon dioxide is shown in Figure 2 (D), which demonstrates that the surface morphology doesn’t change a lot (The SEM image of silicon dioxide sol modified specimen at high magnification is shown in Supporting Information Figure S3 (C)). Obvious fish scale morphology can also be found on the surface. In addition, the XRD pattern of Figure 3 (d) presents almost the same peaks with Figure 3 (c), which means that it is hard to confirm the modified silicon dioxide film by SEM images


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or XRD patterns.

Figure 4 The (A) FTIR and (B) Raman spectra of the prepared specimens (a) before and (b) after silicon dioxide modification. To further investigate the surface chemical composition of the prepared composite coating, the FTIR and Raman spectra were measured and the results were shown in Figure 4. The bands at 1050 cm-1 and 1230 cm-1 in Figure 4 (A) (a) match well with the characteristic bands of P-O and P=O in PO4-3, indicating the formation of Zn2(FeMn)(PO4)2•4H2O by phosphating. The broad vibration bands in the Figure 4 (A) (b) at 1064 cm-1 and 808 cm-1 are corresponded to Si-O-Si stretching vibration of silicon dioxide, which indicates that the surface was covered by silicon dioxide after modification. While the vibration bands at 935 cm-1 and 1217 cm-1 would be characteristics of hydroxyl bands in Si-OH. Meanwhile, the Raman spectrum shown in Figure 4 (B) further illustrates the existence of silicon dioxide. The bands at 917 cm-1, 985 cm-1 and 1017 cm-1 in Figure 4 (B) (a) are corresponded to P-O in PO4-3 and the bands at 600 cm-1 and 1000 cm-1 in Figure 4 (B) (b) are assigned to the silicon dioxide. Therefore, it can be concluded that after modification, the substrate was covered by silicon dioxide with hydroxyl group. Above all, it’s reasonable to assume that the prepared coating was covered by silicon dioxide with hydroxyls after modification by spinning.


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3.2 Anti-wax performance and wetting behaviors. The wetting behavior of surface greatly affects the wax prevention performance17. To better observe the connection between the wetting behavior and wax prevention property, the contact angles (CAs), underwater−oil contact angles (OCAs) and the wax deposition weight on unit area (DWs) of the different specimens were measured and the results were shown in Figure 5. The corresponding values were also shown in Table 2. Figure 5 (A) (a) shows the CAs of different specimens. The carbon steel is hydrophilic with CA of 51.1±1.2o for metal is high surface energy material and the hydroxyl group in water molecules can be easily absorbed on the surface29. After coated by Zn film, the CAs decreased for the Zn film is rougher than the bare carbon steel. Surface roughness is another key factor influencing the wetting behavior and rough surface is prone to decrease the contact angles29, 30. The contact angle further decreased for the phosphating treatment further increase the roughness of the surface, as shown in Figure 2 (C). After coated by silicon dioxide with hydroxyls, the surface morphology was not changed a lot while the CA decreased obviously to 2.0±0.5o. It can be attributed to the existence of hydroxyls on the silicon dioxide film (as shown in Figure 4), which can easily attract the hydroxyl of the H2O and improve the hydrophilicity31. Table 2 the CAs, OCAs and DWs of different specimens: (a) bare A3 carbon steel; (b) Zn coated carbon steel; (c) Phosphating film treated specimen and (d) Silicon dioxide modified specimen. Specimens

(a)

(b)

(c)

(d)

CAs (degree)

51.1±1.2

41.3±1.3

14.0±0.9

2.0±0.5

OCAs(under-water) (degree)

128.2±0.9

135.3±1.1

148.2±1.5

164.5±2.1

DWs(g/dm2)

6.335

2.751

2.072

0.093

The under-water oil contact angles (OCAs) and the wax deposition weight on unit area (DWs)


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were also measured and shown in Figure 5 (B). The bare A3 carbon steel is oleophobic under water with an OCA of 128.2±0.9o (Figure 5 (B) (a)), which is consistent with the results of Jiang32 and Wang16, 17. After coated by Zn, the OCA increased to 135.3±1.1o for surface morphology changing (Figure 5 (B) (b)). After phosphorization process, the OCA increased to 148.2±1.5o and after modified with silicon sol, the specimen turned to superoleophobic with an OCA of 164.5±2.1o, as shown in Figure 5 (B). Meanwhile, the wax prevention performance of the specimens changed a lot with the increase of OCAs, as shown in Figure 5 (B). After the wax deposition test, the bare carbon steel was completely covered by a thick layer of black wax, while the DW was calculated to be 6.335 g/dm2. The DW decreased to 2.752 g/dm2 after the surface was coated by Zn, while the specimen was also covered by thick wax. By further phosphatizating treatment, the DW decreased to 2.072 g/dm2 and the specimen surface presented similar wax deposition with the former one. The anti-wax property improved a lot after modified by silicon dioxide sol. The specimen was nonpolluting with a little wax on its surface and the DW was calculated to be 0.093 g/dm2, which stands for excellent wax prevention property, as shown in Figure 5 (B).

