Antibacterial and Corrosion Inhibition Properties of SA-ZnO@ODA-GO@PU Super-Hydrophobic Coating in Circulating Cooling Water System

Liu Fang; Kong Can; Li Wei; Jiang Guofei; Wang Xueyao

(1. College of Chemical Engineering, China University of Petroleum, Qingdao 266580;2. State Key Laboratory of Petroleum Pollution Control, Beijing 102206)

Abstract: In order to solve the corrosion problem of circulating cooling water system, SA-ZnO@ODA-GO@PU superhydrophobic coating was synthesized for pipeline protection. After hydrophobic modification, the contact angle (CA) of the coating was above 150°. The antibacterial ability of coating was essential for corrosion protection. SA-ZnO@ODA-GO can seriously damage the cell structure, make the cell content outflow, increase the leakage rate of protein, and make the bacteria unable to reach logarithmic growth phase within 24 h. The corrosion inhibition mechanism analysis of SA-ZnO@ODAGO@PU coating indicated that the hydrophobic coating as a physical barrier can prevent the water molecules from entering the carbon steel and prevent the surface charge transfer.

Key words: SA-ZnO@ODA-GO@PU coating; super-hydrophobic; antibacterial; corrosion inhibition

1 Introduction

At present, the circulating cooling water consumption accounts for 60% - 90% of the total industrial water consumption[1]. The equipment surface corrosion becomes a critical issue of circulating cooling water, which makes the entire circulating cooling water system unable to operate normally and can seriously affect the industrial production. In oil industry, the loss caused by corrosion of circulating cooling water system accounts for 6% of the whole production[2]. In the circulating cooling water system, the medium corrosion, the high temperature corrosion and the bacteria corrosion are three main causes for corrosion. Circulating water system is a special ecosystem, which provides nutrient, temperature, acidbase conditions and oxygen for microbial survival[3-4].Electrochemical protection, addition of corrosion inhibitors, and surface coating technology are used to prevent metal corrosion. So far, the coating protection is one of the most cost effective and widely used methods[5-7].

Particularly, the super-hydrophobic surface coating has attracted much attention in theoretical research and practical application[8]. It has wide application[9]in anti-corrosion[10], self-cleaning[11], drag reduction[12],and oil-water separation[13]. Super-hydrophobic surface coating on metal material not only can reduce the contact between corrosive liquid and metal surface,but also can avoid invasion of mold, which has a good protective performance on metal. So the fabrication of superhydrophobic surfaces/films serving as protective films on metal substrate is one of the most important tendency[14]. She Zuxin, et al.[15]has electrodeposited nickel on the surface of magnesium substrate for improving its corrosion resistance. But electrodeposition method consumes energy and the operation process is complex, which would limit its practical application.Therefore, we plan to directly coat the modified PU coating on the metal surface to form a super-hydrophobic surface for further improving its corrosion resistance.

Zinc oxide (ZnO) is an excellent antibacterial material with unique optical characteristic. On the one hand, ZnO has a strong inhibitory effect on bacteria growth[16], and on the other hand, ZnO provides surface roughness in coatings. However, ZnO is easy to agglomerate, which brings great difficulty in its dispersion. Therefore,stearic acid (SA) is considered for surface modification,which has low price coupled with perfect modification effect[17-18].

Graphene oxide (GO) is an excellent carrier with large specific surface area and adsorption capacity, along with a certain antibacterial ability and excellent mechanical strength[19]. However, its dispersion in electrolyte is poor. Since GO surface contains different oxygencontaining groups, it is easy to interact with inorganic metal compounds. Introducing hydrophobic groups on the surface of GO may improve its dispersion. Run Yanling,et al. reported that the modified GO has high water contact angle and water absorption capability[20].

In this work, SA-ZnO was loaded on ODA-GO and the composite was filled into polyurethane (PU) coating for improving its antibacterial and corrosion resistance. The corrosion resistance of the coating was enhanced through the improvement of hydrophobicity and antibacterial property. The structure of the composite coating was analyzed, and the antibacterial mechanism and corrosion inhibition mechanism were investigated.

