Drain cleaner!
Technically it's Sodium Hydroxide (NaOH) but is sold as "Caustic Soda" drain cleaner - just check the contents to get a "cheap" one which is 100% NaOH rather than got lots of nice smelling soap and other additives in it.
Mix around 1 egg cup of caustic soda into each pint of warm water (REMEMBER: always chemicals into water, not the other way round!) in a plastic bucket and the you simply put some wire onto your part and drop it into the bucket. Leave for about 5 mins or until you're happy with the finish, then take out and rinse with plenty of cold water.
You may get some black residue left behind, but this is just the result of the chemical reaction. You should easily be able to rinse this away.
To dispose of the caustic soda you can just flush it down the drain - it's drain cleaner! Best to put a bucket or two of water down the drain after as well to clean it.
Beware that it is actually eating away the aluminium, so don't leave parts in there overnight. It also dulls the surface so you may need to repolish after. Also remember that because you've removed the anodising (the hard outer coating) it will be much softer now so will scratch easily.
Our local Asda sells it for about £2 for 500g (which is actually quite expensive) but it's easy to get and you don't use much. This is the one: