Superhydrophobic surface containing cerium salt and organosilane for corrosion protection of galvanized steel

Journal Pre-proof Superhydrophobic surface containing cerium salt and organosilane for corrosion protection of galvanized steel Chongchong Li, Tingting Liang, Ruina Ma, An Du, Yongzhe Fan, Xue Zhao, Xiaoming Cao PII:

S0925-8388(20)30284-X

DOI:

https://doi.org/10.1016/j.jallcom.2020.153921

Reference:

JALCOM 153921

To appear in:

Journal of Alloys and Compounds

Received Date: 16 December 2019 Revised Date:

15 January 2020

Accepted Date: 17 January 2020

Please cite this article as: C. Li, T. Liang, R. Ma, A. Du, Y. Fan, X. Zhao, X. Cao, Superhydrophobic surface containing cerium salt and organosilane for corrosion protection of galvanized steel, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153921. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Credit authorship contribution statement

Manuscript Title: Superhydrophobic surface containing cerium salt and organosilane for corrosion protection of galvanized steel. Authors and contribution statement, Chongchong Li: Design & Methodology, Investigation, Writing, review&editing. Tingting Liang: Statistical analysis, Software. Ruina Ma: Funding acquisition, review. An Du: Review, Resources. Yongzhe Fan: Funding acquisition, Supervision, Conceptualization. Xue Zhao: Supervision, review&editing, Resources. Xiaoming Cao: Funding acquisition, Conceptualization, Supervision.

Graphical Abstract

Graphical abstract

Superhydrophobic

surface

containing

cerium

salt

and

organosilane for corrosion protection of galvanized steel

Chongchong Li , Tingting Liang, Ruina Ma, An Du , Yongzhe Fan*, Xue Zhao* , Xiaoming Cao *

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China; Key Lab for New Type of Functional Materials in Hebei Province, Tianjin 300130, China; Tianjin engineering and technology center for environmental friendly coating on pipeline；Tianjin 300132, China.

*Corresponding author: School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China. E-mail address: [email protected] (Yongzhe Fan); E-mail address: [email protected] (Xue Zhao); E-mail address: [email protected] (Xiaoming Cao).

1

Abstract: Corrosion-resistant superhydrophobic surface containing cerium salt and organosilane was developed over galvanized steel via a one-step solution-immersion method at ambient temperature. By controlling the deposition time, the maximum water contact angle (WCA) was determined to be 164.2±2°, exhibiting wonderful water-repellency. The morphology and composition were determined by field emission scanning electron microscope (FE-SEM), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), and X-ray photoelectron spectrometer (XPS). The corrosion performance of the superhydrophobic surfaces were evaluated by electrochemical technique. Potentiodynamic polarization curves (PPC) and electrochemical impedance spectroscopy (EIS) results indicated that the corrosion performance of the superhydrophobic surface containing cerium salt and organosilane was significantly enhanced. This process is an attempt to achieve multi-functional surfaces for self-cleaning and corrosion protection of galvanized steel. The method is facile, eco-friendly and cost-saving, making it an attractive strategy potentially used for large scale fabrication and real applications. Keywords: Superhydrophobic; Corrosion resistance; Solution-immersion; Galvanized steel.

2

1. Introduction Zinc and its alloys have been extensively employed as corrosion protection coatings to extend the life of steel structures through cathodic protection.[1] Galvanizing is regard as a good choice for many applications, for instance electricity, transportation, communication, energy, harbor facilities, etc., due to the excellent anti-corrosion performance of zinc coating. The galvanized layer provide effective corrosion resistance for steel substrate when exposed in general atmospheric environment. However, zinc is relatively lively and easy to be corroded, especially in moist and aggressive environments.[2] Damp and corrosive environments can accelerate zinc corrosion, which fail to offer sufficient protection for the steel substrate against corrosion. To further enhance the corrosion performance of the galvanized steel post-treatment is usually adopted. For decades, chromate conversion film is regarded as a cost-effective method to protect metals against fast corrosion. However, it has been strictly controlled for the reason of harmful to human body and environments.[3] To solve this problem, many alternatives have been emerged to avoid using chromates, such as molybdate[4],

silicates[5],

rare

earth

inhibitors[6],

sol-sel

films[7],

organosilanes[8-9], and other techniques[10-11]. Among them, organosilanes, especially modified with corrosion inhibitors or nanoparticles[12-15], are