2 Experimental

2.1 Preparation of hydrophobic GO

GO was prepared by the improved Hummers method[21].Ethanol (150 mL) was slowly added into a vessel(250 mL) containing GO (0.5 g) powder, and the mixture was ultrasonically treated. Then ocadecylamine (ODA, 0.1 g) was poured into the mixture and was then continuously stirred for about 24 h at 80 °C. ODA can be combined with GO by ultrasonic method. At room temperature,the product was washed and dried at 60 °C. Hence, the graphene oxide modified by octadecylamine (ODA-GO)was obtained[22].

2.2 Preparation of SA-ZnO@ODA-GO@PU

Stearic acid (SA, 0.1 g) was added into 150 mL of ethanol solution. ZnO powder was added into the mixture and was stirred at 60 °C for 2 h. The sample was aged naturally for 1 h, centrifuged, and dried for 24 h at 80 °C. Finally, the modified SA-ZnO material was obtained.

A certain amount of ODA-GO (mass fraction of ODAGO is 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%,45%, and 50%, respectively) was dispersed in 20 mL of dimethylacetamide (DMAC) solution. SA-ZnO particle(0.1 g) was treated by ultrasound. The material was separated and washed three times. The SA-ZnO@ODAGO composite was obtained after being freeze-dried at 60 °C. Figure 1 shows the preparation process of SAZnO@ODA-GO composite.

Figure 1 Preparation of SA-ZnO@ODA-GO composite

The SA-ZnO@ODA-GO composite (mass fraction of SA-ZnO@ODA-GO is 0%, 0.3%, 0.6%, 0.9%, 1.2%,1.5%, 1.8%, 2.1 %, respectively) and PU were mixed with defoamer, wax emulsion, wetting agent, water, and other additives. In the process of ultrasound treatment, the mixture was stirred at below 35 °C. The SA-ZnO@ODAGO composite was then brushed onto the surface of the steel, and the thickness of coating was about 50 ± 5 μm.After being dried completely, the SA-ZnO@ODA-GO@PU composite coating was obtained.

2.3 Characterization

An emission scanning electron microscope (SEM,HitachiS-4800) and a transmission electron microscope(TEM, JEM 2100UHR, 120 kV) were applied to analyze the morphology and structure of coating samples. The molecular structure of samples was determined by a Fourier transform infrared spectrometer (FT-IR, Nicolet Nexus670, 400 cm-1to 4000 cm-1). The crystal structure was analyzed by an X-ray diffractometer (XRD, X’Pert Pro MPD). The element and valence state of the sample was analyzed by X-ray photoelectron spectroscopy (XPS,K-Alpha X). The hydrophobic property of the coating was measured by a contact angle tester (CA, XG-CAM).

2.4 Antibacterial and corrosion inhibition experiment

The growth curves ofE. coliandS. aureuswere drawn by photoelectric turbidimetry. A flask containing 107CFU/mL ofE. coli(orS. aureus) suspension and composites(10 mg) was placed in a constant temperature oscillator for bacteria culture. The sample was taken out every 2 h to determine the absorbance using an ultraviolet spectrophotometer (600 nm).

Protein leak experiments ofE. coliandS. aureuswere carried out by photoelectric turbidimetry. 107CFU/mL ofE. coli(orS. aureus) bacteria suspension in the logarithm growth period and the composite (10 mg) were mixed and cultured. Bacterium solution was taken out every 2 h. Coomassie brilliant blue solution (1 mL) was added to the sample and the mixture was shaken successively.The absorbance of the sample was determined by an ultraviolet spectrophotometer (595 nm). The results were compared with the standard curve of protein concentration and protein concentration of the sample could be calculated.

