[Lewis Thomson (OP)]

Donald has sent The Naked Scientists this question.

"Oxygen has vacant orbitals in it's outer electron shell and can bind to two electrons from, say, hydrogen if enough energy is available to overcome 'activation potential' like a spark. So when rusting occurs, where does the activation energy come from. Some metals do explosively react. Why doesn't iron, or aluminium?"

Leave your comments down below...

[Origin]

So when rusting occurs, where does the activation energy come from.

I believe the activation energy is very low for the formation of rust, the ambient temperature supplies enough energy for the reaction.

Some metals do explosively react. Why doesn't iron, or aluminium?

The formation of rust is an exothermic reaction but the amount of energy released in the formation of Al or Fe oxides is quite low.  If there are particles of Al or Fe that are very small then the surface area will be very large relative to the mass of the particles and there can be quite a lot of energy released for a given amount of mass.  Some solid rockets fuels are actually Al particles that are oxidized.

[chiralSPO]

Bulk aluminum and iron don't oxidize explosively because the oxide that gets formed stays on the surface and physically blocks additional oxygen from getting in. (when hydrogen burns, it produces gaseous water, which doesn't interfere with incoming oxygen very much)

Very fine powders of metals (if not already oxidized) will often spontaneously ignite in air, even iron (https://www.amazingrust.com/Experiments/how_to/Pyrophoric_Fe.html)

Alternatively, the protective oxide layer can be disrupted by the presence of other compounds. For instance, significant concentrations of chloride are able to dissolve iron(III), dramatically increasing the rate of corrosion of iron or steel (for instance in seawater).

A striking example of this is the use of mercury (not recommended due to toxicity issues) to disrupt aluminum oxide: