BACKGROUND OF THE INVENTION
[0001] This invention relates to a process for acid descaling ferrous alloys containing
chromium. More specifically, oxide on hot rolled or annealed ferrous alloys containing
chromium is removed by sequentially immersing the alloy in an inorganic acid and then
applying an aqueous solution containing hydrogen peroxide to the pickled alloy.
[0002] One of the most environmentally intensive problems relating to steel manufacturing
is pickling of steel to remove oxide or scale formed during hot processing such as
rolling on a hot strip mill or annealing. Most low carbon steels may be descaled in
hydrochloric acid at high speeds. The scale on stainless steel, however, has a very
fine structure and is tightly adherent usually requiring mechanical scale cracking
such as shot blasting, roll bending or roll leveling of a steel strip to loosen the
scale prior to acid pickling. Additionally, stainless steel pickling acids such as
hydrofluoric, sulfuric, nitric or mixtures thereof generally must be more aggressive
than those required for low carbon steel. The immersion time required for stainless
steel is much longer than that required for low carbon steel and may require electrical
assistance to help remove the scale as well. A major motivation for improving the
scale removing process is the capital and environmental disposal costs associated
with pickling acids. A major disadvantage of chemical descaling using hydrofluoric
and nitric acids is the environmental problems related to their disposal.
[0003] It is known to use acid mixtures containing hydrogen peroxide for pickling and cleaning
stainless steel. For example, US patent 5,154,774 teaches adding an oxygenated agent,
e.g., hydrogen peroxide, potassium permanganate or air, to a hydrofluoric acid for
pickling stainless steel to convert ferrous ions to ferric ions. US patent 5,164,016
teaches adding hydrogen peroxide to an organic acid such as formic, acetic, propionic,
lactic, benzoic, phthalic and naphthoic for pickling stainless steel. Hydrogen peroxide
is added to the acid to control the ratio of ferrous/ferric ions within the range
of 10/90 to 40/60. Japanese patent application 63-20494 discloses a method for chemically
removing scale from stainless steel by adding an adhesive to a solution containing
hydrogen peroxide, phosphoric acid and hydrogen fluoride. The adhesive is not decomposed
by the hydrogen peroxide and gives viscosity to the cleaning solution and forms a
pasty liquid. Japanese patent application 60-243289 discloses reducing smut on steel
using an acid bath containing hydrofluoric acid, hydrogen peroxide and hydrochloric
or sulfuric acid. Japanese patent application 54-64022 discloses providing a viscous
pickling agent for removing stain and scale from stainless steel. Abrasive particles
such as alumina, Cr oxide, Si carbide or silica are added to an acidic solution containing
hydrogen peroxide, sulfuric acid, hydrochloric acid and a surfactant agent. Japanese
patent application 58-110682 discloses pickling hot rolled stainless steel with a
solution containing sulphamic acid, nitric acid, hydrofluoric acid and hydrogen peroxide.
[0004] Although these acids are effective for removing scale from stainless steel, their
use creates certain undesirable problems and have their limitations. For example,
using sulfuric acid alone for removing scale from stainless steel is undesirable because
this acid leaves a black smut on the pickled steel. Using hydrochloric acid alone
results in a bright stainless steel surface but is undesirable because of slow reaction
with the tightly adhering scale. More aggressive acids such as nitric and hydrofluoric
to remove scale from stainless steel are especially undesirable because their use
creates environmental problems requiring fume abatement equipment to handle fumes
from the pickling tanks, special equipment for storing the acids, and their pickling
by-products require special handling and costly disposal. Other disadvantages include
safety and health risks associated from chronic exposure to these acids and limits
on allowable nitrate and fluoride discharge in effluents from treated wastes. An organic
acid is undesirable because it would not be useful for descaling of a stainless steel.
[0005] Accordingly, there remains a need for a process for pickling ferrous alloys containing
chromium that does not include nitric acid, hydrofluoric acid or a fluoride compound.
There remains a further need for a process for pickling ferrous alloys containing
chromium that does not create costly environmental disposal problems of pickling waste
by-products. Another need includes being able to obviate the need for expensive pollution
control and waste treatment facilities associated with using nitric acid, hydrofluoric
acid or a fluoride compound.
