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Facile fabrication of iron-based superhydrophobic surfaces via electric corrosion without bath
Accepted Manuscript Title: Facile Fabrication of Iron-based Superhydrophobic Surfaces via Electric Corrosion without Bath Author: Qinghe Sun Hongtao Liu Tianchi Chen Yan Wei Zhu Wei PII: DOI: Reference:
S0169-4332(16)30238-0 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.069 APSUSC 32587
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-10-2015 5-2-2016 5-2-2016
Please cite this article as: Q. Sun, H. Liu, T. Chen, Y. Wei, Z. Wei, Facile Fabrication of Iron-based Superhydrophobic Surfaces via Electric Corrosion without Bath, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract Abstract: Superhydrophobic surface is of wide application in the field of catalysis, lubrication, waterproof, biomedical materials, etc. The superhydrophobic surface based on hard metal is worth further study due to advantages of high strength and wear resistance. This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion and studys on the properties of as-prepared surface.
The
hydrophobic
properties
were
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superhydrophobic
characterized by a water sliding angle (SA) and a water contact angle
(CA) measured by the Surface tension instrument. A Scanning electron microscope was used to analyze
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the structure of the corrosion surface. The surface compositions were characterized by an Energy Dispersive Spectrum. The Electrochemical workstation was used to measure its anti-corrosion property.
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an
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The anti-icing performance was characterized by a steam-freezing test in Environmental testing
chamber. The SiC sandpaper and 500g weight were used to test the friction property. The research result shows that the superhydrophobic surface can be successfully fabricated by electrocorrosion on
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carbon steel substrate under appropriate process; the contact angle of the as-prepared superhydrophobic surface can be up to 152 ±0.5°, the sliding angle is 1-2°; its anti-corrosion property, anti-icing
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performance and the friction property all show excellent level. This method provides the possibility of industrialization of superhydrophobic surface based on iron substrate as it can prepare massive superhydrophobic surface quickly.
Keywords: superhydrophobic; electric corrosion; hard metal; the contact angle; corrosion resistance
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Highlights
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ability, mechanical durability and anti-icing performance.
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1.This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion without a bath. 2. It has a vital significance to the industrialization of the fabrication of superhydrophobic surface on hard metal due to the advantages such as low cost, high efficiency, can be prepared in a large area, easy to construct in the field. 3. The preparation approach is so facile and time-saving that it delivers an opportunity to construct a superhydrophobic surface on carbon steel substrate and provides the feasibility for industrial application of superhydrophobic surface. 4. The as-prepared surface has many excellent properties, like low adhesive property, anti-corrosion
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Facile Fabrication of Iron-based Superhydrophobic Surfaces via Electric Corrosion without Bath Qinghe Sun1, Hongtao Liu*1, Tianchi Chen2, Yan Wei1 , Zhu Wei1
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1.College of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116; 2. College of Mechanical & Electrical Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116 Corresponding author. Tel.: +86 516 83591916; fax: +86 516 83591916.
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E-mail address: [email protected] (H. Liu)
Abstract: Superhydrophobic surface is of wide application in the field of catalysis, lubrication, waterproof, biomedical materials, etc. The superhydrophobic surface based on hard metal is worth
an
further study due to its advantages of high strength and wear resistance. This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion and studies the properties of as-prepared superhydrophobic surface. The hydrophobic properties were characterized by a water sliding angle (SA) and a water contact angle (CA) measured
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by the Surface tension instrument. A Scanning electron microscope was used to analyze the structure of the corrosion surface. The surface compositions were characterized by an Energy Dispersive Spectrum. The Electrochemical workstation was used to measure its anti-corrosion property. The anti-icing
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performance was characterized by a steam-freezing test in Environmental testing chamber. The SiC
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sandpaper and 500g weight were used to test the friction property. The research result shows that the superhydrophobic surface can be successfully fabricated by electrocorrosion on carbon steel substrate under appropriate process; the contact angle of the as-prepared superhydrophobic surface can be up to 152 ±0.5°, and the sliding angle is 1-2°; its anti-corrosion property, anti-icing performance and the friction property all show an excellent level. This method provides the possibility of industrialization of superhydrophobic surface based on iron substrate as it can prepare massive superhydrophobic surface quickly.
Keywords: superhydrophobic; electric corrosion; hard metal; the contact angle; corrosion resistance
1. Introduction
Superhydrophobic surface is playing an increasingly important role in many fields because of its excellent performance, like self-cleaning anti-corrosion
[5]
[1]
, anti-icing
[2]
, reducing the friction resistance
[3-4]
,
, etc. The researchers have developed two methods to fabricate superhydrophobic
surfaces. One is constructing micro/nano scale rough structure directly on the surfaces with low energy, and the other is constructing micro/nano scale rough structure on the surfaces with high energy first and then reducing its surface energy by modifying the surface with low surface energy material [6-8]. At the present stage, most of the preparation of superhydrophobic surface concentrates on the nonmetal and soft metal, and the prepared surface often has low mechanical properties and poor friction performance. As carbon steel is the most widely used engineering material and steel superhydrophobic surface can reduce the corrosion to save energy and money, fabricating superhydrophobic surfaces on the carbon 3
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steel has a vital significance. Though it’s an age of more and more attention being paid to fabrication of iron-based superhydrophobic surfaces, there are only a few methods of preparing superhydrophobic surface on the steel substrate. Song
[9]
fabricated superhydrophobic PEEK/PTFE composite coating by changing the
curing temperature on 45 steel, the contact angle being up to 161°; Huang
[10]
prepared
superhydrophobic surface via nano composite electrodeposition on 316L stainless steel, the contact angle being up to 174.5°; Using nano-composite electro-brush plating technology, Xu Wenji
[11]
plated
nano-SiO2 coating on Q235 steel substrate, and the contact angle could be up to 169.8°; Liu Hongtao [12]
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fabricated lotus-leaf-like superhydrophobic surfaces via Ni-based nano-composite electro-brush plating, the water contact angle being 155.5°; Superhydrophilic and superhydrophobic TiO2 nanotube (TNT) arrays were fabricated on 316L stainless steel (SS) to improve corrosion resistance and [14]
achieved a superhydrophobic stainless
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hemocompatibility of SS by Huang Qiaoling [13]; MA Frank
steel surface by changing the morphology with a sandblasting process first and then modifying the
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surface energy with myristic acid, during which process a maximum contact angle of 167° was achieved with an average value of 158.3°. A combined electrodeposition and fluorinated modification approach to preparation of stable superhydrophobic film on SS316L substrate was presented by J Liang,
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D Li, et al [15], and results showed that the contact angle with 5µL water droplets on the nickel film after fluorinated modification was higher than 160° and the sliding angle with 10µL water droplets was as low as 1°when the current density fell in the range from 5 to 9 A/dm. A superhydrophobic surface (SHS) was prepared on steel via the synergetic corrosion of H2O2 and H2SO4 by Nan Wang, Dangsheng Xiong, [16]
, and the resultant surface exhibited superior anti-icing property in extremely condensing
condition; Dangsheng Xiong
[17]
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et al
obtained hierarchical structures on steel through a facile method:
combining hydrogen peroxide and anacid (hydrochloric acid or nitric acid), and the surfaces have
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demonstrated excellent anti-icing properties and resistance toward tape peeling 70 times; Using Nano [18]
fabricated low adhesive
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Cu/Al2O3 Ni-Cr composited electro-brush plating, Tianchi Chen
superhydrophobic surfaces whose water contact angle reaches to 156° and sliding angle is less than 2°. Most of the previous methods to fabricate superhydrophobic surface based on carbon steel are plating film or coating on the substrate, whereas the films or coatings are relatively vulnerable and hard to be large-area fabricated. And different methods have different disadvantages, like complex process, time-consumption, strictly limited conditions and high cost. Hence a low-cost, efficient, flexible method suitable for large-area preparation and excellent mechanical property of iron substrate is urgently needed.
Electric corrosion process is a feasible alternative as its required equipment is portable and the process is flexible. To the best of our knowledge, etching the steel substrate to prepare superhydrophobic surface with electric current without a bath is still scarcely reported. In this paper, we first put forward the idea that preparing the coarse micro structure on the steel substrate directly by electric corrosion method in an etching solution composed of NaCl and HCl without a bath. Compared with Chemical bath plating, electric corrosion without a bath greatly enhances the rate of corrosion. The obtained surface has an excellent superhydrophobic property, a mechanical property and a low adhesive property. Moreover, the method isn’t limited to the high requirements of the equipment and the working condition. So it has a vital significance to the industrialization of the fabrication of superhydrophobic surface on carbon steel substrate.
2. Experiments 4
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2.1 Materials Deionized water, NaOH, Na3PO4, NaCl (30%), HCl, Na2CO3. All the materials above were used for preparing electrolytic cleaning solution and electric corrosion solution. Absolute ethanol and stearic acid were used for preparing solution to decrease the free energy of the surface. The substrate is E355DD carbon steel and the dimensions of samples are 30 mm × 40 mm × 7 mm. 2.2 Fabrication of the superhydrophobic surface Preparation process is as follows: Firstly, the E355DD carbon steel substrate was polished mechanically using 400#，800# and 1000#
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metallographic abrasive papers to remove the surface oxide layer and put into a dry box after being degreased in ethanol.
The electrolytic cleaning solutions (Table 1) and electric corrosion solutions (Table 2) required for
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electric corrosion were prepared. Table 1 electrolytic cleaning solutions
Content/g/L
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Chemical composition NaOH
25.0g/L
Na2CO3
21.7g/L 50.0g/L
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Na3PO4 NaCl
2.4g/L
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Table 2 electric corrosion solutions Chemical composition HCl
25g/L 140g/L
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NaCl
Content /g/L
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Electric corrosion was taken in the steps as follows: Electrical cleaning → rinsing with deionized water → electric corrosion → rinsing with deionized water. The parameters of electric corrosion process are shown in Table 3. Table 3 the parameters of electric corrosion
Working procedure
Voltage(V)
Time(s)
Electrode moving speed(m/min)
Electrical cleaning
+6
30
4-8
Electric corrosion
-12
40
8-10
Fig.1 shows the schematic diagram of the electric corrosion method. The electrode is covered by cotton and gauze, and the cotton is used to store the solutions. In the electrical cleaning process, the anode connects the electrode and the cathode connects the sample while the electric corrosion process has the diametric connection.
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Fig.1 Schematic diagram of electric corrosion
The micro/nano scale roughness was built in the electric corrosion process. To obtain hydrophobic
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properties, surface modification by low surface energy materials was needed. The sample was put into stearic acid ethanol solution with a concentration of 0.02mol/L for about 60 min at 60°C, and then dried at 60°C for 30 min in a dry box. Fig.2 shows the schematic illustrations of formation of the
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superhydrophobic surface. The bare steel surface is wetted by water droplet, as is shown in Fig.2 (a). After electric corrosion progress, coarse micro structure is formed , and the surface becomes rough like Fig.2 (b) and presents a superhydrophobic property via the modification of stearic acid. The water droplet can almost keep a spherical on the as-prepared surface, which can be seen in Fig.2 (c); the
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water droplet and the surface are separated by a layer of air.
Fig.2 Schematic view of the formation of the superhydrophobic surface
The schematic view of surface grafting is shown in Fig.3. The metal-etching process always makes the surface bond with hydroxy groups after it is exposed in humid air in the reported literature[19], and –OH groups are excellent substrate for modification[20] because they are crucial for the formation of covalent bond. The coarse micro structure full of –OH groups after electric corrosion process is presented in Fig.3 (a). The covalent bond is formed by stearic acid (CH3 (CH2)16COOH) and the –OH groups when the rough structure full of –OH immerses in stearic acid solution, which can be seen in Fig.3 (b). Therefore, the electric corrosion processing can obtain the rough surface and the surface hydroxylation, to which high adhesion strength between stearic acid molecules and the substrate is attributed.
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Fig.3 Schematic view of surface grafting (b) The formation of covalent bond
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(a) –OH on the rough surface 2.3 Anti-corrosion tests
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Anti-corrosion ability of the as-produced surface was evaluated by a potentiodynamic polarization curve measured in 3.5 wt% NaCl aqueous solution by the electrochemical workstation.
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2.4 Mechanical durability tests
Abrasion test was performed to assess the mechanical durability of as-produced superhydrophobic surfaces. The produced surface was loaded with 500g weight and the weight of the sample was 57g, so
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the gravity of the weight could be calculated as 4641.7 Pa. The abrading surface was 800 grid SiC sandpaper, the sample was moved back and forth in a line for 120 cm distance, and the contact angle and sliding angle were tested every other 20 cm. 2.5 Anti-icing property
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A steam-freezing process was used to measure anti-corrosion performance in a humidity chamber. The original and superhydrophobic surface were put in the chamber, whose temperature was turned to 60°C with a 90% relative humidity. Then the temperature was adjusted to -20°C. Five hours later, the
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after icing for one sample.
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samples were taken out to observe the ice film on the surface and calculate the difference before and 2.6 Characterization
The field emission scanning electron microscopy (SEM, Quanta 250, FEI, America) was used to examine the morphological structure of the as-prepared surface. The surface compositions were obtained by an Energy Dispersive Spectrum (EDS, Quantax 400-10, Bruker, Germany).The contact angle (CA) was measured with approximately 5µL deionized water droplets by the JC2000D2A Surface tension instrument. The average CA value was determined by measuring the same sample at five different positions using angle measurement method. The SA was a critical angle between inclined surface and horizontal plane when droplets on the inclined surface just started to slide.