3

considered to be effective methods for anti-corrosion of galvanized steel and other metals due to the economical, effectiveness, environmental and easy operation characteristics. Although the use of silanes and rare earth salts have shown improved corrosion resistance over the galvanized steel. Nevertheless, a serious disadvantage is that the protection film is thin and easy to absorb moister. Therefore, corrosive liquids are easy to contact with the surface, and corrosive ions (such as H2O, O2, and Cl-) can still penetrate the film through small cracks and defects, leading the corrosion initiation. It is still a challenge for the film to withstand the destruction of aggressive ions in corrosive environment, which fail to provide long-term protection. Therefore, the integrated performances of these films should be further improved. Recently, bionic superhydrophobic surfaces whose water contact angle are greater than 150°, which are inspired by the lotus effect, have emerged as a novel technique to protect metals against corrosion.[11,16-17] These special non-wetting surfaces on metals can prevent the intrusion of corrosive liquids into the coating/metal interface by constructing an air film on the surface of metal substrates. Therefore, it is a desirable choice to construct a superhydrophobic film over the surface of galvanized steel. In addition, the superhydrophobic film can not only enhance the corrosion performance, but also

impart

multi-functional

performances,

4

e.g.

self-cleaning[18],

anti-icing[19], and anti-fouling[20], drug reduction[21], etc. It has shown great potential for anti-corrosion of galvanized steel, and such a functional coating can also reduce the maintenance costs over the life of steel structures. It it generally accepted that suitable roughness and low surface energy are two key factors in achieving superhydrophobic surfaces. Until now, superhydrophobic surfaces have been successfully achieved on variety of metals through multi-step and one-step approaches.[22-28] Among them, the one-step process is much more attractive, by which hierarchical structures and low surface energy can be simultaneously achieved. Many researches have proven that super-hydrophobic films exhibit promising anti-corrosion properties in mild environment. However, when exposed to corrosive solutions, the barrier effect of the superhydrophobic films will rapidly decay or even disappear, failing to meet the long-term corrosion protection requirements for metal substrates. Therefore, how to continue to provide additional

corrosion

protection

to

the

substrate

after

losing

its

superhydrophobicity is also a problem worth considering. As far as we know, there are yet no studies available in literature that report the joint effect of superhydrophobicity with silane and cerium nitrite. The preparation methods, formation mechanism, especially corrosion performances have been rarely reported, and their potentials for improving the corrosion resistance urgently

5

need to be clarified. In the present study, a simple and effective one-step electroless deposition approach for constructing a corrosion-resistant superhydrophobic surface over the galvanized steel is proposed. The morphology, composition, and wetting behavior of the obtained superydrophobic surface were investigated. The corrosion performance as well as its anti-corrosion mechanism were studied. This approach is an attempt to achieve multi-functional surfaces for self-cleaning and corrosion protection of galvanized steel. We believed that this facile, eco-friendly and cost-saving method is an attractive strategy potentially used for large scale fabrication and in various practical applications. 2. Experimental 2.1. Materials and chemicals The Q235 steel (chemical composition: C:0.16%, Mn:0.35%, Si0.30% , S:0.05%, P:0.04%, and Fe:99.10%) were obtained from Tianjin yinchi steel sale Co,. Ltd, China. Zinc alloy (99.995%) was supplied by Yuguang Gold Lead Co., Ltd., China. Vinyltrimethoxysilane(CH2=CH-Si-(OCH3)3), cerium (III) nitrate hexahydrate and silver nitrate, ethanol (anhydrous, 99.5%), nitric acid, sodium hydroxide, acetone, and stearic acid (SA) were procured from Saan Chemical Technology (Shanghai) Co., Ltd.

6

2.2. Fabrication of the hot-dip galvanized (HDG) substrates Before galvanizing, the Q235 steel samples, with dimensions of 3.5 cm×5 cm×0.25 cm, were first undergo shoot blasting to mechanically remove the oxide layer. Afterwards, the specimens were cleaned with running water, fluxed, and dried prior to zinc bath. After pretreatment processes, the samples were dipped in the zinc bath at about 450

for 60s. After dipping process, the

samples were slowly withdraw and cooled in water. The thickness of the prepared zinc coating is about 50µm. 2.3. Preparation of the superhydrophobic surface The preparation of the functional solution is the key process for fabricating of superhydrophobic surface over HDG. First, a certain amount of vinyltrimethoxysilane was dissolved into a mixture of ethanol and deionized water, where silane : water : ethanol = 4: 6 : 90 (v/v). To ensure sufficient hydrolysis of the silane solution, it was magnetically stirred for 48h at ambient temperature (~25℃) before use. Adjust the pH value of the solution to 4 with 1mol/L acetic acid. Then, a certain amount of stearic acid, as well as a water solution containing silver nitrite and cerium (III) nitrate hexahydrate were added into the prepared silane solution under magnetic stirring at 60 ℃ for 20min. The corresponding concentration for stearic acid, silver nitrite and cerium (III) nitrate hexahydrate of the resulted functional solution were 0.05

