Electrochemical Corrosion Behavior of Magnesium Alloys in Biological Solutions

Abstract

Magnesium (Mg) is the fourth most abundant cation in the human body [Fekry & El-Sherief, 2009]. It is very abundant in the Earth being considered the fourth highest, following iron, oxygen and silicon. The raw ores of Mg are dolomite (MgCO3.CaCO3) and magnesite (MgCO3), and Mg is the second most abundant metal in seawater following sodium. It is therefore a comparatively low cost material. Magnesium is the lightest of all metals in practical use, and has a density (1.74 g cm-3) of about two thirds of aluminum and only one quarter that of iron. Pure magnesium metal has useful properties such as shielding against electromagnetic waves, vibration damping, dent resistance, machinability and low toxicity in humans, in addition to its recyclability as it has a lower specific heat and a lower melting point than other metals. On the other hand, magnesium has shortcomings such as insufficient strength, elongation and heat resistance as well as being subject to corrosion. To put Mg to practical use, it is necessary to deal with its shortcomings and improve its performance through alloying with various elements. Alloying magnesium improves its strength, heat resistance and creep resistance (creep is defined as deformation at a high temperature and under load). However, the addition of alloying elements modifies the corrosion behavior of magnesium in such a way that it can be beneficial or deleterious. Some advantages of magnesium alloys are their high stiffness-to-weight ratio, great ease of machinability, good casting qualities suitable for high pressure die-casting, high damping capacity and good weldability under controlled atmosphere. Magnesium can form intermetallic phases with most alloying elements, the stability of this phase increases with the electronegativity of the other element [Kainer, 2003]. Aluminum (Al) had already become the most important alloying element for significantly increasing the tensile strength, specifically by forming the intermetallic phase Mg17Al12. Similar effects can be achieved with zinc (Zn) and manganese (Mn), while the addition of silver (Ag) leads to improve hightemperature strength. The identification of magnesium alloys is standardized worldwide in the ASTM norm; each alloy is marked with letters indicating the main alloy elements, followed by the rounded figures of each (usually two) weight in percentage terms. The last letter in each identification number indicates the stage of development of the alloy. However, according to the elemental composition two major magnesium alloy systems are available to the designer. The first includes alloys containing 2 – 10 wt% Al, combined with minor additions of zinc and manganese. These alloys are widely available at moderate cost,