In Situ Ellipsometric Study on Corrosion of Magnesium Alloys

Magnesium is the 8th most abundant element on the earth. Moreover, it is the lightest structural metal. Magnesium and alloys have numerous advantageous properties such as high specific strength, high thermal conductivity, high dimensional stability, high damping capacity, good machinability, good electromagnetic shielding characteristics and easilyrecycling characteristics. These excellent properties make them an ideal choice for a number of applications including automobile parts, computer and mobile components, sporting goods, aerospace equipment, household equipment, and implanted materials (Zhang & Zhang, 2004). However, magnesium and alloys are chemically active (the standard potential of Mg2+/Mg is -2.356Vvs. N.H.E. at 25°C(Bard & Faulkner, 2001).). Hence, they are extremely susceptible to corrosion whether in wet atmosphere or acid, neutral and weak alkaline solutions. The poor corrosion resistance seriously hinders their widespread use in many applications. Hence, a lot of researches have been performed on the corrosion of magnesium and alloys (Song & Atrens, 2003; Song, 2005; Song, 2006; Wei at al, 2007; Li et al, 2008a-c). In the investigations on the corrosion of magnesium alloys, the hydrogen evolution and weight loss measurements, the electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy (EIS), the corrosion morphology characterization techniques such as optical microscopy and scanning electron microscopy (SEM), the corrosion products identification methods such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS), etc., are often used. These conventional methods can provide the information of corrosion rate, corrosion resistance, corrosion morphology, corrosion products composition (Song, 2006). However, they fail to provide the micro-dynamic information of the "magnesium alloy corrosive medium" interfaces, which is critical for understanding the corrosion mechanism and hence developing anti-corrosive magnesium alloys. Moreover, these conventional investigations are always carried out in such a way producing much perturbation of the "magnesium alloy corrosive medium" interfaces. Therefore, employing appropriate methods to clarify the micro-dynamic details of the "magnesium alloy corrosive medium" interfaces with minimal electrochemical and physical perturbation is very necessary.