7

mol/L, 0.02mol/L, and 0.005mol/L, respectively. The chemically cleaned HDG samples were first immersed into 1mol/L sodium hydroxide solution for 1min under ultrasound conditions to hydroxylate the surface. Then the specimens were soaked in the functional solution for various duration time at room temperature, and withdraw slowly, washed with ethanol and dried in air. Then the prepared samples were cured in an oven at 100

during 30 min. For comparison, samples treated with

cerium-modified silane solution and superhydrophobic surface without modified silane were also prepared. In addition, the galvanized plates without any treatment were also used as a reference. The schematic of the fabrication process of the superhydrophobic coatings were shown in Fig.1. The samples prepared at different condition in this paper were also listed in Table 1 for ease of understanding. 2.4. Characterization The topography of the superhydrophobic surfaces were observed via FESEM (Quanta 450 FEG). The crystal structures and chemical composition of the prepared surfaces were determined by XRD and XPS (radiation: Al Kα), respectively. The C1s peak (284.6 eV) was used as standard to correct the original data. The chemical grafting and functional groups from the superhydrophobic surface was determined by FTIR spectroscopy (V80,

8

Bruker, Germany). DAS30 contact angle testing system (KRUSS, Germany) was used to characterize the water contact angles (WCA) and sliding angles (SA) at ambient temperature (~25℃). The volume of water droplet used for testing was 10 µL. Measure at least five different locations on the same sample and average them. 2.5. Electrochemical corrosion tests The PPC and EIS testing of the prepared surfaces were evaluated by CHI-660E (Chenhua Instruments, China) electrochemical instrument, using 3.5wt% NaCl solution as electrolyte. A three-electrode system were used for testing, where the obtained samples acted as working electrode with testing area of about 1cm2, platinum sheet supplied as the control electrode, and saturated calomel electrode (SCE) as a reference electrode. Before EIS measurements, the samples were put into the electrolyte solution for 30min to obtain a stable open circuit potential (Eocp). The PPC curves were measured at scan rate of 0.001V/s and range of ±0.2V vs. Eocp. The frequency ranges used for the EIS measurements from 10-2 Hz 100 kHz. At least three samples were tested to confirm the repeatability of the result. 3. Results and discussion 3.1. Morphology and Wettability One major advantage of this one-step self-deposition approach,

9

comparing with the conventional two-step methods or other one-step technique, due to the fact that the fabrication process is greatly simplified, no special equipment is required, low-cost and high efficiency, and it is beneficial to industrialization. Fig.2 depicts the typical SEM pictures of the bare sample and the samples after deposited in the functional solution for various time. One can clearly see from Fig.2a that the surface of the bare sample is relatively smooth, except for some etching holes and defects. After deposited in the solution for a short time (Fig.2b 30s), a thin film with flower-like structures can be found on the substrate surface. Prolonging the deposition time leads to a rougher surface, and some dendritic structures begins to appear (the white areas in Fig.2c 60s). However, these dendritic structures may have been nucleated at the beginning of the reaction. As the deposition time is further extended, the substrate is almost covered by the dendritic structures with hierarchical roughness (Fig.2e 300s). Extend the deposition time to 600s, more porous and coarse structures are formed, which provide sufficient roughness to achieve superhydrophobicity. The illustrations in the SEM pictures are the water contact angle photos on the corresponding surfaces. It shows that the surfaces treated by the functional solution becomes more and more water-repellent with lengthening deposition time. Super water-repellent surface is achieved when the

10

deposition time is upon 600s as illustrated in the insets of Fig.2f. However, the achievement of superhydrophobicity is generally considered to be the collective effect of high surface roughness and low-surface-energy, which is similar to the lotus leaf. Fig. 3 quantitatively illustrates the time-dependent film thickness and roughness. As depicted in Fig.3, the thickness constantly increased with the duration time, and similar trend was also observed for the surface roughness. After deposited in the solution for 600s, a film with a thickness of 8 µm is achieved. Fig.4a describes the evolution of contact angles and sliding angles with deposition time. It reflects that the WCA increase with the extension of deposition time. While, the sliding angle exhibits an opposite trend. Fig.4b depicts the state of droplets with different sizes on real lotus leaf (top) and the superhydrophobic coating (bottom) obtained at 600s. It can be seen that the droplets are spherical due to the capture of air in the rough structures, and this liquid-solid contact mode is also called Cassie state.[29] The results showed that a superhydrophobic surface similar to the water-repellent lotus leaf was successfully prepared by this simple one-step self-deposition technique. In Cassie-Baxter mode[29], the solid-liquid interface is considered as a complicated surface constituted of air and solid. The liquid-solid contact ratio

11

can be calculated by Cassie-Baxter equation:[30-31] cos θ = Ф1cos θ0 - Ф2

(1)

