A Study on the Corrosion Resistance of Hydrophobic Coatings on 65Mn Steel

Abstract

Calcium stearate hydrophobic coatings with a hierarchical micro/nanostructure were prepared on 65Mn steel using direct current electrodeposition. The deposition time has a visible influence on the morphology, surface wettability and thickness of the coatings, but little effect on the phase composition. The corrosion behavior of the coated samples in 3.5 wt.% NaCl solution was also investigated. The prepared coatings at different deposition times show different corrosion resistance. The coating fabricated at 30 min has the best corrosion resistance due to the highest water contact angle and thicker coating.

1. Introduction

Corrosion of steels poses a serious threat in numerous industrial and economic fields. Thus, various methods have been developed to prevent steels from corroding. The methods for steel corrosion protection mainly include electrochemical protection, the use of corrosion inhibitors and applied protective coatings [1,2,3,4]. Among these approaches, applied protective coatings represent an effective and economical way of protecting steel substrates. Hydrophobic coatings such as the lotus leaf, water strider leg or mosquito compound eye, and so on have attracted much attention worldwide [5,6]. Due to their hydrophobicity, these coatings show tremendous value in corrosion protection [7,8,9], self-cleaning [10,11], anti-icing [12,13], drag reduction [14], oil/water separation [15,16,17] and other fields [18,19]. Hydrophobic coatings are very effective against metal corrosion. The reason for the corrosion protection effects of hydrophobic coatings is that a large amount of air is trapped in the valleys between the rough structures of the surfaces forming an air cushion that will inhibit the migration of water and aggressive ions.

Various methods have been developed to fabricate hydrophobic coatings on steel surfaces. It is well known that hierarchical micro/nanostructure and low surface energy material are two crucial factors to prepare hydrophobic coatings [20,21]. Usually, two steps are involved: creating a rough micro/nanostructure and modifying the surface with a low surface energy material. Many techniques have been used to create micro/nanostructures, including in situ crystallization method [22], etching [23] and electrodeposition [20,24,25]. Fluorosilanes or long fatty acids are usually employed as low surface energy materials to fabricate hydrophobic coatings [23]. However, most of these techniques and materials are complicated, expensive or toxic. Cai et al. [22] developed the hydrophobic Mobil five–type zeolite coatings on stainless steel via an in situ crystallization method and subsequently modifying them with hexadecyltrimethoxysilane. These coatings exhibited good self-cleaning, antifouling and anticorrosion properties. In addition, coated meshes showed high separation efficiency for oil-water mixtures. Zhang et al. [23] fabricated superhydrophobic surfaces on 304 stainless steel via etching and stearic acid modification treatment. The surface displayed a maximum static water contact angle of 162.45° and a minimum sliding angle of 4.8°. The prepared surface possessed an excellent corrosion resistance and a good mechanical stability. Xue et al. [20] prepared Ni-Co coating with hierarchical micro-spherical structures on carbon steel by electrodeposition. After modification with low surface energy materials, the coating deposited at −1.8 V displayed high superhydrophobicity with a water contact angle around 165°. The superhydrophobic Ni-Co coating had an excellent anti-corrosion ability.

Recently, electrodeposition has been used to prepare hydrophobic coatings on metals due to its simple, inexpensive, time-saving and high efficiency nature [21,26]. Furthermore, it gives hope that a rapid one-step electrodeposition method can replace two-steps fabrication process. Electrodeposition, as a one-step method to obtain the micro/nanostructure and low surface energy simultaneously, has been used to construct hydrophobic/superhydrophobic coatings on aluminum [27,28], copper [29], magnesium alloys [30,31,32,33], 316L stainless steel [34] and mild steel [35]. Nevertheless, there are few studies about hydrophobic coatings on 65Mn steel (a low-alloy spring steel), which is often used to manufacture soil-engaging tools working in humid or corrosive environments with pesticides and fertilizers [36]. The soil-engaging tools working in this environment tend to corrode faster, therefore, it is necessary to investigate hydrophobic coatings to protect 65Mn steel against corrosion.

In this paper, hydrophobic coatings are fabricated on a 65Mn steel substrate by direct current electrodeposition to improve the corrosion resistance. The corrosion behaviors of the coated specimens in 3.5 wt.% NaCl aqueous solution are investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves. The influences of deposition time on the corrosion resistance of the coatings are evaluated. Moreover, the corrosion mechanism of the coated specimens will be discussed in detail.

