Think of the aluminium-alumina junction as a junction
diode, with a
very high resistance in one direction and a very low resistance the
other way.
About 80 years ago, there was a thing called an 'electrolytic rectifier'
which IIRC used this sort of principle. They were used for low voltages
in battery charagers, etc. I think I have the instructions for making
them somewhere, but I've never tried it!
-tony