3. Results and discussion
3.1 Influence of the parameters of electric corrosion on hydrophobic properties The design of the experiment was based on the factors of the parameters of electric corrosion, including the corrosion voltage, corrosion time and electrode moving speed. The voltage ranged from 8V to 14V, and the electrode moving speed varied from 4m/min to 10m/min. The corrosion time selected 20s-50s in the premise of corrosion efficiency. The deionized water used to measure the contact angles was 5µL. 3.1.1 Influence of corrosion voltage on hydrophobic properties Fig.4 shows contact angles and sliding angles at different corrosion voltage. The corrosion time was 40s, the electrode moving speed was 8m/min, and the modifier concentration was 0.02mol/L (process 7
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parameter of electrolytic cleaning step was the same, as is shown in Table 1). The deionized water used to measure the contact angles was 5µL. As is subtly revealed in Fig.3, the contact angles increase with the increase of corrosion voltage and then decrease when the voltage varies from 8V to 14V. The CA is 144°when the voltage is 8V. It reaches the maximum (approximately 152°) at 12V, and then begins to reduce, being 147° at 14V. Fig.5 shows the surface topography at different corrosion voltage, and it can
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be seen that the surface topography presents straw shaped structure after electric corrosion.
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Fig.4 Contact angles and Sliding angles at different corrosion voltage.
Fig.5 Surface topography at different corrosion voltage (at 2000× magnifications). (a) 8V, (b) 10 V, (c) 12 V, (d) 14V 3.1.2 Influence of different electrode moving speed on hydrophobic properties Fig.6 shows contact angles and sliding angles at different electrode moving speed. The corrosion time was 30s, the corrosion voltage was 12V, and the modifier was 0.02mol/L (process parameter of electrolytic cleaning step was the same, as is shown in Table 1). The deionized water used to measure the contact angles was 5µL. Fig.7a, b, c, d are surface topography at the moving speed of 4m/min, 8
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6m/min, 8m/min and 10m/min, respectively. As is illustrated in Fig.6, the contact angle rises at first and then tends to be steady. The CA is 143.5°at the moving speed of 4m/min, which is because the workpiece will be heated seriously when electrode moving speed is too slow . As a result, the formation of microstructure is affected as is seen in Fig.7 (a). The CA changes a little, starting from the speed of 6m/min and reaching the maximum (about 150.5°) at the speed of 8m/min-10m/min. At this moment, the difference of their microstructure is very small, which makes no sense to increase again. Therefore,
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the appropriate moving speed is 8-10m/min.
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Fig.6 Contact angles and Sliding angles at different electrode moving speed
Fig.7 Surface topography at different electrode moving speed (at 2000×magnifications). (a) 4m/min, (b) 6m/min, (c) 8m/min, (d) 10m/min. 3.1.3 Influence of corrosion time on hydrophobic properties Fig.8 shows contact angles and sliding angles at different corrosion time. The electrode moving speed was 8m/min, the corrosion voltage was 12V and the modifier concentration was 0.02mol/L (process parameter of electrolytic cleaning step was the same, as was shown in Table 1). The deionized 9
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water used to measure the contact angles was 5µL. It can be seen that the contact angle increases with the increase of corrosion time and then tends to be stable, and there is no significant change of the microstructure (Fig.9). The CA is 146° when the corrosion time is 20s while it is 152° when the corrosion time is 40s. It neatly illustrates that it takes only 40s to construct rough structure which enables the surface of carbon steel to have superhydrophobic properties. This is just the advantage of
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electric corrosion method.
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Fig.8 Contact angles and Sliding angles at different corrosion time
Fig.9 Surface topography at different corrosion time (at 2000×magnifications). (a) 20s, (b) 30s, (c) 40s, (d) 50s. 3.2 Wettability analysis The EDS components analysis is shown in Fig.10, revealing that the surface mainly consists of carbon and iron, and the content of iron in corroded part reduces while the carbon content increases. The lamellar microstructure formed by carbon and iron can store a certain amount of air pockets which forms a Cassie state, which is an important reason why the as-prepared surface could have hydrophobic 10
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properties.
Fig.10 The EDS component analysis (the corrosion voltage was 12V, corrosion time was 40s, the electrode moving speed was 8m/min).
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As is illustrated in Fig.11, the bottom of water droplets appears bright reflection. It is because there is a thin air layer between the surface and the bottom of water droplets. Fig.12 is the CCD image (1000×) of the bare steel (a) and the as-prepared surface (b) in the water. It can be seen that water
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immerses in the gap of the bare surface and covers most of the surface (Fig.12a) while there are air
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pockets (the white part) composed of a large amount of air bubble on the as-produced surface. The air pockets cover most of the area (Fig.12b), which just explains why we can see bright reflection on the bottom of water droplets on the superhydrophobic surface. This phenomenon shows that the contact of the water and the rough structure is a solid-liquid/gas-liquid composite contact, being consistent with the Cassie model. The Cassie equation is:
In the formula, steel surface, and in this paper,
is the apparent contact angle of the interface,
is the contact angle on the bare
is the occupied percentage of solid phase. Fig.12 shows calculated values of
,
is 97°. It can be found that the air occupies 86.7 percent between the solid phase and
the liquid phase when the contact angle is 152°, and this also applies to the fact that the water on the as-prepared surface is Cassie contact model.
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Fig.11 Optical images of water droplets on the surface (at 1× magnifications).
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Fig.12 The image of the as-prepared surface in the water (at 1000× magnifications)
Fig.13 Calculated values of
at different contact angles.
The screenshots of dynamic contact angle are shown in Fig.14. From Fig.14a, we can see that the water droplet gradually changes from sphere to ellipsoid on the compression and lifting process, and finally falls to the surface, keeping wetting condition. As to the superhydrophobic surface , the water droplet changes from sphere to ellipsoid and finally remains sphere in the compression and lifting process, as is illustrated in Fig.14b. In addition, the volume of the water droplet barely changes. In other words, the as-prepared superhydrophobic surface has a much lower adhesive property than the bare steel surface.
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Fig.14 Screenshots of dynamic contact angle. (a) The bare steel surface. (b) The superhydrophobic surface.
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3.3 Anti-corrosion ability and chemical stability of the as-prepared superhydrophobic
As is shown in Fig.15. In a typical electrochemical polarization curves, a positive corrosion
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potential and low corrosion current density corresponds to a better corrosion resistance. As is seen in table 4, the corrosion potential of superhydrophobic surface is -0.27V while that of bare steel is -0.7V. Thus the superhydrophobic surface corrosion current density (1.259×10-7 A·cm-2) is far lower than bare
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steel (1.0×10-5 A·cm-2)，showing that superhydrophobic surface achieves an excellent anti-corrosion ability compared with bare steel. It is due to the fact that there are air pockets existing between water and the superhydrophobic surface, which resists Cl ion contacting the surface. However, the Cl ion can
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easily contact the bare steel surface.
