Science

Stainless magnesium breakthrough bodes well for manufacturing industries

Stainless magnesium breakthrou...
2.5 liter V6 magnesium alloy engine block (Photo: US Department of Energy)
2.5 liter V6 magnesium alloy engine block (Photo: US Department of Energy)
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BMW R6 Inline-6 magnesium-aluminum engine block (Photo: 160sx(talk) via Wikipedia)
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BMW R6 Inline-6 magnesium-aluminum engine block (Photo: 160sx(talk) via Wikipedia)
2.5 liter V6 magnesium alloy engine block with aluminum heads (Photo: US Department of Energy)
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2.5 liter V6 magnesium alloy engine block with aluminum heads (Photo: US Department of Energy)
2.5 liter V6 magnesium alloy engine block (Photo: US Department of Energy)
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2.5 liter V6 magnesium alloy engine block (Photo: US Department of Energy)
Magnesium alloy transfer case (Photo: US Department of Energy)
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Magnesium alloy transfer case (Photo: US Department of Energy)
Corrosion chemistry with and without arsenic as cathodic poison (Image: Monash University)
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Corrosion chemistry with and without arsenic as cathodic poison (Image: Monash University)

Magnesium alloys are very attractive for a range of weight-sensitive applications. They have the largest strength-to-weight ratio of the common structural metals, are lighter than aluminum and are particularly favored for being easy to machine and for their ability to be die cast to net shape. Unfortunately, magnesium alloys tend to corrode too easily. A team at Monash University in Australia has now discovered a novel and potentially game-changing approach to the problem: poisoning the chemical reactions leading to corrosion of magnesium alloys by adding a dash of arsenic to the recipe.

Magnesium alloys are of great interest as lightweight replacements for aluminum, titanium, and steel components in a range of transportation and aerospace applications. However, such alloys corrode easily, and this often prevents their use as replacements for noncorroding metals, particularly in applications requiring high reliability over a range of environments. As a result, the use of magnesium alloys at present is less than a million tons per year, while nearly 50 million tons of aluminum alloys are used each year.

Research and development carried out over the past decade have solved certain problems presented by magnesium alloys. Their tendency toward high-temperature creep was tamed by inclusion of scandium and gadolinium, and their flammability has been greatly reduced by introducing a small amount of calcium into the mix.

Corrosion resistance in magnesium alloys has not improved to the same degree. The main discovery is that the presence of iron, nickel, copper, and cobalt in a magnesium alloy strongly activates corrosion. This is due to their low solid solubility limits (meaning that above a very small percentage they precipitate out as intermetallic compounds within the alloy structure) and the fact that they have the right electrochemistry to behave as active cathodic sites that reduce water while causing the loss of magnesium from the alloy.

If a magnesium alloy has small enough quantities of these metals, it will have improved corrosion resistance. Also, the presence of iron can be overcome by the presence of a larger amount of manganese. Maintaining such precise control over the composition of structural magnesium alloys, unfortunately, forces the price skyward, and doesn't really solve the corrosion problem.

Corrosion chemistry with and without arsenic as cathodic poison (Image: Monash University)
Corrosion chemistry with and without arsenic as cathodic poison (Image: Monash University)

Led by Associate Professor Nick Birbilis, the Monash team attempted to apply an additive known as a cathodic poison to a standard magnesium structural alloy. Cathodic poisons act by capturing atomic hydrogen within the structure of a metal. This prevents the formation of free hydrogen gas which is required to balance the corrosive chemical processes. A number of alloying elements, including arsenic, antimony, sulfur, selenium, and tellurium, are known to act in this manner in other alloy systems.

The result was that addition of about one-third of a percent of arsenic to the magnesium alloy reduced its corrosion rate in a salt solution by a factor of nearly ten. In this initial study the intent was to prove the principle of the use of cathodic poisoning. Prof. Birbilis' lab is currently working with corporate sponsors on developing a series of commercially practical stainless magnesium alloys.

"This is a very important and timely finding," says Prof. Birbilis. "In an era of light-weighting for energy and emissions reductions, there is a great demand for magnesium alloys in everything from portable electronics to air and land transportation. Magnesium products are rapidly evolving to meet the demands of industry, but presently are hindered by high corrosion rates. The arsenic effect we discovered is now being trialed as a functional additive to existing commercial alloys. Our breakthrough will help develop the next generation of magnesium products, which must be more stainless.”

Considering the enormous impact of stainless steels on our society, the game-changing potential of stainless magnesium is clear.

The University of Wales and CSIRO also took part in the research, the findings of which are published in the journal Electrochemistry Communications.

Source: Monash University

25 comments
MBadgero
Interesting stuff. Wonder whether the arsenic becomes less soluble when alloyed with the magnesium. And could you get arsenic poisoning if you cut yourself on a sharp corner made from this alloy? Might be a way to lock arsenic up into a non-toxic form. May sound strange, but nickel and chromium are toxic, and much less so when locked up in stainless steel alloys.
Anthony Parkerwood
The foundry workers would probably get sick.
Grumpyrelic
This is indeed a potential game changer. Outboard motors, marine gas turbines (aircraft use mag but marine has to use aluminium) even gas weed trimmers would benefit. The list is really endless.
squigbobble
"poisoning the chemical reactions leading to corrosion of magnesium alloys by adding a dash of arsenic to the recipe" ...and anybody who uses a grinder on it. This sounds like H&S nightmare for anyone processing it.
Ken Tuck
This is a great development. I have one question, can this alloy be extruded into structural components like aluminum?
MBadgero
@squigbobble, this is also true for sawing, milling or grinding stainless steel. That's why they give you the MSDS when you buy it in stock forms (sheet, tube, rod, etc.).
limbodog
Would I want an engine block made of magnesium? I don't know how hot it would have to get to ignite, but I recall magnesium fires as being quite a thing.
Sergius
Promising discovery ...
George Wilson
Cathodic poisoning sounds like a surface finishing process after processing via machining, casting, stamping and extruding.. Even lighter electric motors? Will it allow a 3d laser sintered printing process? What are the best alloys? Immersive CAD machining?
PDS
@limbodog Magnesium fires are quite a thing, but you have to have a very high surface area (like magnesium tape or shavings) to ignite it. As a block I believe it would melt before it burned. Aluminum burns, too, but so far as I know no Al engine blocks have spontaneously combusted. One-third of one percent is 0.00333 repeating. That's not a whole lot of anything. I imagine if you worked around it long enough you might accumulate significant Arsenic exposure. Arsenic is encountered in copper smelting so it isn't as if industry doesn't have experience working with it in a (hopefully) safe manner.