BRIEF SUMMARY OF THE INVENTION
[0006] A principal object of the invention is to provide a ferrous alloy containing chromium
having a bright, oxide free surface, using a hydrochloric or sulfuric pickling solution
whose by-products do not cause an environmental disposal problem.
[0007] Another object of the invention is to provide a ferrous alloy containing chromium
having a bright, oxide free surface without using nitric acid, hydrofluoric acid or
a fluoride compound.
[0008] Another object of the invention includes providing a hydrochloric or sulfuric pickling
process wherein the chemical cost is no greater than that otherwise required for nitric,
hydrofluoric acid or a fluoride compound.
[0009] Another object of the invention is to pickle a ferrous alloy strip containing chromium
at a speed of at least 30 m/min.
[0010] The invention relates to a hot rolled or annealed ferrous alloy strip containing
chromium being descaled with an acid. The hot rolled or annealed ferrous alloy strip
is pretreated to crack the scale and then immersed into at least one pickling tank
containing an inorganic acid from the group consisting of hydrochloric or sulfuric
acid to remove the cracked scale. Thereafter, an aqueous solution containing hydrogen
peroxide is applied to the pickled alloy strip wherein any residual scale becomes
activated by the peroxide so that the residual scale can be removed by the inorganic
acid thereby providing a clean chromium ferrous alloy strip.
[0011] Another feature of the invention is for the aforesaid aqueous solution to contain
at least about 10 g/l hydrogen peroxide.
[0012] Another feature of the invention is for the aforesaid aqueous solution to contain
the inorganic acid for removing the residual scale.
[0013] Another feature of the invention is for the aforesaid aqueous solution to contain
at least about 5 g/l of the inorganic acid.
[0014] Another feature of the invention is for the aforesaid aqueous solution being disposed
of in the pickling tank.
[0015] Another feature of the invention is for the aforesaid pickling tank to contain at
least about 50 g/l of the inorganic acid.
[0016] Another feature of the invention is for the aforesaid pickling tank acid to have
a temperature of at least about 60
oC.
[0017] An advantage of the invention includes using hydrochloric or sulfuric acid for removing
hot roll mill scale or annealing scale from a ferrous alloy strip containing chromium
rather than using nitric acid, hydrofluoric acid or fluoride compounds. Another advantage
of the invention includes increased pickling speeds without using nitric acid, hydrofluoric
acid or fluoride compounds. Other advantages include fewer environmental concerns,
a hydrogen peroxide containing waste solution being compatible with hydrochloric or
sulfuric acid waste by-products, a smut free chromium alloyed strip, obviating the
need for electrical assistance for removing scale and a more passive corrosion resistant
pickled ferrous chromium alloyed surface.
[0018] The above and other objects, features and advantages of the invention will become
apparent upon consideration of the detailed description and appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
FIG. 1 schematically illustrates a pickling line incorporating the process of the
invention,
FIG. 2 schematically illustrates another embodiment of a pickling line incorporating
the process of the invention, and
FIG. 3 schematically illustrates another embodiment of means for applying an inorganic
acidic solution containing hydrogen peroxide of the invention to a pickled ferrous
alloy strip containing chromium.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] This invention relates to a process using inorganic acid for descaling a ferrous
alloy containing chromium, such as ferritic stainless steel strip. More specifically,
oxide or scale, hereafter referred to as scale, on a hot rolled or an annealed ferrous
alloy containing chromium is removed by immersing the alloy into an inorganic acid
of hydrochloric (HCl) or sulfuric acid (H
2SO
4), and then rinsing the pickled strip with an aqueous solution containing hydrogen
peroxide (H
2O
2). Any residual scale remaining on the strip is activated by the hydrogen peroxide
contained in the aqueous rinsing solution and then removed by inorganic hydrochloric
or sulfuric acid. The inorganic acid for removing residual scale can be sprayed onto
the strip after activation by the hydrogen peroxide, the activated strip can be immersed
into a tank containing the inorganic acid or preferably the inorganic acid is contained
in the aqueous solution containing the hydrogen peroxide rinsing.
