I appreciate that this is an old thread but though I might reply for interest of future users.
So a couple of comments, phosphoric acid does passivate steel surfaces. Leaving a layer of iron phosphate, however the theory behind this method is mainly using it as an electrolyte.
The tinfoil, doesn't actually contain any tin. It's aluminium foil. Contact between the aluminium metal and the iron oxide rust in the presence of the acidic electrolyte, in theory sets up an electrolytic cell, where the iron oxide is reduced to iron metal and the aluminium is oxidised to aluminium ions. Does it work ? Well it looks as if it's somewhat effective, but iron oxide is also somewhat soluble in phosphoric acid so there could be a process of rust removal there. You could do the same thing with other sources of phosphoric acid, but you would have to dilute the acid down to a sensible concentration.
Coca Cola is also useful for some other cleaning applications but it's generally a more expensive way of doing things although it is sometimes readily available where other cleaning products are not and it's safe to handle, although whether it's healthy to drink is another question ;).
