Pickling or brightening/passivating solution and process for steel and stainless steel

A process solution for pickling steel comprising:

• a) one or more strong acids other than the complex fluoro acids of group c), and different from nitric acid, in a total concentration of at least 10 g/l and at most 200 g/l,
• c) one or more complex fluoro acids of Si and/or anions thereof in concentrations from 50 to 500 mmoles per liter,
• e) iron(III) cations in concentrations from at least 3 g/l to at most 100 g/l,
• f) a total of from 0.1 to 10 g/l of chloride ions and/or hydrochloric acid, and, optionally,
• d) a hydrogen peroxide stabilizer,
• the process solution containing in addition an amount of fluoride ions and/or hydrofluoric acid such that at least 1% and up to 100% of the iron(III) ions are present in the form of fluoride complexes, but such that the process solution contains less than 10 g/l of free fluoride ions and/or free hydrofluoric acid.

A process for pickling steel, wherein the steel is brought into contact with a process solution comprising:

• a) one or more strong acids other than the complex fluoro acids of group c), and different from nitric acid, in a total concentration of at least 10 g/l and at most 200 g/l.
• c) one or more complex fluoro acids of Si and/or anions thereof in concentrations from 50 to 500 mmoles per liter,
• e) iron(III) cations in concentrations from at least 3 g/l to at most 100 g/l, and, optionally,
• d) a hydrogen peroxide stabilizer,
• the process solution containing in addition an amount of fluoride ions and/or hydrofluoric acid such that at least 1% and up to 100% of the iron(III) ions are present in the form of fluoride complexes, but such that it contains less than 10 g/l of the total of free fluoride ions and/or free hydrofluoric acid.