Process for pickling and passivating stainless steel without using nitric acid
AssigneeITB S.r.l.
InventorMarco Bianchi
Life of this patent
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Abstract
A process for pickling and passivating stainless steel using a bath of the following initial composition: H.sub.2 SO.sub.4 at least 150 g/l Fe.sup.3+ at least 15 g/l HF at least 40 g/l stabilized H.sub.2 O.sub.2 1-20 g/l non-ionic surfactants and acid attack inhibitors 1 g/l, into which an air flow and an H.sub.2 O.sub.2 quantity of between 0.3 and 1 g/l per hour, controlled to maintain the bath Redox potential at .gtoreq.350 mV, are fed in continuously.
Claims5
A process for pickling and passivating stainless steel, said process consisting of contacting the material to be treated with a bath maintained at a temperature of between 30.degree. and 70.degree. C. and preferably between 45.degree. and 55.degree. C. and having the following initial composition:
- a) H.sub.2 SO.sub.4 at least 150 g/l
- b) Fe.sup.3+ at least 15 g/l
- c) HF at least 40 g/l
- d) H.sub.2 O.sub.2 1-20 and preferably 2-5 g/l
- e) additives of the non-ionic surfactant type acid attack inhibitor type about 1 g/l in total;
- continuously feeding into said bath:
- an air flow of at least 3 m.sup.3 /h per m.sup.3 of bath, using a suitable distributor device for diffusing the flow into the liquid mass;
- a quantity of stabilized H.sub.2 O.sub.2 of between 0.3 and 1 g/l per hour, controlled on the basis of the Redox potential of the bath, which must be maintained at .gtoreq.350 mV;
- and possibly sufficient quantities of ingredients a), c) and e) to maintain their concentration in the bath at optimum levels and the bath pH between 0 and 0.5.
Dependencies are inferred from the claim text.