[0021] By activating any residual scale with a solution containing hydrogen peroxide, it
has been determined the use of nitric or hydrofluoric acids and/or fluoride containing
compounds is not required to adequately remove scale from ferrous alloys containing
chromium during high speed pickling. Not being bound by theory, what is meant by activating
the scale is that hydrogen peroxide reacts with the base metal of the steel alloy
to loosen and/or decompose the scale tightly adhering thereto thereby aiding in the
removal of the scale from the substrate by the inorganic acid.
[0022] By a ferrous alloy containing chromium is meant an alloy of iron and chromium, e.g.,
chromium alloyed steel, stainless steel, in which the chromium content is at least
about 5% Cr, preferably at least 10% Cr and up to about 30% Cr. The alloy preferably
is a ferritic stainless steel including up to about 0.5% Al, up to about 0.3% of C,
up to about 1% of one or more of Si, Ti, Nb, Zr; up to about 5% of Ni and/or Mo and
up to about 1.5% Mn. All percentages are by wt.%. These alloys also may include purposeful
additions of one or more of Ta, Ca, Cu, B and N as well.
[0023] After a scale is formed during hot processing such as by rolling on a hot strip mill
or in a continuous annealing furnace, continuous stainless steel strip or foil or
cut to length sheets, referred to hereafter as strip, is given a mechanical scale
cracking treatment such as shot blasting or roll bending to loosen the scale. Thereafter,
the strip is immersed into a pickling tank containing an inorganic acid to remove
the cracked scale. For this invention, the inorganic acid is defined to include either
of sulfuric acid or hydrochloric acid. An important feature of the invention is to
thereafter apply an aqueous solution containing hydrogen peroxide onto the pickled
strip wherein any remaining residual scale, smut, dirt, and the like becomes activated
by the hydrogen peroxide. Preferably, the aqueous solution contains inorganic acid
and any activated residual scale is simultaneously removed by the inorganic acid in
the aqueous solution containing the hydrogen peroxide. Preferably, the pickling tank
contains the same inorganic acid as that used to remove the residual scale. This remaining
scale then becomes removed when the strip is rinsed with the solution containing the
inorganic acid and hydrogen peroxide and when the strip then is brushed and rinsed
with water.
[0024] The pickling tank preferably contains the same inorganic acid as that used to remove
residual scale so that the spent aqueous solution containing hydrogen peroxide and
inorganic acid can be disposed of in the pickling tank after being used to activate
and aid in the removal of any residual scale on the strip. Considerable make up solution
is required in the pickling tank because of evaporation when the acid is hot. The
aqueous solution advantageously can be disposed of by being sent to the pickling tank
as part of this make up requirement.
[0025] Hydrogen peroxide rapidly reacts with ferrous iron (Fe
+2) resulting in ferric iron (Fe
+3) when iron is removed from steel strip and becomes dissolved into a solution containing
the hydrogen peroxide. For this reason, the hydrogen peroxide of the invention preferably
is dissolved into a hydrochloric or sulfuric acid and applied directly to the surface
of the strip rather than being stored within an immersion tank. If the peroxide were
dissolved within the inorganic acid stored within an immersion tank, the peroxide
would break down and become ineffective after a relatively short period of time no
longer activating any residual scale remaining on the pickled strip. Hydrogen peroxide
is consumed when ferrous ions (Fe
+2) dissolved in the inorganic acid are oxidized to ferric ions (Fe
+3).
[0026] It was surprisingly discovered any residual scale advantageously need only remain
in contact with an aqueous solution containing hydrogen peroxide a very short period
of time to become sufficiently activated by the hydrogen peroxide thereby easily being
removed by the inorganic acid. If the inorganic acid is contained in the aqueous solution,
any residual scale is simultaneously removed by the acid. By simultaneously removing
any residual scale from the strip is meant having an activation time as short as about
1 second, preferably at least 3 seconds and more preferably less than 10 seconds.
A major advantage of this invention is that it is not necessary to apply the inorganic
acid to the steel strip for removing residual scale apart from the hydrogen peroxide.
[0027] FIG. 1 illustrates one embodiment of a pickling line incorporating the process of
the invention. More specifically, reference numeral 10 schematically illustrates a
ferrous alloy strip containing chromium such as stainless steel covered with a scale
such as from rolling on a hot strip mill. The scale on strip 10 would have been cracked
such as being passed through a shot blasting machine or roll leveler (not shown).
