Applied Surface Science 486 (2019) 371–375

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Applied Surface Science

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A ceramic coating on carbon steel and its superhydrophobicity T

a,b,⁎ a,b a a
Song Luo , Li Zheng , Hong Luo , Changsen Luo
a
College of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China
b
Key Laboratory of Material Corrosion and Protection of Sichuan Province, Zigong 643000, PR China

ARTICLE INFO ABSTRACT

Keywords: In this work, we present a combined method to produce a superhydrophobic ceramic coating on carbon steel. A
Ceramic coating hot-dip aluminizing approach is proposed for fabricating an aluminum coating on the carbon steel surface. This
Anodization aluminum coating is anodized to produce a ceramic oxide layer. The ceramic coating is identified as a mixture of
Steel α-AlOOH, γ-AlOOH and γ-Al2O3. The XRD analysis indicates that the ceramic coating is poorly crystalline. The
Superhydrophobic surface
ceramic coating exhibits a patterned surface with nano-pores and nano-wires. Due to this structured surface, the
ceramic coating shows superhydrophobicity after reducing surface energy by using a chemical treatment.

1. Introduction superhydrophobic surface [10–13]. Only a few of them are low-cost and
convenient, such as elector-deposition and chemical etching. We would
The concept of “Lotus effect” is first reported by Barthlott [1,2], and like to proposed a low-cost and convenient approach to create a rough
stimulated research interest in superhydrophobic surface. Barthlott re- surface on steel for preparing superhydrophobic surfaces.
vealed that the superhydrophobic plant leaves are covered with micro- As to that, anodization is a low-cost and convenient approach and is
nano scale asperities, and the micro-nano structure plays a vital role for used to produce an oxide layer on the metal surface. And anodization is
their superhydrophobicity. also a way to produce a superhydrophobic surface on metal such as
According to the Young's Equation, Wenzel model and Cassie-Baxter aluminum and aluminum alloy because it can create ordered nano-
model [3–5], wetting property of solid materials is related to two main structures [14,15]. However, not all metallic materials can be anodized.
aspects: surface structure (i.e. surface texture) and surface energy. Aluminum and its alloys are easy to be anodized. Carbon steel is not
Hence, a strategy for preparing superhydrophobic surface is to fabricate able to be anodized. We have reported a method to anodize medium
a rough surface with a geometric pattern, and then to reduce the surface carbon steel and fabricate a superhydrophobic metal oxide layer on
energy. The geometric pattern of the obtained rough surface plays an carbon steel [16,17]. We would like to propose another approach re-
important role for wetting model. Wetting model plays an important lated to anodization for fabricating a superhydrophobic metal oxide
part in determining the wetting property. layer on carbon steel.
Many methods have been developed to fabricate a rough surface The purpose of this study is to produce a ceramic coating on carbon
with a geometric pattern for preparing superhydrophobic surface. steel and to modify this coating to repel water. As mentioned above,
Shankar and coworkers [6] presented a method to fabricate a robust anodization is an effective method to produce a structured oxide layer
superhydrophobic surface on Ti foil. Zhang et al. [7] reported a method on the metal surface. It means that anodization can provide the metal
to produce a self-healing superhydrophobic coating on epoxy-based with a patterned rough surface. Since carbon steel is not suitable to be
shape memory polymer. Song's group [8] proposed a method to fabri- anodized, we would like to find a way to coat the carbon steel with an
cate hierarchical CuO spheres on polyimide for producing a super- aluminum layer, and then anodized it for creating a structured surface.
hydrophobic and flexible film with self-cleaning function. Cao et al. [9]
prepared a superhydrophobic surface with controllable periodic struc- 2. Materials and methods
tures on stainless steel by using picosecond laser. For steel, a very im-
portant engineering material, superhydrophobicity is also a significant 2.1. Preparation of substrate
property. Several approaches, such as electro-deposition, chemical
etching, plasma etching, laser machining and chemical vapor deposi- Commercial medium carbon steel specimens with dimensions of
tion, have been used to build a rough surface on steel for preparing 20 mm × 20 mm × 5 mm were used as substrates. The composition of

