Note: Descriptions are shown in the official language in which they were submitted.
9~
FMC 1750
METHOD FOR CONTROLLING PICKLING SOLUTION OF STAINLESS STE~L
This invention relates to a method for controlling the
composition of a pickling solution of stainless steel.
More particularly, in the pickling for stainless steel
using a ferric sulfate-hydrofluoric acid bath, this in-
vention is concerned with a method of controlling the com-
position of the pickling solution through adjustment of the
oxidation-reduction potential of the same solution.
In the conventional descaling pickling method for
stainless steel, two to four inorganic acids are selected
10 from such inorganic acids as sulfuric acid, hydrochloric
acid, nitric acid, hydrofluoric acid and phosphoric acid
according to the kind of the material to be pickled, the
degree of adhesion of annealing scale, and the purpose of
pickling.
Above all, a pickling solution consisting of a mixed
nitric-hydrofluoric acid is generally used widely. This
pickling solution has a sufficient pickling ability, but
its repeated use tends to lower the pickling ability com-
paratively in an early stage. To prevent this, it is re-
20 quired to replenish nitric acid and hydrofluoric acid
intermediate in pickling, or to newly make the same bath.
Thus, it is very troublesome to control the pickling solu-
tion.
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f~
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In the case of such conventional pickling solution,
moreover, an environmental pollutlon due to the generation
of NOx (nitrogen oxide gases) gas now causes a serious social
problem. Various methods of treating NOx gas have been pro-
posed, but at present there is no method that is technicallyfree of drawback and economical. Therefore, such a pickling
solution as does not generate ~70X gas is requested and many
proposals have been made, among which a pickling method
using a ferric sulfate-hydrofluoric acid bath is known.
However, such pickling solution ages in a short
period, so that many operations are needed, Eor example,
replenishing chemicals or making a new bath. It is more
troublesome in the control of pickling solution and less
economical than the mixed nitric-hydrofluoric acid bath
15 which is now generally used widely. Therefore, such con-
ventional pickling solution is not practical.
Moreover, as the method of preparing a ferric sulfate
solution, it is publicly known to obtain ferric sulfate by
oxidizing a ferrous sulfate solution with hydrogen per-
20 oxide and sulfuric acid. From this, it is inevitably con-
ceivable and publicly known that in a ferric sulfate-
hydrofluoric acid bath, the ferrous sulfate which is pri-
marily produced in the pickling solution for stainless
steel when pickled is oxidized using hydrogen peroxide
25 and sulfuric acid to regenerate ferric sulfate, and the
latter is used repeatedly in the pickling. Even in this
method, however, it is very difficult to control the con-
centration of ferric sulfate when aged after pickling and
the concentration thereof after regeneration by the oxi-
30 dation with hydrogen peroxide and sulfuric acid. Thisdifficulty causes a problem, and so this method is not
actually adopted.
The foregoing problems associated with the conven-
tional pickling methods, are solved according to the
35 present invention characterized in that in the pickling
of stainless steel using a ferric sulfate-hydrofluoric
--2--
30~0
acid bath, the oxidation-reduction potential of the pickling
solution is held in a predetermined range by adding hydrogen
peroxide and sulfuric acid into the pickling solution. The
addition of the hydrogen peroxide and sulfuric acid can be
done continuously while pickling stainless steel.
An excellent feature of the present invention resides
in that by only a simple operation of adding hydrogen per-
oxide and sulfuric acid so as to maintain the oxidation-
reduction potential in a ;onstant range, ferric sulfate can
10 be held in a constant range of concentration without the
need for a chemical analysis of the composition with the
result that uniform surface finish and pickling time can
be achieved.
Furthermore, according to the method of the present
15 invention, it becomes possible to make a complete auto-
mation, continuation and solution closing, with ~Ox gas
not generated and the amount of waste liquor to be
treated largely decreased. Thus, the method of the
present invention is an epoch-making method which has
20 remedied the drawbacks encountered in the prior art pick-
ling methods. Besides, as compared with the conventional
ferric sulfate-hydrofluoric acid bath, the pickling cost
can be reduced to a large extent; and even in comparison
with the nitric acid-hydrofluoric acid bath presently in
25 use, a low cost comparable thereto can be attained.