2. Experimental

2.1. Materials and Coating Preparation

The composition of the 65Mn steel plate substrate used in this experiment is shown in

Table 1. Chemical composition of the received material.

Element	C	Si	Mn	S	P	Cr	Ni	Cu	Fe
wt.%	0.62–0.70	0.17–0.37	0.90–1.20	≤0.035	≤0.035	≤0.25	≤0.25	≤0.25	Balance

. Each sample was cut to the dimensions of 100 mm × 40 mm × 0.4 mm. The plates were successively polished using 60, 180, 400, 800 and 1200 grit SiC sandpapers. Then, they were rinsed in deionized water and ethanol, respectively, and dried in cold air.

The coatings were cathodically electrodeposited on the 65Mn substrate using a DC power supply (PSW 160-14.4, Gwinstek, Suzhou, China). The solution for electrodeposition was composed of stearic acid (0.05 mol/L) and calcium chloride (0.025 mol/L), and the solvent was ethanol (≥99.7%). The 65Mn steel plate and graphite plate are used as cathode and anode, respectively. The electrodes were placed vertically and parallel to each other with an immersed area of 24 cm² and 30 mm apart, in a 200 mL cylindrical beaker at ambient temperature. The working voltage applied between the electrodes was controlled at 80 V, and the deposition duration was set at 3, 15, 30 and 60 min, respectively. The solution was stirred with a magnetic stirrer during the whole electrodeposition process.

2.2. Characterization of the Coatings

Surface and cross-sectional morphology were imaged by scanning electron microscopy (SEM JSM-6490LV, JEOL, Tokyo, Japan). X-ray diffraction (X’Pert PRO, Philips, Amsterdam, The Netherlands) was employed to identify the phase compositions of coatings. It works at 40 kV and 40 mA with Cu Kα radiation source. Grazing incidence method with 2° of incident beam angle was used to collect the data. The scanning range was from 5° to 90° with a step size of 0.08°.

Static water contact angles were measured at room temperature using a contact angle meter (Dataphysics OCA20, Filderstadt, Germany). The volume of water droplets used in the measurement was 3 μL. In order to get a reliable result, the value of the static water contact angle was read after the water droplet stayed on the surface for 60 s and the static contact angle measurements were repeated at least three times for each specimen.

2.3. Electrochemical Measurements

The corrosion properties of 65Mn substrate and coated samples in 3.5 wt.% NaCl solution at room temperature were investigated using an electrochemical workstation (Reference 600+, Gamry, Philadelphia, PA, USA). A three-electrode cell was adopted with the specimen as the working electrode, a saturated Ag/AgCl electrode and graphite rods as the reference and counter electrode respectively. The exposed surface area of the specimen was 1 cm². The corrosion behaviors were evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). Prior to potentiodynamic polarization, the specimen was kept at the open circuit potential (OCP) for 2000 s. Potentiodynamic polarization curve was measured by scanning the potential from −0.2 V (vs. OCP) with a sweep rate of 0.5 mV·s⁻¹. EIS measurements were conducted at the frequency ranged from 100 kHz to 10 mHz, applying a sinusoidal potential perturbation of 10 mV RMS (at OCP), and the data were fitted with Zsimpwin software 3.50. All the electrochemical measurements were repeated three times to ensure reproducibility.

3. Results and Discussion

3.1. Phase Composition and Morphology of the Coatings

Figure 1 shows the XRD patterns of the bare 65Mn substrate and coatings at 80 V after deposition times of 3, 15, 30 and 60 min, respectively. It can be seen that the diffraction peaks located at 82.33°, 65.02° and 44.67° are attributed to the element Fe from the substrate. There are peaks corresponding to calcium stearate in the deposited coatings, which is the main composition of the coatings. Furthermore, an evolution of preferred orientation is observed. The coating deposited at 3 min shows the strongest diffraction peak at 20.12° compared to other coatings at 21.66°. This might be due to the influence of 65Mn substrate on the crystal growth in the first 3 min deposition process [5,37]. With the deposition time increasing, the coating deposits on the previously formed calcium stearate coating, and the effect on the 65Mn substrate is weakened.