The scale of a stainless steel should be loosened whenever nitric, hydrofluoric acid
and/or fluoride compounds are not used to enhance the descaling effect. Thereafter,
the strip is immersed into sulfuric or hydrochloric acid contained within one or more
pickling tanks such as tanks 12 and 14. If the pickling line of the invention includes
a plurality of pickling tanks, the acid in the pickling tanks preferably is counter
current flowed such as through a pipe 15 through the tanks in a direction opposite
the direction of travel of the strip. Thereafter, the strip normally may have residual
amounts of tightly adherent scale 11. This tightly adherent residual scale is activated
by being contacted with an aqueous solution containing hydrogen peroxide and then
removed by hydrochloric or sulfuric acid. This aqueous solution may be sprayed onto
the strip such as by a spray header 16 extending completely transversely across and
positioned above the strip and another spray header 18 extending completely transversely
across and positioned below the strip. Preferably, another pair of spray headers 24
and 26 extending transversely completely across the strip is provided. Using multiple
spray headers above and below the strip increases the activation time of the tightly
adherent residual scale by the hydrogen peroxide. Ferrous alloy strip containing chromium
having a very clean surface was pickled at a speed of at least 30 m/min. Any peroxide
spray dripping from the strip may be collected onto a catch pan 30 and flowed into
a tank 28. If it is desired to apply the hydrochloric or sulfuric acid to the strip
separately from the aqueous solution, another pair of spray headers can be positioned
a short distance down stream from spray headers 24 and 26 for this purpose. Any hydrochloric
or sulfuric acid spray dripping from the strip may then also be collected onto catch
pan 30 and flowed into tank 28. After the hydrogen peroxide rinse, it is desirable
to abrade the pickled steel strip by one or more pairs of brushes 20 and 22. These
brushes are of a grit impregnated polymer construction. The strip also will be rinsed
with water. If the acid in tanks 12 and 14 is sulfuric, a black smut may remain on
the strip surface exiting from tank 14. This smut is easily removed from the strip
by the inorganic acid contained in the hydrogen peroxide solution to improve the cleanliness
of a cleaned strip 42.
[0028] When tanks 12 and 14 contain hydrochloric acid, a collected sprayed hydrogen peroxide
solution containing hydrochloric acid can be disposed of in either of tanks 34 and
35 as makeup for liquid lost to evaporation through pipes 32 and 33 respectively.
The used acidic hydrogen peroxide solution can flow by gravity to tanks 34 and 35
through a line 37 by opening a valve 44. After the acid becomes saturated with iron,
this acid is replaced with fresh acid. Spent acid may be periodically withdrawn from
tanks 34 and 35 through a line 50 and sent to an acid recovery plant (not shown).
Fresh acid would be returned to tanks 34 and 35 through a return line 48. The fresh
acid, along with the used aqueous solution originally containing the hydrogen peroxide,
would be pumped from tank 34 to pickling tank 14.
[0029] FIG. 2 illustrates another embodiment of a pickling line incorporating the process
of the invention. In this embodiment, components that are the same as in the embodiment
illustrated in FIG. 1 have like numerals. If scale 11 on strip 10 is extremely adherent
and/or it is desired to operate the pickling line at very high speeds, it may be necessary
to repickle the strip by passing the pickled strip through another tank 38 containing
inorganic acid. Thereafter, if the strip still has any residual scale 11, this remaining
scale may be reactivated by the aqueous solution containing the inorganic acid and
hydrogen peroxide sprayed onto the strip by a second set of multiple spray headers
52, 54 extending completely transversely across and positioned above the strip and
another set of multiple spray headers 56, 58 extending completely transversely across
and positioned below the strip. By using a second set of multiple spray headers above
and below the strip, the activation time of the residual scale by the hydrogen peroxide
may be twice as long as that illustrated in FIG. 1 and insures a very bright strip
42 so that a ferrous alloy strip containing chromium can be pickled to a very clean
surface at speeds in excess of 60 m/min. Any peroxide spray dripping from the strip
at this second set of spray headers may be collected onto a catch pan 60 and flowed
into a tank 62. Thereafter, the pickled steel strip is abraded by brushes 20 and 22
and rinsed with water. Used peroxide solution can flow by gravity from tank 62 to
a tank 36 by opening a valve 68. Alternatively, the spent peroxide solution can be
pumped to a waste water holding tank 66 though a line 64 by opening a valve 70. The
waste water in holding tank 66 may then be sent to a waste water treatment plant (not
shown) though a line 46. The acid solution in tank 36 may be pumped to the acid recovery
plant though a line 40.