Figure 1 shows the results of measurement of the oxi-
dation-reduction potential in the form of a graph which
results serve as basic data for working the method of
controlling a pickling solution of the present invention
30 in which measurement the concentration of ferrous sulfate in
the pickling solution was 30 grams per litre and the concen-
tration of ferric sulfate and that of hydrofluoric acid (HF)
were varied, and the oxidation-reduction potential of each
was measured at the solution temperature of 50C.
In working the present invention, the concentration
range of ferric sulfate and that of hydrofluoric acid are
predetermined according to the kind of material to be
pickled, the degree of annealing scale and the purpose of
pickling, and further the range of oxidation-reduction po-
tential in balance therewith is decided. And hydrogen per-
oxide and sulfuric acid are added and pickling is carriedout within such range of oxidation-reduction potential.
It is desired that the molar ratio of hydrogen per-
oxide and sulfuric acid to be added be about 1:1. The con-
centration of hydrogen peroxide and that of sulfuric acid
l0 to be used are not specially restricted.
If the pickling for stainless steel is carried out in
a ferric sulfate-hydrofluoric acid bath, the ferric sulfate
is consumed for the dissolving of stainless steel and there
are produced ferrous sulfate, chromium sulfate and nickle
15 sulfate according to the composition of the material to be
pickled. On the other hand, the hydrofluoric acid is
partially consumed as hydrogen ion in the dissolving of
stainless steel, but fluoride ion is not consumed and
exists in the pickling solution. The consumed hydrogen
20 ion can be replenished by slightly increasing the amount
of sulfuric acid to be added in the oxidation-regeneration
operation. If pickling is continued, there accumulate in
the solution ferrous sulfate in an amount exceeding the
necessary amount for the regeneration of ferric sulfate,
25 and also unoxidized chromium sulfate and nickel sulfate.
It is mainly the concentration of ferric sulfate and
that of hydrofluoric acid that affect the oxidation-reduction
potential of the pickling solution. Ferrous sulfate somewhat
affects the same potential. Chromium sulfate and nickel sul-
30 fate have no influence. It has become clear that in case ameasurement of oxidation-reduction potential is made by
changing the concentration bf ferric sulfate and that of hy-
drofluoric acid, there is nearly a straight-line relationship
between the concentration of ferric sulfate and the oxidation-
35 reduction potential, and that the straight line shifts in a
~Ll9~
parallel manner by the change in hydrofluoric acid concen-
tration. As previously noted, fluoride ion in the solution
is not consumed and exists, so that if the concentration of
hydrofluoric acid in the pickling solution is maintained
constant, the oxidation-reduction potential becomes cor-
responding to the change in ferric sulfate concentration.
The concentration of ferrous sulfate affects the oxidation-
reduction potential in such a manner that in case the con-
centration of ferric sulfate and that of hydrofluoric acid
10 are made constant, the oxidation-reduction potential tends
to decrease as the concentration of ferrous sulfate becomes
higher. In this case, however, the decreasing degree of
the oxidation-reduction potential is not so large.
Therefore, with these facts taken into account in ad-
15 vance, it is necessary to be aware of the minimum concen-
tration of ferric sulfate required for pickling, regard the
oxidation-reduction potential at that time as the minimum
potential and maintain the potential always above the
minimum value during pickling and regeneration, whereby even
20 if the oxidation-reduction potential somewhat decreases due
to the presence of ferrous sulfate which gradually ac-
cumulates in the solution, it has no influence upon process
control, and uniform pickling finish and pickling time can
be achieved. It has become clear that the change in concen-
25 tration of ferrous sulfate due to its accumulation does notgreatly affect the oxidation-reduction potential up to its
concentration of about 200 g/l. At a concentration of fer-
rous sulfate above 200 g/l, it is deposited as crystals
tmainlY FeSO4 . 7H2O) on standing at room temperature.
30 Therefore, through combination with this method it is also
possible to keep the concentration of ferrous sulfate below
200 g/l. The amount of ferrous-, chromium- and nickel-
sulfate which accumulate in the pickling solution differs
according to the amount of solution brought out of the
35 system in an adhered state of steel after pickling. The
larger the amount of solution brought out of the system,
0~0
the smaller becomes the amount of sulfates which accumulate.
