Method at treatment of metals

Details Method at treatment of metals Method at treatment of metals

1998-07-01

2001-01-16

Markoff, Alexander (Department: 1746)

Cleaning and liquid contact with solids

Processes

Metal base work, acid treating

C134S002000, C134S003000, C134S010000

Reexamination Certificate

active

06174383

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method for pickling of steel in an acidic aqueous pickling solution containing Fe
3+
and Fe
2+
. The pickling capability of the bath is maintained by continuous supply of hydrogen peroxide.
In manufacturing of steel, particularly stainless steel, an oxide layer forms at the surface during the annealing, and this layer must be removed. This is normally done by pickling which means that the steel is treated in an acidic oxidizing pickling bath to effect some dissolution of metal under the oxide layer which then comes loose. For a ling time, pickling of stainless steel has often been performed in pickling baths based on nitric acid as an oxidizing agent which, however, has involved emissions of nitrous fumes and nitrates that are detrimental to the environment.
U.S. Pat. No. 4,938,838 discloses addition of hydrogen peroxide for oxidizing nitrite to nitrate in nitric acid based pickling baths. The emissions of nitrous fumes are significantly reduced but are not totally eliminated, and the emissions of nitrates are not reduced at all.
Pickling without nitric acid is disclosed in the U.S. Pat. Nos. 5,154,774 and 5,354,383 and in GB-A-2000196. These processes are based on the fact that Fe
3+
in the pickling bath acts as an oxidizing agent and is reduced to Fe
2+
at the same time as metallic iron in the steel is oxidized to Fe
2+
. In order to maintain the oxidation potential in the pickling bath hydrogen peroxide is added to reoxidize Fe
2+
to Fe
3+
. A disadvantage of these processes is that the cost for hydrogen peroxide is rather high since a great deal of it does not just react with Fe
2+
but also with other metal ions in the pickling bath, such as Fe
3+
, and is then consumed to no use. It is also hard to achieve a sufficiently high pickling rate.
SUMMARY OF THE INVENTION
The present invention concerns a method for pickling steel, preferably stainless steel, with an acidic aqueous pickling solution containing Fe
3+
and Fe
2+
. The steel is contacted with pickling solution that continuously circulates through a conduit into which hydrogen peroxide is supplied to oxidize Fe
2+
to Fe
3+
.
The object of the present invention is to provide an efficient and environmentally friendly process for pickling steel with low consumption of hydrogen peroxide.
It has surprisingly been found that the consumption of hydrogen peroxide is significantly lower if, instead of being supplied directly to a bath, it is fed into a special circulation conduit. It is assumed that the reaction between hydrogen peroxide and Fe
2+
is considerably faster than the corresponding undesired reactions with other metal ions. By feeding the hydrogen peroxide in a circulation conduit, there is always Fe
2+
present to come in contact with the hydrogen peroxide, while it has been found that in a pickling bath, even with vigorous agitation, always may exist zones depleted of Fe
2+
. In order to minimize the consumption of hydrogen peroxide it is preferably supplied in such an amount that the pickling solution the steel is contacted with is substantially free from hydrogen peroxide.


REFERENCES:
patent: 3622478 (1971-11-01), Beyer
patent: 3962005 (1976-06-01), Lerner
patent: 4747907 (1988-05-01), Acocella et al.
patent: 4938838 (1990-07-01), Dalin et al.
patent: 5154774 (1992-10-01), Bousquet et al.
patent: 5354383 (1994-10-01), Bianchi
patent: 5518131 (1996-05-01), Chen et al.
patent: 0 582 121 A1 (1994-02-01), None
patent: 2000196 (1979-01-01), None
patent: 50-133125 (1975-10-01), None
patent: WO 95/35397 (1995-12-01), None
“Nonpolluting and regeneratable rust-removing agent for iron and steel” (abstract) Zhu et al., (CN 1,054,102) Aug. 28, 1991, Chemical Abstracts, vol. 116, No. 22, Jun. 1, 1992 (116:219062u).
“Descaling Method for Steel or Stainless Steel”, (abstract of Japanese Patent No. 55-138081, Oct. 28, 1980) Tsuyoshi et al., Patent Abstracts of Japan.
“High-speed Pickling Method for Low CR Steel”, (abstract of Japanese Patent 63-216986, Sep. 9, 1988) Hideaki et al., Patent Abstracts of Japan.
“Pickling of Austenitic Stainless Steel”, Ryozo et al., abstract of Japanese Kokai No. 75,133,125, Oct. 22, 1975, Chemical Abstracts, vol. 84 (1976) p. 256 (84:139369e).

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