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ISSN: 2329-6798
Modern Chemistry & Applications
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Development of Anti-Corrosion Processing on Magnesium Alloys

Sun Rujian, Guan Yingchun* and Zhu Ying
Beihang University, 37 Xueyuan Road, Beijing 100191, P.R. China
Corresponding Author : Guan Yingchun
Beihang University, 37 Xueyuan Road
Beijing 100191, P.R. China
Tel: 86-082-339-961
Fax: 86-823-399-61
E-mail: [email protected]
Received April 30, 2015; Accepted May 25, 2015; Published May 29, 2015
Citation: Rujian S, Yingchun G, Ying Z (2015) Development of Anti-Corrosion Processing on Magnesium Alloys. Mod Chem appl 3:153. doi: 10.4172/2329-6798.1000153
Copyright: © 2015 Rujian S et al., This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

Magnesium alloys have been increasingly used in the industries and biomaterial fields due to their low density, high specific strength and biodegradability. However, poor surface-related properties are major factors that limit their practical applications. This paper will begin with a brief review of conventional methods for anti-corrosion of Mg alloys, and demonstrate the feasibility of laser shock processing in achieving the enhanced corrosion resistance, especially for stress corrosion cracking.

Keywords
Stress corrosion; Metal corrosion; Aerospace; Organic coating
Introduction
Metal corrosion has been considered as common phenomenon, ranging from daily life to wide industrial applications including metallurgy, chemical industry, energy industry, transportation, aeronautics, agriculture, marine engineering. For instance, annual economic loss due to corrosion accounts for 2 to 4 percent of the gross domestic product in China. The economic losses caused by corrosion reached to 500 billion Yuan in 1999, while the economic losses jumped to 819 billion Yuan in 2004, which stands for an increase of 64 percent only within five years [1]. Therefore, the impact of corrosion damage to the national economy is critical, thus is no doubt a serious problem in urgent need of being resolved.
Magnesium is one of the lightest metallic element, and magnesium alloys have unique properties including high strength to weight ratio, low density, effective damping qualities, good absorbing electromagnetic radiation performance, as well as more than 80% recyclable rate. This makes it consistent with the concept of “21st Century Green Engineering Material” [2]. The need for lightweight structural as well as functional parts, particularly in the aerospace, automotive and electronic industries, has stimulated engineers to be more adventurous in their choice of materials. Mg and its alloys, with one quarter of the density of steel and only two thirds that of aluminum, and a strength to weight ratio that far exceeds either, fulfills the role admirably as an “ultralight” alloy [3]. Moreover, Mg-based metal implants have attracted much attention in the fields of bio-medicals nowadays due to their biodegradability [4]. Witte claimed that this groundbreaking approach to temporary metallic implants was one of the latest developments in bio-materials science that was being rediscovered [5] (Figure 1).
However, Mg is a highly active element, which is susceptible to becoming corroded within its working condition, especially when Mg alloy serving as structural materials are immersed in corrosive media and combined with stress [5-7]. This limits the wide use of Mg alloys; therefore magnesium alloy anti-corrosion becomes an important issue. This review starts with looking back to conventional surface treatment methods for improving corrosion resistance of Mg alloy, subsequently, provides a new approach using laser shock processing technique for stress corrosion cracks.
Brief Review of Conventional Anti-Corrosion Technologies
According to previous study, conventional surface treatment technologies for anti-corrosion of Mg alloys mainly include chemical conversion, anodic oxidation, micro-arc oxidation, organic coating, thermal spraying, physical vapor deposition (PVD), etc. [8]. The advantages and disadvantages of these methods can be mainly summarized in Table 1.
Laser Shock Processing for Mg Alloy
Laser surface processing has been considered as an advanced alternative over conventional counterparts to improve the surface properties of materials [7,9]. It has been recognized that laser surface engineering can produce surface layers with a fine microstructure that reduce the size of galvanic couples and expand the solid solution range of alloying elements [6,9]. Such factors can potentially improve surface performance of laser-treated metallic alloys. Moreover, short processing time, flexibility in operation, economy in time/energy/ material consumption, shallow heat affected zone and precision are the important advantages of laser processing methods [10].
