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why lye and aluminum shouldn't be mixed


For a high school Chemistry course I made soap for a project. Before doing this, thank god, i learned that one should never use an aluminum container to mix the lye (NaOH) and water. Now, I need to know why.
I know it eats away at the container but i can't figure out why. It goes against everything I've learned so far. At first i thought it was because Aluminum was, for some strange reason  :o, more reactive than Sodium therefore replacing it in the reaction but that turned out not to be true. Does anyone have an idea of why?

Edit: edited title for better indexing. Mitch

This quoted from a German website:

"Under normal circumstances, aluminum does not react with water, as an impermeable protective layer composed of aluminum hydroxide either forms within seconds or is already in place. With the addition of sodium hydroxide, the formation of a protective layer is prevented. With the production of aluminates [ Al(OH)4 ]-, the amphoteric (capable of acting as either an acid or a base) aluminum hydroxide Al(OH)3 goes in solution:

2 Al + 6 H2O --> 2 Al(OH)3 + 3 H2

Al(OH)3 + NaOH --> Na+ + [ Al(OH)4 ]-

A layer of aluminum oxide previously formed by passive corrosion is dissolved by the addition of sodium hydroxide. For this reason, the reaction takes place at the beginning relatively slowly:

Al2O3 + 2 NaOH + 3 H2O --> 2 Na+ + 2 [ Al(OH)4 ]-

The aluminum completely dissolves and the water acts here too as an acid (for an analog, see Experiment 4.4.1).

This reaction is used in drain cleaners. They are mostly made out of strong alkalis, to which alumunim or zinc has been added. The alkalis break down organic residues chemically. In addition, the formation of hydrogen leads to a bubbling effect which adds an additional mechanical cleaning mechanism."

The big problem here is the formation of hydrogen gas (as well as some heat).  The hydrogen gas is flammable, and if it's in a closed container, will explode.

Donaldson Tan:
one more point to note is that this protective layer is not only impermeable but also adheres to the surface of the Al metal, unlike other metal oxides

thanks so much


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