The one-step electroposition of superhydrophobic surface on AZ31 magnesium alloy and its time-dependence corrosion resistance in NaCl solution

Highlights

• A superhydrophobic coating is electrodeposited on AZ31 by one-step method.

• Deposition time affects morphology, wettability and thickness of the coatings.

• Deposition times do not affect coating’ phase which changes hydration in NaCl.

• EIS evolution shows the time dependence of the disappearance of the air layer.

• Superhydrophobic surface has a temporal limitation to the corrosion resistance.

Abstract

A calcium myristic superhydrophobicity coating with a hierarchical micro-nanostructure was fabricated on AZ31 magnesium alloy by one-step electroposition. The effects of deposition time on the coating structure, such as morphology, thickness, wettability and phase composition of the coating were studied. The corrosion behavior of the coated samples in 3.5% NaCl solution was also investigated and the corrosion mechanism was discussed. It was found that the deposition time has a visible effect on the morphology, thickness and wettability, which distinctly affects the corrosion resistance of coatings. The corrosion resistance of the coating gradually decreases with the increase in the immersion time due to the disappearance of the air layer which exists on the coating surface. The superhydrophobic surfaces present the temporal limitations to the corrosion resistance of AZ31 magnesium alloy.

Introduction

Magnesium has many advantages, including high strength-to-weight ratio, excellent structural performance and ease of recycling, which make it be widely used in lightweight equipment, aerospace and automobiles [1], [2]. Nevertheless, magnesium is a very active metal and is easily corroded in aqueous solution or humid atmosphere. Therefore, how to solve the corrosion problem has become a key point in the increasingly diverse practical applications of Mg alloys [3], [4]. Surface treatment is considered a simple and efficient approach. Chemical conversion coatings [5], anodic oxidation [6], microarc oxidation [7], chetablemical conversion coatings [8] and sol-gel process [9] are typical examples of surface modification processes.

Superhydrophobic materials, which with a water contact angle (CA) above 150° and a sliding angle (SA) below 10°, are inspired by the self-cleaning lotus leaf. Its unique properties, including transparency, self-cleaning, antireflection, mechanical robustness and resistance to icing [10], [11], have attracted much attention. Barthlott W and Neinhuis C [12] studied over 3000 plant surfaces in 1970s, and finally discovered that the “lotus effect” came from the complex microstructure on the surface. Surface roughness and the epicuticular wax’s hydrophobic properties lead to the water- repellency on lotus [13]. So hydrophobicity comes from the combination of a hierarchical micro-nanostructure and a low surface energy material. A superhydrophobic coating has high surface roughness (micro/nanostructure), and many peaks and valley structures present on the coating surface. This micro-nano hierarchical texture can trap air within the space in valley between peaks and staggered sheets [14]. Then the actual contact area of the corrosive solution and the substrate is greatly reduced to enhance the corrosion resistance. Herein, a superhydrophobic film can inhibit the solution’s permeation by providing an effective barrier to the water molecules and ions. And superhydrophobic coatings are considered a promising method to improve the corrosion performance of magnesium alloys.

Recently, various methods have been explored to fabricate superhydrophobic coatings, such as chemical vapor deposition [15], hydrothermal synthesis [16], chemical etching [17], anodic oxidation [18] and electrochemical deposition, etc. Superhydrophobic coatings were fabricated successfully on copper [19], [20], [21], titanium [22], [23] and other metal materials [24], [25], [26]. The common process route includes two steps: preparing a rough micro-nano structure surface and modify the surface with a low free energy material [27], [28]. However, most of the mentioned methods usually require complex processes, strict process control and expensive equipment, limiting their extensive applications. Gao et al. [29] fabricated hierarchical structure fibrous szaibelyite films by a simple template-free hydrothermal synthesis method on AZ31 magnesium alloy. After modification with fluoroalkylsilane (FAS), the films exhibited properties with a static water contact angle of 166° and a sliding angle less than 5°. Yufen Zhang [30] electrodeposited a hydrophobic coating (calcium stearate) on an anodized magnesium to decrease the degradation rate of the substrate in simulated body fluid. Although the water contact angles of the coatings were less than 150°, the better barrier properties of the coated substrate are consistent with their hydrophobicity.

As more research going on, some researchers begin to use electrodeposition technique to fabricate superhydrophobic coatings. It has many advantages such as simple operation, low cost, and ability to make large-area surfaces. Additionally, it gives hope that a rapid one-step electrodeposition method can take place of two-steps preparation technology. With an appropriate solution, the coating can simultaneously get surface roughness and low surface energy. Then the process route could be simplified and the fabrication time could be reduced. Moreover, the nanostructure of coatings can be easily controlled by adjusting deposition parameters. But few papers reported about this method and the obtained results were still limited. Liu [31] used cerium (III) nitrate hexahydrat and myristic acid ethanol solution to deposit coatings on Mg−Mn−Ce magnesium alloy. All static contact angles were above 155°, and sliding angles were below 10°. Polarization curves and EIS measurements demonstrated that the superhydrophobic surface greatly improved the corrosion properties in a series of solutions. But the effects of Ce⁴⁺ on coatings did not give an explanation and cerium was still nocuous.

The air layer is the key factor of superhydrophobic coatings’ corrosion resistance, which mainly depend on the surface states. Although many papers have researched the successful preparation processes of superhydrophobic coatings, the effects and disappearance of the air layer in corrosion environment are rarely reported. Yi Qi [26] obtained superhydrophobic surfaces with Cu²⁺ ion-assisted chemical etching treatment, whose static water contact angle on the surface remains approximately constant at a value of 157 ± 2° almost without any fluctuation for 7 months. This showed that the hydrophobicity of the superhydrophobic surfaces has enough stability in the air, but it had better applied value to evaluate its chemical stability in solution. Takahiro Ishizaki [14] prepared a superhydrophobic coating on AZ31 magnesium alloy by immersion in cerium nitrate hexahydrate and FAS. The water contact angle decreased from 152 ± 2° to 50 ± 2° after immersion in 5 wt% NaCl aqueous solution for 24 h, and the R_air||C_air elements are deleted in the equivalent circuit model at this time. The authors thought that the water contact angles on the superhydrophobic surface after 24 h show hydrophilic properties and the air layer formed between many minute pores on the superhydrophobic surface disappears at this time. In their other work [15], after EIS measurement within 24 h, the water contact angle of superhydrophobic surface which deposited on AZ31 magnesium alloy by microwave plasma-enhanced chemical vapor deposition was still 141°. The authors thought that the air layer was stabilized within the roughness grooves in the whole experiment. The authors don’t give longer EIS measurements, and the EIS measurements don’t connect with the decrease of the water contact angles and the disappearance of the air layer. According to the above analysis, it can be found that a detailed research about time dependence of the disappearance of the air layer and the corrosion mechanism of superhydrophobic coatings is absent and a further investigation should be necessary.

In this paper, a superhydrophobic coating is prepared on AZ31 magnesium alloy by cathodic electrodeposition. The corrosion behavior of the coatings in 3.5% NaCl aqueous solution is evaluated by polarization curves and electrochemical impedance spectroscopy (EIS). The effects of deposition time on corrosion resistance of the coatings and immersion behavior in NaCl are investigated. The corrosion mechanism of the coated samples and effects of the air layer to its time-dependence corrosion resistance are also discussed in detail.