PI stained experiments ofE. coliandS. aureuswere carried out by a fluorescence microscope. The SA-ZnO@ODA-GO composite (10 mg) was added into the 107CFU/mL ofE. coli(orS. aureus) bacteria suspension and was shaken for 2 h. The bacterial suspension was centrifuged to remove the supernatant. Phosphate buffer solution and PI dye solution were poured into the solution and then was placed in a dark room for 15 min. Bacteria solution was transferred to prepare an observation sample,and photos were taken by the fluorescence microscope to obtain the PI stained images.

The plate counting method was used to count the viable microorganisms, and the antibacterial rate could be calculated. The corrosion inhibition properties of composite coating were measured by the CHI660E electrochemical workstation. The corrosion inhibition mechanism of the coating could be further analyzed.

3 Results and Discussion

3.1 Characterization analysis

Interlayer variations and crystalline properties of composites were obtained by the XRD patterns (Figure 2). GO shows a diffraction peak at around 2θ= 9.2°.According to the Bragg’s law[23], the crystal plane spacing is 0.98 nm. Compared to graphite, the GO layer spacing has increased[24], because of the ‘intercalation effect’ resulting from oxidation. To form the oxygencontaining functional groups, the crystalline structure of graphite is damaged and substituted gy GO. After modification, a new diffraction peak appears at 2θ= 7.4°.The corresponding crystal plane spacing of ODA-GO is 1.226 nm, which is larger than GO. The appearance of the graphite (002) crystal plane diffraction peak indicates that hydrophobic chains appear on the edge or surface of GO[25]. By contrast, the diffraction peaks at 31.5°，34.5°，36.17°，47.51°，56.53°，62.77°，66.31° and 67.78° on SA-ZnO and SA-ZnO@ODA-GO pattern belong to (002), (101), (102), (110), (103), (200),and (112) crystal planes of hexagonal ZnO[26-27]. After modification, SA-ZnO is loaded on ODA-GO, and there is no new diffraction peak observed.

Figure 2 XRD patterns of GO, ODA-GO, ZnO, SA-ZnO and SA-ZnO @ ODA-GO composites

Figure 3 shows the FT-IR spectra of different composites.Due to a small number of residual H2O molecules on the surface of GO, -OH stretching vibration absorption peak and C-O vibration absorption peak in GO appear at 3 435 cm-1and 1 056 cm-1, respectively. The characteristic peaks of C=C vibration and C=O stretching vibration of carboxyl group (-COOH) in GO skeleton correspond to 1 627 cm-1and 1 731 cm-1. Compared with GO, a new N-H absorption peak of ODA-GO at 1 465 cm-1indicates that ODA has been successfully loaded on GO[25]. The peak at 425 cm-1corresponds to the skeleton of ZnO.The stretching vibration absorption peak of -OH in ZnO is at 3 418 cm-1and the characteristic peak at 1260 cm-1corresponds to the bending vibration absorption peaks of -OH. The former is a wide absorption peak, which indicates that there are considerable hydroxyl groups on the surface of ZnO. It is attributed to a large amount of oxygen vacancies existing on the surface of ZnO, which makes a part of zinc atoms combine with oxygen to form physical or chemical adsorption products. At the same time, it combines with water molecules in the air to form a large amount of -OH. The strength of characteristic peak of -OH stretching vibration at 1 620 cm-1in SAZnO composite material decreases, which indicates that the hydroxyl groups on the surface of SA-ZnO decrease.Moreover, the appearance of the asymmetric stretching vibration of C-H at 2 942 cm-1and the symmetric stretching vibration of C-H at 2 847 cm-1shows that there are -CH3hydrophobic groups in ODA-ZnO.Two characteristic peaks at 1 397 cm-1and 1 545 cm-1correspond to symmetric and asymmetric stretching vibration of COO-, respectively[28]. It indicates that SA existing on the surface of ZnO in the form of acid radical and ZnO can react with SA. The -OH bending vibration peak in the SA-ZnO@ODA-GO composite at 3435 cm-1is weakened, indicating that the number of hydroxyl groups on GO surface has reduced.