Figure 5 (A) Digital CAs images of: (a) Bare A3 carbon steel; (b) Zn coated specimen; (c)


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phosphating film coated specimen and (d) Silicon dioxide sol modified specimen. (B) Digital images of OCAs and specimens surface after wax deposition test, and the DW values of different specimens. From the results of the CAs, OCAs and DWs, we can easily know that the wax prevention property of a specimen is closely related to its wetting behavior. Hydrophilic surface usually presents oleophobic property underwater, which is contributing to improve wax prevention property. Furthermore, superhydrophilic surfaces in water present superoleophobic property and demonstrate excellent wax prevention property, as shown in Figure 5. This special underwater oil wetting behavior of the prepared composite coatings is very important for the potential application in water-contained crude oil transition. To prove that fish scale like phosphating film played an important role in preventing wax deposition, we designed contrast experiments that spinning the silicon dioxide sol directly on the Zn film, and found out that there were no such special wettability properties and good wax prevention performance (Supporting Information Figure S4).

Figure 6 The (a) FTIR and (b) Raman spectra of the prepared specimens modified with different silicon dioxide sols. Generally, the wetting behavior of the prepared coatings is mainly determined by the surface chemical composition and the surface microstructures. In this work, the different chemical groups


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with specific properties on the coating surface greatly influence the surface energy and interface property. To further confirm the inference, the following experiments were carried out. By adjusting the composition proportion of the silicon dioxide sol, the hydroxyl group number could be changed and resulted in the changing of wetting behaviors33. The surface composition of the prepared coating on A3 carbon steel was determined by FTIR and Raman spectroscopy, and the results were shown in Figure 6. The obvious broad vibration bands in the FTIR spectrum (Figure 6 (A)) at 935 cm-1 and 1217 cm-1, corresponding to Si-OH, presented gradually decrease trend from specimen I to specimen IV. The results indicate that the hydroxyl group number of specimen IV is smaller than specimen I. In addition, the bands at 1380 cm-1 in Raman spectra of Figure 6 (B) confirm the presence of hydrophobic group, which is assigned to the Si-CH3. In conclusion, from specimen I to specimen IV, the numbers of hydrophilic groups (Si-OH) decreased while the numbers of hydrophobic groups (Si-CH3) increased. The CAs, OCAs and DWs results are consistent with the previous analysis, as shown in Figure 7.

Figure 7 (A) Digital images of CAs and OCAs of different specimens; (B) Digital images of different specimens after wax deposition test and the DW values of different specimens. A further decrease of hydroxyl groups leads to an increase of contact angle, as shown in Figure 7.


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The surface of specimen I was superhydrophilic with a CA of about 2o, and the corresponding OCA was about 164o, as shown in Figure 7 (A). Thus the specimen presents superoleophobicity under water. After the hydrophobic group increased and hydrophilic group number decreased, the CA increased and the corresponding OCA decreased. The specimen IV showed hydrophobicity with CA of about 135o and the OCA decreased greatly to 110o, as shown in Figure 7 (A). The DW values and the corresponding digital images of the specimens after wax deposition test were also shown in Figure 7 (B). The specimen I was almost undefiled with a low DW value of 0.085 g/dm2, indicated an excellent anti-wax property. The change of the silicon dioxide sol composition proportion resulted in the change of the wetting behavior, and further resulted in the wax deposition increasing in specimen IV with a DW of 4.152 g/dm2. That is to say, a smaller CA and larger OCA is prone to better anti-wax property. As controlling the wetting behavior is a fundamental issue in many applications for special functional surfaces, the above method based on changing the surface composition may offer new approaches. The bare carbon steel is hydrophilic in air and turns to oleophobic in water. But the OCA is only 90±2.0o and not large enough. Thus, wax is prone to deposit on its surface and present high wax deposition. After coated with Zn film, phosphating film and modified by silicon dioxide sol, the surface morphology and composition are greatly changed. The silicon dioxide film with massive hydroxyl groups greatly improves the hydrophilicity for the hydroxyl group presents strong attraction to water molecules. Besides, the phosphating process provides a rougher fish-scale like surface, which further improves the hydrophilicity of the surface, according to Wenzel’s theory34. After immersed in crude oil, the water in the crude oil can be easily attracted by the hydroxyl groups and form a water film on the surface of the specimen, which stop the wax from depositing on the surface.


