As a strong, lightweight and easily machined material, magnesium alloy holds much promise as an alternative to heavier metals like aluminum, particularly when it comes to transportation. One attribute holding it back, however, is the fact that it corrodes easily. But Australian researchers have discovered an ultra-low density and corrosion-resistant magnesium-lithium alloy that could greatly reduce the weight of cars and planes, in what they describe as the first step toward mass production of stainless magnesium.

Monash University's Professor Nick Birbilis has carried out very calculated research in pursuit of a corrosion-resistant magnesium alloy. In 2013, a team he led discovered that they could better preserve the metal in the lab by adding a dash of arsenic, which ultimately cut its corrosion rate in salt solution by a factor of nearly 10.

But there would be an element of luck involved in his latest, even more promising breakthrough, when a team of collaborating researchers from the University of New South Wales (UNSW) spotted a heat-treated magnesium alloy sitting inert in a beaker of water.

Normally when testing magnesium alloys for corrosiveness, researchers will place the samples in solutions like salt water and return a day later to see how much of it remains. But to their surprise, they found that, though this particular sample had been resting in the salty water for some time, it was completely intact, with no corroded surfaces. So the team began to investigate the structural detail of the alloy, turning to scientists on the Powder Diffraction beamline at the Australian Synchrotron to uncover its secrets.

What they found was a unique nanostructure that gives rise to a protective layer of carbonate-rich film upon atmospheric exposure. They liken this to the way a protective film of chromium oxide forms on stainless steel, and report that it made the magnesium alloy immune to corrosion in the lab setting.

"This is the first magnesium-lithium alloy to stop corrosion from irreversibly eating into the alloy, as the balance of elements interacts with ambient air to form a surface layer which, even if scraped off repeatedly, rapidly reforms to create reliable and durable protection," explains Professor Michael Ferry, who led the UNSW team.

The magnesium-lithium alloy weighs half as much as aluminum and is 30 percent lighter than magnesium. This could see it find its way into lighter cars, trucks and aeroplanes that use significantly less fuel, which would put a huge dent in the transport sector's carbon emissions.

"These panels will make many vehicles and consumer products much lighter and, eventually, just as durable as today's corrosion-resistant stainless steel, another example of how advanced manufacturing is unlocking the potential of materials that have been under investigation, in too narrow a manner, for centuries," says Birbilis.

The team is now studying the molecular composition of the alloy and carbonate-rich film to better understand how the corrosion process is averted.

The research was published in the journal Nature Materials.

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