Figure 3 FT-IR spectra of GO, ODA-GO, ZnO, SA-ZnO and SA-ZnO @ ODA-GO composites

The chemical bonds of composites, the oxygencontaining functional groups and the state of elements were analyzed by XPS. The XPS survey spectrum (Figure 4(a)) shows that all spectra are calibrated using the C1s standard binding energy of 284.8eV and the composites contain C, N, O, and Zn. The XPS spectrum (Figure 4(b)) shows two different characteristic peaks, which are at 1 022.08 eV and 1 045.18 eV, corresponding to Zn2p3/2and Zn2p1/2, respectively. Figure 4(c) is the XPS spectrum of O1s, showing that the broad binding energy of Zn-O bond is at 545.15 eV. On the surface of ZnO, the wide binding energy peak at near 531.58 eV relates to OH-adsorption[29]. There are three different characteristic peaks in the XPS spectrum of C1s (Figure 4(e)), which are at 284.78 eV, 285.18 eV and 288.68 eV, respectively,corresponding to the carbon-carbon single bond (C-C),epoxy group and alkoxy group (C-O-C) and carboxyl(-COOH) in the carbon skeleton[30]. The N1s peak at 401.98 eV exhibits that the ZnO lattice is doped with nitrogen element, forming nitrogen doping[31].

Figure 5(a) shows that GO demonstrates a sheet-like structure with obvious wrinkles[23]. This is the stacking effect between GO monolayers due to the strong interaction of a large number of oxygen-containing functional groups with negative charges on the GO surface. The morphology and structure of ODA-GO are obviously different. The surface of ODA-GO becomes uneven with obvious undulation. The addition of ODA improves the surface roughness of the material and presents a layered stacking structure, which is conducive to the formation of superhydrophobic interface. It can be seen from Figure 5(c) that ZnO is plate-like, closely arranged and ordered. Figure 5(d) demonstrates that the distribution of SA decorated on the surface of ZnO is relatively uniform. Less agglomeration phenomenon appears due to the existence of a large amount of SA. Figure 5(e) shows that ZnO modified by SA is successfully loaded on the GO surface. The TEM image of GO (Figure 5(f)) displays the smooth lamella, wrinkled edges and thin gauze structure. The appearance of GO folding increases specific surface area of GO, which is conducive to further loading ZnO on its surface.Differently, the color of ODA-GO is darker, because some oxygen-containing functional groups have reduced.A large number of ZnO particles are closely arranged and an obvious agglomeration phenomenon appears. The results show that ZnO is loosely distributed on the surface of SA, and no obvious aggregation occurs. Figure 5(j)indicates that the modified ZnO uniformly disperses on the surface of ODA-GO, and the material is successfully prepared. These phenomena correspond to the results of SEM analysis.

Figure 4 XPS spectra of SA-ZnO @ ODA-GO composites: (a) survey spectra, (b) Zn 2p core level spectra, (c) O 1s core level spectra, (d) N 1s core level spectra, (e) C 1s core level spectra

3.2 Performance analysis of SA-ZnO@ODA-GO@PU coating

Figure 5 SEM and TEM images of (a, f) GO, (b, g) ODA-GO , (c, h) ZnO, (d, i) SA-ZnO and (e, j)SA-ZnO @ ODA-GO composites