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Thus the prepared coating presents a higher oleophobicity and better wax prevention property, as shown in Figure 5 (D). By decreasing the numbers of hydroxyl groups and increasing the methyl groups, the wetting behavior of the surface is dramatically changed35, which resulted in the wax prevention property deterioration, as illustrated in Figure 6.

Figure 8 Digital images of: (A) The water droplets taken at different stages during the wetting process on the prepared coating; (B) Underwater oil rolling angle of the phosphating film and (C) Underwater oil rolling angle of the prepared coating. The rapid wetting process and underwater oil rolling angles (ORA) were captured and the results were shown in Figure 8. The Figure 8 (A) shows that upon the water droplet contact the specimen surface, the water would rapidly spread out and cover the whole surface. The prepared coating was surperhydrophilic with a contact angle of about 2o. The digital images of underwater oil rolling angles of the phosphating film and the prepared composite coating were shown in Figure 8 (B) and


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(C), indicating that the underwater oil rolling angle decreased from 15.2o to 2.8o after modification by silicon dioxide sol. This change is advantage to wax prevention in the water-contained crude oil. Above all, it can be concluded that the wax prevention property of a surface is closely related to its wetting behavior. Good hydrophilicity and under-water oleophobicity is conducive to excellent wax prevention property. A superhydrophilic surface usually presents underwater superoleophobic property36, which provides inspiration for fabricating efficient anti-wax coatings to prevent wax deposition in water-contained crude oil. The consistency of hydrophilicity in air and oleophobicity under water can be explained by Eq. (3):

cosθ 3 =

γ o− g cosθ1 − γ w− g cosθ 2 γ o−w (3)

Where γo-g is the oil/gas interface tension, θ1 is the contact angle of oil in air, γw-g is the water/gas interface tension, θ2 is the contact angle of water in air, γo-w is the water/oil interface tension, and θ3 is the contact angle of oil in water. The related validation can be found in previous literature23. The mechanism of under-water oleophobicity and wax prevention property are also discussed. It’s known that the water-contained crude oil is a composite liquid system composed of two incompatible phase, oil and water. In this work, the used crude oil containing 50% water flows as o/w emulsion, indicates that water exists as linked phase and oil as dispersed phase13. Crystallization of wax is a kinetic process, the onset of which can be described by classical homogeneous nucleation theory. In this o/w form of emulsion, the crystallization of wax can be divided into two stages: nuclear formation and crystal growth. The nuclear formation speed (ν1) and the crystal growth speed (ν2) can be calculated according to Eq. (4) and (5), respectively:

ν1 = k

( c − ce ) ce

(4)


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ν2 =

D

δ

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A(c − ce ) (5)

Where c is the concentration of supersaturated solution, ce is the concentration of the saturated solution, k is a constant, D is the solute diffusion coefficient, δ is the diffusion distant and A is the surface area of the nuclear. In oil pipeline, temprature near the pipeline inwall is much lower than the pipeline center, which resulted in a lower ce, and further leaded to higher ν1 and ν237. That is the reason why the pipeline inwall is easier to deposit wax than pipeline center. While in the water-contained crude oil, the water would rapidly cover the whole superhydrophilic surface and form a continuous water film, as shown in Figure 8 (A), which can stop the wax nuclear formation on the surface or bonding in the surface. The new generated wax nuclear in the flowing crude oil would be taken away rather than depositing on the pipeline inwall for the smaller underwater oil rolling angle of 2.8o, as shown in Figure 8 (C). In this way, a superhydrophilic surface is prone to good wax prevention property. In this work, the modification of silicon dioxide sol with hydrophilic groups greatly increased the bonding force of the surface and water film, which resulted in a stronger water film and further improve the anti-wax property. 3.3 Corrosion resistance, high-temperature resistance and mechanical resistance tests.


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Figure 9 (A) Potentiodynamic polarization curves of (a) bare A3 carbon steel, (b) Single phosphating film coated specimen and (c) prepared composite coating; (B) CA values of prepared coating after different heating time; (C) CA images of prepared coating (a) before and (c) after mechanical test, (b) Schematic illustration of the mechanical test. Table 3 The parameters derived from the potentiodynamic polarization curves in 3.5 wt.% NaCl solution. Specimen

Ecorr(V/SCE)

Icorr(A/cm2)

Rp(Ω/cm2)