⁎
Corresponding author at: College of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China.
E-mail address: luosong@suse.edu.cn (S. Luo).

https://doi.org/10.1016/j.apsusc.2019.04.235
Received 3 February 2019; Received in revised form 15 April 2019; Accepted 25 April 2019
Available online 26 April 2019
0169-4332/ © 2019 Elsevier B.V. All rights reserved.
S. Luo, et al. Applied Surface Science 486 (2019) 371–375

Table 1
The composition of the carbon steel.
Element C Mn P S Fe

Weight % 0.42–0.50 0.6–0.9 0.04 max 0.05 max Balance

the steel is given in Table 1. Surfaces of all specimens were mechani-

cally ground using SiC abrasive paper with grit size from 120 to 1200,
and then polished using a diamond suspension followed by cleaned in
distilled water. All polished specimens were ultrasonically cleaned in
acetone for removing residual grease, and then dried in air.
A flux treatment is required prior to hot dipping. The main purpose
of the flux treatment is to facilitate metallurgical interaction between
steel and molten aluminum during hot dipping. Specimens were im-
mersed in an aqueous flux solution (K2ZrF6, 70 g L−1, 70 °C) for 3 min.

2.2. Hot dip aluminizing procedure

This procedure aims to prepare an aluminum coating on steel. Fig. 2. XRD patterns for (a) bare steel and (b) steel coated with a hot dip
During hot dip aluminizing, the steel substrate was completely im- aluminizing layer.
mersed in a molten bath of pure aluminum (Al, 99.7 wt%). The tem-
perature of the molten aluminum was maintained at about 700 °C. The 10 min. The temperature of oxalic acid was maintained at 25 °C.
dipping time was 3 min. After dipping, the specimens were cooled to Thereafter, all anodized samples were rinsed under running water, and
room temperature in air. then cleaned with distilled water followed by drying.

2.3. Anodization 2.4. Chemical treatment

This procedure aims to prepare an anodic aluminum oxide (AAO) The anodized surfaces were modified with a solution of
layer. Anodization was conducted on the hot dip aluminized steel 0.01 mol L−1 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (C10H4Cl3F17-
specimens in 0.3 mol L−1 oxalic acid at a constant voltage of 60 V for Si) in n-hexane to reduce surface energy. In this procedure, anodized

Fig. 1. SEM image and EDS spectra for the cross section of the hot dip aluminized steel. (a) an SEM image; (b) an EDS spectrum for the outer layer; (c) an EDS
spectrum for the inner layer; (d) an EDS spectrum for the substrate.

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Fig. 3. SEM image and EDS spectra. (a) an SEM image for the cross section of the steel coated with a ceramic coating (a multi-layer); (b) an EDS spectrum for the
anodic aluminum oxide (AAO) layer; (c) an SEM image for the AAO layer and the hot dip aluminizing layer; (d) an EDS spectrum for the hot dip aluminizing layer.

Fig. 4. Surface topography of the anodic aluminum oxide layer (i.e. the ceramic coating). (a) Low-magnification SEM image; (b) high-magnification SEM image.

samples were immersed in the solution at room temperature for 30 min.

2.5. Characterization

A scanning electron microscope (SEM, S-3400, Hitachi, Japan) was

used to characterize the cross section of the aluminum coating, as well
as the surface topography of the anodic aluminum oxide (AAO) layer.
Compositions of the AAO layer and the aluminum coating were ana-
lyzed using an energy dispersive spectrometer (EDS) equipped with
SEM. Phases of the AAO layer and the aluminum coating were identi-
fied using an X-ray diffractometer (XRD, D/MAX-Ultima, Rigaku,
Japan). The Raman spectrum of the AAO layer was recorded using a
laser confocal micro-Raman spectrometer (LabRAM HR800, Horiba,
Japan). The IR spectrum of the AAO layer was recorded using an FTIR
spectrometer (Tensor 27, Bruker, Germany) in ATR mode. Contact
angle measurements were performed using a contact angle system
(OCA, Data physics, Germany).