Generally, the mass ratio of SA-ZnO@ODA-GO in PU coating has a remarkable effect on the hydrophobicity and antibacterial activity. Figure 6(a) indicates that when the mass ratio of ODA-GO is 15%, the antibacterial rate increases from 80% to 95% and the contact angle reaches 150.5°. The addition of ODA-GO can further improve the antibacterial and hydrophobic performance of the SA-ZnO@PU coating[32-33]. It can effectively prevent the adhesion of biofouling in circulating cooling water system. Figure 6(b) shows that the hydrophobic angle of PU coating is only 45.9°. The PU coating without modification has poor hydrophobicity without antibacterial effect. However, the antibacterial rate of ZnO@ODA-GO@PU coating exceeds 97%. According to previous research conducted by our team[22,33-34], it was found that after the composite coating is applied onto the metal surface, the properties of the metal are significantly enhanced, such as hardness, heat resistance, weather resistance, adhesion, etc. These excellent properties of composite coating are conducive to the practical application in circulating cooling water. Meanwhile, the contact angle of the SA-ZnO@ODA-GO@PU coating increases obviously with the increase of the mass ratio of SA-ZnO@ODA-GO in the coating. The increase of contact angle indicates the better hydrophobicity.According to the Wenzel model, it is inferred that the water drops enter the gaps of the coating because of gravity depression. However, with the increase of mass ratio of SA-ZnO@ODA-GO, the roughness of the coating increases and the surface gap decreases. When the mass ratio of SA-ZnO@ODA-GO is 1.8%, the contact angle reaches up to 150.8° and the coating forms a superhydrophobic structure. The contact state between the composite coating and the water droplets changes from the Wenzel model to the Cassie model and the contact surface between the composite coating and the water droplets would decrease[35]. This phenomenon is caused by the existence of air layer between the composite coating and the water droplets. The contact angle of the composite coating does not continue to increase with the addition of more SA-ZnO@ODA-GO.

3.3 Analysis of antibacterial mechanism

Figure 6 Effect of (a) ODA-GO mass fraction and (b) SAZnO@ODA-GO mass fraction on antibacterial rate and CA of coatings

Figure 7(a) and Figure 7(b) reflect the effect of SA-ZnO@ODA-GO composite onE. coliandS. aureusgrowth.In the absence of SA-ZnO@ODA-GO composite, the bacteria enter a logarithmic growth period within 4 h. By contrast, the growth ofE. coliis seriously inhibited after adding SA-ZnO@ODA-GO composite, and the growth delay period is extended to 24 h. It is found from Figure 8b thatS. aureusdoes not have a delayed growth period and almost directly enters into the logarithmic growth period. After adding the SA-ZnO@ODA-GO composite,the growth ofS. aureusshows a delayed growth period,and it enters into a logarithmic growth period about 16 h later. It indicates that SA-ZnO@ODA-GO composite can delay the growth ofE. coliandS. aureus.

Figure 7(c) reflects the effect of SA-ZnO@ODA-GO on cell membrane. Under the normal growth conditions,it cannot detect any protein leakage, which is less than 10 mg/L within 12 h. The leakage of a small amount of protein is attributed to the bacteria excreting a part of the protein from the cells through exocytosis, the normal aging and death of bacteria, and the bacterial cell autolysis. After adding the SA-ZnO@ODA-GO, the protein leakage rate is faster. This is true, because the cell membrane ofE. coliis damaged and the cells could not prevent the content of cytoplasm from flowing out. As shown in Figure 7d, the protein leakage ofS. aureusis insignificant in a normal growth state. SA-ZnO@ODAGO increases the leakage rate of protein. Furthermore,the leakage rate ofS. aureusprotein is less than that ofE.coliprotein. The main reason is thatS. aureusbelongs to the Gram positive bacteria. Peptidoglycan is an important component in the cell wall of Gram positive bacteria.Hence, the cell wall is thick (20 - 80 nm) and has high mechanical strength. Meanwhile,S. aureusalso has strong ability to resist external damage, and can keep the original cell morphology well when the cell is damaged by external environment.E. colipertains to the Gram negative bacteria. There are several main characteristics about cell wall of Gram negative bacteria, viz.: less peptidoglycan content, thin cytoderm (10 - 15 nm),multilayer structure, and loose arrangement between layers. Therefore, cells are easy to damage and deform,when they are subject to extracellular damage.