Based on the analysis above, the phase composition of the deposited coatings is calcium stearate. When the DC voltage is applied to the two electrodes, some calcium ions (Ca²⁺) near the cathode react with stearic acid, generating calcium stearate ($\mathrm{Ca}{\left[{\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_{16}\mathrm{COO}\right]}_2$) and hydrogen ions (H⁺). Meanwhile, free hydrogen ions receive electrons and then form H₂. The reaction processes can be proposed as follows:

$${\mathrm{Ca}}^{2+}+2{\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_{16}\mathrm{COOH}\to \mathrm{Ca}{\left[{\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_{16}\mathrm{COO}\right]}_2+2{\mathrm{H}}^{+}$$

(1)

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-} \to { \mathrm{H}}_2\uparrow$$

(2)

The current density as a function of time was monitored during the electrodeposition process, as shown in Figure 2. The current density decreases rapidly during the first few minutes due to an increase in the cathode resistance, demonstrating the fast nucleation rate of the coating at the beginning of the electrodeposition. During the succedent deposition process, the current density keeps a lower value and eventually remains constant because the surface resistance of the 65Mn substrate (cathode) stops changing. This implies that the 65Mn substrate is entirely covered by the deposited coating and the nucleation rate of the coating is lower.

To explore the coating formation process and growth mechanism, electrodeposition at 80 V with different electrodeposition times were conducted as shown in Figure 3. After 3 min, it can be seen that the 65Mn substrate is fully covered by calcium stearate deposits of micron-scale protrusions, which display an uneven size distribution. The average size is around 10 μm in diameter (Figure 3a). From the inset in Figure 3a, the protrusions are composed of nanoscale pore structures. By extending the electrodeposition time to 15 min (Figure 3b), cracks in the image become obvious (indicated with arrows), which might be caused by internal stress from the fast nucleation and growth rate of the coating [38]. The average size of protrusions increases to about 15 μm and some of them grow into honeycomb-like structures. In addition, flake-like structures appear on the surface of the coating. Compared to the coating deposited for 15 min, the volume of flake structures seems to decrease when the coating is deposited for 30 min (Figure 3c). The honeycomb-like structures evolve into nanoscale villiform structures (see the inset in Figure 3c). The size of the protrusions does not change significantly. Figure 3d shows that the average size of the protrusions increases to about 19 μm with the extension of the electrodeposition time to 60 min. In addition, the size of the protrusions is found to be extremely uneven at the micro scale and significant cracks appear on the coating surface (indicated with an arrow in Figure 3d). In general, the images in Figure 3 demonstrate that hierarchical micro/nanostructures appear on the surface of the coating, which may contribute to the hydrophobicity of the deposited coatings.

The relative thicknesses of the deposited coatings were compared using the 45° rotation cross-sectional morphologies of the coated specimens, shown in Figure 4. The observed cross-sections are natural fracture surfaces of the obtained coatings from the substrate without any artificial scraping. It is obvious that the coating thickness increases greatly from about 5 μm (Figure 4a) to 9 μm (Figure 4b) as the deposition time changes from 3 to 15 min. By contrast, the coating thickness increases slowly from about 11 μm (Figure 4c) to 12 μm (Figure 4d) with an increasing deposition time from 30 to 60 min. This suggests a larger growth rate of coating during the initial stage of deposition but a slower one during the late stage of deposition.

3.2. Hydrophobic Properties of the Coatings

To evaluate the wetting behavior of the deposited coatings at different times, static water contact angle measurements were conducted and results are shown in Figure 5. For bare 65Mn substrate after polishing, the contact angle is around 73.0°. With the deposition times of 3, 15, 30 and 60 min, the water contact angles of the deposited surfaces are 140.8°, 140.8°, 146.4° and 142.8°, respectively. Compared with the 65Mn substrate, the water contact angles of deposited coatings are greater than 140°, demonstrating hydrophobic properties. This illustrates that the deposited coatings enhance the hydrophobicity of the surface.

