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 a s structural materials in the automotive, aerospace and electronic industri es. Magnesium, being one third lighter than an equal volume of Al, offers g reat 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 signif icant advantages in terms of electromagnetic shielding and recycling. Howev er, Mg is very reactive to the environment, above all in the presence of wa ter 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 or iginal performance during environmental exposure over extended periods of s ervice. Magnesium alloys for structural applications are processed by casting (die, sand and mold) or used as wrought products (extrusions, forgings, sheet an d plate). In addition to the high specific strength, Mg alloys exhibit exce llent die castability, superior machinability, good ductility and damping c apacity. The most common recent die casting Mg alloy is the AZ91D (high pur ity alloy containing typically 8.3-9.7% Al + 0.35-1% Zn + >0.13% Mn [1]; un less otherwise indicated in the text additions to Mg are given in mass perc entage). This alloy combines the above properties with good corrosion resis tance, 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 an d 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 instrum ent panels. Alloys such as WE43 (3.7-4.3% Y + 2.4-4.4% RE (1.5-2% Nd + heav y RE) + 0.4-1% Zr + 0.2% Zn), QE22 (2-3% Ag + 1.8-2.5% RE (> 70% Nd, any re mainder 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, airframe s and landing gear) due to their improved creep and corrosion resistance. The present review concentrates on the environmental deterioration of Mg an d its alloys by oxidation, corrosion and stress corrosion cracking. Commonl y 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.