Figure 7 Effect of SA-ZnO @ ODA-GO composites on growth curves of: (a) E. coli; (b) S. aureus; effect of SA-ZnO @ ODAGO composites on protein leakage for: (c) E. coli; and (d) S. aureus

The PI dye is a kind of substance that can be fused with DNA double strand structure.In physiological experiments, the cell membrane is destroyed and PI dye can enter the cell. Figure 8(a) indicates that in normal growth state, the PI staining solution cannot enterE. colito stain DNA due to the integrity of cell membrane, and the field of vision is dark. After adding SA-ZnO@ODAGO to theE. colisuspension, red fluorescence appears in the field of vision (Figure 8(b)). The fluorescence shape is an irregular shape, and the size is different. This illustrates that the cell membrane function ofE. coliis damaged,and the original cell morphology collapses. As shown in Figure 8(c), the normalS. aureuscannot be stained by the PI staining solution. With the addition of SA-ZnO@ODA-GO, red fluorescence appears. However, Figure 8(d)shows that only a part of the red spherical fluorescence appears in the field of vision, which proves that the SAZnO@ODA-GO composite cannot completely destroy the cell structure ofS. aureus,sincethe bacterial cell frame structure remains and the DNA leakage is small.

The SA-ZnO@ODA-GO composite is an excellent antibacterial material against both Gram-negative and Gram-positive bacteria. There are two main antibacterial mechanisms. On the one hand, SA-ZnO@ODA-GO is added to inhibit the two bacteria from achieving the logarithmic growth. Bacteria cannot proliferate normally,which affects the normal physiological activity of bacteria and reduces the number of bacteria. On the other hand,SA-ZnO@ODA-GO can damage the cell membrane structure of the two kinds of bacteria, and makes the bacterial cell membrane lose the ability to selectively control substances, resulting in the accelerated leakage of cell contents. Hereby the cell structures would change.

Figure 8 Fluorescence images of E. coli : (a) control, (b) SA-ZnO @ ODA-GO composites;fluorescence images of S.aureus: (c) control, (d) SA-ZnO @ ODA-GO composites

3.4 Analysis of corrosion inhibition mechanism

In order to estimate the corrosion inhibition property of SA-ZnO@ODA-GO@PU coating, polarization curves of the coating with different SA-ZnO@ODAGO concentration are tested. Based on the Tafel curve in Figure 9, the epitaxial method is used to calculate corrosion current density of the specimen in circulating cooling water. It is known that only by means of adding PU coating on the surface of the specimen, the corrosion potential shifts to the positive direction, and the anodic corrosion current density decreases. This indicates that the PU coating mainly inhibits the anodic dissolution and reaction, and delays the reduction of hydrogen ion in the cathode. After adding SA-ZnO@ODA-GO composite to PU coating, the corrosion potential continues to shift to the positive direction, and the corrosion current density decreases significantly. In the meantime, the inhibition rate increases from 61.9% to 96.5% (Table 1). This illustrates that the SA-ZnO@ODA-GO@PU coating can inhibit the chemical reaction between the anode and the cathode, and its corrosion resistance is stronger than that of PU coating.The electrochemical impedance spectroscopy (EIS)of SA-ZnO@ODA-GO@PU coating is used to make a thorough study on the corrosion behavior of the composite. Judging from the Bode diagram in Figure 10(a-b), the longitudinal intercept of superhydrophobic coating is larger than that of blank carbon steel. The impedance mode value at low frequency decreases in the following order: SA-ZnO@ODA-GO@PU coating＞ PU coating ＞ blank. The larger impedance mode value indicates a better corrosion resistance. When the frequency is 0.01 Hz, the low frequency impedance mode value of carbon steel is 527.23 ohm·cm2, and that of superhydrophobic coating is 5.01×104ohm·cm2. The|Z| value proves that the property of superhydrophobic coating improves the corrosion resistance of carbon steel.This result is consistent with the polarization curve.

Figure 9 Tafel curves of SA-ZnO @ ODA-GO @ PU composite coatings at different concentrations

Blank carbon steel has only one capacitance compression arc in the Nyquist diagram (Figure 10(c)) and one time constant in the Bode diagram. Therefore, the charge transfer process is the main reason of corrosion. In circulating cooling water, diameter of the capacitive arc in the Nyquist diagram (Figure 10(c)) increases significantly,indicating that the product molecules can be adsorbed on the surface of the test piece to form a protective film.

