Micca -

The pH alone isn't a very good indicator of "stronger" or "weaker." Before the current plastic liners were developed, concentrated sulphuric acid was shipped in iron tanks, which it will not corrode, but diluted sulphuric acid (as in early batteries) had to be kept in glass. Particularly with iron, weak (dilute) acids are often more effective than stronger ones.

Carbonic acid, as in carbonated drinks, may be "effective," but we have some very fine and beautiful subterranean caverns that demonstrate that it also attacks the calcium/limestone components of the vitreous glazes often used on/in porcelain.

The are a number of fairly effective "rust removers" available at my local hardware/lumber/drug-store outlets, and my recollection is that the ones intended for use on rusted iron have "oxalic acid" as the main active ingredient. One that I've used that's quite handy was a "gel" that stayed where you put it for "spot cleaning."

In order to dissolve the iron, it needs to be "reduced" by something that can "suck the oxygen" out of it. If you have black iron stains, quite a few things may work; but if they're reddish, the red-iron-oxide is very stable, and will resist most of the "weak acid" treatments - and some of the weak acids can "damage" the adjacent glaze, if there's much of it left.

If the commercial rust removers are not available, you might have some luck talking to your pharmacist, a jeweler who has some experience in "restorations," or even a local (auto) repair/body shop. Cautions by previous posters do apply to handling oxalic acid, which is one of the reasons I don't have any left in the house to check the label. I'll try to look some up to re-check the ingredients if I get out of the house soon.

John