Recent Advances in Corrosion Protection of Magnesium
Magnesium is the 8th most widely spread chemical element on the earth (Gray & Luan 2002). Magnesium accounts for nearly 2% of the earth’s crust and makes up approximately 0.13% of the ocean mass (Gray & Luan 2002). The benefits and properties of magnesium are well-documented: these include but are not limited to low density and high straight-to-weight ratio, excellent electromagnetic shielding properties, and perfect dimensional stability (Hamdy 2006). Given these characteristics, it comes as no surprise that magnesium remains one of the most popular materials in engineering. Unfortunately, magnesium is extremely susceptible to corrosion. For this reason, professional engineers seek effective ways to improve corrosion protection of magnesium.
The current state of literature provides a wealth of information relating to the recent improvements in corrosion protection of magnesium and its alloys. Voulgaris, Amanatides, Mataras, Grassini, Angelini and Rosalbino (2006) analyzed the effects of RF power and SiOxCyHz deposition on corrosion properties of magnesium alloys. The researchers applied TEOS/O2/He gas and 27.12 MHz power to thin silicon-based films, to protect magnesium alloys against corrosion (Voulgaris et al 2006). Silicon films deposited at intermediate voltages proved to be an effective element of protecting magnesium alloys from the risks of corrosion (Voulgaris et al 2006). The use of intermediate RF voltages resulted in a better adhesion of the films to magnesium and led to the subsequent removal of –C and –OH groups (Voulgaris et al 2006). It was concluded that the effectiveness of corrosion protection depended upon the degree to which SiOxCyHz films adhered to the Mg substrate, rather than on their chemical composition (Voulgaris et al 2006).
In a similar vein, Zhu and Song (2006) analyzed corrosion resistance of AZ91D magnesium alloy under the influence of an aluminum-alloyed coating. Zhu and Song (2006) applied aluminum-alloyed coating to the surface of AZ91D alloy. It was found that aluminum-coated magnesium alloys exhibited remarkable corrosion-resistance characteristics (Zhu & Song 2006). The researchers ascribed better corrosion resistance to the presence of β phase in the aluminum coating (Zhu & Song 2006). Birbilis, Howlett, MacFarlane and Forsyth (2007) used ionic liquid pretreatment to facilitate effective protection against corrosion in magnesium. Surprisingly or not, only Hamdy (2006) mentioned the importance of using eco-friendly materials to protect magnesium from corrosion: cerate conversion coatings and the use of NaCl solution proved to be an effective eco-friendly solution to the problem of corrosion in magnesium alloy AZ91D.
Despite the rapid advances in engineering, no single method can “adequately protect magnesium from corrosion in tough service conditions” (Gray & Luan 2002, p.111). Most protection methods are extremely complex and involve several pre-treatment stages. Thus, the future research must focus on the development of a single, universally applicable method of protecting magnesium and magnesium alloys from corrosion. The current knowledge of corrosion in pure magnesium and magnesium alloys will create a foundation for developing effective methods of protection against corrosion.
Birbilis, N, Howlett, PC, MacFarlane, DR & Forsyth, M 2007, ‘Exploring corrosion protection of Mg via ionic liquid pretreatment’, Surface & Coatings Technology, vol.201, pp.4496-4504.
Gray, JE & Luan, B 2002, ‘Protective coatings on magnesium and its alloys – a critical review’, Journal of Alloys and Compounds, vol.336, pp.88-113.
Hamdy, AS 2006, ‘Enhancing the corrosion resistance of magnesium alloy AZ91D in 3.5 per cent NaCl solution by cerate conversion coatings’, Anti-Corrosion Methods and Materials, vol.53, no.6, pp.367-373.
Voulgaris, C, Amanatides, E, Mataras, D, Grassini, S, Angelini, E & Rosalbino, F 2006, ‘RF power and SiOxCyHz deposition efficiency in TEOS/O2 discharges for the corrosion protection of magnesium alloys’, Surface & Coatings Technology, vol.200, pp.6618-6622.
Zhu, L & Song, G 2006, ‘Improved corrosion resistance of AZ91D magnesium alloy by an aluminum-alloyed coating’, Surface & Coatings, vol.200, pp.2834-2840.