Process for the acid pickling of stainless steel products

The invention relates to a process for pickling stainless steel products, in which the pickling bath used has as its initial composition, HF.gtoreq.10 g/l, dissolved ferric iron (Fe.sup.3+).gtoreq.15 g/l, and water=balance. During the pickling operation(s), the redox potential of the bath is monitored and maintained between +100 mV and +350 mV by oxidation of the bath, if necessary, by at least one or several addition(s) of oxidant to the bath. The process of the invention applies particularly to the industrial pickling of stainless steel sheets and strips, in which it is possible to avoid the use of nitric acid and the resulting pollution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to processes for the surface treatment, and more 
particularly for the acid pickling or descaling, of stainless steel 
products. 
2. Discussion of the Background 
The acid pickling of stainless steels is usually performed using fluonitric 
baths, in which the disadvantage of using nitric acid is that it leads to 
the formation of nitrous vapors which pollute the atmosphere and soluble 
nitrates which pollute the liquid effluent. In the context of the 
continuous acid pickling of stainless steel sheet, the inventors have 
sought to develop a modified pickling process which, while remaining 
industrially economical, limits or, better still, avoids such pollution. 
In "Stainless Iron and Steel" (Chapman & Hall Ltd, London 1951), J. H. G. 
Monypenny indicates (pp. 183-4) that, to minimize the problem of vapors 
from fluonitric pickling baths, stainless steel sheets have been pickled 
in baths containing from 6 to 12% of a 90% ferric sulphate solution and 
1.5 to 3% of hydrofluoric acid, e.g., at 70.degree. to 80.degree. C., to 
descale a hot-rolled sheet. The initial concentration of ferric iron in 
the baths used hitherto is thus from about 16.5 to 33 g/l. The inventors' 
tests have shown that when successive samples of stainless steel sheets 
are pickled in baths of this kind, the rate and quality of pickling 
deteriorate rapidly. These acid pickling baths are therefore not 
satisfactory as such for serial or continuous pickling of stainless steel 
products. 
It is also known to use pickling baths containing hydrofluoric acid and 
oxygenated water. Industrial pickling tests on strips of stainless steel 
have been carried out by the inventors, who noticed temperature surges in 
the baths as well as a considerable consumption of oxygenated water, which 
made the process very expensive compared with the fluonitric process for 
pickling stainless steels. In this process, the replacement of nitric acid 
by oxygenated water therefore does not appear to be suitable for 
industrial application. 
There is accordingly a strongly felt need for a process for acid pickling 
stainless steel products which does not suffer the above-noted 
disadvantages. 
SUMMARY OF THE INVENTION 
Accordingly, objects of the present invention include providing a novel 
process for the acid pickling of stainless steel products, which process 
is economical and minimizes, or avoids, pollution-related problems. 
The inventors have now discovered a process which satisfied the above 
objects of the invention and other objects which will become apparent from 
the description of the invention given herein below. This process for 
pickling stainless steel products uses a pickling bath having, as an 
initial composition, (i) HF present in an amount of at least 10 g/l and up 
to 100 g/l or more, including 150 g/l, (ii) dissolved ferric iron 
(Fe.sup.3+) in an amount of at least 15 g/l, and (iii) water as the 
balance. No particular temperature needs to be used, but a temperature of 
between 15.degree. and 70.degree. C. is convenient to use. In accordance 
with the invention, during the pickling operation or operations, the 
ferric iron content of the bath is maintained at at least 15 g/l. This may 
be achieved, if necessary, by oxidation of the bath. 
Oxidation of the bath in accordance with the present invention is achieved 
by addition to the bath of at least one member selected from the group 
consisting of ozone (O.sub.3), chlorine (Cl.sub.2), water vapor, inorganic 
and organic peroxides including hydrogen peroxide, peracids, or persalts 
such as chlorates, perchlorates, persulfates, permanaganates (e.g. 
KMnO.sub.4) or chlorites of the alkaline earth and alkali metals. 
Combinations of two or more of these oxidants may be used in accordance 
with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For practical industrial use, and particularly for repeated or continuous 
pickling of stainless steel products in at least one large vat, typically 
one or more pickling baths are used, initially containing at least 10 g/l 
and up to 100 g/l or more including 150 g/l, preferably 10 to 50, g/l, of 
HF and .gtoreq.15, preferably .gtoreq.20, g/l of Fe.sup.3+. During the 
pickling operation or operations the Fe.sup.3+ content of this bath or 
these baths is maintained at at least 15 g/l, preferably at least 20 g/l. 
This can be achieved, if necessary, by oxidation of the or each bath, 
comprising one or more addition of oxidant into the pickling bath. 
The oxidant seems to participate in the process of the invention to 
regenerate Fe.sup.2+ into Fe.sup.3+, whereas F.sup.3+ constitutes an 
oxidizing agent acting on the base metal to dissolve it. The essential 
reactions appear to be as follows: 
(i) A dissolution reaction: 
##STR1## 
in which the equilibrium is almost totally shifted in direction 1 under 
normal pickling conditions; 
(ii) Another dissolution reaction: 
EQU Fe.sup.3+ +2HF.revreaction.H.sub.2 .uparw.+FeF.sub.2 (B) 
also possible in an oxidizing medium, which is the case; 
(iii) And oxidation of Fe.sup.2+ by the oxidant in the pickling solution, 
possibly supplemented by another means of oxidation: 
##STR2## 
in which the equilibrium is strongly shifted in direction 3 if the 
solution is correctly oxidized and if the pH of the pickling bath is 
between about 1 and 3. 