Fig.15 Potentiodynamic polarization curve of superhydrophobic surface and bare steel substrate. Table.4 The Ecorr and Icorr values of the bare steel and superhydrophobic surface
Samples Ecorr/V
-2
Icorr/A·cm
Bare steel
Superhydrophobic surface
-0.7
-0.27
1.000×10
-5
1.259×10-7
The contact angles of different PH value droplets were used to assess the chemical stability of the sample. The water droplets was 5µL, whose results were shown in Fig.16. We can found that when the PH value varied from 1 to 14, the contact angles did not change a lot and the sample’s superhydrophobicity was still exhibited.
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Fig.16 Contact angles at different PH values.
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3.4 Mechanical durability
The results of the mechanical durability test are shown as Fig.18, it can be seen that 60cm is the turning point. In other words, when the pressure is 4641.7 pa, the abrasion distance is less than 60 cm
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and the sample can retain its superhydrophobic properties. As far as we know, there are few reports about the mechanical durability test on the steel substrate. The iron-based surface after electro-brush plating and blackening process can endure 80 cm of abrasion at 4000 pa on 800 grid sandpaper. [21] The superhydrophobicity of the surface based on 1045 steel decreased largely after dragging in one [17]
It’s a fact that the as-prepared surface can’t endure long
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direction for 110 cm under 16000 pa.
distance abrasion, and it’s indeed a limitation to be applied to extremely hard condition. Though the structure is destroyed, we can rebuild it neatly and quickly in a large scale. In addition, we don’t need
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to move it to special working condition. Therefore, the superhydrophobic surface fabricated by electric
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corrosion process has a good application in mechanical durability aspects.
Fig.17 The abrasion test for the sample
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Fig.18 Contact angles and sliding angles at different abrasion distance.
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The surface topographies of the samples after abrasion for 0.4 m at 4641.7Pa are displayed in Fig.19. Compared with the original superhydrophobic sample, there are scratches on the surface of the sample after abrasion (Fig19.b, c) and part of microstructure is worn by sandpaper. However, new
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rough structure is generated. In other words, the hierarchical structure is still existed. That’s why the surface remains superhydrophobicity. Besides, as a side-effect of the abrasion, microstructures that affect the sliding angle were partially damaged by the sandpaper. As a consequence, the sliding
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performance decreased relatively more than the contact property.
Fig.19 (a) Surface topography of the sample at 1000×magnification (b) Surface topography of the sample after abrasion at 600×magnification (c) Surface topography of the sample after abrasion at 1000×magnification 3.5 Anti-icing property Fig.20 shows the results of the steam-freezing test of the bare steel surface and the superhydrophobic surface respectively. It can be easily found that there is large ice film on the bare 15
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steel surface while small ice film can be found on the superhydrophobic surface. The weight of the bare steel sample varied from 57.469g to 57.619g, and for the superhydrophobic sample, the mass varied from 57.553g to 57.633g. That is, the superhydrophobic surface can reduce almost half ice film
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compared with bare steel surface.
Fig.20 Optical images of frost films after the steam-freezing and melt process at room temperature (a) bare steel surface (b) as-prepared superhydrophobic surface
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In order to quantify the superhydrophobicity of the surface during the anti-icing process, the periodic icing/melting process was carried out. The sample was dried before every cycle. The results are shown in Fig.21. After our test, the CA and SA only changed a little after 15 times icing/melting,
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especially for the contact angle. It dropped slightly, still keeping an excellent superhydrophobicity.
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SEM images of the surfaces of the sample before and after 15 icing/melting cycles are shown in Fig.22. We can see that the micro/nano scale rough structure barely changed. In other words, the robustness of the surface morphology is excellent in the application of anti-icing. It is clearly illustrated that the
contact angle drops slightly even after a number of cycles. So the as-prepared surface has a very excellent anti-icing property.
Fig.21 The CA and SA as a function of freezing/melting cycle times
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icing/melting cycles (b).
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4. Conclusions
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Fig.22 SEM images of the superhydrophobic surface before icing/melting test (a) and after 15
In summary, we have fabricated a superhydrophobic surface on carbon steel substrate via electric corrosion process which is easy to carry out as well as time-saving, and studied on the hydrophobic
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properties of the as-prepared surface. The experiments shows that the corrosion voltage has a dominant influence on the hydrophobic properties. When the voltage varies from 8V to 14V, the CA increases first and then decreases, while the corrosion time and electrode moving speed have relatively small effects on it. When the corrosion voltage is 12V, the electrode moving speed is 8-10m/min and the
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corrosion time is 40s, the contact angle of the surface after modified with stearic acid is the largest which is up to 152° and the sliding angle is 1-2°. The as-prepared surface also has been demonstrated that it has excellent low adhesive property, anti-corrosion ability and anti-icing performance. Although
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the as-produced surface can’t endure long distance abrasion and it’s indeed not easy to be applied in
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extremely hard condition, we can rebuild it neatly and quickly in a large scale without moving it to special working condition. Above all, the preparation approach is so facile and time-saving that it delivers an opportunity to construct a superhydrophobic surface on carbon steel substrate and provides the feasibility for the industrial application of the superhydrophobic surface.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (No.51475457) and Qing Lan Project.
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Physics a-Materials Science&Processing, 2008, 91:73-76.
[10] Huang Siya, Hu Yawei, Pan Wei. Relationship between the structure and Hydrophobic Performance of
3872-3876.
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Ni-TiO₂ Nanocomposie Coatings by Electrodeposiiong [J]. Surface&Coatings Technology. 2011, 205:
[11] Xu W, Zhao Y, Sun J, et al. Experimental study of super-hydrophobic surfaces obtained on steel matrix by
d
brush plating technique [J]. Zhongguo Jixie Gongcheng (China Mechanical Engineering), 2013, 24(1): 1-5. [12] Hongtao Liu, Xuemei Wang, Hongmin Ji. Fabrication of lotus-leaf-like superhydrophobic surfaces via
Ac ce pt e
Ni-based nano-composite electro-brush plating [J]. Applied Surface Science 288(2014)341-348.
[13] Qiaoling Huanga, Yun Yanga, Ronggang Hub, et al. Reduced platelet adhesion and improved corrosion resistance of superhydrophobic TiO2-nanotube-coated 316L stainless steel [J]. Colloids and Surfaces B: Biointerfaces (2015) 134-141.
[14] Frank M A, Boccaccini A R, Virtanen S. A facile and scalable method to produce superhydrophic stainless steel surface [J]. Applied Surface Science, 2014, 311(9): 753–757.
