Stella?
Jed[/quote]
Jed wins the prize

....well nearly, closest so far
Its the base product of virtually all rust removers, H3PO4 - Phosphoric acid (check out wikipedia for more info). most rust converters are about 10% or less, the industrial version is 60-85%. Its certainly not Sulfuric acid (H2SO4) that's useless in rust removal.
The industrial version can be found used on Dairy farms to clean out the stainless steel tanks of the calcium or 'milkstone' as its known in the business. Its often known as milkstone remover and can be purchased from dairy product suppliers. Its well known in the Triumph, Landy and Morris circles - see moggies do have their uses, spread the love I say.....
Obviously when using this stuff treat with great caution and apply all relevant COSHH rules as its nasty stuff (sorry boring stuff over), and no you cannot put this stuff into a dishwasher - Doh

.. (Though I do like the idea as a parts washer)
you should see what it does to concrete (imagine it on the front room carpet)....did i tell you its also used as concrete remover in the building trade....I'm a mine of useless info
So if the thought of industrial acid does not float your boat then there is an alternative, not quite as fast but just as effective and its safe... This is called DEOX-C and its made by bilt-hamber, this is really good stuff for small ish parts that can be soaked over night or for a few days if really bad. For more info
http://www.bilthamber.com/corrosion-treatments/deox-c" onclick="window.open(this.href);return false;
I can vouch for both as I've used them extensively, heres a few shots of a moggie that was left outside after being sand-blasted,
and then de-rusted using milkstone remover
Enjoy