The ferric iron content of the bath can be calculated as the difference 
between the total iron concentration, measured by atomic absorption, for 
example, and the Fe.sup.2+ concentration measured by its oxidation into 
Fe.sup.3+ in the presence of permanganate, e.g., KMnO.sub.4. Oxidation of 
the pickling bath in accordance with the invention, by the addition of 
oxidant, makes it possible to maintain the quality of pickling in the 
course of successive pickling operations or continuous pickling of 
stainless steel products, while regenerating Fe.sup.3+. 
Peroxides which may be used in accordance with the presentinvention 
include, in addition to hydrogen peroxide, peroxyurea (i.e., 
CO(NH.sub.2).sub.2.H.sub.2 O) or percarbonic acid (i.e., H.sub.4 
(CO.sub.3).sub.2.3H.sub.2 O.sub.2). Peracids which may be used in 
accordance with the present invention include perboric acid, peracidic 
acid, percarbonic acid, perbenzoic acid, persulfuric acid, perphosphoric 
acid, periodic acid, and perphthalic acid. Persalts which may be used in 
accordance with present invention conclude sodium percarbonate and 
magnesium percarbonate. 
The total amount of oxidant added into the pickling bath depends 
essentially on the quantity of stainless steel pickled, and is 
proportional to the surface area pickled and the duration of pickling of 
this surface. So that the pickling bath is properly oxidized, it is 
preferred to add a good proportion of the oxidant, typically at least half 
of the amount to be added to the bottom of the bath (i.e., in the lower 
half of the bath). The oxidant added is preferably preheated to a 
temperature similar to that of the bath, i.e. typically between 35.degree. 
and 60.degree. C. 
For industrial use of the pickling bath, recharging with HF is carried out 
in the usual manner. And, rather than determining the Fe.sup.3+ 
concentration of the bath, it is practical to determine the redox 
potential of the bath and regulate it between 0 mV and +800 mV, preferably 
between 100 and +350 mV, and more preferably between +100 mV and 300 mV, 
if necessary, by adjusting the oxidation of the bath through addition of 
the oxidant. The reference redox potential is selected in accordance with 
the grade and surface condition of the strip and readjusted, if necessary, 
in accordance with observations of the surface condition after pickling. 
The redox potential may be measured between a platinum electrode and an 
Ag/AgCl reference electrode or a reference electrode with a fixed 
potential, reproducible and with zero power of irreversibility. A device 
for measuring this redox potential can be suitably made leaktight so that 
continuous measurements can be taken in the bath. 
Depending on the Fe.sup.3+ concentration found, or more conveniently 
depending on the value of the redox potential, there may be a need for an 
oxidation means which temporarily and/or locally supplements the action of 
the oxidant used in order to arrive more quickly at the desired Fe.sup.3+ 
concentration or the set redox potential, so as to achieve good pickling. 
In this case, the addition of a stronger oxidizing agent is used as the 
supplementary oxidizing means. 
The inventors have also found that it was possible to modify the solubility 
of the sludge or precipitate from the spent bath by regulating the redox 
potential of the bath during pickling. The "sludge" is not very soluble 
when the bath is regulated below +100 mV or above +300 mV to +350 mV, and 
its solubility is greatly improved at between +100 mV and +300 mV, more 
particularly between +190 mV and +260 mV, whilst the optimum setting for 
the bath is +220 mV.+-.20 mV. 
For a spent bath which has thus been used to pickle strips of stainless 
steel with a redox potential of between +200 and +240 mV, and containing 
about 60 g/l of iron in the form of "sludges" of precipitated fluorides, 
this sludge may be recycled into a new bath as follows. The liquid is 
removed from (e.g., sucked out of the) spent bath, then hot water (e.g., 
50.degree. to 60.degree. C.) is run onto the sludge to solubilize it, then 
the HF content is adjusted by adding free HF (15 to 20 g/l) and the whole 
is agitated. Then a little oxidant is added to adjust the potential to 
about +220 mV and a fresh bath is obtained. This possibility of recycling 
the sludge is particularly valuable on an industrial scale. It appears 
that this favorable dissolution of the sludge is linked to the 
precipitation of a mixed iron fluoride, the majority of which is formed 
between +100 mV and +300 mV and more particularly between +190 mV and +260 
mV. 
The pickling bath is generally prepared using ferric fluoride, ferric 
sulphate or ferric chloride, with a ferric iron concentration of between 
15 and 40 g/l, with a preference for ferric fluoride, so that there is 
only one acid radical in the bath. 
The pickling process according to the invention is used for stainless steel 
sheets or strips, typically with the following initial HF concentrations 
and pickling temperatures: 
ferritic stainless steels: HF=10 to 25 g/l, 35.degree.-50.degree. C. 
austenitic stainless steels: HF=20 to 35 g/l, 40.degree.-60.degree. C. 
Apart from solving the pollution problem set forth above, the pickling 
process according to the invention brings about major advantages for 
industrial exploitation including: 
regulation of the quality of the bath is all the more convenient and 
accurate as the majority of the oxidation is effected by the or each 
addition of oxidant; 
regulation of the level of the oxidation reduction potential makes it 
possible to obtain "sludges" which can be re-used directly in the form of 
a new bath. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.