[15] Liang J, Li D, Wang D, et al. Preparation of stable superhydrophobic film on stainless steel substrate by a combined approach using electrodeposition and fluorinated modification [J]. Applied Surface Science, 2014, 293(4): 265-270.
[16] Wang N, Xiong D, Deng Y, et al. Superhydrophobic surface on steel substrate and its anti-icing property in condensing conditions [J]. Applied Surface Science, 2015, 355.
[17] Wang N, Xiong D, Deng Y, et al. Mechanically Robust Superhydrophobic Steel Surface with Anti-Icing, UV-Durability, and Corrosion Resistance Properties[J]. ACS applied materials & interfaces, 2015, 7(11): 6260-6272. [18] Tianchi Chen, Shirong Ge, Hongtao Liu. Applied Surface Science Fabrication of Low Adhesive Superhydrophobic Surfaces Using Nano Cu/Al 2 O 3 Ni-Cr Composited Electro-brush Plating [J]. Applied Surface Science 356(2015)81-90. [19] Saleema N, Sarkar D K, Paynter R W, et al. Superhydrophobic Aluminum Alloy Surfaces by a Novel One-Step Process[J]. Acs Appl.mater.interfaces, 2010, 2(9):2500-2502. 18
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[20] Pujari S P, Luc S, Marcelis A T M, et al. Covalent surface modification of oxide surfaces.[J]. Angewandte Chemie, 2014, 53(25):6322–6356. [21] Wei Y, Hongtao L, Wei Z. Preparation of anti-corrosion superhydrophobic coatings by an Fe-based micro/nano composite electro-brush plating and blackening process [J]. RSC Advances, 2015, 5(125):
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103000-103012.
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S0169-4332(16)30238-0 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.069 APSUSC 32587
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-10-2015 5-2-2016 5-2-2016
Please cite this article as: Q. Sun, H. Liu, T. Chen, Y. Wei, Z. Wei, Facile Fabrication of Iron-based Superhydrophobic Surfaces via Electric Corrosion without Bath, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract Abstract: Superhydrophobic surface is of wide application in the field of catalysis, lubrication, waterproof, biomedical materials, etc. The superhydrophobic surface based on hard metal is worth further study due to advantages of high strength and wear resistance. This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion and studys on the properties of as-prepared surface.
The
hydrophobic
properties
were
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superhydrophobic
characterized by a water sliding angle (SA) and a water contact angle
(CA) measured by the Surface tension instrument. A Scanning electron microscope was used to analyze
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the structure of the corrosion surface. The surface compositions were characterized by an Energy Dispersive Spectrum. The Electrochemical workstation was used to measure its anti-corrosion property.
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an
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The anti-icing performance was characterized by a steam-freezing test in Environmental testing
chamber. The SiC sandpaper and 500g weight were used to test the friction property. The research result shows that the superhydrophobic surface can be successfully fabricated by electrocorrosion on
d
carbon steel substrate under appropriate process; the contact angle of the as-prepared superhydrophobic surface can be up to 152 ±0.5°, the sliding angle is 1-2°; its anti-corrosion property, anti-icing
Ac ce pt e
performance and the friction property all show excellent level. This method provides the possibility of industrialization of superhydrophobic surface based on iron substrate as it can prepare massive superhydrophobic surface quickly.
Keywords: superhydrophobic; electric corrosion; hard metal; the contact angle; corrosion resistance
1
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Highlights
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ability, mechanical durability and anti-icing performance.
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1.This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion without a bath. 2. It has a vital significance to the industrialization of the fabrication of superhydrophobic surface on hard metal due to the advantages such as low cost, high efficiency, can be prepared in a large area, easy to construct in the field. 3. The preparation approach is so facile and time-saving that it delivers an opportunity to construct a superhydrophobic surface on carbon steel substrate and provides the feasibility for industrial application of superhydrophobic surface. 4. The as-prepared surface has many excellent properties, like low adhesive property, anti-corrosion
2
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Facile Fabrication of Iron-based Superhydrophobic Surfaces via Electric Corrosion without Bath Qinghe Sun1, Hongtao Liu*1, Tianchi Chen2, Yan Wei1 , Zhu Wei1
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1.College of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116; 2. College of Mechanical & Electrical Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116 Corresponding author. Tel.: +86 516 83591916; fax: +86 516 83591916.
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E-mail address: [email protected] (H. Liu)
Abstract: Superhydrophobic surface is of wide application in the field of catalysis, lubrication, waterproof, biomedical materials, etc. The superhydrophobic surface based on hard metal is worth
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further study due to its advantages of high strength and wear resistance. This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion and studies the properties of as-prepared superhydrophobic surface. The hydrophobic properties were characterized by a water sliding angle (SA) and a water contact angle (CA) measured
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by the Surface tension instrument. A Scanning electron microscope was used to analyze the structure of the corrosion surface. The surface compositions were characterized by an Energy Dispersive Spectrum. The Electrochemical workstation was used to measure its anti-corrosion property. The anti-icing
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performance was characterized by a steam-freezing test in Environmental testing chamber. The SiC
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sandpaper and 500g weight were used to test the friction property. The research result shows that the superhydrophobic surface can be successfully fabricated by electrocorrosion on carbon steel substrate under appropriate process; the contact angle of the as-prepared superhydrophobic surface can be up to 152 ±0.5°, and the sliding angle is 1-2°; its anti-corrosion property, anti-icing performance and the friction property all show an excellent level. This method provides the possibility of industrialization of superhydrophobic surface based on iron substrate as it can prepare massive superhydrophobic surface quickly.
Keywords: superhydrophobic; electric corrosion; hard metal; the contact angle; corrosion resistance
1. Introduction
Superhydrophobic surface is playing an increasingly important role in many fields because of its excellent performance, like self-cleaning anti-corrosion
[5]
[1]
, anti-icing
[2]
, reducing the friction resistance
[3-4]
,
, etc. The researchers have developed two methods to fabricate superhydrophobic
surfaces. One is constructing micro/nano scale rough structure directly on the surfaces with low energy, and the other is constructing micro/nano scale rough structure on the surfaces with high energy first and then reducing its surface energy by modifying the surface with low surface energy material [6-8]. At the present stage, most of the preparation of superhydrophobic surface concentrates on the nonmetal and soft metal, and the prepared surface often has low mechanical properties and poor friction performance. As carbon steel is the most widely used engineering material and steel superhydrophobic surface can reduce the corrosion to save energy and money, fabricating superhydrophobic surfaces on the carbon 3
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steel has a vital significance. Though it’s an age of more and more attention being paid to fabrication of iron-based superhydrophobic surfaces, there are only a few methods of preparing superhydrophobic surface on the steel substrate. Song
[9]
fabricated superhydrophobic PEEK/PTFE composite coating by changing the
curing temperature on 45 steel, the contact angle being up to 161°; Huang
[10]
prepared
superhydrophobic surface via nano composite electrodeposition on 316L stainless steel, the contact angle being up to 174.5°; Using nano-composite electro-brush plating technology, Xu Wenji
[11]
plated
nano-SiO2 coating on Q235 steel substrate, and the contact angle could be up to 169.8°; Liu Hongtao [12]
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fabricated lotus-leaf-like superhydrophobic surfaces via Ni-based nano-composite electro-brush plating, the water contact angle being 155.5°; Superhydrophilic and superhydrophobic TiO2 nanotube (TNT) arrays were fabricated on 316L stainless steel (SS) to improve corrosion resistance and [14]
achieved a superhydrophobic stainless
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hemocompatibility of SS by Huang Qiaoling [13]; MA Frank
steel surface by changing the morphology with a sandblasting process first and then modifying the
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surface energy with myristic acid, during which process a maximum contact angle of 167° was achieved with an average value of 158.3°. A combined electrodeposition and fluorinated modification approach to preparation of stable superhydrophobic film on SS316L substrate was presented by J Liang,
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D Li, et al [15], and results showed that the contact angle with 5µL water droplets on the nickel film after fluorinated modification was higher than 160° and the sliding angle with 10µL water droplets was as low as 1°when the current density fell in the range from 5 to 9 A/dm. A superhydrophobic surface (SHS) was prepared on steel via the synergetic corrosion of H2O2 and H2SO4 by Nan Wang, Dangsheng Xiong, [16]
, and the resultant surface exhibited superior anti-icing property in extremely condensing
condition; Dangsheng Xiong
[17]
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et al
obtained hierarchical structures on steel through a facile method:
combining hydrogen peroxide and anacid (hydrochloric acid or nitric acid), and the surfaces have
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demonstrated excellent anti-icing properties and resistance toward tape peeling 70 times; Using Nano [18]
fabricated low adhesive
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Cu/Al2O3 Ni-Cr composited electro-brush plating, Tianchi Chen
superhydrophobic surfaces whose water contact angle reaches to 156° and sliding angle is less than 2°. Most of the previous methods to fabricate superhydrophobic surface based on carbon steel are plating film or coating on the substrate, whereas the films or coatings are relatively vulnerable and hard to be large-area fabricated. And different methods have different disadvantages, like complex process, time-consumption, strictly limited conditions and high cost. Hence a low-cost, efficient, flexible method suitable for large-area preparation and excellent mechanical property of iron substrate is urgently needed.
Electric corrosion process is a feasible alternative as its required equipment is portable and the process is flexible. To the best of our knowledge, etching the steel substrate to prepare superhydrophobic surface with electric current without a bath is still scarcely reported. In this paper, we first put forward the idea that preparing the coarse micro structure on the steel substrate directly by electric corrosion method in an etching solution composed of NaCl and HCl without a bath. Compared with Chemical bath plating, electric corrosion without a bath greatly enhances the rate of corrosion. The obtained surface has an excellent superhydrophobic property, a mechanical property and a low adhesive property. Moreover, the method isn’t limited to the high requirements of the equipment and the working condition. So it has a vital significance to the industrialization of the fabrication of superhydrophobic surface on carbon steel substrate.
2. Experiments 4
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2.1 Materials Deionized water, NaOH, Na3PO4, NaCl (30%), HCl, Na2CO3. All the materials above were used for preparing electrolytic cleaning solution and electric corrosion solution. Absolute ethanol and stearic acid were used for preparing solution to decrease the free energy of the surface. The substrate is E355DD carbon steel and the dimensions of samples are 30 mm × 40 mm × 7 mm. 2.2 Fabrication of the superhydrophobic surface Preparation process is as follows: Firstly, the E355DD carbon steel substrate was polished mechanically using 400#，800# and 1000#
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metallographic abrasive papers to remove the surface oxide layer and put into a dry box after being degreased in ethanol.
The electrolytic cleaning solutions (Table 1) and electric corrosion solutions (Table 2) required for
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electric corrosion were prepared. Table 1 electrolytic cleaning solutions
Content/g/L
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Chemical composition NaOH
25.0g/L
Na2CO3
21.7g/L 50.0g/L
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Na3PO4 NaCl
2.4g/L
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Table 2 electric corrosion solutions Chemical composition HCl
25g/L 140g/L
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NaCl
Content /g/L
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Electric corrosion was taken in the steps as follows: Electrical cleaning → rinsing with deionized water → electric corrosion → rinsing with deionized water. The parameters of electric corrosion process are shown in Table 3. Table 3 the parameters of electric corrosion
Working procedure
Voltage(V)
Time(s)
Electrode moving speed(m/min)
Electrical cleaning
+6
30
4-8
Electric corrosion
-12
40
8-10
Fig.1 shows the schematic diagram of the electric corrosion method. The electrode is covered by cotton and gauze, and the cotton is used to store the solutions. In the electrical cleaning process, the anode connects the electrode and the cathode connects the sample while the electric corrosion process has the diametric connection.
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Fig.1 Schematic diagram of electric corrosion
The micro/nano scale roughness was built in the electric corrosion process. To obtain hydrophobic
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properties, surface modification by low surface energy materials was needed. The sample was put into stearic acid ethanol solution with a concentration of 0.02mol/L for about 60 min at 60°C, and then dried at 60°C for 30 min in a dry box. Fig.2 shows the schematic illustrations of formation of the
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superhydrophobic surface. The bare steel surface is wetted by water droplet, as is shown in Fig.2 (a). After electric corrosion progress, coarse micro structure is formed , and the surface becomes rough like Fig.2 (b) and presents a superhydrophobic property via the modification of stearic acid. The water droplet can almost keep a spherical on the as-prepared surface, which can be seen in Fig.2 (c); the
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water droplet and the surface are separated by a layer of air.
Fig.2 Schematic view of the formation of the superhydrophobic surface
The schematic view of surface grafting is shown in Fig.3. The metal-etching process always makes the surface bond with hydroxy groups after it is exposed in humid air in the reported literature[19], and –OH groups are excellent substrate for modification[20] because they are crucial for the formation of covalent bond. The coarse micro structure full of –OH groups after electric corrosion process is presented in Fig.3 (a). The covalent bond is formed by stearic acid (CH3 (CH2)16COOH) and the –OH groups when the rough structure full of –OH immerses in stearic acid solution, which can be seen in Fig.3 (b). Therefore, the electric corrosion processing can obtain the rough surface and the surface hydroxylation, to which high adhesion strength between stearic acid molecules and the substrate is attributed.
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Fig.3 Schematic view of surface grafting (b) The formation of covalent bond
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(a) –OH on the rough surface 2.3 Anti-corrosion tests
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Anti-corrosion ability of the as-produced surface was evaluated by a potentiodynamic polarization curve measured in 3.5 wt% NaCl aqueous solution by the electrochemical workstation.
