[cimon9999]

Hi,

This may or may not be a stupid question. Chemistry isn't my area but I need to know what's going on at a molecular level to understand material behaviour.

The reaction is this:

Al + 3H₂O --> Al(OH)₃ +3H₂

This reaction does not occur with ordinary aluminium because of the thin (roughly 50Angstroms - 40 layers of atoms or so) but very adherent oxide layer which forms as soon as it is exposed to air. The oxidation reaction is:

2Al + 3/2 O₂ --> Al₂O₃

This oxide layer is very inert and surrounds the aluminium, preventing any further reaction.

If, however, the aluminium is mixed with 20 weight % Gallium, the reaction with water does proceed. I want to understand at a molecular level how that might arise.

Clearly the oxygen wants to react with the aluminium at the surface but somehow the gallium prevents it. Yet while the gallium prevents Al₂O₃ from forming it allows Al(OH)₃ to form.

I was looking at van der Waals' radii & the polarity of water as possible causes but I'm really at a loss.

Any guidance would be appreciated!

Thanks,
Eamon

[Astrokel]

Simple googling: http://en.wikipedia.org/wiki/Gallium

"Gallium also attacks most other metals by diffusing into their metal lattice. Gallium for example diffuses into the grain boundaries of Al/Zn alloys[1] or steel.[2] making them very brittle. Also, gallium metal easily alloys with many metals, and was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.[3]"

"As part of an energy storage mechanism:

Aluminium is reactive enough to reduce water to hydrogen, being oxidized to aluminium oxide. However, the aluminium oxide forms a protective coat which prevents further reaction. When gallium is alloyed with aluminium, the coat does not form, thus the alloy can potentially provide a solid hydrogen source for transportation purposes, which would be more convenient than a pressurized hydrogen tank. Resmelting the resultant aluminium oxide and gallium mixture to metallic aluminium and gallium and reforming these into electrodes would constitute most of the energy input into the system, while electricity produced by a hydrogen fuel cell could constitute an energy output.[10][11] The thermodynamic efficiency of the aluminium smelting process is said to be approximately 50 percent.[citation needed] Therefore, at most no more than half the energy that goes into smelting aluminium could be recovered by a fuel cell. "

[cimon9999]

Thanks for the reply, Astrokel.

You'll have to walk me through it a bit further, I'm afraid!

Here's a picture of a typical grain boundary:



So we have gallium in red and aluminium in yellow. The aluminium wants to react with both O₂ and H₂O, correct? The presence of the gallium in the grain boundaries prevents the aluminium from reacting with O₂, how does it do that?

Thanks,
Eamon