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Vol. 8. Issue 5.
Pages 3878-3890 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3878-3890 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.051
Open Access
Hierarchical micro/nano structure surface fabricated by electrical discharge machining for anti-fouling application
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Z.R. Hea,b, S.T. Luoa, C.S. Liua,
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liucs@gdut.edu.cn

Corresponding authors.
, X.H. Jiea,
Corresponding author
cnxyyz3@gdut.edu.cn

Corresponding authors.
, W.Q. Liana
a School of Materials and Energy, Guangdong University of Technology, Guangzhou, PR, 510006, China
b School of Mechanical and Engineering, Guangdong University of Petrochemical Technology, Maoming, PR, 525000, China
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Tables (7)
Table 1. Chemical compositions of substrate and tool electrode by weight (%).
Table 2. Machining parameters.
Table 3. Roughness and contact angle values of the HMNS.
Table 4. Chemical compositions of the HMNS.
Table 5. Electrochemical and Tafel parameters from polarization curves of the polished and HMNS before anti-fouling property experiment.
Table 6. Parameters of equivalent circuit model of sample obtained by ZsimpWin in 3.5 wt% NaCl solution.
Table 7. Electrochemical and Tafel parameters from polarization curves of the polished and HMNS after anti-fouling property experiment.
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Abstract

Surface that durably and anti-fouling are of interest for fundamental research and industrial application in the heat exchanger equipment. In the present study, hierarchical micro/nano structure (HMNS) is fabricated by electrical discharge machining (EDM) method. The morphology, metallographic and crystalline structures as well as corrosion resistance of the HMNS are characterized. The micro/nano structure improves the corrosion resistance and hydrophobic property of the surface. The HMNS, with the grain size dimension of 1.5–11 μm, shows an improved contact angle of 124.9° ± 5.4°. The co-effect of hydrophobic and corrosion resistance endows the HMNS with a superior anti-fouling property than that of polished surface. Fouling adhesion experiments shows that, after being immersed in CaCl2 + Na2CO3 solution for 72 h at 50 ℃, there are only a few fouling adhere on the HMNS and the adhered fouling is loose and easy to fall off. Moreover, after 50 times of semitransparent tape adhesion, the HMNS still in the hydrophobic status with slightly decreases (4.09%–9.38%) of contact angle, which indicates that the HMNS is durably and suitable for industrial application. There is an industrial prospect for fabricating anti-fouling heat transfer surface by a low cost and environmentally friendly one-step EDM process.

Keywords:
Electrical discharge machining
Hierarchical micro/nano structure
Hydrophobic
Anti-fouling
Corrosion resistance
Full Text
1Introduction

Fouling is an important problem for the heat exchanger equipment. Fouling, such as inorganic crystallization fouling (ICF) [1], organic fouling [2], biofouling [3], etc. adheres on the heat exchanger surface will lead to the increases of the fouling resistance, resulting in the largely increase of energy consumption [4]. Fouling also cause other problems, such as corrosion, pressure loss and flow misdistribution [5], leading to severe negative impact on cost, safety, health and environment. Researchers have investigated the use of the most widespread chemical agents to inhibit the adhesion of fouling on the heat exchanger surface [5]. However, the corrosion of chemical agents will bring some dangerous heavy metal to the environment.

Recently, researches showed that the typical ICF adhered on the heat transfer surface can be avoided by controlling the surface characteristics, such as the surface roughness, contact angle and surface energy [4]. Cheng et al. [6] found that there existed an effective range of surface energy to obtain the better anti-fouling property. The anti-fouling property can be improved by adjusting the surface energy of heat transfer surface by surface modified technologies [7,8]. A wide range of anti-fouling coatings have been produced by different surface modified technologies. The amorphous coating [9] and metallic composite coating [6,10–14] fabricated on the heat transfer surface via electroless technology are the most common anti-fouling coatings. The coatings synthesized by chemical vapor deposition (CVD) [15,16] and sol–gel method [17] are also reported in recent years. Nevertheless, the effective working life of the coatings fabricated by chemical treatment gradually decrease with the increased temperature of heat transfer surface. For example, TaN [8] and TiN [18] coatings prepared by magnetron sputter are easy to fall off from the heat transfer surface. Compared with chemical treatment, the anti-fouling coatings prepared by physical method like microstructure machining [4,19], displayed superior mechanical strength and longer working life. However, the high cost for the equipment and low production efficiency of the physical method make it necessary to develop a new idea to manufacture anti-fouling coatings.

In recent years, electrical discharge machining (EDM) method is used to fabricate micro/nano structure with low surface energy and hydrophobic property on the metal surfaces [20,21]. The micro/nano structure can also be changed by adjusting the machining parameters, which indicates the surface characteristics, including surface energy and hydrophobic property, can be controlled by the machining parameters. During the EDM process, the metal is melt under the high temperature from the discharge current between the tool electrode and surface and then solidified on the surface to form the micro/nano structure. Moreover, when compared to other anti-fouling fabrication methods, EDM method is a green method with less pollutions and lower cost, exhibiting a potential for producing surface with remarkable anti-fouling performance.

In the present study, micro/nano structures with different surface energy are modified on the heat transfer surface by EDM method. The anti-fouling and corrosion resistance properties of the resulting surface are investigated. The anti-fouling mechanism of the micro/nano structure surface is revealed and the relationship between the anti-fouling property and the surface characteristic is also discussed. This study gives a new idea for the design of anti-fouling coating or devices in heat transfer surface area.

2Experimental procedures2.1Preparation of modified surface

Pure copper (99.7 wt.% with size of 10 × 10 × 3 mm and 99.9 wt.% with diameter of 20 mm) were supplied by Shenzhen Hongwang Mold Co. Ltd., China and used as the heat transfer surface and tool electrode respectively. Their compositions are listed in Table 1. The common die-sinking EDM machine (D7145, Jiangsu Sanxing machinery manufacture Co. Ltd., China) was shown in Fig. 1. The dielectric fluid was EDM fluid (GREATWALL M0251, SINOPEC Group, China). The surfaces of substrates were first polished by SiC abrasive paper from 300–1000 P and then machined by EDM machine. EDM machining parameters are shown in Table 2. The modified substrates were respectively ultrasonic cleaned by ethanol, acetone and deionized water for degreasing and removing surface borne impurities, and then dried in a vacuum oven at 35 ℃ for 2 h.

Table 1.

Chemical compositions of substrate and tool electrode by weight (%).

  Cu  Ag  Bi  Sb  As  Fe  Pb 
Substrate  99.7  –  0.002  –  –  –  0.01  –  – 
Tool electrode  99.9  0.08  0.001  0.002  0.002  0.005  0.003  0.005  0.002 
Fig. 1.

Structures of the electrical discharge machine.

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Table 2.

Machining parameters.

Sample.  Current (A)  Pulse width (μs)  Duty ratio (%)  Gap voltage (V) 
Polished  –  –  –  – 
1#  180  60  80 
2#  10  180  60  80 
3#  12  180  60  80 
2.2Anti-fouling property experiment

The CaCl2 + NaHCO3 mixed solution was used to characterize the anti-fouling performance of the polished and modified surfaces [1,10,19]. Concentration of fouling solution was 0.1 mol L−1 for accelerating fouling processes. All chemical reagents were analytical grade and the solvent was deionized water. The anti-fouling experiment was carried out by a self-made heat transfer test device, as shown in Fig. 2. The polished and modified surfaces were immersed in the fouling solution at 50 ℃ and keep in the self-made heat transfer test device for 72 h. The adhering fouling weights on the surface were measured every 8 h by an electronic analytical balance with an accuracy of 10−3 g (BSA223S-CW, Sartorius, Germany) and five replicate tests were carried out for each sample.

Fig. 2.

Structure of self-made heat transfer test device.

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2.3Characterization

The morphology of the modified surface and the fouling were characterized by SEM (JSM-6510LV, JEOL, Japan) and the chemical compositions were analyzed via EDS (X-act, OXFORD Instruments, UK) equipped with SEM. The chemical compositions of modified surface were also detected by XPS (Escalab 250Xi, Thermo Fisher Scientific, USA). The 3D structures of modified surfaces were evaluated by AFM (CSPM5000, Being Nano-Instruments, China) in continuous measure mode and the measure range was 16 μm × 16 μm. The metallographic structures of the test samples section were observed via metallographic microscope (Axio Observer A1m, Zeiss, Germany). The crystalline structure of modified surface before anti-fouling experiment was investigated by X-ray Diffraction using Cu Kα radiation with a wavelength of 0.15418 nm (Ultima Ⅳ, Rigaku, Japan). The surface roughness (Ra, arithmetic mean roughness) was measured via surface roughness measuring instrument (JB-1C, Shanghai Cany precision instrument Co. Ltd., China) and the average value from three random regions for each sample was reported with their error. Contact angle test was carried out by contact angle measuring instrument (SD-CAZ2, Dongguan Chengding precise instrument Ins., China) and the deionized water was used as the testing medium. Test drops (4 μL) were placed on the surface and the average contact angle values measured at five different positions of the same sample were reported. The grain size of modified surface was measured, each sample for three times in the same test region, by image processing software (Nano Measurer V 1.2.5, Fudan University Jie XU) and the measured data was also statistically analyzed via this software.

Corrosion resistance was performed by electrochemical workstation (Zennium, Zahner, Germany). The test mode was three-electrode configuration. The samples, platinum electrode and saturated calomel electrode (SCE) were respectively used as working electrode, counter electrode and reference electrode [22]. The test samples were sealed up around by silica gel with an exposed surface area of 10 mm2. The samples were immersed firstly in the 3.5 wt.% NaCl solution for 30 min without stirring at room temperature until the open-circle potential of samples reached the stable values. The scan range of the polarization plotting was from −0.6 V to 0.2 V with 1 mV scan rate. The frequency range of electrochemical impedance spectroscopy (EIS) was 10−2–105 Hz. The EIS curves were analyzed and fitted to equivalent circuits by using the software ZsimpWin. All electrochemical measurements were conducted at least three times to confirm reproducibility.

3Results and discussion3.1Surface morphology and wettability

Figs. 3 and 4 respectively shows the SEM, optical images and AFM morphologies of polished and modified surfaces before anti-fouling experiment. As shown in igs. 3 and 4, the polished samples show a flat surface, and the modified samples display a rough surface with different morphologies from each other. Typical EDM characteristics, such as micro/nano porous, molten ball, recast region, are appeared on the modified surface [23]. As shown in Fig. 3(e), these typical EDM characteristics are uniformly distributed on the sample surface. And the nano porous shown in the red frame of Fig. 3(b) are also distributed homogenously and hierarchically on the surface. That is, the nano porous combines the micro scale structures to form the hierarchical micro/nano structure (HMNS), which is uniformly distributed on the modified surface.

Fig. 3.

SEM morphologies and optical images polished surface and HMNS samples before anti-fouling experiments: (a) polished surface, (b) 1#, (c) 2#, (d) 3# and (e) optical image of HMNS samples. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

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Fig. 4.

AFM morphologies of (a) polished and HMNS (b) 1#, (c) 2# and (d) 3#.

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When the tool electrode discharges to machine the copper surface, the high temperature come from the discharge current is up to thousands degrees [24]. The surface metal is melted immediately, some metal and dielectric fluid are even gasified. The melted metal are scoured away by the dielectric fluid and then cooled down and solidified later. During the solidifying process of the melted metal, the co-effect of bubbles from the gasified dielectric fluid and the spark impacts the metal surface and then the micro/nano porous are formed [25,26]. Besides, the number and dimension of nano porous are increase with the increase of current, uniformly distributed on the micro region of modified surface, as shown in Fig. 3(b–d). Thus, the roughness of modified surfaces is also increase with the increase of current (Table 3).

Table 3.

Roughness and contact angle values of the HMNS.

Sample  Ra (μm)  Contact angle (°)  Roughness factor (f
Polished  0.32 ± 0.40  70 ± 1.2 
1#  1.99 ± 0.26  124.9 ± 5.4  1.67 
2#  3.21 ± 0.15  131.7 ± 2.8  1.95 
3#  5.64 ± 0.31  138.4 ± 2.2  2.19 

Because of the micro/nano structures, the copper surface is changed from hydrophilic to hydrophobic surface [27]. The contact angles of 1#, 2# and 3#, are respectively 124.9° ± 5.4°, 131.7° ± 2.8° and 138.4° ± 2.2° (Table 3). Base on the Wenzel hydrophobic mode [28], the contact angle can be expressed as

where θw represents the actual contact angle of the surface of solid. θc represents the intrinsic contact angle of the surface of solid, which can be measured on the polished surface of solid. f is roughness factor and its value can be calculated by the proportion of actual surface to geometric surface. The roughness factor f stands for the contribution of surface roughness to hydrophobic property. It is obvious that the value of f is more than 1. According to Figs. 3 and 4, the surface morphologies of the HMNS become much more complex and the contact angle increase with the increase of current. The increased contact angle indicates the corresponding f of the HMNS is also increased and the effect of micro/nano structure on hydrophobic property is increased with the increase of current. Thus, the hydrophobic property of the HMNS can be improved by increasing the current.

The chemical compositions of the HMNS are shown in Table 4. The existence of O element indicates some oxidations are adhered on the HMNS. The C, Cl and Al elements come from the decomposed dielectric fluid due to the high temperature.

Table 4.

Chemical compositions of the HMNS.

Sample  C (at.%)  O (at.%)  Cu (at.%)  Cl (at.%)  Al (at.%)  Cu/O ratio 
1#  24.06  7.29  68.65  –  –  9.42 
2#  20.16  14.51  65.33  –  –  4.50 
3#  32.47  12.96  49.20  1.27  4.10  3.80 
3.2Crystalline structure of the HMNS

The XRD results of the polished, 1#, 2# and 3# before anti-fouling experiment are shown in the Fig. 5. All samples display typical X-ray diffraction pattern of pure copper, indicating that there is little change of crystalline structures and no nanocrystalline phase formed in the solidified layer of the HMNS. In addition, elements (C, Cl and Al) from dielectric fluid are absent. For more details of the chemical compositions on HMNS, the Cu 2p XPS spectrums of polished and HMNS samples are shown in Fig. 6. As shown in Fig. 6(b–d), there are two main peaks (Cu 2p3/2 and Cu 2p1/2) and some satellite peaks with weak intensity in all XPS spectrum of HMNS samples. The peak of Cu 2p3/2 of HMNS samples is at the binding energy of 933.6–934 eV and the peak of Cu 2p1/2 appears in the binding energy range of 952.1–952.8 eV. The satellite peaks are at a binding energy of 942.8 eV and 944.2 eV. According to the previous literatures [29,30], the XPS results mean the oxidations on the HMNS samples are copper passive film. And the contents of CuO on HMNS samples are different due to the different ratio of O/Cu ratio [30], which is affected by the EDM current shown in Table 4.

Fig. 5.

X-ray diffraction patterns of polished, 1#, 2# and 3# before anti-fouling experiment.

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Fig. 6.

Cu 2p XPS spectrums of (a) polished, (b) 1#, (c) 2# and (d) 3# samples.

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Fig. 7 shows the metallographic images of cross sections and statistical diagram of surface average grain sizes of different samples. As shown in Fig. 7, the crystalline structure of the HMNS is different from that of untreated matrix. Fig. 7(b–d) shows an obvious undulated interface on the HMNS, which is similar to the AFM morphology shown in Fig. 4. Three distinctive regions, namely, HMNS, Heat affected zone and Copper matrix, are observed from the cross section. The thickness of the HMNS increases with the increase of current and the average thickness value of 1#, 2# and 3# samples are respectively 10 μm, 13 μm and 18 μm. Besides, there are no obvious cracks appeared in the HMNS from the cross section images shown in Fig. 7(a–d). From Fig. 7(e), the crystalline structures of the HMNS are micro grains with grain size of 1.5–11 μm, which are smaller than that of the polished and the average grain sizes of the HMNS increases with the increase of current. It is because the increased current brings high temperature and more melted metal, which formed larger scale grain size after solidification.

Fig. 7.

Metallographic images of cross sections, and statistical diagram of surface average grain sizes of (a) polished, (b) 1#, (c) 2# and (d) 3#; (e) statistical diagram of surface average grain sizes for each sample.