Laser shock processing (LSP) is a surface engineering process in which the base metal covered by an absorbing coating (aluminum foil) and transparent constrained layer (water), is hit by a high power density (GW/cm2) short pulse (nanosecond) laser. After the coating absorbs laser energy rapidly, ultimately forms a large number of dense hightemperature (>10 K) high pressure (>1GPa) plasma. Correspondingly, it produces an explosive shock wave on the metal surface. When the peak pressure of the shock wave exceeds the dynamic yield strength of the material, it generates plastic deformations which finally impart beneficial residual stress within the material. Such deep high magnitude compressive residual stresses induced by LSP increase the resistance of materials to surface-related failures, such as fatigue, fretting fatigue and stress corrosion cracking [11] (Figure 2).
In 2008, Jiangsu University firstly conducted LSP on Mg alloys to form dense dislocations, vacancies and vacancy clusters, in turn changing microstructure and mechanical properties of Mg alloys [12]. After this, more researchers mainly examined mechanical properties of Mg alloys after LSP, and they found that surface micro-hardness was increased by more than 50%, surface residual stress up to hundreds of MPa, local plastic deformation within 1mm, and the enhanced fatigue properties [13-17]. Table 2 shows the residual stress of different magnesium alloys after LSP, indicating that the compressive residual stress up to hundreds MPa is achieved, which will be beneficial for surface properties enhancement including anti-corrosion.
Laser Shock Processing on Anti-corrosion
Effect of LSP on corrosion behavior of Mg alloy has not been widely studied. The first paper was published in 2008 [14], and only a few papers covering electrochemical corrosion, stress corrosion cracking, and hot corrosion exist can be found to the limit of our knowledge [15-18]. It has been claimed that LSP induces compressive residual stress on the surface of Mg alloys, resulting in grain refinement, a large number of twins, and a high density of dislocation, thus improving the stability of passive film and the corrosion resistance [15-18].
Electrochemical Behavior
The capability of corrosion resistance of Mg alloys depends on the surface passive film, Mg(OH)2, which determines the properties of the effect of corrosion control [17]. Good passive film should be able to prevent the outflow of metal ions from the metal surface, while also preventing the inflow of harmful ions from the external film [18]. At the same time, the impedance is significantly increased due to uneven surface because of plastic deformation generated by LSP, while corrosion current density reduced tremendously and improved the corrosion resistance of the base material effectively. Zhang and his co-workers measured the polarization and EIS curves under different power density by LSP, showing that Tafel curve of the metal moves towards positive direction and EIS increases at lower laser density, and EIS begins to decrease when laser density reaches 0.7 GW/cm2, as shown in Figure 3a and 3b. Moreover, Figures 3c and 3d indicates that subtle shifts causes change of Tafel curve and EIS curve when laser density is more than 1GW/cm2 [18]. Guo and his co-workers studied biodegradable Mg alloy by LSP, and indicated that LSP slowed down corrosion rate to a large extent in Figure 4a, and laser-peened surface had less potential to initiate corrosion in Figure 4b [19] (Figures 3 and 4).
Stress Corrosion Cracking (SCC)
LSP induces plastic deformation on the surface and elastic deformation on the sub-surface. At the end of the shock, the surface layer material is compressed, material particles away from the equilibrium position. In the mean time, the plastic layer blocks the elastic deformation layer from returning to the equilibrium position, so that compressive residual stress is formed on the surface layer of the material, which can then be used to balance tensile stresses when it is in working condition, thereby preventing the generation of stress corrosion cracking, and preventing the growth of stress corrosion cracks, as shown in Figure 5 [20].
Hot Corrosion
It has been found that LSP can improve the hot corrosion resistance by improving its oxidation film composition in titanium alloy [21]. The hardened layer formed with high level of compressive residual stress enables it to endure larger stresses, and prevents the penetration of salts into the scale. Thereby, the layer has excellent resistance for hot corrosion. On basis of previous study, refinement of coarse grains occurs in the plastic deformation layer during LSP of Mg alloy [20]. Accordingly, a large amount of twins and highly tangled dense dislocation arrangements will be formed in the microstructure, which can delay crack initiation. Therefore, it is suggested that LSP can also be applied to Mg alloy for preventing hot corrosion.
Conclusion
LSP has been regarded as a useful method to improve mechanical properties of materials. Little work has been conducted for corrosion behavior. On basis of current analysis, LSP can play a significant role in improving surface anti-corrosion; therefore there is far more necessary research space to be filled. It is proposed that LSP is a promising technique for enhancing corrosion resistance of Mg alloys, especially for stress corrosion cracking, which will be very useful for practical applications in industries.
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Table 1   Table 2

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