Bare A3 carbon steel

-0.7098±0.02

4.22±0.30×10-7

2.24±0.40×106

Single phosphating film coated specimen

-0.5968±0.02

2.1±0.25×10-7

5.02±0.40×106

Prepared composite coating

-1.0120±0.02

5.35±0.10×10-8

3.16±0.50×107

During the crude oil transition, good corrosion resistance, high-temperature resistance and mechanical property are always required for the damp and salty environment38, 39, which causes reinforcement rusting and physical damage. To investigate the anti-corrosion property of the prepared composite coatings, we characterized


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bare A3 carbon steel, single phosphating treated specimen (silicon dioxide sol modified) and prepared composite coatings with potentiodynamic polarization in 3.5 wt.% NaCl solution40. The potentiodynamic polarization curves and the fitting values of electrical parameters of the corrosion potentials (Ecorr), corrosion current densities (icorr) and polarization resistance (Rp) are presented in Figure 9 (A) and Table 3, respectively. From the Figure 9 (A) we can observe that all the specimen present active dissolution behavior in anodic region for a high current density is measured as the potential increases, indicating no passivation occurs. Comparing the results of bare A3 carbon steel, single phosphating treated specimen and prepared composite coating, a corrosion current densities decrease can be observed, indicating the increase of corrosion consistent. Furthermore, the results of Figure 9 (A) (c) and Table 3 indicated that with the Zn coating, the corrosion current density (icorr) is about 1 order lower than the bare A3 carbon steel and the single phosphating treated specimen, and the polarization resistance (Rp) is about 1 order greater, indicating that the Zn coating is able to offer higher corrosion protection. Besides, it can be found that the corrosion potential shifts 0.3022 V in a negative direction after coated by Zn. In conclusion, the increasing of corrosion resistance can be contributed to the Zn film and phosphating film, which are all reported to have anti corrosion ability41, 42. The Zn film and phosphating film are all intact and compact, which can effectively prevent air and solution infiltration and further increase the anti corrosion stability. A high-temperature test was also carried out to investigate the high-temperature resistance. The prepared specimen was put in 200 oC for several hours and then the contact angles were measured. The results in figure 9 (B) shows that after a high-temperature test of 48 hours, the composite coating can still stay superhydrophilicity with a contact angle of about 0o, which indicates a good high-temperature resistance.


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To further observe the mechanical property of the prepared composite coating, a scratch test was carried out with 800# SiC sandpapers served as abrasive surfaces29. The coating surface to be tested was faced with the rough side of the sand paper. A 50 g load was applied on the coating surface and then the surface was moved 10 cm on the sandpaper longitudinally and transversely, which was defined as an abrasion cycle. Figure 9 (C) shows the contact angle images before and after 20 abrasion cycle. The water contact angle changed slightly within 20 cycles and presented excellent superhydrophilicity. The corresponding OCA (under-water) was also tested and the result was shown in Supporting Information for review (Supporting Information Figure S5), which presents good oleophobicity. It indicates that the prepared composite coating possessed good mechanical property. Above all, it can be concluded that the prepared composite coating present extreme stability which is very meaningful in industry application. 4 Conclusions In this work, a silicon dioxide sol modified Zn coating was employed to inhibit wax deposition in petroleum industry. The coating contains three layers: a Zn film prepared by electrodeposition, a phosphating film and a silicon dioxide film prepared by spinning. The coating surface exhibited flake morphology, which is favorable for preventing wax deposition. Many hydroxyl groups exist on the coating surface, which are proved to be hydrophilic and further improve the wax prevention property. Influenced by the special morphology and composition, the prepared coating was superhydrophilic in air with CA of 2.0±0.5o and superoleophobic under water with OCA of 164.5±2.1o. The special wetting behavior endows the coating excellent wax prevention property. Corrosion tests were also carried out and good anti corrosion property was investigated. This bio-inspired composite coating presents better wax prevention property and wider application potential than traditional approaches


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commonly used.


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ASSOCIATED CONTENT Supporting Information. The schematic diagrams of the “cold-finger” based wax deposition test device and the underwater oil contact angle test device, the high-resolution SEM images of the Zn film and the digital image of the silicon dioxide sol modified Zn coating after wax deposition test. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Huicong Liu Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China *E-mail address: [email protected] Tel: +86 1082317113. Fax: +86 1082317113.

Author Contributions All authors contributed to the development of the experimental design and discussion of the results, and preparation of the manuscript. All authors have given approval to the final version of the manuscript.

Acknowledgment


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For Table of Contents Only


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(A) Digital image of the real fish scales; (B) SEM image of prepared fish-scale like composite coating (insets are the digital images of water contact angle and oil contact angle); (C) Schematic diagram of the prepared composite coating. 199x202mm (300 x 300 DPI)


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Bioinspired Composite Coating with Extreme Underwater

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