At the same time, however, fluoride ion is also brought out
of the system, so it is necessary to replenish hydrofluoric
acid on the basis of a calculated amount, or alternatively
measure the concentration of fluoride ion using an ion
meter or the like and replenish hydrofluoric acid.
Conversely, in case the amount of solution brought out
of the system is small and the amount of sulfates which ac-
cumulate becomes large, the amount of solution to be drawn
10 out is determined so as to qive a concentration not affecting
the oxidation-reduction potential. In this case, there is
adopted a method in which the solution is drawn out con-
tinuously or intermittently, or a method in which the bath
is cooled for removal as crystals outside the system.
These operations can be made regular once pickling is
experienced. The oxidation-reduction potential of the
pickling solution is affected by temperature, so when
measuring such potential it is necessary to pay attention
so that the temperature is the same.
With respect to the electrodes used in the measurement
of the oxidation-reduction potential, as a reference elec-
trode there may be used those which are normally in use,
for example, a saturated calomel electrode and a silver
chloride electrode, and as a measuring electrode there
25 may be used any electrodes if only they are inert to the
pickling solution, for example, noble metal electrodes
such as platinum, gold and rhodium.
The following examples are given to further illustrate
the present invention:
Example 1
h~ith the concentration of ferrous sulfate of 30 g/l,
the concentration of ferric sulfate and that of hydro-
fluoric acid were varied and the oxidation-reduction po-
tential of each was measured at the solution temperature
35 of 50-C, the results of which are shown in Figure 1 hereto
attached.
In the above measurement of the oxidation-reduction
potential, a saturated calomel electrode was used as a
reference electrode and a platinum electrode as a measuring
electrode.
Example 2
Into pickling solutions containing ferric sulfate and
hydrofluoric acid in amounts set out in Table 1 below were
added ferrous sulfate in concentrations of 30 g/l, 100 g/l
and 200 g/l and chromium- and nickel-sulfate in concentra-
lO tions of 20 g/l and 40 g/l as chromium plus nickel (whose
ration was 1:1j. Using the same electrodes as in Example 1,
the oxidation-reduction potential of each pickling solution
at 50DC was measured, the results of which are shown in
Table 1.
Example 3
A pickling solution containing 150 g/l of ferric sulfate,
5% of hydrofluoric acid and 30 g/l of ferrous sulfate was pre-
pared. With the pickling solution thus prepared, annealing
scales respectively of pipe, bar wire and plane sheet of SUS
20 304, 308 and 316 were pickled continuously. Using the same
electrodes as in Example 1 and with the pickling temperature
held at 50 C, the minimum value of the oxidation-reduction
potential was set to +300mV and hydrogen peroxide (35~) and
sulfuric acid t98%) were added at times in the molar ratio
25 of 1:1 so that the oxidation-reduction potential was kept
above +300mV at all times during pickling. The amount of
the pickling solution was adjusted-according to the amount
of hydrogen peroxide and sulfuric acid used for the oxi-
dation-regeneration and the amount of solution brought out
30 of the system in an adhered state to the samples after
descaling. And a small amount of hydrofluoric acid was
replenished at times.
As the pickling proceeded in a continuous manner,
ferrous-, chromium- and nickel-sulfate accumulated grad-
35 ually and reached the respective concentrations of 180g/l, 14 g/l as chromium and 16 g/l as nickel. Even at
this moment, a constant pickling time just the same as
that in the initial bath and a good finish were achieved.
Table 1
Experiment No. 1 2 3 4
Concentration of ferric 150 g/l 150 g/l 150 g/l 150 g/l
sulfate
Concentration of errous 30 g/l 100 g/l 200 g/l 30 g/l
sulfate
Concentration of chromium - - - 20 g/l
+ nickel
Concentration of hydro-5% 5% 5~ 5%
fluoric acid
Oxidation-reduction+415mV +395mV+370mV +410mV
potential
Experiment No. 5 6 7 8
Concentration of ferric 150 g/l 150 g/l 100 g/l 100 g/l
sulfate
Concentration of ferrous 30 g/l 200 g/l 30 g/l 200 g/l
sulfate
Concentration of chromium 40 g/l 40 g/1
+ nickel
Concentration of hydro-5% 5% 5% 5%
fluoric acid
Oxidation-reduction +410mV+370mV+335mV+295mV
potential
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