The hydrophobicity of the deposited coatings can be explained with the Cassie-Baxter model, which is expressed as follows:

$$cos\theta =f_{sl}\left(cos{\theta }_y+1\right)-1$$

(3)

where, ${\theta }_y$ and $\theta$ represent the water contact angles on surfaces of the 65Mn substrate and electrodeposited coatings, respectively. The term $f_{sl}$ is the area fraction of the solid interface in contact with the water droplets. The sum of fractions of solid and air at the solid/water interface is 1. The calculated $f_{sl}$values related to solid-liquid interface are 17.4%, 17.4%, 12.9% and 15.7%, respectively, for coatings deposited at 3, 15, 30 and 60 min. Obviously, the coating formed at 30 min deposition shows the lowest solid fractional area, implying that this coating has the highest air fractional area. The air layer entrapped among the surface micro/nanostructures can effectively impede water penetration into the coating. However, the calculated $f_{sl}$values are typically less than 10% for superhydrophobic surfaces [39], suggesting higher air fractional area compared with the hydrophobic surfaces.

3.3. Corrosion Protection of the Deposited Coatings

For simplification, the coated specimens deposited for 3, 15, 30 and 60 min are named HC-3, HC-15, HC-30 and HC-60, respectively. The open circuit potential (OCP) of all samples in this study were recorded for 2000 s to reach a relatively stable state to obtain further potentiodynamic polarization curves, and the results are presented in Figure 6a. It is observed that the OCP values of the coated samples are more positive than those of 65Mn substrate, meaning that hydrophobic coatings improve the corrosion protection of the 65Mn steel. During the first 1200 s, the OCPs of the coated samples show fluctuations, subsequently they maintain relatively stable values until 2000 s.

To estimate the corrosion resistance of the hydrophobic coatings prepared at 80 V with different deposition times, potentiodynamic polarization curves of 65Mn substrate and hydrophobic surface coated substrates were measured in 3.5 wt.% NaCl aqueous solution. The obtained results are given in Figure 6b. The values of corrosion potential (E_corr), corrosion current density (i_corr), cathodic Tafel slope (β_c), anodic Tafel slope (β_a) and breakdown potential (E_bd) for each polarization curve are summarized in

Table 2. Electrochemical data of 65Mn substrate and the coated samples as read from potentiodynamic polarization curves.

Samples	Ecorr (V vs. Ag/AgCl)	icorr (A/cm2)	Ebd (V vs. Ag/AgCl)	βc (V/dec)	βa (V/dec)
65Mn substrate	−0.58 ± 0.01	(2.74 ± 0.12) × 10−5	—	0.99 ± 0.03	0.08 ± 0.01
HC-3	−0.31 ± 0.01	(3.14 ± 0.38) × 10−8	−0.20 ± 0.01	0.17 ± 0.01	0.16 ± 0.01
HC-15	−0.27 ± 0.01	(1.58 ± 0.03) × 10−9	−0.11 ± 0.01	0.66 ± 0.04	0.10 ± 0.01
HC-30	−0.11 ± 0.02	(7.46 ± 0.60) × 10−10	0.50 ± 0.02	0.44 ± 0.02	0.23 ± 0.02
HC-60	−0.18 ± 0.01	(1.67 ± 0.13) × 10−9	0.37 ± 0.02	0.18 ± 0.01	0.24 ± 0.01

. It is worth noting that the i_corr is reduced by about three orders for HC-3, more than three orders for HC-15, HC-30 and HC-60 in comparison with 65Mn substrate, illustrating that the corrosion resistance of the 65Mn substrate is dramatically improved by the hydrophobic coatings. Another influence of the hydrophobic coating on the polarization curves compared to the bare 65Mn substrate is the presence of a passive region with the potential (E_bd − E_corr) of approximately 0.11 V, 0.16 V, 0.61 V and 0.55 V for HC-3, HC-15, HC-30 and HC-60 on the anodic branch respectively. It can be stated that HC-30 sample has the best corrosion resistance.

Electrochemical impedance spectroscopy was used to evaluate the corrosion resistance and analyze the corrosion mechanism. Figure 7 shows the Nyquist spectrum and Bode plots of 65Mn steel substrate after immersion in 3.5 wt.% NaCl solution for 30 min. The values of impedance modulus are very low (around 463 Ω·cm² at 0.01 Hz) in this case, suggesting that 65Mn has a poor corrosion resistance in 3.5 wt.% NaCl solution. Inductive behavior can be observed at low frequency, which may be caused by the corrosion of 65Mn steel.

