Not a very helpful response, nov. The link was particularly unhelpful since, fortunately, I am not a chemist.
Nevertheless, Google did produce quite a bit of information.
Tea does contain oxalic acid, but not much of it. But enough to produce the chelation effect: it converts non-soluble iron oxide into a soluble compound that can be flushed away with water. Contrary to certain beliefs, tea does not contain tannic acid, though it does contain tannins. Tannic acid mostly comes from oak bark. No information on how tannic acid would work to remove rust. Some talk of other organic acids --citric, acetic, etc. but nothing very interesting. The best acidic rust remover is phosphoric acid, the stuff in naval jelly. Also in Coca Cola. And apparently in molasses.
A more robust source of oxalic acid would be rhubarb leaves, though not always as readily available as tea. Oxalic acid has numerous industrial uses; but they don't get it from tea, they manufacture it with chemical magic. You could probably buy some, and whomp up a solution without wasting tea bags.
The tea method generally mentioned is quite simple. Brew up a batch of very strong tea--5 or 6 teabags per quart of water. Put it in a bucket or other container, and submerge the rusted object in it for 8-12 hours. Take it out and wipe it dry. I didn't learn whether this leaves a protective coating on the de-rusted steel. Or whether the tea can be re-used, or quickly wears out.
Anyhow, that's what I learned today.
