Magnesium vs Aluminum

For a part that has to be cast, has to be light, and lives indoors or under paint, magnesium. For anything exposed to weather, salt or a steel bolt, aluminum — and the margin there is not close.

That is an unusually clean split for two metals that sit next to each other in the table, and it comes down to the fact that magnesium is the most electrochemically active metal anyone builds structures out of, while aluminum is one of the best-behaved.

The lightest structural metal, and what it costs

Magnesium is about 36% lighter than aluminum, which makes it the lightest metal anyone uses for load-bearing parts. In the die-cast alloys that dominate both markets the strengths are broadly comparable, so magnesium delivers a genuine weight saving of roughly a third for the same job.

The catch arrives with the first drop of salt water. Magnesium's position in the galvanic series is so far to the active end that it is used deliberately as a sacrificial anode — specifically in high-resistivity soils and fresh water, where less active anode metals cannot drive enough current. A metal chosen for its willingness to corrode on someone else's behalf is not one you want in an unprotected structural role.

The practical consequence is that magnesium parts are always coated — conversion coatings, plasma electrolytic oxidation, or paint — and that every joint with a steel or aluminum fastener needs deliberate electrical isolation. A magnesium housing bolted directly to a steel bracket will eat itself around the bolt.

Corrosion turned out to be a purity problem

For decades magnesium's reputation for dissolving in salt spray was accepted as intrinsic. It is not, or at least not entirely.

Work in the 1980s established that the culprit was largely contamination. Iron, nickel and copper present above quite small tolerance limits form cathodic sites inside the metal that drive rapid local corrosion, and dropping iron content below roughly fifty parts per million improves salt-spray performance by orders of magnitude rather than percentages. High-purity die-cast alloy grades were the result, and they turned magnesium from a material that failed corrosion testing into one that merely needs respect.

It is still not aluminum. Aluminum's surface film reforms instantly whenever it is broken and needs no help from a coating in most atmospheres. Magnesium's does not passivate anything like as effectively, and its protection is applied rather than intrinsic.

Where magnesium is unambiguously better: the die

The strongest argument for magnesium has nothing to do with the finished part's weight. It is about how cheaply and precisely the part can be made.

Magnesium alloys carry less heat into the tooling than aluminum alloys, so magnesium can be run in hot-chamber die-casting machines that aluminum would destroy. Cycle times are shorter, die lives run to hundreds of thousands of shots rather than tens of thousands, and wall sections below a millimetre are achievable. Magnesium also machines with less cutting force than any other structural metal, so secondary operations are fast.

Add the highest damping capacity of the common structural metals — magnesium absorbs vibration that aluminum transmits — and the pattern of real applications makes sense. Steering wheel armatures, instrument panel cross-beams, camera bodies, power tool housings and laptop chassis are magnesium because of quietness and thin walls, not because a few hundred grams were the design driver.

Heat, creep, and the long road to an engine bay

Conventional magnesium-aluminum-zinc alloys begin to creep under sustained load somewhere around 120 to 130 °C. That is well below any engine bay temperature, and it kept magnesium confined to cabin and cosmetic parts for a long time.

Creep-resistant alloys containing rare earth additions changed that, and magnesium engine cradles did eventually reach production cars. But the temperature ceiling remains materially lower than aluminum's, and any part that runs hot and carries a sustained load has to be checked for creep in a way an aluminum designer would never think about.

The fire question, and the rule that changed

Bulk magnesium is difficult to ignite. Thin sections, swarf and powder are not, and once burning it reaches temperatures that ordinary extinguishing agents cannot deal with — water and carbon dioxide both react with it. Those are facts about the metal, and they were enough for magnesium alloys to be effectively excluded from aircraft cabin interiors for decades.

That exclusion was lifted in stages from around 2015, after flammability testing under realistic cabin-fire conditions showed that specific alloys — the rare-earth-containing grades in particular — performed acceptably. Magnesium seat frames are now permitted under the relevant industry standards, which is a rare case of a materials ban being reversed by evidence rather than by a substitution.

Aluminum cannot be made without magnesium

The relationship between these two metals is not really competitive. Each is the other's main alloying element.

Magnesium's own alloys are named for their aluminum content. And on the other side, magnesium is the defining addition in two of the most important aluminum alloy families: the 5000 series used for marine plate and can bodies, and the 6000 series that most structural extrusion is made from. There is no substitute for it in those alloys.

That dependence became visible in the autumn of 2021. Around 85 to 90 per cent of the world's primary magnesium is produced in China, largely by a coal-fired silicothermic route, and energy curtailment in the producing region cut output sharply. European magnesium stocks fell to a few weeks, and European aluminum producers publicly warned that they would have to stop making alloy — not because of any shortage of aluminum, but because they could not get the magnesium to put in it.

The supply concentration is worth understanding correctly, because it is not geological. Magnesium is the eighth most abundant element in the crust and the third most abundant ion in seawater, and it was extracted from seawater at industrial scale for much of the twentieth century. There is no shortage of magnesium anywhere on Earth. The concentration is a story about where energy is cheap and where emissions are tolerated, and it can in principle be undone by building plants elsewhere.

Casting, cladding or extruding

• A thin-walled cast housing that will live indoors — magnesium, for the tool life as much as the weight.
• Anything that will see salt, weather or standing water — aluminum, unless magnesium is fully coated and fully isolated.
• A part where vibration is the complaint — magnesium, which damps far better.
• An extrusion — aluminum; magnesium extrudes poorly and slowly by comparison.
• A hot, continuously loaded component — aluminum, or a creep-resistant magnesium grade chosen explicitly for the temperature.
• A sacrificial anode in high-resistivity soil or fresh water — magnesium, doing on purpose what it does accidentally everywhere else.
• Marine plate or a structural extrusion in aluminum — you are buying magnesium too, whether you meant to or not.