[0030] In the embodiment of FIG. 2, the aqueous solution containing spent hydrogen peroxide
collected in tank 28 is flowed into and disposed of in acid tank 14 through a line
17. Since the inorganic acid in pickling tank 14 contains dissolved iron, any hydrogen
peroxide remaining in the aqueous solution will break down into water and oxygen.
[0031] Other means for applying the aqueous solution containing hydrogen peroxide to the
pickled steel may include using laminar flow or an absorbent contact roller for contacting
each side of a steel strip. FIG. 3 illustrates another embodiment for applying the
aqueous solution containing peroxide to the pickled steel using laminar flow. The
pickled strip is passed through means 72 for laminar flowing the aqueous solution.
Laminar flow means 72 includes a pair of juxtaposed panels 74 and 76 sealably joined
to a strip entry end 78 and a strip exit end 80. Ends 78 and 80 include squeegee type
wipers for sealing the ends of the applicator. The aqueous solution containing hydrogen
peroxide is pumped into laminar flow means 72 through a line 82. The steel strip would
be immersed into the aqueous solution. Spent aqueous solution would be continuously
withdrawn from laminar flow means 72 through a line 84 for disposal to one of the
acid tanks to prevent accumulation of dissolved iron.
[0032] An important feature of this invention is that the aqueous solution containing the
hydrogen peroxide must be metered onto the pickled strip such as by a spray header,
laminar flow or using a contact roller rather than being contained within an immersion
tank. Hydrogen peroxide readily oxidizes ferrous iron to ferric iron. If a pickled
steel strip were to be continuously immersed into a tank containing the aqueous peroxide
solution, the solution would continuously dissolve iron from the steel strip thereby
continuously consuming the hydrogen peroxide. This would result in a very inefficient
use and wasting of hydrogen peroxide. It is important not to contaminate the aqueous
solution containing hydrogen peroxide with iron, e.g., ferrous ions, prior to the
aqueous solution being applied to the pickled steel strip. Accordingly, it is important
that the aqueous solution containing hydrogen peroxide remain free of ferrous ions
prior to the aqueous solution being applied to the pickled steel strip.
[0033] Another important feature of the invention is that any waste waters containing the
spent aqueous solution containing the hydrogen peroxide and inorganic acid not contain
free hydrogen peroxide. When hydrogen peroxide is present and the pH of the solution
is at least equal to about 7, trivalent chromium, i.e., Cr
+3, is readily oxidized to hazardous hexavalent chromium, i.e., Cr
+6. Trivalent chromium can be readily precipitated as an environmentally safe insoluble
chromium hydroxide whereas hazardous hexavalent chromium tends to remain soluble and
can not be safely disposed such as in an unsecured landfill. Accordingly, the spent
aqueous solution containing the inorganic acid and hydrogen peroxide will be mixed
with waste water containing dissolved ferrous iron or spent inorganic acid containing
dissolved ferrous iron to break down the spent hydrogen peroxide into water and oxygen.
[0034] The strip is initially pickled in tanks 12, 14 and 38 in a hot inorganic acid such
as hydrochloric or sulfuric maintained at a temperature of at least 60
oC. Preferably, the strip is pickled at a temperature of at least 77
oC, more preferably at least 82
oC and most preferably at least 88
oC in hydrochloric acid in pickling tanks 12, 14 and 38. Preferably, the hydrochloric
acid is maintained at 50 g/l, more preferably at least 75 g/l and most preferably
at least 100-200 g/l.