Fig. 5. XRD patterns for (a) the aluminum coating and (b) the ceramic coating.

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3. Results and discussion

3.1. Hot dip aluminizing layer

Fig. 1a shows a SEM image of the cross section of the steel coated
with a hot dip aluminizing layer. This aluminum coating consists of an
outer aluminum layer and an FeeAl intermetallic layer near the steel
substrate. The interface between the steel substrate and the FeeAl in-
termetallic layer is irregular. It is called as a tongue-like or a finger-like
interface. It has been confirmed that the intermetallic compound is
mainly Fe2Al5 [18]. Fig. 1b shows an EDS spectrum for the outer layer.
The result confirms that the outer layer is an aluminum coating. Fig. 1c
shows an EDS spectrum for the inner layer. The result confirms that the
inner layer contains Al and Fe. As shown in Fig. 1d, the substrate is a
carbon steel.
The XRD pattern of the bare steel is different from that of the steel
coated with a hot aluminizing layer, as shown in Fig. 2. The bare steel is
a carbon steel. Three peaks of iron are observed in the XRD pattern for
Fig. 6. The Raman spectrum for the anodic aluminum oxide layer.
the bare steel. Peaks of aluminum are observed in the XRD pattern for
the steel coated with a hot aluminizing layer. It demonstrates that the
hot dip aluminized steel has an outer aluminum coating.

3.2. AAO layer

It can be seen from Fig. 3 that an added layer is coated on the

surface of the hot dip aluminized steel after anodization. EDS analysis
shows that this added layer is an aluminum oxide coating. It indicates
that a ceramic layer forms on the surface of the hot dip aluminized steel
after anodization.
Fig. 4 shows surface topography of the AAO layer. The AAO layer
exhibits a patterned surface with nano-pores and nano-wires. The AAO
layer is formed on the hot aluminizing layer. The hot aluminizing layer
is coated on the steel substrate. It means that the steel substrate has a
nano structured surface, after applying hot dip aluminizing and sub-
sequent anodization process.
Fig. 5a shows an XRD pattern for the hot dip aluminizing layer. The
hot dip aluminizing layer has an outer aluminum layer. Thus, the XRD
pattern exhibits several peaks of aluminum. After anodization, an AAO
Fig. 7. The IR spectrum for anodic aluminum oxide layer (ceramic coating). layer forms on the surface of the aluminum layer. However, only sev-
eral peaks of aluminum appear in the XRD pattern of the AAO layer, as
shown in Fig. 5b. Oxide peaks doesn't appear in this XRD pattern. There
is a decrease in peak intensity compared with the XRD pattern in
Fig. 5a. According to the XRD analysis, it can be deduced that the
anodic oxide is poorly crystalline or amorphous.
Fig. 6 shows the Raman spectrum of the obtained AAO layer. It was
reported by [19] that oxide formed by anodic oxidation of aluminum is
γ-Al2O3. Gamma-Al2O3 does not exhibit any bands in its Raman spec-
trum. Only a fluorescence background can be observed in the Raman
spectrum of γ-Al2O3 [19–22]. Three broad, weak bands at 470, 592, and
1276 cm−1 emerge in the Raman spectrum (Fig. 6). These three bands
are due to α-AlOOH [23].
Fig. 7 shows the IR spectrum of the obtained AAO layer. Bands at
2926, 2856 and 420 cm−1 are attributed to α-AlOOH. Bands at
1461 cm−1 is attributed to γ-Al2O3. Bands at 458, 482, 547 and
591 cm−1 are attributed to γ-AlOOH.
According to the Raman spectrum and the IR spectrum, the anodic
aluminum oxide is a mixture of α-AlOOH, γ-AlOOH and γ-Al2O3.
Fig. 8. Contact angles for several samples before and after reducing surface
energy. (A) The bare steel; (B) the steel coated with an aluminum coating; (C) 3.3. Wetting of the ceramic coating
the steel coated with the ceramic coating.
Fig. 8 shows results of contact angle measurements. Before chemical

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