The electrochemical impedance spectra of coated samples immersed into 3.5 wt.% NaCl for different periods are displayed in Figure 8. The symbols are experimental data and solid lines are fitting data. For all the coated samples, EIS data at high frequency are removed for the fitting because they cannot be fitted reliably. Compared to the steel substrate (Figure 7), the coated samples show larger diameter of the capacitive loop (Figure 8a,d,g) and much higher impedance modulus at 0.01 Hz after 1 h of immersion (Figure 8b,e,h), implying the higher barrier properties of hydrophobic coatings during the initial immersion stage. It is worth noting that the sample HC-30 shows significantly improved corrosion inhibition with a modulus value of 2.98 × 10⁶ Ω·cm² at a low frequency (0.01 Hz), which is comparable to previously reported superhydrophobic coatings, such as the superhydrophobic dodecyltrimethoxysilane (DTMS) coatings on mild steel with modulus value of 2.7 × 10⁵ Ω·cm² [40]. However, the capacitive arcs shrink rapidly and the impedance moduli drop gradually with the increase of immersion time. This suggests that water and aggressive ions have penetrated into the coating, and corrosion happens continuously during immersion in NaCl solution.

The phase angles of HC-15 (Figure 8c) show the asymmetrical peak in the entire frequency range, meaning two time constants appear during the immersion process. Two well defined time constants are also observed from the Bode plots for HC-60 (Figure 8i). The time constant at medium and high frequency (around 10–10⁴ Hz) is related to the response from the hydrophobic coating and the other one at low frequency (around 10⁻²–1 Hz) can be ascribed to the electrochemical process at the steel surface. For HC-30 (Figure 8f) within 12 h of immersion, there is only one time constant due to the hydrophobic coating, indicating that the initial corrosion process is greatly inhibited by the hydrophobic coating. After longer immersion (16–24 h), another time constant appears at low frequency (about 10⁻²–1 Hz), which is associated with the electrochemical process at the steel/corrosive solution interface. This demonstrates that the corrosive medium has penetrated into the hydrophobic coating and reached the substrate. The time constant related to the electrochemical process at the steel/solution interface appears after 1 h of immersion for HC-15 and HC-60, however, it appears after 16 h of immersion for HC-30. This illustrates that HC-30 has the best corrosion resistance in 3.5 wt.% NaCl solution, which is consistent with the results of the polarization curves.

For quantitative estimation of the corrosion resistance of the coating, the EIS spectra are fitted using equivalent circuits. In order to get a better fitting quality, varied equivalent circuit models have been tried to fit the EIS spectra. Figure 9 shows the equivalent circuits most suitable for the experimental data of hydrophobic coatings. The standard test values (χ²), shown in

Table 3. The elements parameters of EIS fitted by the equivalent circuit.

Sample	Time	Rs (Ω cm2)	Rcoat (Ω cm2)	CPEcoat		Rct (Ω cm2)	CPEdl
				Y0 (Ω−1 cm−2 sn)	n		Y0 (Ω−1 cm−2 sn)	n	χ2 (×10−3)
HC-15	1 h	16.8	1.41 × 104	1.59 × 10−5	0.59	2.10 × 105	1.94 × 10−6	0.98	3.64
	16 h	21.1	1.04 × 104	3.21 × 10−5	0.60	6.45 × 104	1.09 × 10−5	0.95	2.22
	24 h	31.3	1.19 × 104	4.30 × 10−5	0.56	2.76 × 104	1.13 × 10−5	0.94	1.29
HC-30	1 h	38.5	4.33 × 106	6.36 × 10−7	0.60	—	—	—	5.14
	12 h	20.7	4.61 × 106	1.47 × 10−6	0.53	—	—	—	4.92
	16 h	30.8	9.94 × 105	1.87 × 10−6	0.47	4.35 × 106	3.12 × 10−7	0.96	1.94
	24 h	42.7	3.66 × 105	4.40 × 10−6	0.28	1.43 × 106	7.72 × 10−6	0.94	0.46
HC-60	1 h	21.4	1.47 × 105	1.64 × 10−6	0.44	6.16 × 105	9.58 × 10−6	0.77	0.39
	8 h	43.1	3.93 × 104	6.45 × 10−6	0.41	3.41 × 105	1.15 × 10−5	0.90	1.43
	16 h	39.3	2.04 × 104	1.65 × 10−5	0.29	1.98 × 105	1.47 × 10−5	0.80	0.94
	24 h	15.7	1.38 × 104	1.57 × 10−5	0.17	7.30 × 104	3.65 × 10−5	0.74	0.47

