JiK, pH is the most reliable indicator of acid strength,(and what Chemists normally use as such) what you are referring to above is different acids havean abilty to react with substances because of the nature of the materials, it is NOT a function of acid Strength. Conc Sulphuric does attack Iron but after the initial reaction it forms an (unreactive and insoluble) skin on the surface and the reaction ceases, Phosphoric acid( or any other you may wish to use) needs to produce a soluble product fron the oxide(rust) and not (or minimally) attack the pure metal, This is the case as the product is soluble Iron Phosphate. Many of the commercial "Rust removers" contain Phosphoric in varying amounts for this reason.