In Eq. (1), the Ф1 and Ф2 represents the solid and air ratio of the complicated interface respectively with Ф 1 + Ф 2 = 1; θ and θ0 stand for the WCA of the prepared SH@MS sample and the flat surface, respectively. The air captured in the rough structures reduce the contact area of liquid-solid and increase the liquid-air fraction. The SH@MS sample exhibits a WCA of 164.2° and the flat surface offers a WCA of 72°. According to Eq.1, the results for Ф1 and Ф2 are computed as 0.029 and 0.971, respectively. It means that the liquid-vapor ratio is 97.1% of the contact area and solid-liquid contact area is merely2.9%, thus resulting in the low adhesion and water-repellent property. 3.2. Composition analysis of the water-repellent surface 3.2.1 EDS analysis To determine the elemental composition of the SH@MS, EDS was conducted. EDS results of the untreated substrate confirms that it was mainly composed of zinc element. Upon treated with the functional solution (30s), the elements C , O, Si, Ce, Zn and Ag were detected, which suggested that a series of complicate reactions occurred between the solution and substrate. Prolonging the deposition time, the same elements were detected with an

12

increase in C and Ag element, and a decrease in Zn element. However, there was no significant change in the mass fraction of the Si and Ce elements. The Ag element may due to the displacement reaction between zinc and silver ions, where silver ions is reduced to its metal form (with micro-nano porous pattern) and deposited on the substrate. Fig.5 shows the elemental mapping of the SH@MS obtained at deposition time of 600s. One can clearly see that the distribution characteristics of the C, O and Si elements are similar to those of Ag. This may be due to the coarse porous structure of silver and its high specific surface area on which a massive of stearic acid and silane can be adsorbed. Due to the presence of many hydroxyl groups (-OH) on the porous silver, and dehydration condensation reaction may occurred between carboxyl group (-COOH) from stearic acid and hydroxyl groups (-OH) to form a covalent bond by COOMe [32]. The Ce element relatively evenly distributed on the surface, which indicated that cerium compounds maybe formed and deposited on the surface. The zinc element in the image may due to the formation of zinc compounds, which was further confirmed by XRD and XPS techniques. 3.2.2. FTIR analysis FTIR was used to determine the bonding property of the obtained superhydrophobic surface. The FTIR results of superhydrophobic surface,

13

vinyltrimethoxysilane, and stearic acid at 4000-400 cm-1 were presented in the Fig.6, respectively. Fig.6a illustrates the representative adsorption peaks of the vinyltrimethoxysilane. The adsorption peaks appeared at 2976 cm-1 and 2843 cm-1 ascribed to the C-H stretching vibration from Si-OCH3 groups.[33] The peaks appeared at 1456 cm-1 agree to the C=C stretching vibration and the peak at 1082 cm-1 ascribed to Si-O-C from vinyltrimethoxysilane (VTES), respectively.[34] The adsorption peaks that belong to asymmertric vibration of -CH3 (2953 cm-1), asymmertric and symmetric vibrations of -CH2 (2918cm-1, 2849 cm-1, 740 cm-1, and 720 cm-1) of alkyl chain, can be observed in both spectra of stearic acid and superhydrophobic surface.[32] Furthermore, the adsorption peak at 1702 cm-1, which belonged to the carboxyl group (-COOH) of stearic acid, was also disappeared in the spectrum of superhydrophobic surface, suggesting that deprotonation occurred and new chemical bond was formed[35]. It was found that two new peaks centered at 1538 cm-1 and 1394 cm-1 emerged in the spectrum of superhydrophobic coating, which ascribe to the asymmertric and symmetric stretch vibrations of -COO group.[22] Moreover, the Si-O-Si bond at bout 1107 cm-1 also appeared in the spectrum of superhydrophobic coating, suggesting that silane participates in the film formation process and dehydration condensation reaction occurred to form a crosslinked polymer structure.[36] The results

14

demonstrated that stearic acid and silane were grafted on the surface through chemical bonding during the formation process of superhydrophobic surface.The substrate was covered with long alkyl chain groups, which provide low surface energy to the surface for achieving superhydrophobicity. 3.2.2. XRD analysis Further analyses of the prepared superhydrophobic surface was performed

using

XRD.

Fig.7

depicts

the

XRD

patterns

of

the

superhydrophobic surface prepared at 600s. It can be clearly seen that three strong peaks at 2 theta angle of 36.3°, 43.2°, and 54.3° were detected in the spectrum, which correspond to the (002), (101), and (102 ) planes of zinc (JCPDS Card No. 04-0831), respectively. The small peaks appearing in the spectra at 21.4°, 39.1°, and at 41.9° are assigned to Zn(OH)2 and ZnO, respectively.[37] The peak located at 38.1° were also observed in the spectrum, which correspond to Ag (111) crystal plane, confirming that the porous structures in SEM image of the surface were Ag.[38] This is also a complementary with EDS results, where Ag element show a similar shape of the porous structures. The peaks at low diffraction angle (5-20°) with 2 theta 6.5°,8.73°,10.9°, and 19.6° were observed in the spectrum, which derived from zinc stearate (Zn(SA)2),[32] resulting from a reaction of zinc and stearic acid upon immersed in the functional solution.The appearance of zinc stearate

