Corrosion resistance and chemical stability of super-hydrophobic film deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition
Introduction
Magnesium and its alloys have excellent physical and mechanical properties such as low density, good electromagnetic shielding, and high strength/weight ratio [1], [2]. Thus, they are expected to be applied to various industries such as the aerospace, automobile, and railway industries [3], [4], [5]. Their poor corrosion resistance, however, hinders their use on a larger scale. The corrosion of magnesium and magnesium alloys occurs via the following reactions [6], [7]:2Mg → 2Mg+ + 2e−2Mg+ + 2H2O → 2Mg2+ + 2OH− + H22H2O + 2e− → H2 + 2OH−2Mg2+ + 4OH− → 2Mg(OH)2According to Eqs. (1), (2), (3), (4), the contact of magnesium alloy with water triggers the corrosion reaction. Thus, it is important to prevent the contact of magnesium alloy with water to suppress the progress of corrosion. In order to realize this, various methods such as chemical conversion, electroplating, and anodic oxidization have been developed [8], [9], [10], [11]. The films prepared by these surface treatments always contain pores, pine-holes, cracks etc. The porosity and defect density are critically important to the quality of a film. In addition, the film thickness and film density are important factors from the viewpoint of anticorrosion performance. However, the advantage offered by the lightweight properties of magnesium alloy might be lost if the thickness and density of the applied coating are increased. Thus, it is crucial to develop a coating technology to improve anticorrosion performance, while maintaining the advantage offered by the lightweight properties of magnesium alloys. A corrosion resistant hydrophobic film formed on AZ91D Mg alloy through a simple dipping process has recently been reported [12]. This hydrophobic film improved the corrosion resistance of magnesium alloy AZ91. Thus, a super-hydrophobic coating would be also a promising technology for improving anticorrosion performance because it would inhibit the contact of a surface with water and environmental humidity. A super-hydrophobic surface would be of great importance to many industrial applications and could present a solution to the long-standing problems of environmental contamination and corrosion of metals and metal alloys [13], [14], [15].
Recently, super-hydrophobic treatments have been applied to various engineering material surfaces such as steel, copper, zinc, and aluminum, to improve their corrosion performances [16], [17], [18], [19], [20]. Liu et al. demonstrated the preparation of a super-hydrophobic surface on copper by chemical etching and surface modification with n-tetradecanoic acid and investigated the corrosion resistant performance in seawater by electrochemical measurements [21]. They concluded that the super-hydrophobic surface considerably reduced the corrosion rate of copper due to its special morphology. He et al. fabricated a super-hydrophobic surface on anodized aluminum by modifying myristic acid and estimated the corrosion resistant performance by electrochemical impedance spectroscopy [19]. The super-hydrophobic surface greatly improved the corrosion resistance of aluminum. In this way, a super-hydrophobic surface has been shown to be effective for improving the corrosion resistance of engineering materials. However, it was widely assumed that the advantages of a super-hydrophobic surface would disappear if it was completely immersed in a solution; thus, there was little attention paid to the application of super-hydrophobic surfaces equipped with anticorrosion performances to metal surfaces. As a consequence, there are few reports on the application of a super-hydrophobic surface to magnesium alloys. Jiang et al. only reported the fabrication of a bioinspired super-hydrophobic surface on Mg–Li alloy [22]. The super-hydrophobic surface was fabricated by chemical etching and surface modification with a CF3− terminated self-assembled monolayer (fluoroalkyl silane self-assembled monolayer: FAS-SAM). They estimated the corrosion resistant performance by investigating the relationship between the time of exposure to air and the change in static water contact angles on the super-hydrophobic surface of the Mg–Li alloy. The surface showed stably super-hydrophobic properties for over 3 months. However, the surface was not exposed to a corrosive environment such as diluted NaCl aqueous solution, so it is difficult to conclude that the super-hydrophobic property provides high corrosion resistance to the magnesium alloy.
In this paper, we report corrosion resistant performance of the super-hydrophobic film coated magnesium alloy AZ31 by electrochemical measurements. The super-hydrophobic film was deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition (MPECVD). In addition, the chemical stabilities of a super-hydrophobic surface on magnesium alloy AZ31 in aqueous solutions of various pHs were also investigated.
Section snippets
Preparation of a super-hydrophobic surface on magnesium alloy AZ31
Magnesium alloy AZ31 (composition: 2.98% Al, 0.88% Zn, 0.38% Mn, 0.0135% Si, 0.001% Cu, 0.002% Ni, 0.0027% Fe, and the rest is Mg) with a size of 10 mm × 10 mm × 1.5 mm was used as the substrate. The substrates were ultrasonically cleaned in absolute ethanol for 10 min. Each cleaned substrate was placed on the substrate stage in the MPECVD system. The MPECVD system consisted of a Vycor glass discharge tube attached with a microwave cavity and a deposition chamber made of stainless steel.
Fabrication of a super-hydrophobic surface on magnesium alloy AZ31
Fig. 1(a)–(c) shows FE-SEM images of the film on magnesium alloy AZ31 surfaces deposited for 10 min, 20 min, and 30 min, respectively, by MPECVD. Fig. 1(d)–(f) shows enlarged images of Fig. 1(a)–(c), respectively. The film coverage increases with an increase in preparation time. When the deposition time was less than 10 min, the surface coverage of the film was low and the film surface was smooth compared to those deposited for 20 min and 30 min. When the preparation time was prolonged to more than 20
Conclusions
We successfully deposited a super-hydrophobic film on magnesium alloy AZ31 through the MPECVD process. The film surface showed a water contact angle of more than 150°. The hydrophobicity of the film surface increased with an increase in deposition time. The anticorrosion resistance of the deposited film was estimated by potentiodynamic and EIS measurement. The potentiodynamic curves revealed that the coating of super-hydrophobic film decreased significantly both the anodic and cathodic current
Acknowledgements
This study was partially supported by the Iketani Science and Technology Foundation and Aichi Science & Technology Foundation program “Nagoya Nano-Technology Cluster of Innovative Production System of the Ministry of Education, Culture, Sports, Science and Technology, Japan”. This work was carried out by the joint research program of the EcoTopia Science Institute, Nagoya University.
References (32)
- et al.
J. Alloys Compd.
(2002) - et al.
Appl. Surf. Sci.
(2008) - et al.
Surf. Coat. Technol.
(2006) - et al.
Mater. Sci. Eng. A
(2001) - et al.
Corros. Sci.
(1997) - et al.
Surf. Coat. Technol.
(2006) - et al.
Surf. Coat. Technol.
(2006) - et al.
Electrochim. Acta
(2007) - et al.
Electrochim. Acta
(2007) Electrochim. Acta
(2010)
ACS Appl. Mater. Interfaces
Corros. Sci.
Electrochim. Acta
Thin Solid Films
Electrochim. Acta
Corros. Sci.
Cited by (283)
The mechanisms and advances in magnesium-based materials protection against corrosion by the superhydrophobic coatings
2024, Surface and Coatings TechnologyThermo-driven oleogel-based self-healing slippery surface behaving superior corrosion inhibition to Mg-Li alloy
2023, Journal of Magnesium and AlloysThe synthesis and mechanism of superhydrophobic coatings with multifunctional properties on aluminum alloys surface: a review
2023, Progress in Organic Coatings