I would like to give a brief overview of those inorganic acids, which are traditionally used in pickling applications.
Sulfuric acid H2SO4:
Sulfuric acid has a molecular weight of 98.08 g/mol and a density of 1,836 t/m³. 100% sulfuric acid is liquid, color- and odorless. It´s melting point is 10.4 °C and the boiling point is 279.6 °C.
Commercially available concentrated sulfuric acid has a concentration of 96 to 98% and its molecules are not dissociated. Due to this fact it is non-corrosive and could be stored in steel tanks. Concentrated sulfuric acid is hygroscopic and absorbs humidity. Contact with water results in a very strong exothermic reaction, which could heat the acid up to its boiling point. This is due to hydrate formation which released 85.4 kJ of heat per mole of sulfuric acid. Dilution procedure requires cooling and generally special care which is especially very important in large-scale applications.
The dissociation of sulfuric acid takes place in two stages. The second occurs at higher water contents.
H2SO4 + H2O <-> H3O+ + HSO4-
HSO4- + H2O <-> H3O+ + SO4–
It should be noted that sulfuric acid and water are completely miscible. The concentration of the acid can be determined easily via its density. The following table shows the relationship between density and concentration of sulfuric acid. The above mentioned, figures were taken from the PERRY’S CHEMICAL ENGINEERS HANDBOOK SEVENTH EDITION.
Sulphuric acid is an oxidizing acid and can dissolve also noble metals such as copper.
As a strong dibasic acid H2SO4 dissolves all base metals by forming hydrogen and the corresponding sulfates and hydrogen sulfates.
Fe + H2SO4 -> FeSO4 + H2
A further essential feature is that sulfuric acid could decompose the salts of most other acids by forming the original acid.
FeCl2 + H2SO4 -> FeSO4 + 2 HCl
Sulfuric acid is often used for pickling of metals, pure or as a mixture with other acids.
In contrast to hydrochloric acid there is no industrial scale regeneration technology for the recovery of H2SO4 molecules available. At sulfuric acid pickling applications only chemically not spent acid can be recovered and used again.
Data source: Beizen von Metallen, Dr. Ralf Rituper, Eugen G. Leuze Verlag