15

in the spectrum also confirm the existence of low surface energy groups (CH3 and CH2). These results indicated that the SH@MS surface were mainly comprised of silver, zinc stearate and Zn(OH)2/ZnO. However, some other phase structures may also exist in the coating, but was not detected due to its limited content. 3.2.3 XPS analysis Fig.8 presents the XPS spectra of the superhydrophobic surface prepared at 600s . It can be seen that the C 1s, Ag 3d, O 1s, Si 2p, Ce 3d, and Zn 2p were detected in Fig. 8a. Fig. 8b-g are the corresponding peak fitting results of the detected elements at high-resolution. The C1s region can be fitted by two distinct peaks. The main peak at 284.7 eV belonged to the C-C/C-H groups, which was sourced from -(CH2)n groups in stearic acid.[39] Another peak at 288.4 eV may ascribe to the C=O/O-C=O groups derived from stearic acid.[39] These results suggested that the functional groups from stearic acid was successfully bonded on the substrate. The O 1s spectrum can be decomposed into three peaks. The main peak at 531.6 eV is ascribed to C=O group and the peak at 532.5 eV can be ascribed to the C-O group. The peak located at 530.4 eV is ascribed to the SiO2 species, suggesting that a Si-O-Si network structure was formed in the obtained coating. For Ag 3d region, the peaks positioned at 368.2 eV and 374.2 eV can ascribe to silver, indicating

16

that silver was deposited on the substrate. These results were also in consistent with XRD result, where Ag (111) was detected. For Si 2p, three fitted peaks can be observed, the main peak at 102.6 eV was attributed to the Si-O-Si bond, and the peaks at 102.1 eV and 103.1 eV were ascribed to the Si-O-Me and SiO2 bond [40]. This result confirmed that the silane was successfully grafted in the obtained superhydrophobic coating, and dehydration condensation reaction occurred between silane and silane, and between silane and substrate, forming a crosslinked Si-O-Si network structure. Fig.8f depicts the typical spectrum of Ce 3d detected from the prepared surface. The XPS spectrum of cerium compounds is complicated owing to the existence of 4f electrons.[41] The Ce 3d spectrum usually composed of two multiples (u and ν), which were related to the spin-oribit split of 3d5/2 and 3d3/2 , respectively.[42] The peaks labled as ν-u ( 883.7 eV and 901.7 eV ), v"-u" ( 888.3 eV and 907.9 eV ), and v‴-u‴ ( 897.9 eV and 917.6 eV) were derived from Ce4+. Another groups, i.e., ν0-u0 (881.9 eV and 900.4eV) and ν′-u ′ (885.7 eV and 904.4 eV) were arised from Ce3+. It can be inferred that the Ce element present in the obtained coating as a trivalent or tetravalent oxide or hydroxide, in which it will act as corrosion inhibitor. For Zn 2p region (Fig.8g), three peaks can be fitted in the spectrum. The peak centered at about 1045.1eV is ascribed to Zn-O bond of Zn 2p1/2 , and the peaks at

17

1021.7 eV and 1022.2 eV are due to the Zn-O bond of Zn 2p3/2 [39]. The spin-energy between Zn 2p3/2 and Zn 2p1/2 is about 23eV, suggesting that Zn element presents in its ion form [32]. Meanwhile, the atomic% ratio of C, O, Zn determined from the superhydrophobic surface was analogously 36:4:1, which was consistent with the molecule structure of zinc stearate [Zn(CH3(CH2)16COO)2]. The XPS results indicated that many complicated reactions occurred in the film formation process. The XPS results also confirmed the presence of low surface energy groups due to the detection of silver (covered with stearic acid) and zinc stearate, which contribute to the realization of superhydrophobicity. 3.3. Formation process of the superhydrophobic surface Based on the comprehensive analysis (SEM, EDS, FT-IR, XRD and XPS) above, a possible schematic diagram was proposed to illustrate the formation mechanism of the superhydrophobic surface, as shown in Fig.9. It can be speculated that the formation of the superhydrophobic surface is a complex process, accompanied with the dissolution of the zinc layer, the deposition of silver, the formation of an alkaline passive layer, the adsorption of silane, and the formation of stearates. One of the main reactions can be ascribed to the spontaneous chemical displacement deposition reaction between galvanized layer and Ag+ due to the electrochemical potential difference, where the silver

18

ions were reduced to metal silver with porous micro/nano structures and zinc was oxidized to zinc ions (Zn2+), according to Eq(2). This process can provide hierarchical

rough

structures,

which

is

indispensable

to

its

superhydrophobicity. Zn + 2Ag+ → 2Ag + Zn2+

(2)