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3.3Corrosion resistance of the HMNS

The corrosion resistance results of polished surface and HMNS samples in 3.5 wt.% NaCl solution are shown in Fig. 8. Table 5 is the electrochemical and Tafel parameters from polarization curves of the polished and HMNS samples, including the corrosion potential (Ecorr), corrosion current density (Icorr), anode Tafel slope (βa), cathode Tafel slope (βc) and corrosion rate. The Tafel parameters were calculated by using the Tafel extrapolation method [31]. As shown in Table 5, the Ecorr values of 1#, 2# and 3# are respectively 0.092 V, 0.077 V and 0.056 V, which are much positive than that of polished sample. The Ecorr indicates the resistance of material for corrosion behavior and the more positive Ecorr value of HMNS sample reflects the better corrosion resistance. Besides, the anode polarizability value is increases from 1# to 3# with the increase of current density. The more positive Ecorr and lower anode polarizability value suggest that the HMNS samples show better corrosion resistance in 3.5 wt.% NaCl solution than that of polished sample. The Ecorr value of HMNS samples is decrease from 1# to 3#, which indicates the micro size grain can enhance the corrosion resistance of HMNS. On the other hand, the Icorr value of HMNS sample, which reflects the corrosion rate, is increase with the increase of EDM current and lower than that of polished sample. It is due to the different content of CuO passive film on HMNS sample (Fig. 6), which can affect the charge transfer resistance of passive film. According to the result of Icorr, it means that the charge transfer resistance of passive film on HMNS sample decreases with the increasing EDM current. The different corrosion rates are caused by the different sizes and distribution of micro grains in HMNS. The different size of distribution of micro grains brought different grain boundary length in HMNS, which could affect Icorr due to the variety of grain boundary conduction and reactivity [32]. Moreover, the anode polarizability value reflects the corrosion resistance, which increased from 1# to 3# with the increasing Icorr, and means 1# with smallest size micro grain shows the best corrosion resistance among the HMNS samples. Thus, the overall corrosion resistance and corrosion rate of HMNS sample can be altered by the grain size [32], which was similar to pure Mg [33] and 316 L steel [34].

Fig. 8.

Corrosion resistance results of polished surface and HMNS samples in 3.5 wt.% NaCl solution: (a) polarization curves, (b) Nyquist plot, (c) (d) Bode plots.

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Table 5.

Electrochemical and Tafel parameters from polarization curves of the polished and HMNS before anti-fouling property experiment.

Sample  Ecorr (V)  Icorr (μA/cm2βa (mV/decade)  βc (mV/decade)  Corrosion rate (mm/y) 
Polished  0.022  24.2  0.064  0.219  0.278 
1#  0.092  8.07  0.033  0.582  0.093 
2#  0.077  9.24  0.035  0.593  0.106 
3#  0.056  11.4  0.046  0.651  0.131 

Fig. 8(b–d) shows the Nyquist and Bode plots of the polished and HMNS samples in 3.5 wt.% NaCl solution. The Nyquist plot shows that the radiuses of capacitive arc of the HMNS samples are much larger than that of the polished surface and their radius decrease with the increase of current. The result indicates the HMNS samples display better corrosion resistance than that of polished sample, and the corrosion resistance of HMNS samples are decrease with the increase of current, which is corresponding to the result of polarization curves. In addition, there are two capacitive arcs in the Nyquist plot (Fig. 8b), which indicates the polished surface and HMNS have two time constants (Fig. 8b and d). The equivalent circuit model shown in Fig. 8(b) is used to study the more details of the corrosion procedure of samples in NaCl solution. The equivalent circuit model of Nyquist plot for samples can be described as Rs(RfQf)(RctQdl), where Rs is the solution resistance, Rf is the charge transfer resistance of passive film, Rct is the charge transfer resistance, Qf and Qdl is the constant phase element. Table 6 gives the parameter of equivalent circuit model, the Rf and Rct of HMNS are higher than that of polished surface and their value decreases with the increasing current. The reduction of Rf of HMNS is due to the change of content of CuO passive film on HMNS samples [35], which is shown in Fig. 6 and Table 4. The decrease of Rct of HMNS reflects the corrosive rate of the HMNS in NaCl solution is increase, which may be caused by the increasing grain size of HMNS [32]. The ions in corrosion medium are more difficult to react with the HMNS due to the small average grain size and high density of grain boundaries allowing for faster diffusion and oxide formation [36,37].

Table 6.

Parameters of equivalent circuit model of sample obtained by ZsimpWin in 3.5 wt% NaCl solution.

Sample  Rs (Ω cm2Rf (Ω cm2Rct (Ω cm2Qf−1 cm−2 · s−n 10−4nf  Qdl−1 cm−2 s−n 10−4ndl 
Polished  9.077  683  2017  1.906  0.68  3.397  0.73 
1#  9.085  18110  14080  0.054  0.84  4.489  0.79 
2#  9.067  6190  11380  0.781  0.82  2.505  0.59 
3#  9.087  4649  3611  0.166  0.84  2.081  0.85 

Usually, the passive film is easily formed on the surface with a large amount grain boundaries [38]. In the NaCl solution, the main corrode ion is OH+ ion, which reacts with the metal surface to form oxide. According to the result in Table 6, the reduction of Rf and the increment of Qf from 3# to `# indicate the passive film on HMNS sample become thickening and compaction [35]. The corrode ion the NaCl solution is difficult to pass through the compaction passive film to corrode the surface. As a result, the compaction passive film provide the protection effect thoughtfully and lead to increase the charge transfer resistance of HMNS, which affects the radius of capacitive arc in the Nyquist plot (Fig. 8b). In addition, the passive film reduces the Icorr and helps to inhibit the ions from corroding the substrate [39,40].

3.4Anti-fouling property performance of HMNS

Fig. 9 shows morphologies of the polished, 1#, 2# and 3# after anti-fouling experiments for 72 h. As shown in Fig. 9(a), the polished sample displays a flat layer with a lot of cube like crystals with a dimension range of 5–70 μm and needle like crystals with a length about 40 μm. When compared to the polished samples, it is clearly see that there fewer fouling crystalline adhered on the samples surface, that is, fewer cube like or needle like crystals with smaller size observed on the surface.

Fig. 9.

Fouling morphologies of the (a) polished, (b) 1#, (c) 2# and (d) 3# after anti-fouling experiments for 72 h.

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The XRD results of the polished and HMNS after anti-fouling property experiment are shown in Fig. 10. For HMNS, characteristic peaks of CaCO3 fouling appear and the XRD peak intensity of fouling is lower than that of the polished. It indicates that the amount of fouling adhered on the polished surface is larger than that on the HMNS and the fouling types adhered on all test surfaces are calcite and aragonite. Besides, the XRD peak intensity of fouling gradually reduces from 1# to 3# fouling. It means the amount of fouling adhered on the HMNS gradually reduces from 1# to 3# fouling, and these results are consistent with the observations in Fig. 9.

Fig. 10.

X-ray diffraction patterns of polished, 1#, 2# and 3# after anti-fouling experiment.

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After anti-fouling property experiment, HMNS were sealed up around by silica gel with a test area of 10 mm2 for electrochemical test. The polished fouling, 1# fouling, 2# fouling and 3# fouling respectively represent the polished, 1#, 2# and 3# after anti-fouling property experiment. Fig. 11 shows the polarization curves of polished, 1#, 2# and 3# after anti-fouling property experiment in 3.5 wt.% NaCl solutions. As shown in Fig. 11, there is an obvious passivation area in polarization curve for all test samples. Besides, for the polished fouling, there are other two peaks appeared in the passivation area of polarization curve, which is different from that of the original polished sample showing in Fig. 8(a). The electrochemical and Tafel parameters of the polished fouling, 1# fouling, 2# fouling and 3# fouling are shown in Table 7. According to the Fig. 11 and Table 7, the Ecorr of polished fouling decreases significantly when compares to that of polished before anti-fouling property experiment. The Ecorr of all HMNS fouling slightly offset to negative potential direction. It means the polished surface is corroded seriously by fouling solution and the HMNS is slightly corroded, showing superior corrosion resistance. For the polished surface, the adhered fouling on the polished surface likes a layer and combines with the original copper passive film to affect the heat transfer surface. Hence, when the dynamic potential starts to scan from −0.6 V, the ions of CaCO3 fouling diffuses into the NaCl solutions due to the instability of the fouling and the process is a dynamic balance process. When the dynamic potential increases to −0.2092 V, the fouling layer dissolves and form many cracks in the layer. Then the Cl in the solution embed the cracks and react with the substrate, leading to the current peak appears in the polarization curve. When the dynamic potential continues to increase toward 0.2 V, the original copper passive film dissolves and the current peaks appear again in the polarization curve. For the HMNS fouling, there are few fouling adhered on the surface as shown in Fig. 9 and the corrosion behavior of fouling to the HMNS is inhibited due to the superior corrosion resistance of HMNS, which slightly affect the polarization curve of HMNS fouling. Moreover, after anti-fouling experiment, the HMNS exhibits better corrosion resistance than the polished according to the results of Ecorr and corrosion rate in Table 7. This phenomenon indicates the chemical stability of HMNS in the fouling solution and the influence of corrode ions to HMNS is little.

