What Is Magnesium?

Magnesium is a silvery-white alkaline earth metal with atomic number 12 and a density of 1.74 g/cm³, making it the lightest structural metal in practical engineering use. Its combination of low density, reasonable specific strength, good machinability, and electromagnetic shielding capability has made it an attractive material for applications where mass reduction is a primary design objective. The element is the eighth most abundant in Earth's crust and the third most abundant in seawater, yet it is never found in its pure form in nature, occurring instead in mineral compounds including magnesite (MgCO₃) and dolomite (CaMg(CO₃)₂).

The engineering use of magnesium spans pure metal applications and, more commonly, magnesium alloys in which aluminum, zinc, manganese, rare earth elements, or zirconium are added to improve mechanical strength, ductility, and creep resistance at elevated temperatures. The disciplinary foundations of magnesium engineering lie in physical metallurgy, electrochemistry, and surface science, with manufacturing processes including casting, extrusion, forging, and sheet rolling all requiring adaptations to account for the metal's hexagonal close-packed crystal structure and its tendency toward anisotropic deformation behavior.

Physical and Chemical Properties

Pure magnesium melts at 650 °C and has a Young's modulus of approximately 45 GPa, lower than that of aluminum (69 GPa) but sufficient for many structural applications when combined with the density advantage. Its hexagonal close-packed crystal structure limits the number of available slip systems at room temperature, which reduces ductility and makes room-temperature forming more difficult than for face-centered cubic metals such as aluminum. Magnesium burns in air when finely divided or when heated above its ignition temperature, a property that demands careful handling in powder or chip form during machining operations. The element is highly reactive electrochemically, sitting near the negative end of the galvanic series, which makes it vulnerable to galvanic corrosion when in contact with most other structural metals in the presence of an electrolyte.

Production and Alloy Development

Magnesium is produced commercially by two principal routes: electrolytic reduction of anhydrous magnesium chloride derived from seawater or brine, and the Pidgeon process, a thermal reduction of calcined dolomite by ferrosilicon in a vacuum retort. China accounts for the large majority of global primary magnesium production. Alloy development has focused on overcoming the intrinsic limitations of pure magnesium; the AZ series (Mg-Al-Zn), the AM series (Mg-Al-Mn), and the WE series (Mg-Y-RE) are widely used families, with the choice among them governed by the service temperature, corrosion environment, and forming requirements of the application. A review of magnesium alloys for aerospace applications documents how strategic alloying element selection and surface coatings are being developed to extend the use of magnesium in structural and thermal management roles in aircraft and spacecraft.

Corrosion and Surface Protection

Corrosion resistance is the most significant barrier to wider deployment of magnesium alloys in structural roles. In humid or salt-laden environments, failure rates due to galvanic corrosion and stress corrosion cracking can be three to five times higher than for comparable aluminum alloys. Protective strategies include anodizing, plasma electrolytic oxidation, chemical conversion coatings, and organic top coats, with NASA technical work on magnesium alloys for space hardware addressing the specific requirements for corrosion protection and thermal control coatings in the space environment. The ScienceDirect overview of magnesium alloy developments surveys advances in alloy design aimed at improving inherent corrosion resistance through microstructural refinement and secondary phase control.

Applications

Magnesium and its alloys have applications across a range of industries, including:

• Aerospace interior panels, seat frames, and lightweight structural components
• Automotive gearbox housings, instrument panel beams, and steering wheel cores
• Consumer electronics and portable computer housings
• Medical implants and biodegradable orthopedic fixation devices
• Pyrotechnic and incendiary materials in defense applications
• Sacrificial anodes for cathodic protection of steel structures