Environmental behavior of magnesium and magnesium alloys

Because of their low density, magnesium (Mg) alloys exhibit higher specific strength compared to other metals and alloys and are of growing interest as structural materials in the automotive, aerospace and electronic industries. Magnesium, being one third lighter than an equal volume of Al, offers great possibilities to reduce the vehicle weight, which is essential to save energy. Further applications include e.g. computer and cellular telephone housings. Mg-housings are an attractive alternative to polymers with significant advantages in terms of electromagnetic shielding and recycling. However, Mg is very reactive to the environment, above all in the presence of water and oxygen. Corrosion and oxidation are major shortcomings of Mg parts and these are still poorly understood, Hence, a widespread industrial usage of Mg alloys will ultimately depend on their capability to maintain the original performance during environmental exposure over extended periods of service. Magnesium alloys for structural applications are processed by casting (die, sand and mold) or used as wrought products (extrusions, forgings, sheet and plate). In addition to the high specific strength, Mg alloys exhibit excellent die castability, superior machinability, good ductility and damping capacity. The most common recent die casting Mg alloy is the AZ91D (high purity alloy containing typically 8.3-9.7% Al + 0.35-1% Zn + >0.13% Mn [1]; unless otherwise indicated in the text additions to Mg are given in mass percentage). This alloy combines the above properties with good corrosion resistance, comparable to that of Al casting alloys. For applications requiring improved ductility and fracture toughness a series of die casting Mg alloys with reduced content of Al was developed. AM50 and AM60 alloys (4.4-5.4 and 5.5-6.5% AL respectively, <0.22% Zn and >0.25% Mn) have found application in automotive safety parts such as steering wheels, seat parts and instrument panels. Alloys such as WE43 (3.7-4.3% Y + 2.4-4.4% RE (1.5-2% Nd + heavy RE) + 0.4-1% Zr + 0.2% Zn), QE22 (2-3% Ag + 1.8-2.5% RE (> 70% Nd, any remainder substantially Pr) + 0.4-1% Zr) andZE41 (0.75-1.75% RE + 3.5-5% Zn + 0.4-1%Zr) are extensively used in the aircraft industry (engines, airframes and landing gear) due to their improved creep and corrosion resistance. The present review concentrates on the environmental deterioration of Mg and its alloys by oxidation, corrosion and stress corrosion cracking. Commonly used structural Mg alloys as well as new alloys that are being developed in order to improve properties, e.g. alloys containing rare earth elements (RE) or produced by rapid solidification (RS), are included.