Materials

Discovery of corrosion-resistant "stainless magnesium" to enable lightweight vehicles

Discovery of corrosion-resista...
A new magnesium-lithium alloy (not pictured) has been discovered that was shown to be corrosion-resistant in the lab
A new magnesium-lithium alloy (not pictured) has been discovered that was shown to be corrosion-resistant in the lab
View 2 Images
A new magnesium-lithium alloy (not pictured) has been discovered that was shown to be corrosion-resistant in the lab
1/2
A new magnesium-lithium alloy (not pictured) has been discovered that was shown to be corrosion-resistant in the lab
The surface condition of two samples of the magnesium-lithium alloy following immersion for 20 hours in salt water, one after being heat-treated and water quenched, and the other without processing. In previous magnesium-lithium alloys, irreversible corrosion would have set in after such time, but clearly visible in this optical profilometer image, the surface of the processed alloy remains in near pristine condition
2/2
The surface condition of two samples of the magnesium-lithium alloy following immersion for 20 hours in salt water, one after being heat-treated and water quenched, and the other without processing. In previous magnesium-lithium alloys, irreversible corrosion would have set in after such time, but clearly visible in this optical profilometer image, the surface of the processed alloy remains in near pristine condition

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.

The surface condition of two samples of the magnesium-lithium alloy following immersion for 20 hours in salt water, one after being heat-treated and water quenched, and the other without processing. In previous magnesium-lithium alloys, irreversible corrosion would have set in after such time, but clearly visible in this optical profilometer image, the surface of the processed alloy remains in near pristine condition
The surface condition of two samples of the magnesium-lithium alloy following immersion for 20 hours in salt water, one after being heat-treated and water quenched, and the other without processing. In previous magnesium-lithium alloys, irreversible corrosion would have set in after such time, but clearly visible in this optical profilometer image, the surface of the processed alloy remains in near pristine condition

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.

Source: Australian Synchrotron

9 comments
jimbo92107
Combine this Mg-Li alloy with the composite from this article:
http://www.gizmag.com/new-metal-composite-light-float-on-water/37522/
This combined material would be a magnesium / lithium alloy matrix foam, reinforced with hollow particles of silicon carbide. Better than either material alone, it is lighter than water, and it reacts with air to form its own protective layer against atmospheric and salt water corrosion.
Whoever makes this owes me a case of really good beer.
Adrien
Cool! I wonder how flammable it is though.
Alfredo
#Adrian ... I was just wondering the same thing.
From wikipedia "Lithium is flammable, and it is potentially explosive when exposed to air and especially to water, though less so than the other alkali metals."
And they disallowed magnesium alloy vehicles in Formula 1 cars, after Joe Schlesser's fatal crash, where the firemen tried to put out a magnesium fire with WATER --- it just added oxygen to the fire!
Bob Stuart
As usual, I came looking for numbers, but only found "just as durable as today's corrosion-resistant stainless steels." That sure sounds better than the equally accurate "half the strength of aircraft aluminum" but also suggests extreme ductility, which seems unlikely. As for stiffness, I'll guess it is proportional to density, as usual in structural metals, but I want to see the alloy proportions, or test results on both properties. Writers, structural properties are numbers you should quote precisely, even if you don't understand them. The difference between stiffness and strength, the beginning of modern engineering, was unknown to the staff at The Big Bang Theory (8-2).
Techtwit
Ah, well, there goes a few more tons of lithium that the "electric car" future dreamers won't have for their golf cart batteries. Shame lithium is so useful when we have so little of it.
Bruce Miller
No real shortage of Li! And: Recent Super Capacitor developments may just shock the world! We seek high voltage resistant dielectrics? This will come form Asian intelligentsia as Big Oil cripples U.S. research in this direction? Should Electrical Storage other than low volt, internal resistance strapped, chemical Li batteries be exploited, Li may find its way into these alloys? Are these alloys reformable? do they have a high scrap mental value? Will a substantial "Ball of Wax" or store of these alloys make remanufacturing a viable route? For that matter: Li batteries can be made fully recyclable, and once a nation has a stock of theses in circulation, Li demands will drop!
Dr.Veritas
Bruce Miller, I liked what you were saying right up until you used the term "Big Oil". It significantly deflates the credibility of your comments. Such a shame. Perhaps I'm the only one that feels this way but if there's a specific charge against a particular industry then cite the case wherein collusion was proven and the specific target of these multi-national corporate fiends.
I'm not suggesting there aren't corrupt individuals in most if not all industries but really, if there's proof your comments will be better accepted by sharing the story.
I've heard from so many people about a friend's uncle's brother's cousin who invented a carburettor that got 150 mpg but how "Big Oil" bought the idea and stuffed it away in their vault where they keep all technologies that threaten their dominance in the world.
Anyway...I always meant to ask how does a carburettor get 150 miles per gallon and how can people ride in or on a carburetor anyway?
darkstar01
What happens during a fire? Would this turn into thermite during a really nasty gasoline fire? Last time I checked... Magnesium + oxygen + iron or other metal = very thermogenic chemical reaction? (or would it be endothermic?)
J4rH43d
If you think a magnesium fire is bad, you should see a titanium fire.
It sucks the oxygen out of water AND CO2. It also burns with nitrogen. So let's stop using titanium
Thermite, BTW, is finely divided aluminum mixed with iron oxide. The aluminum burns hotter than magnesium in this scenario.
Any metal will burn in air if the fire is hot enough to begin with. Think flame-cutting of steel.