According to the theory of cathode film formation proposed by Hinton[43], upon the galvanized substrate was immersed in the functional solution, numerous micro cells were formed at the solution/substrate interface. The micro-anode reaction was the dissolution of zinc, and the cathodic reaction was the reduction of oxygen (or H+), resulting in the formation of zinc ions Zn2+ and the OH- at the solution/substrate interface.These electrochemical reactions caused a large number of hydroxides (OH-) at the electrode interface, leading to a local increase in pH value. Therefore, the OHrich area will become the core of the hydroxide layer. With the further increase in pH value, the zinc ions(Zn2+) and cerium ions(Ce3+) would be orderly deposited at the metal/solution interface according to the solubility rule.[44] Thus, an alkaline passive layer (including zinc hydroxides and cerium hydroxides) was formed at the interface. According to the following reactions (Eq.3~Eq.9): Zn - 2e- → Zn2+

19

(3)

2H2O + O2 + 4e- → 4OH-

(4)

or 2H+ + 2e- → H2

(5)

Zn2+ + 2OH- → Zn(OH)2

(6)

Zn(OH)2 → ZnO + H2O

(7)

Ce3+ + 3OH- → Ce(OH)3

(8)

2Ce(OH)3 → Ce2O3 + 3H2O

(9)

Simultaneously, the hydrolyzed silane molecules in the mixed solution were able to frame hydrogen bonds between Si-OH groups from the solution and metal hydroxyls (Me-OH, where Me stands for metal atom) on the substrate[36]. Meanwhile, the silane molecules can also cross-linked to each other through hydrogen bonding. During curing process, the Si-OH groups and Me-OH groups further condensed to form metallo-siloxane (Me-O-Si) and cross-linked to form a network layer through Si-O-Si covalent bonds over the substrate, according to Eq.10 and Eq.11.[45] As a result, the Me-O-Si /Si-O-Si chemical bonds were formed between the silane and the substrate. Si-OH(solution) + Me-OH(metal) → Me-O-Si(interface) +H2O Si-OH(solution) + Si-OH(solution) → Si-O-Si(film) + H2O

(10) (11)

Based on the reactions above, the activity of the reactive areas was reduced due to the formation of a passive layer. Therefore, the galvanic

20

reaction by Eq.2 was inhibited. However, the passive layer can provide very limited protection to the substrate. When the galvanic reaction occurred, the zinc ions (Zn2+) were released from the substrate and combines with CH3(CH2)16COO- to generate zinc stearate [Zn[CH3(CH2)16COO]2] according to Eq.12. In addition, the stearic acid in the mixed solution may also react with zinc hydroxides and cerium hydroxide to generate low surface energy compounds, such as zinc stearate and cerium stearate. This was verified by the XRD results, where zinc stearate was detected. As there existed many hydroxyl groups over the metal and metal oxides, the porous dendritic silver structures were able to adsorb a massive of stearic acid and silane due to its high specific surface area, and dehydration condensation reactions occurred to form a covalent bonding (Eq.13).Therefore, the prepared superhydrophobic surface maybe fully covered with the long chain alkyl groups.[32] Zn2+ + CH3(CH2)16COOH → Zn[CH3(CH2)16COO]2 + 2H+ Me-OH + nCH3(CH2)16COOH → Me[CH3(CH2)16COO]n + nH2O

(12) (13)

In a word, the superhydrophobic surface enhanced with silane and corrosion inhibitor was successfully developed over the galvanized steel by this facile one-step solution-immersion process. 3.4. Anti-corrosion performance Generally, superhydrophobic surfaces usually worked for corrosion

21

protection of metals under relatively mild conditions (such as atmospheric corrosion). To shorten the evaluation time, strong corrosion media (3.5 wt% NaCl solution) was used as service environment. 3.4.1. Potentiodynamic polarization curves (PPC) PPC testing is an efficacious method to assess the corrosion property of the obtained superhydrophobic coating. Fig.10 depicts the potentiodynamic polarization plots of bare sample, MS, SH, and SH@MS, respectively. The corrosion potential (Ecorr) and corrosion current density (icorr) are illustrated in Table 2. The icorr of bare sample is determined to be 1.158×10-4 A/cm2. The modified silane (MS) presents the icorr of 2.724×10-5A/cm2, which is one order of magnitude lower than that of bare sample. However, the SH (without addition of silane and cerium salt ) shows a much lower icorr (5.238×10-7A/cm2). The icorr of SH@MS sample is 3.042×10-8A/cm2, which decrease for about one order of magnitude when compared with the SH one and four order of magnitude when compared with bare sample. Since both the SH@MS and SH are superhydrophobic, the samples cannot be wetted by the corrosive medium, protecting the zinc from corrosion. However, for the MS sample the corrosive medium can easily contact and penetrate through the film, leading corrosion onset. Moreover, it can be obviously found that the Ecorr of the SH@MS sample positively shifted from -1.086V to -0.888V,

