Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: lightarrow on 22/01/2009 12:19:46

Title: Methanol attacks aluminum oxide?
Post by: lightarrow on 22/01/2009 12:19:46
wiki says that CH3OH attacks some metals, e.g. aluminum:
One of the drawbacks of methanol as a fuel is its corrosivity to some metals, including aluminum. Methanol, although a weak acid, attacks the oxide coating that normally protects the aluminium from corrosion:

    6 CH3OH + Al2O3 → 2 Al(OCH3)3 + 3 H2O

The resulting methoxide salts are soluble in methanol, resulting in clean aluminum surface, which is readily oxidized by some dissolved oxygen. Also the methanol can act as an oxidizer:

    6 CH3OH + 2 Al → 2 Al(OCH3)3 + 3 H2

This reciprocal process effectively fuels corrosion until either the metal is eaten away or the concentration of CH3OH is negligible. Concerns with methanol's corrosivity have been addressed by using methanol compatible materials, and fuel additives that serve as corrosion inhibitors.
My question is: is that really true or it's more a legend? How the heck methanol attacks aluminum oxide?
Title: Methanol attacks aluminum oxide?
Post by: Bored chemist on 22/01/2009 19:12:04
I think that the reaction would be, at best, very slow except in the presence of some acid or base to form an aluminuim salt or an aluminate.
Title: Methanol attacks aluminum oxide?
Post by: lancenti on 23/01/2009 15:49:10

If that link is correct, then I would go with Myth since if you can study it adsorbing onto the surface, that means it doesn't react spontaneously. Then again, you probably still could but it'd be pointless since it'd be a kinetics-like experiment rather than anything else.
Title: Re: Methanol attacks aluminum oxide?
Post by: topcat1957 on 13/04/2019 13:25:46
Adding aluminum foil, sodium hydroxide (red devil lye) and water together creates a vigorous reaction and generates hydrogen gas, which is extremely flammable (remember the Hindenburg!).

You can add those constituents to a glass bottle (only use a little of each, about an inch at the bottom of the bottle) and use the gas to inflate balloons.
But remember, those things are flammable. You can put a match or an ember on the end of a long pole, such as those used to trim small branches from trees, and ignite such balloons. Even at that distance you will feel the heat from the exploding balloon.
I would only ignite one balloon at a time however. They really do make a large fireball in the night sky.
Title: Re: Methanol attacks aluminum oxide?
Post by: topcat1957 on 22/05/2019 22:28:13
Thanks.  I don't recall what I was thinking when I posted my reply above about hydrogen generation with NaOH and aluminum; it has nothing to do with the original post and I apologize for going off topic.

As far as methanol removing the aluminum oxide film on aluminum, I agree with the fellow who wrote that it would likely be a slow reaction.  There is likely a catalyst which would help speed the reaction rate, but I cannot think what an appropriate agent would be for that. I have not used my chemistry degree in decades and my mind is rusty.

Anyone else have an idea for a catalyst?
Title: Re: Methanol attacks aluminum oxide?
Post by: evan_au on 22/05/2019 22:57:44
My car has a warning about the maximum ethanol concentration that should be used in fuel.

But I was under the impression that this limit had more to do with the ethanol attacking rubber gaskets than aluminium metal in the petrol tank?