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2.4 Mechanical durability tests
Abrasion test was performed to assess the mechanical durability of as-produced superhydrophobic surfaces. The produced surface was loaded with 500g weight and the weight of the sample was 57g, so
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the gravity of the weight could be calculated as 4641.7 Pa. The abrading surface was 800 grid SiC sandpaper, the sample was moved back and forth in a line for 120 cm distance, and the contact angle and sliding angle were tested every other 20 cm. 2.5 Anti-icing property
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A steam-freezing process was used to measure anti-corrosion performance in a humidity chamber. The original and superhydrophobic surface were put in the chamber, whose temperature was turned to 60°C with a 90% relative humidity. Then the temperature was adjusted to -20°C. Five hours later, the
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after icing for one sample.
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samples were taken out to observe the ice film on the surface and calculate the difference before and 2.6 Characterization
The field emission scanning electron microscopy (SEM, Quanta 250, FEI, America) was used to examine the morphological structure of the as-prepared surface. The surface compositions were obtained by an Energy Dispersive Spectrum (EDS, Quantax 400-10, Bruker, Germany).The contact angle (CA) was measured with approximately 5µL deionized water droplets by the JC2000D2A Surface tension instrument. The average CA value was determined by measuring the same sample at five different positions using angle measurement method. The SA was a critical angle between inclined surface and horizontal plane when droplets on the inclined surface just started to slide.
3. Results and discussion
3.1 Influence of the parameters of electric corrosion on hydrophobic properties The design of the experiment was based on the factors of the parameters of electric corrosion, including the corrosion voltage, corrosion time and electrode moving speed. The voltage ranged from 8V to 14V, and the electrode moving speed varied from 4m/min to 10m/min. The corrosion time selected 20s-50s in the premise of corrosion efficiency. The deionized water used to measure the contact angles was 5µL. 3.1.1 Influence of corrosion voltage on hydrophobic properties Fig.4 shows contact angles and sliding angles at different corrosion voltage. The corrosion time was 40s, the electrode moving speed was 8m/min, and the modifier concentration was 0.02mol/L (process 7
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parameter of electrolytic cleaning step was the same, as is shown in Table 1). The deionized water used to measure the contact angles was 5µL. As is subtly revealed in Fig.3, the contact angles increase with the increase of corrosion voltage and then decrease when the voltage varies from 8V to 14V. The CA is 144°when the voltage is 8V. It reaches the maximum (approximately 152°) at 12V, and then begins to reduce, being 147° at 14V. Fig.5 shows the surface topography at different corrosion voltage, and it can
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be seen that the surface topography presents straw shaped structure after electric corrosion.
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Fig.4 Contact angles and Sliding angles at different corrosion voltage.
Fig.5 Surface topography at different corrosion voltage (at 2000× magnifications). (a) 8V, (b) 10 V, (c) 12 V, (d) 14V 3.1.2 Influence of different electrode moving speed on hydrophobic properties Fig.6 shows contact angles and sliding angles at different electrode moving speed. The corrosion time was 30s, the corrosion voltage was 12V, and the modifier was 0.02mol/L (process parameter of electrolytic cleaning step was the same, as is shown in Table 1). The deionized water used to measure the contact angles was 5µL. Fig.7a, b, c, d are surface topography at the moving speed of 4m/min, 8
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6m/min, 8m/min and 10m/min, respectively. As is illustrated in Fig.6, the contact angle rises at first and then tends to be steady. The CA is 143.5°at the moving speed of 4m/min, which is because the workpiece will be heated seriously when electrode moving speed is too slow . As a result, the formation of microstructure is affected as is seen in Fig.7 (a). The CA changes a little, starting from the speed of 6m/min and reaching the maximum (about 150.5°) at the speed of 8m/min-10m/min. At this moment, the difference of their microstructure is very small, which makes no sense to increase again. Therefore,
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the appropriate moving speed is 8-10m/min.
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Fig.6 Contact angles and Sliding angles at different electrode moving speed
Fig.7 Surface topography at different electrode moving speed (at 2000×magnifications). (a) 4m/min, (b) 6m/min, (c) 8m/min, (d) 10m/min. 3.1.3 Influence of corrosion time on hydrophobic properties Fig.8 shows contact angles and sliding angles at different corrosion time. The electrode moving speed was 8m/min, the corrosion voltage was 12V and the modifier concentration was 0.02mol/L (process parameter of electrolytic cleaning step was the same, as was shown in Table 1). The deionized 9
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water used to measure the contact angles was 5µL. It can be seen that the contact angle increases with the increase of corrosion time and then tends to be stable, and there is no significant change of the microstructure (Fig.9). The CA is 146° when the corrosion time is 20s while it is 152° when the corrosion time is 40s. It neatly illustrates that it takes only 40s to construct rough structure which enables the surface of carbon steel to have superhydrophobic properties. This is just the advantage of
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electric corrosion method.
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Fig.8 Contact angles and Sliding angles at different corrosion time
Fig.9 Surface topography at different corrosion time (at 2000×magnifications). (a) 20s, (b) 30s, (c) 40s, (d) 50s. 3.2 Wettability analysis The EDS components analysis is shown in Fig.10, revealing that the surface mainly consists of carbon and iron, and the content of iron in corroded part reduces while the carbon content increases. The lamellar microstructure formed by carbon and iron can store a certain amount of air pockets which forms a Cassie state, which is an important reason why the as-prepared surface could have hydrophobic 10
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properties.
Fig.10 The EDS component analysis (the corrosion voltage was 12V, corrosion time was 40s, the electrode moving speed was 8m/min).
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As is illustrated in Fig.11, the bottom of water droplets appears bright reflection. It is because there is a thin air layer between the surface and the bottom of water droplets. Fig.12 is the CCD image (1000×) of the bare steel (a) and the as-prepared surface (b) in the water. It can be seen that water
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immerses in the gap of the bare surface and covers most of the surface (Fig.12a) while there are air
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pockets (the white part) composed of a large amount of air bubble on the as-produced surface. The air pockets cover most of the area (Fig.12b), which just explains why we can see bright reflection on the bottom of water droplets on the superhydrophobic surface. This phenomenon shows that the contact of the water and the rough structure is a solid-liquid/gas-liquid composite contact, being consistent with the Cassie model. The Cassie equation is:
In the formula, steel surface, and in this paper,
is the apparent contact angle of the interface,
is the contact angle on the bare
is the occupied percentage of solid phase. Fig.12 shows calculated values of
,
is 97°. It can be found that the air occupies 86.7 percent between the solid phase and
the liquid phase when the contact angle is 152°, and this also applies to the fact that the water on the as-prepared surface is Cassie contact model.