22

which presents remarkably higher Ecorr than that of bare substrate and more positive than that of SH. It is known that corrosion potential represents the corrosion susceptibility of the substrate, and a shift in corrosion potential toward positive direction conveys that it is less prone to corrosion. Obviously, the SH@MH provides the best corrosion resistance, demonstrating that the addition of organosilane and corrosion inhibitor can significantly improve the corrosion performance of superhydrophobic surfaces. 3.4.2. Electrochemical impedance spectroscopy (EIS) EIS, a effective complementary electrochemical technique, is also used as a supplement to PPC technique for further evaluation of its corrosion behavior. Fig.11 shows the typical spectra of (a) Bode modulus|Z| and (b) Bode-phase angle versus frequency for bare sample, MS, SH, and SH@MS after immersion for 0.5h in 3.5wt% NaCl aqueous solution. It can be found from Fig.11a that the SH@MS sample offers the maximum impedance value at low frequency (0.01Hz), which is about four orders of magnitude greater than that of bare sample and three orders of magnitude larger than the MS. The impedance modulus |Z| of SH@MS is nearly two orders of magnitude greater than that of SH, demonstrating that the corrosion performance of the surface was greatly enhanced due to the presence of the silane and cerium salt.

23

One can see from Fig.11b that the SH@MS sample presents a significantly higher phase angle at high frequency region, which is associated with the superhydrophobic film and its excellent water-repellency. In addition, the phase angle at low frequency region moves to a much lower frequency, suggesting better barrier property of SH@MS. However, for the bare sample, the first constant at high frequency area is related to the diffusion process of chloride ions to the matrix and the second one at low frequency region is ascribed to the corrosion process.[46] For the MS sample, these two time constants are associated to the silane film and flaws within the film where corrosion may occur.[47] Fig.11c shows the variation of low-frequency(0.01Hz) impedance modulus |Z| with immersion time for SH@MS sample. One can see that the impedance modulus |Z| decreases rapidly in the initial period of immersion, and then gradually stabilizes. However, the impedance modulus |Z| of SH@MS is still more than an order of magnitude higher than that of bare sample after soaking for 168h. Although the introduction of silane and cerium salt enhances the resistance of the superhydrophobic film, there still has some tiny pores, and this pores connected in series with each other to form some sub-channels, through which the corrosive molecule and ions (H2O, O2, and Cl- etc.) can reach to the internal matrix during immersion. With the

24

prolonged immersion time, the repeated invasion of corrosive ions will lead to the destruction of the air layer trapped in the rough structure, and the corrosive medium will penetrate the substrate through the weak area (defects) of the surface, resulting in a reduction in the protective performance. The results demonstrate that the SH@MS sample provides excellent corrosion resistance and can protect the substrate more effectively in such aggressive solution. Fig.11d depicts the schematic of the anti-corrosion mechanism of the prepared superhydrophobic surface (SH@MS) in corrosive solution. When a galvanized steel is submerged in the corrosive solution, the corrosive ions and water will immediately contact with the surface, leading to the corrosion onset. However, when a superhydrophobic surface is soaked in the corrosive solution, the superhydrophobic surface can create a composite interface composed of solid and air, as illustrated in Fig.11d. The hierarchical rough structures can capture a massive of air and form an air cushion, which served as a physical barrier[48] to prevent corrosive medium (such as Cl-, H2O, O2, etc.) from contacting the surface, inhibiting the penetration process of corrosive medium to reach to the substrate at the initial period. When the air film is gradually decayed, the coating its self can also serve as a resistant barrier, inhibiting the penetration of corrosive medium. The

25

hydrolyzed silanol groups can establish a stable covalent bond with the metal surface (Me-O-Si) by the dehydration condensation reaction. Meanwhile, the silanol groups (Si-OH) can condense and cross-link to form a silicon network layer (Si-O-Si). These Me-O-Si/Si-O-Si bonds guarantee their excellent barrier protection over the metal substrate.[49]. In addition,the cerium salt exists in the coating in its oxide and hydroxide form, and these poorly soluble particles can fill in the gaps of the Si-O-Si network structure and the porous structures of the coating, which can effectively prevent the corrosive medium (such as H2O, Cl-, O2) from penetrating to the substrate. Moreover, previous studies have shown that the cerium ions present a self-healing ability by migrating to the corrode area to form insoluble hydroxide or oxide compounds, thereby inhibiting the corrosion process.[15,50] The combination of silane and cerium salt provides a resistant composite layer, which acts as an inner protective layer.[45] Therefore, the prepared superhydrophobic surface can provide multiple protections for the galvanized substrate, maximizing its service life. It is believe that such a protective coating system is capable to be used as a protection barrier for protection of galvanized steel. 3.5. Evaluation of stability and durability The chemical stability and durability of the prepared SH@MS samples were evaluated by exposure to outside environment and immerse in 3.5 wt%

