Patent ID: 6554908
Filing Date: 2003-04-29
Classification: C23C,C23G,Y10T

Abstract:
Process for pickling stainless steels selected from the group consisting of the austenitic, ferritic and martensitic series, duplex steels, superaustenitic and superferritic steels, and Ni or Ni/Cr based superalloys, carried out at a temperature of between 20Â° and 70Â° C., with the use of a pickling solution containing the following basic ingredients:H2SO4 (free acid): 50 to 200 g/l HF (free acid): 0 to 60 g/l Fâˆ’ anion (total): 5 to 150 g/l SO42âˆ’ anion (total): 50 to 350 g/l Fe3+ in a quantity of at least 15 g/l, Cl-anion in a quantity of between 0.1 and 10 g/l in said solution the total free acid (H2SO4+HF) is between 1 and 7 g equivalent/l where by â€œfree acidâ€  is meant the acid that does not constitute the anion bound in the form of salts or complexes with the metal cations present in the solution: and feeding a sufficient amount of oxidant into said pickling solution during the pickling process in order to oxidize at least part of the Fe2+ ions, formed in the pickling process, to Fe3+ ions in order to maintain the Fe3+/Fe2+ ratio at a value of at least 0.2 and the redox potential of the solution at a value of between +230 and +800 mV.