The formation of the protective film reduces the active surface area and enhances the corrosion resistance of the test piece. It can be found from the Nyquist diagram that the coating with SA-ZnO@ODA-GO@PU has larger capacitance arc. It manifests that the shielding effect of SA-ZnO@ODA-GO@PU coating is stronger than that of PU coating. The capacitance arc is larger, as a higher ratio of SA-ZnO@ODA-GO in the coating can result in a larger capacitive arc. Meanwhile, coverage and thickness of the coating on the metal surface is more,the hydrophobic layer is denser, which make it difficult for water molecules to enter the metal surface through the coating. If the content of SA-ZnO@ODA-GO in the coating is excessive, the distribution of the composite on the metal surface may be uneven and may even agglomerate. At this moment, the anti-corrosion coating is easy to crack, and water molecules will corrode the metal.The capacitance between metal and solution is determined by the slope value in the intermediate frequency region of the Bode diagram. The interface structure is not ideal, because ions are much larger than electrons, and a differential capacitance will be formed at the metal/solution interface. Figure 10(d)-(e) shows the equivalent circuit. The fitting parameter of EIS in Table 2 shows that theR2value increases from 450 Ω·cm2to 4092 Ω·cm2after adding PU coating. The resistance of the coating increases to 43030 Ω·cm2after adding SA-ZnO@ODAGO to the PU coating. Especially, the resistance of the coating is as high as 168283 Ω·cm2with a SA-ZnO@ODA-GO content of 1.8%. The results show that the corrosion rate of carbon steel can effectively decrease by adding the composite coating. The PU coating is more complete and compact, which can effectively prevent the charge transfer on the surface of carbon steel.

In addition, the coating with good hydrophobicitycan separate carbon steel from water and reduce the possibility of corrosion. The coating can shield too many polar groups and reduce the ‘new polar electrolyte channel’, which would greatly reduce the penetration of corrosive medium to the coating. The coating can have a dense structure and perfect cross-linking property,after SA-ZnO@ODA-GO is combined with PU coating.The corrosion reaction between coating and metal interface and the ‘transverse and longitudinal diffusion’of corrosion products are limited, which reflects the shielding mechanism of coating/metal interface to the substrate corrosion. The strong antibacterial property of the composite coating prevents bacteria from contacting the test piece, which is also one of the reasons for the excellent anti-corrosion ability of the SA-ZnO@ODAGO coating.

Table 1 Electrochemical reaction parameters of SA-ZnO @ ODA-GO @ PU composite coatings at different concentrations

Figure 10 EIS diagram of SA-ZnO @ ODA-GO @ PU composite coatings with different amounts

Table 2 EIS fitting results of carbon steel coupons in circulating cooling water with and without composite antibacterial coating

4 Conclusions

In this work, two monomers (ZnO and GO) with hydrophobic modification were compounded and added to the PU coating. The composite coating has excellent superhydrophobicity and cross-linking property. When the content of SA-ZnO@ODA-GO compisite was 1.8%in PU coating, a superhydrophobic interface was formed.Since SA-ZnO@ODA-GO had remarkable antibacterial properties,E. coliandS. aureuswere highly sensitive to SA-ZnO@ODA-GO composite. SA-ZnO@ODA-GO can severely damage the cell structure, make the cell content outflow, increase the leakage rate of protein, and make the bacteria unable to achieve a logarithmic growth within 24 hours. Besides, SA-ZnO@ODA-GO @PU coating can arrest the charge transfer on specimen surface. It also makes the corrosion potential of the specimen move forward and can inhibit the anodic reaction and cathodic reaction of the specimen at the same time. The formation of superhydrophobic interface and the antibacterial property of the coating are also conducive to the protection of metal.

Acknowledgments:This work was supported by the CNPC Safety and Environmental Protection Key Technology Research and Promotion Project (2017D-4613) and the Sub Project of National Science and Technology Major Project (2016ZX05040-003).