, are low, representing good fitting to Nyquist and Bode diagrams. In these circuits, the constant phase element (CPE) is employed instead of the ideal electrical capacitance in order to compensate the non-homogeneity and obtain a good fitting result. The impedance of CPE is described as:

$$Z_{CPE}=\frac{1}{Y_0{\left(jw\right)}^n}$$

(4)

where w is the angular frequency, $Y_0$is the CPE constant, j is the imaginary number, and n is the dimensionless index (−1 ≤ n ≤ 1) [40,41]. The CPE is an ideal capacitor for n = 1. The deviation of n is due to the heterogeneous effect [41]. When n is close to zero, the CPE describes a resistor. The CPE behaves like an inductor for n = −1 [42].

The circuit in Figure 9a is used to fit the EIS data of HC-30 within 12 h of immersion. The circuit in Figure 9b is proposed for 16 h and 24 h of HC-30 along with HC-15 and HC-60 from 1 h to 24 h. In the circuits, R_s is the solution resistance, CPE_coat is the coating capacitance, R_coat is the coating resistance, CPE_dl is the electrochemical double layer capacitance, and R_ct stands for the charge transfer resistance.

The fitting parameters are listed in Table 3. In the immersion process, R_coat of all the coatings decrease more or less with the immersion time, while CPE_coat increase in this process, demonstrating the penetration of corrosive medium into the coating gradually. Overall, the sample HC-30 shows the highest R_coat value and lowest CPE_coat value, suggesting the best corrosion resistance. Similar behavior has been observed in the case of Ca/Ce coating of AZ31 magnesium alloy [43]. R_ct is an essential factor to reflect the protection effect of the coatings on the underlying 65Mn steel. As shown in Table 3, the R_ct value of HC-30 is obviously the highest within the immersion process, suggesting that HC-30 has the best corrosion resistance. This coincides well with the potentiodynamic polarization results (Figure 6b). Correspondingly, the sample HC-30 maintains a relatively low CPE_dl value during the immersion process.

According to the analysis above, the deposition time affects the surface morphology, thickness, wettability, and other characteristics of the coatings. It is believed that the corrosion resistance of the coating is due to the combination of several factors. The hydrophobicity plays an important role in the corrosion protection at the initial stage of immersion. With the increase of immersion time, the water contact angle on the coating surface reduces and the coating turns into hydrophilicity. In this case, water and aggressive ions can penetrate into the coating and reach the 65Mn substrate easily along the coating defects.

The hydrophobic coatings obtained at different deposition times have different corrosion resistance. Sample HC-15 shows the thinnest coating (Figure 4) and the lowest water contact angle of the surface (Figure 5). Furthermore, obvious cracks (Figure 3b) appear on the coating surface and the corrosive media can easily penetrate the coating along these cracks. Thus, HC-15 can only provide limited corrosion protection for the 65Mn substrate. Sample HC-30 presents the best corrosion resistance due to the highest water contact angle up to 146.4° and thicker coating. Though sample HC-60 has the thickest coating, the corrosion resistance is not the highest due to the lower water contact angle (Figure 5) and the appearance of cracks on the coating surface (Figure 3d).

In summary, this work demonstrates a simple and effective pathway to prepare calcium stearate hydrophobic coatings, which improve the corrosion resistance of the 65Mn substrate. These coatings applied on the surface of magnesium alloys have been studied for use in the biomedical area [26,38], where they show superior corrosion resistance as well. However, 65Mn steel used for tools should have good mechanical stability, like friction and wear properties, adhesive strength, which shall be the focus of our future research.

4. Conclusions

We present a simple, environment friendly and single-step method to fabricate hydrophobic coatings with improved protective properties on 65Mn steel. The influences of deposition time on the phase composition, morphology, thickness, surface wettability and corrosion resistance of the coatings have been investigated. The following conclusions can be drawn:

(1)

The deposition time obviously affect the morphology, wettability and thickness of the coatings. However, it has no influence on the phase composition which is calcium stearate.

(2)

The deposited coatings exhibit hydrophobicity thanks to the hierarchical micro/nanostructure of the coating surface and the low surface energy of calcium stearate.

(3)

The sample HC-30 has the best corrosion resistance due to a combination of superior hydrophobicity and thicker coating.