[0035] The concentration of the hydrogen peroxide in the aqueous solution should be at least
10 g/l. If it is not at least 10 g/l, the peroxide will not effectively activate stainless
steel scale. Preferably, the hydrogen peroxide concentration in the aqueous solution
will be at least 25 g/l, more preferably, at least 30 g/l and most preferably at least
40 g/l. The hydrogen peroxide preferably is dissolved in an aqueous solution containing
at least 5 g/l of inorganic acid. Preferably, the aqueous solution will contain at
least 20 g/l inorganic acid, more preferably, at least 40 g/l inorganic acid and most
preferably at least 50 g/l inorganic acid.
Example 1
[0036] In an example, a 409 grade stainless steel was hot rolled on a continuous strip mill
and then shot blasted. Thereafter, the steel strip was cut into coupons which were
pickled in a solution containing 280 g/l sulfuric acid at 99
oC and then pickled in 150 g/l hydrochloric acid at 88
oC. The coupons then were removed from the acid, rinsed with water, brushed and dried.
The coupons contained small amounts of scale and a large amount of smut. The dirty
appearance of the coupons would result in the steel being unacceptable for many exposed
applications.
Example 2
[0037] In another example, the hot rolled stainless steel of Example 1 was processed in
accordance with the invention. The samples were processed in a manner similar to that
described in Example 1 except as noted herein. After being pickled in the hot sulfuric
acid, the coupons were immersed for 5 seconds into an aqueous solution at 88
oC containing 20 g/l of sulfuric acid and 40 g/l H
2O
2. The coupons were removed from the aqueous solution, rinsed with water, brushed and
then pickled again in hydrochloric acid at 88
oC. The coupons then were removed from the second acid, rinsed with water, brushed
and dried. Unlike the coupons of Example 1, this time the coupons contained no scale
and no smut. These samples processed according to the invention had a very bright
appearance and resulted in a steel acceptable for all exposed applications. This demonstrated
the importance of adding the hydrogen peroxide to the aqueous solution to obtain a
clean surface free of smut and scale.
Example 3
[0038] In another example, a 409 type stainless steel was hot rolled on a continuous strip
mill and then pretreated in a shot blasting machine. Thereafter, the steel strip was
processed in accordance with the invention by being pickled in a solution containing
150 g/l hydrochloric acid heated to 82
oC and processed at a speed of 20 m/min. After being pickled in the hot hydrochloric
acid, the strip was sprayed for about 2 seconds with an aqueous solution containing
50 g/l of hydrochloric acid and 50 g/l H
2O
2. This activated strip then was brushed and rinsed with water. The strip contained
no visible scale or smut. The strip processed according to the invention had a very
bright appearance and resulted in a steel acceptable for all exposed applications.
This example demonstrates the importance of adding the hydrogen peroxide to the aqueous
hydrochloric acid solution to obtain a clean surface free of smut and scale. This
trial also demonstrates that had the strip been pickled using three acid tanks instead
of just one, the line speed could have been increased to 60 m/min.
Example 4
[0039] In another example, a 409 type stainless steel was hot rolled on a continuous strip
mill and then pretreated in a shot blasting machine. Thereafter, the steel strip was
processed in accordance with the invention by being pickled in two acid tanks each
containing a solution containing 250 g/l sulfuric acid heated to 112
oC and processed at a speed of 40 m/min. After being pickled in the hot sulfuric acid,
the strip was sprayed for about 2 seconds with an aqueous solution containing 40 g/l
of sulfuric acid and 40 g/l H
2O
2. This activated strip then was brushed and rinsed with water. The strip contained
no visible scale or smut. The strip processed according to the invention had a very
bright appearance and resulted in a steel acceptable for all exposed applications.
Example 5
[0040] In another example, a 409 type stainless steel was hot rolled on a continuous strip
mill and then pretreated in a shot blasting machine. Thereafter, the steel strip was
processed in accordance with the invention by being pickled in two acid tanks each
containing a solution containing 250 g/l sulfuric acid heated to 112
oC and processed at a speed of 60 m/min. After being pickled in the hot sulfuric acid,
the strip was sprayed for about 2 seconds with an aqueous solution containing 40 g/l
of sulfuric acid and 40 g/l H
2O
2. After being brushed and rinsed with water, this activated strip then was immersed
again in an acid tank containing a solution containing 150 g/l hydrochloric acid heated
to 82
oC. After being pickled in the hot hydrochloric acid, the strip was sprayed again for
about 2 seconds with an aqueous solution containing 40 g/l of sulfuric acid and 40
g/l H
2O
2. This activated strip then was brushed and rinsed with water. The strip contained
no scale or smut. The strip processed according to the invention was fully descaled,
totally free of smut, with a bright appearance and resulted in a steel acceptable
for all exposed applications.