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Fig.11 Optical images of water droplets on the surface (at 1× magnifications).
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Fig.12 The image of the as-prepared surface in the water (at 1000× magnifications)
Fig.13 Calculated values of
at different contact angles.
The screenshots of dynamic contact angle are shown in Fig.14. From Fig.14a, we can see that the water droplet gradually changes from sphere to ellipsoid on the compression and lifting process, and finally falls to the surface, keeping wetting condition. As to the superhydrophobic surface , the water droplet changes from sphere to ellipsoid and finally remains sphere in the compression and lifting process, as is illustrated in Fig.14b. In addition, the volume of the water droplet barely changes. In other words, the as-prepared superhydrophobic surface has a much lower adhesive property than the bare steel surface.
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Fig.14 Screenshots of dynamic contact angle. (a) The bare steel surface. (b) The superhydrophobic surface.
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3.3 Anti-corrosion ability and chemical stability of the as-prepared superhydrophobic
As is shown in Fig.15. In a typical electrochemical polarization curves, a positive corrosion
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potential and low corrosion current density corresponds to a better corrosion resistance. As is seen in table 4, the corrosion potential of superhydrophobic surface is -0.27V while that of bare steel is -0.7V. Thus the superhydrophobic surface corrosion current density (1.259×10-7 A·cm-2) is far lower than bare
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steel (1.0×10-5 A·cm-2)，showing that superhydrophobic surface achieves an excellent anti-corrosion ability compared with bare steel. It is due to the fact that there are air pockets existing between water and the superhydrophobic surface, which resists Cl ion contacting the surface. However, the Cl ion can
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easily contact the bare steel surface.
Fig.15 Potentiodynamic polarization curve of superhydrophobic surface and bare steel substrate. Table.4 The Ecorr and Icorr values of the bare steel and superhydrophobic surface
Samples Ecorr/V
-2
Icorr/A·cm
Bare steel
Superhydrophobic surface
-0.7
-0.27
1.000×10
-5
1.259×10-7
The contact angles of different PH value droplets were used to assess the chemical stability of the sample. The water droplets was 5µL, whose results were shown in Fig.16. We can found that when the PH value varied from 1 to 14, the contact angles did not change a lot and the sample’s superhydrophobicity was still exhibited.
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Fig.16 Contact angles at different PH values.
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3.4 Mechanical durability
The results of the mechanical durability test are shown as Fig.18, it can be seen that 60cm is the turning point. In other words, when the pressure is 4641.7 pa, the abrasion distance is less than 60 cm
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and the sample can retain its superhydrophobic properties. As far as we know, there are few reports about the mechanical durability test on the steel substrate. The iron-based surface after electro-brush plating and blackening process can endure 80 cm of abrasion at 4000 pa on 800 grid sandpaper. [21] The superhydrophobicity of the surface based on 1045 steel decreased largely after dragging in one [17]
It’s a fact that the as-prepared surface can’t endure long
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direction for 110 cm under 16000 pa.
distance abrasion, and it’s indeed a limitation to be applied to extremely hard condition. Though the structure is destroyed, we can rebuild it neatly and quickly in a large scale. In addition, we don’t need
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to move it to special working condition. Therefore, the superhydrophobic surface fabricated by electric
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corrosion process has a good application in mechanical durability aspects.
Fig.17 The abrasion test for the sample
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Fig.18 Contact angles and sliding angles at different abrasion distance.
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The surface topographies of the samples after abrasion for 0.4 m at 4641.7Pa are displayed in Fig.19. Compared with the original superhydrophobic sample, there are scratches on the surface of the sample after abrasion (Fig19.b, c) and part of microstructure is worn by sandpaper. However, new
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rough structure is generated. In other words, the hierarchical structure is still existed. That’s why the surface remains superhydrophobicity. Besides, as a side-effect of the abrasion, microstructures that affect the sliding angle were partially damaged by the sandpaper. As a consequence, the sliding
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performance decreased relatively more than the contact property.
Fig.19 (a) Surface topography of the sample at 1000×magnification (b) Surface topography of the sample after abrasion at 600×magnification (c) Surface topography of the sample after abrasion at 1000×magnification 3.5 Anti-icing property Fig.20 shows the results of the steam-freezing test of the bare steel surface and the superhydrophobic surface respectively. It can be easily found that there is large ice film on the bare 15
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steel surface while small ice film can be found on the superhydrophobic surface. The weight of the bare steel sample varied from 57.469g to 57.619g, and for the superhydrophobic sample, the mass varied from 57.553g to 57.633g. That is, the superhydrophobic surface can reduce almost half ice film
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compared with bare steel surface.
Fig.20 Optical images of frost films after the steam-freezing and melt process at room temperature (a) bare steel surface (b) as-prepared superhydrophobic surface
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In order to quantify the superhydrophobicity of the surface during the anti-icing process, the periodic icing/melting process was carried out. The sample was dried before every cycle. The results are shown in Fig.21. After our test, the CA and SA only changed a little after 15 times icing/melting,
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especially for the contact angle. It dropped slightly, still keeping an excellent superhydrophobicity.
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SEM images of the surfaces of the sample before and after 15 icing/melting cycles are shown in Fig.22. We can see that the micro/nano scale rough structure barely changed. In other words, the robustness of the surface morphology is excellent in the application of anti-icing. It is clearly illustrated that the
contact angle drops slightly even after a number of cycles. So the as-prepared surface has a very excellent anti-icing property.
Fig.21 The CA and SA as a function of freezing/melting cycle times
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icing/melting cycles (b).
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4. Conclusions
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Fig.22 SEM images of the superhydrophobic surface before icing/melting test (a) and after 15
In summary, we have fabricated a superhydrophobic surface on carbon steel substrate via electric corrosion process which is easy to carry out as well as time-saving, and studied on the hydrophobic
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properties of the as-prepared surface. The experiments shows that the corrosion voltage has a dominant influence on the hydrophobic properties. When the voltage varies from 8V to 14V, the CA increases first and then decreases, while the corrosion time and electrode moving speed have relatively small effects on it. When the corrosion voltage is 12V, the electrode moving speed is 8-10m/min and the
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corrosion time is 40s, the contact angle of the surface after modified with stearic acid is the largest which is up to 152° and the sliding angle is 1-2°. The as-prepared surface also has been demonstrated that it has excellent low adhesive property, anti-corrosion ability and anti-icing performance. Although
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the as-produced surface can’t endure long distance abrasion and it’s indeed not easy to be applied in
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extremely hard condition, we can rebuild it neatly and quickly in a large scale without moving it to special working condition. Above all, the preparation approach is so facile and time-saving that it delivers an opportunity to construct a superhydrophobic surface on carbon steel substrate and provides the feasibility for the industrial application of the superhydrophobic surface.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (No.51475457) and Qing Lan Project.
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