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Reaction of Aluminum with Water

May 17, 2011

When I was a graduate student, I investigated the reaction of metallic elements in the formation of alloys. One interesting effect that I noticed (and I was likely not the first to notice this) was that reactions were initiated prior to the melting point of the lowest melting metal and progressed quickly.

Since I was experimenting with intermetallic compounds that had a large exothermic heat of formation, the theory was that once the reaction started in the solid state, it quickly propagated to heat the metal mixture, forming a liquid that enhanced the reaction rate. Having a liquid phase was the key to rapid reaction.

Aluminum has the nice property of forming a protective oxide coating of alumina (Al2O3) that stops growing after a shallow thickness has been formed. This oxide protects the surface from further oxidation or corrosion upon immersion in water. Aluminum will still be corroded by some chemicals, such as ferric chloride and sodium hydroxide. I've done some art etching of aluminum using both of these chemicals.[1]

Researchers at Purdue University have been investigating the rapid reaction of some particular aluminum alloys with water at room temperature.[2-5] These alloys react by the same mechanism as my alloy reactions; that is, the reactions are enabled by a liquid metal phase. They've prepared aluminum-gallium-indium-tin alloys in compositions ranging from one that's Al50Ga34In11Sn5 (weight percent) to one with 95% aluminum.

The products formed by immersion of these alloys in water are aluminum hydroxide and hydrogen gas; viz.,
Al + 3H2O → Al(OH)3 + (3/2)H2

The aluminum hydroxide is in the form, α-Al(OH)3, or bayerite.

This alloy is not a renewable energy source. It's more like a battery. Some of the energy that was initially spent in refining aluminum from its ores is liberated, and the aluminum hydroxide that's produced can be refined back to aluminum to remake the alloy and "recharge" this energy source.

For the Al50Ga34In11Sn5 alloy, the aluminum is solvated in a reaction-enabling liquid phase at 9.38 °C, which pumps the aluminum through to a reaction interface with water.

Deutsches Reich, 50 Pfennig Aluminum Coin (1920)Deutsches Reich, 50 Pfennig Aluminum Coin (1920).

The composition is 99% Al, 1% Cu.

(Via Wikimedia Commons)

A particular heat treatment is required to produce the two-phase mixture responsible for the aluminum reactivity. If the alloy mixture is cooled fast, the other elements are incorporated into the aluminum matrix. Slow cooling, however, causes a phase separation of a gallium-indium-tin alloy that's responsible for the reactivity. Understanding this process allowed the redesign of the alloy from a 50-weight percent aluminum content to 95%.

In brainstorming applications for this novel alloy, the Purdue team, led by Jerry Woodall, a distinguished professor of electrical and computer engineering, has proposed that this technology could be used to generate clean drinking water from saltwater or contaminated water sources, while simultaneously producing hydrogen to generate electricity. In these applications, the alloy would be useful for things such as emergency relief in the wake of disasters, and for military operations.

The clean water would be produced from steam generated in the exothermic reaction, and the hydrogen would be pumped into a fuel cell to produce electricity. The steam, of course, would kill any bacteria in the water. Woodall estimates that the generated electricity could be obtained for about 35 cents per kilowatt hour, and the water for about a dollar a gallon.[5] Woodall has high hopes for these alloys:
"Because aluminum is a low-cost, non-hazardous metal that is the third-most abundant metal on Earth, this technology promises to enable a global-scale potable water and power technology, especially for off-grid and remote locations."[5]

One problem is that indium is a rare metal that's become quite expensive because of the demand for computer and consumer displays. As I wrote in a previous article (Indium, January 08, 2008), the price of indium jumped from about $90 per kilogram to about $1,000 per kilogram from 2002 to 2005. The price of indium is now about $700 per kilogram due to increased production. Because of its combined utility and scarcity, Indium is listed as a "most critical material" in a US Department of Energy, December, 2010, report on Critical Minerals.[6]

A patent has been filed on this invention, and AlGalCo LLC, an Indiana startup, has a license for commercialization. Now that we understand this reaction, what other alloys can you imagine will have this same property? If I didn't spend all my time writing this blog, I might be able to discover a few. It's not rocket science - It's materials science.

References:

  1. Standard Disclaimer - Only technically trained individuals should use chemicals; and those should always read the material safety data sheet (MSDS) and wear proper personal protective equipment, such as safety glasses and gloves.
  2. Emil Venere, "New aluminum-rich alloy produces hydrogen on-demand for large-scale uses," Purdue University Press Release, February 19, 2008.
  3. Jeffrey Thomas Ziebarth, "Use of the aluminum gallium indium tin system for energy storage and conversion," Purdue University, Department of Electrical and Computer Engineering, Ph. D. Dissertation, April 26, 2010.
  4. Go Choi J.T. Ziebarth, J.M. Woodall, R. Kramer, D. Sherman and C.R. Allen, "Mechanism of Hydrogen Generation via Water Reaction with Aluminum Alloys," Micro/Nano 2010 18th Biennial University/Government/Industry Symposium, June 28 2010-July 1 2010.
  5. Emil Venere, "Portable tech might provide drinking water, power to villages," Purdue University Press Release, May 3, 2011.
  6. Critical Minerals Strategy (US Department of Energy, December, 2010).