[0041] It will be understood various modifications may be made to the invention without
departing from the spirit and scope of it. Therefore, the limits of the invention
should be determined from the appended claims.
1. A process for removing scale from a ferrous alloy containing chromium, comprising:
providing a ferrous alloy strip containing chromium covered by scale,
pretreating the strip to crack the scale,
immersing the pretreated strip into at least one pickling tank containing an inorganic
acid from the group consisting of hydrochloric and sulfuric acid to remove the cracked
scale, and
applying an aqueous solution containing hydrogen peroxide to the pickled strip wherein
any residual scale on the strip becomes activated by the hydrogen peroxide and removed
by the inorganic acid thereby forming a clean strip.
2. The process of claim 1 wherein the aqueous solution contains at least about 10 g/l
of the hydrogen peroxide.
3. The process of claim 1 wherein the aqueous solution contains the inorganic acid for
removing any residual scale.
4. The process of claim 3 wherein the aqueous solution contains at least about 5 g/l
of the inorganic acid.
5. The process of claim 3 wherein the aqueous solution contains at least about 20 g/l
of the inorganic acid and at least about 25 g/l of the hydrogen peroxide.
6. The process of claim 5 wherein the aqueous solution after activating the strip is
disposed of in the pickling tank.
7. The process of claim 5 wherein the pickling tank contains at least about 50 g/l of
the inorganic acid at a temperature of at least 60oC.
8. The process of claim 1 wherein the pickling tank contains 100-200 g/l of the inorganic
acid.
9. The process of claim 1 including a second pickling tank,
the second tank containing at least about 50 g/l of the inorganic acid at a temperature
of at least 60oC.
10. The process of claim 9 wherein the second tank contains 100-200 g/l of the inorganic
acid and a temperature of at least 77oC.
11. The process of claim 10 wherein the acid in the pickling tanks is counter current
flowed in a direction opposite the direction of travel of the strip.
12. The process of claim 1 including the additional step of immersing the activated strip
into another tank containing inorganic acid.
13. The process of claim 12 wherein the other tank also includes hydrogen peroxide.
14. The process of claim 1 wherein the aqueous solution after activating the strip is
mixed with a waste water containing dissolved ferrous iron to break down any free
hydrogen peroxide.
15. The process of claim 1 wherein the aqueous solution after activating the strip is
mixed with the inorganic acid containing dissolved ferrous iron to break down any
free hydrogen peroxide.
16. The process of claim 1 wherein the pickled strip is brushed to loosen any residual
scale not removed by the inorganic acid.
17. The process of claim 1 wherein the strip is hot rolled or annealed ferritic stainless
steel.
18. The process of claim 1 wherein the pretreatment is shot blasting.
19. The process of claims 1-4, wherein
the aqueous solution contains at least about 20 g/l hydrogen peroxide and wherein
any residual scale on the strip becomes activated by the peroxide and simultaneously
removed by the inorganic acid contained in the aqueous solution.
20. The process of claim 1, wherein a stainless steel strip covered by scale is provided,
the strip is pretreated to crack the scale, and
after immersing the pretreated strip into at least one pickling tank containing at
least 50 g/l of inorganic acid at a temperature of at least 77 °C to remove the cracked
scale, applying an aqueous solution containing at least about 20 g/l of hydrogen peroxide
and at least about 20 g/l of the inorganic acid to activate any residual scale on
the strip,
the activated strip is immersed into another pickling tank containing at least 50
g/l of the inorganic acid at a temperature of at least 77 °C to remove any activated
residual scale, and
an aqueous solution containing at least about 20 g/l of the hydrogen peroxide and
at least about 20 g/l inorganic acid is applied to the pickled strip wherein any residual
scale on the strip becomes activated by the hydrogen peroxide and simultaneously removed
by the inorganic acid contained in the aqueous solution thereby forming a clean strip.