Fig. 11.

Polarization curves of polished, 1#, 2# and 3# after anti-fouling property experiment in 3.5 wt.% NaCl solution.

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Table 7.

Electrochemical and Tafel parameters from polarization curves of the polished and HMNS after anti-fouling property experiment.

Sample  Ecorr (V)  Icorr (μA/cm2βa (mV/decade)  βc (mV/decade)  Corrosion rate (mm/y) 
Polished fouling  −0.078  9.50  0.294  0.354  0.109 
1# fouling  0.089  7.39  0.062  0.338  0.085 
2# fouling  0.058  7.77  0.078  0.390  0.089 
3# fouling  0.039  9.27  0.108  0.315  0.107 

Fig. 12 shows the variations of fouling weight adhered on the polished, 1#, 2# and 3# surface. The fouling weight on the HMNS is significantly lower than that of the polished surface. The fouling weight of the HMNS is fluctuant. Combining the result shown in Fig. 9, the fouling adhered on the HMNS is loose and easy to fall off. Owing to the hydrophobic property of HMNS, air cushion trapped in the micro/nano structures of the HMNS, preventing the fouling crystals in the solution adhere and grow up on the HMNS. As for the polished surface, the fouling crystalline could adhere firmly on the oxidation layer and grow up rapidly with experimental time. As a result, the HMNS fabricated by EDM shows a better anti-fouling performance than the polished surface.

Fig. 12.

Curves of variations of fouling weight adhered on the polished, 1#, 2# and 3# surface.

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3.5Durability test of HMNS

The durability is an important factor for the industrial application of HMNS. According to the ASTM standard (ASTM D3359-09) [41], the hydrophobic property of HMNS after many times of semitransparent tape adhesion is measured by contact angle measuring instrument. The tape test of the HMNS sample per condition was repeated three times. The contact angle of HMNS after the tape adhesion is shown in Fig. 13. According to Fig. 13, the contact angle values of HMNS samples decrease with the increase of tape adhesion frequency, the drop range of contact angle for 1#, 2# and 3# is, respectively, 6.97%, 9.38% and 4.09%. It maybe because the micro/nano structure, such as the molten ball and nano porous, is destroyed when the tape is torn off from the surface. In addition, the HMNS after the durability test remains in the hydrophobic surface state, which can improve the anti-fouling property of surface. That is, the anti-fouling property of HMNS is durability and it is suitable for industrial application.

Fig. 13.

Curves of contact angle of HMNS after the tape adhesion experiment.

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4Conclusion

The hierarchical micro/nano structure (HMNS) fabricated by EDM shows the hydrophobicity and better corrosion resistance property and enhanced inhibition of fouling adhesion than that of polished surface. Only a few fouling adhered on the HMNS after being immersing in CaCl2 + Na2CO3 solution for 72 h at 50 ℃ and the adhered fouling on the HMNS is loose and easy to fall off. Moreover, the HMNS remains in the hydrophobic surface state after 50 times of semitransparent tape adhesion and the durability of HMNS is suitable for industrial application. There is an industrial prospect by EDM method to fabricate anti-fouling surface for heat transfer surface in one-step process with low cost and no pollution during machining procedure.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (NSFC, grant No. 51075075); Science and Technology Planning Project of Guangdong Province, China (grant No. 2014A010105046); Guangdong Colleges and Universities Create Innovative and Strong Natural Science Project, China (grant No. 2017KTSCX126); Opening Foundation of Guangdong Petrochemical Equipment Engineering and Technology Research Center, China (grant No. 2017JJ517010) and Training Programs of Innovation and Entrepreneurship for Undergraduates, China (grant number 201811845150).

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