26

NaCl solution, respectively. Fig.12 depicts the variation of static contact angles during exposure in air and NaCl aqueous solution. It can be seen from Fig.12a

that

the

superhydrophobic

coating

offers

excellent

superhydrophobicity (WCA > 150°) and very low sliding angle after exposure in outdoor environment for nearly 6 months, indicating good stability of the coating in atmospheric environment. Superhydrophobic surface is well known for its self-cleaning property, where the rust on the surface can be removed during the rolling-off process of droplets. The high water contact angle and low sliding angel are beneficial for surface cleaning, suggesting good self-cleaning ability. Fig.12b shows the variation of static water contact angle (WCA) with immersion time in 3.5wt% NaCl solution. It was found that the the WCA gradually decreases with the immersion time. However, it still maintains a very high water contact angle (~152±3°) even after 24h immersion in such a corrosive solution, indicating robust durability of the coating in corrosive solution environment. These results demonstrate that the prepared coating provides excellent chemical stability and durability during exposure in open air and immersion in solution. 4. Conclusions In conclusion, superhydrophobic surface containing organosilane and

27

cerium salt was successfully prepared via a facile one-step immersion process at ambient temperature. The obtained superhydrophobic surface with deposition time for 10min provides the maximum WCA of 164.2±2°, exhibiting wonderful water-repellency (similar to the lotus leaf). The prepared superhydrophobic surface was composed of porous micro/nano structures, which were covered by low surface energy groups. Electrochemical tests of the superhydrophobic surface demonstrate that the corrosion resistance of galvanized steel is significantly improved, which benefited from the addition of modified silane. Moreover, the superhydrophobic surface provides excellent chemical stability and durability during exposure in open air and immersion in solution. It is a novel, eco-friendly, and cost-effective strategy potentially used for the protection of galvanized steels and such surfaces can be further improved for future practical applications. Acknowledgments We would like to appreciate the NNSF of China (grant numbers 51501055, 51601056), the NSF of Hebei Province of China (grant number E2017202012), and Hebei province science and technology support program (grant number 19274009D) for financial support. References [1] A. R. Marder, The metallurgy of galvanized steel, Prog. Mater. Sci. 45

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Figure captions Fig. 1 Schematic of the fabrication process of the superhydrophobic coatings.

Fig.2 SEM images of (a) untreated sample and evolution of morphology at various deposition times: (b) 30 s, (c) 60 s, (d) 180s, (e) 300s, (f) 600s.

Fig.3 Variation of film thickness and roughness with deposition time.

Fig.4. Characterization of wettability: (a) The variation of contact angles and sliding angles with deposition time, (b) Droplets with different sizes on lotus leaf (top) and the superhydrophobic coating (bottom) prepared at 600s.

Fig. 5 EDS mapping of the superhydrophobic surface prepared at 600s.

Fig.6 FTIR spectra of the (a) Vinyltrimethoxysilane; (b) superhydrophobic coating; (c) stearic acid.

Fig.7 XRD patterns of the prepared superhydrophobic surface; the inset is at high magnification of the low diffraction angles.

37

Fig. 8 XPS spectra of the prepared superhydrophobic coating: (a) survey region; (b) C1s; (c) O1s; (d) Ag 3d; (e) Si 2p; (f) Ce 3d; (g) Zn 2p.

Fig. 9 Schematic diagram of the formation mechanism for the superhydrophobic surafce.

Fig.10 Potentiodynamic polarization plots for bare sample, MS, SH, and SH@MS in 3.5 wt% NaCl solution.

Fig. 11 Bode modulus|Z| (a) and Bode-phase angle (b) versus frequency for bare sample, MS, SH, and SH@MS after immersion for 0.5h in 3.5wt% NaCl aqueous solution. (c) The evolution of low-frequency(0.01Hz) impedance modulus |Z| with immersion time for SH@MS sample. (d) The schematic of the anti-corrosion mechanism for superhydrophobic surface (SH@MS) in corrosive medium.

Fig. 12 Evaluation of the stability and durability of the superhydrophobic coating (a) Water contact angles and sliding angles as a function of exposure time in air, the inset is the image of water contact angle before and after exposure for 6month in air; (b) Water contact angles as a function of

38

immersion time in 3.5 wt% NaCl aqueous solution.

39

Tables Table 1 Samples prepared at different conditions.

Samples

Treating conditions

Bare sample original HDG without any post-treatment MS

silane solution containing 0.005M cerium nitrate (modified silane, MS)

SH

functional solution except modified silane (SH)

SH@MS

the prepared functional solution containing silane and cerium nitrate

Table 2 Parameters resulted from the potentiodynamic polarization curves.

Ecorr (V)

icorr (A/cm2)

-1.086

1.158×10-4

MS

-1.081

2.724×10-5

SH

-0.988

5.238×10-7

SH@MS

-0.888

3.042×10-8

Sample Bare sample

40

Highlights 1. A corrosion-resistant superhydrophobic surface was developed by one-step process. 2. The developed coating displays improved corrosion resistance. 3. The improved corrosion performance benefits from the addition of modified silane. 4. The method is facile, eco-friendly and cost-saving, attractive for practical applications.

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

We confirm that no interests to declare.