Title of Invention	A PROCESS FOR CHEMICAL DESCALING TO REMOVE OXIDE LAYER FROM A PIECE OF STEEL SUCH AS A STRIP OF STAINLESS STEEL
Abstract	This invention relates to a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 gilitre and 300 gilitre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 m V, the potential being measured between a platinum electrode and a reference AG/ AGCI electrode which are placed in the solution.

Title of Invention

A PROCESS FOR CHEMICAL DESCALING TO REMOVE OXIDE LAYER FROM A PIECE OF STEEL SUCH AS A STRIP OF STAINLESS STEEL

Abstract

This invention relates to a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 gilitre and 300 gilitre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 m V, the potential being measured between a platinum electrode and a reference AG/ AGCI electrode which are placed in the solution.

Full Text	The invention relates to a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel. The Japanese patent N° JP S 56-171638 discloses a process for descaling a steel wire in which the wire is pickled in a bath of hydrochloric acid for 20 to 40 minutes, the bath having a concentration of hydrochloric acid of between 10 and 20% by weight. In order to accelerate the pickling, the acid concentration of the bath is modified. However, it is taught that an excessively concentrated solution results in fumes and requires increasing the size of the installations with increased cost. According to the technique proposed in this document, an amount of FeCl2 at a concentration which might be as much as to cause saturation is added together with an amount of FeCl3 so that the oxidoreduction potentials of Fe^+/ Fe2+ are measured in the course of the descal-ing. The potential is adjusted by addition of FeC13 through an injection nozzle. The pickling procedure consists in the dissolution by HC1 of the ferrous substrate which gives off hydrogen. It is also mentioned in this document that, generally, pickling baths based on hydrochloric acid result in a corrosion of the steel of the pinhole type. A bath is also known, for example from the patent JP H2 205692 for pickling stainless steels in which products hot rolled or subjected to an annealing are pickled in a solution of sulphuric acid containing Fe3+ and Fe2+ ions, said solution containing at least 10 g/litre of iron, the pickling being carried out by aerating the solution with air blown at a rate of at least 100 ml/min and per litre of solution. In this type of bath, the blowing of air has for sole function to stir the solution since it is well known that, in a sulphuric medium, the potential of oxidation of the iron is such that a simple stirring with air cannot oxidize the Fe2+ ions into Fe3+ ions. Further, the Utility Certificate No FR 2551465 associated with the patent EP N° 0236354 disclose a process for continuously pickling a strip of stainless steel in which there is employed a pickling bath comprising principally hydrofluoric acid containing ferric ions, the pickling of the steel strip being carried out by maintaining the ferric iron content at at least 15 g/1 by oxidation of the bath by an injection of air or the addition of another oxidizer while maintaining the REDOX potential of said bath within a range of between 0 and 800 mV. These documents teach the oxidizing function of the air which oxidizes the Fe2+ions into Fe3+ions, the pickling reaction occurring without giving off hydrogen. arranged in succession on the strip production line. A large number of options in the pickling processes have been envisaged in taking into account the most important criterion of the pickling which becomes the pickling rate or the minimum time during which the strip must be in contact with the pickling solution. An object of the invention is to propose a pickling process which effects the pickling of a piece of steel and in particular a strip of stainless steel, within a very short period of time, the pickling time being compatible with the imposed periods on a production line producing the piece or strip of sheet. The invention therefore provides a process for pickling a piece of steel and in particular a sheet strip of stainless steel, comprising applying an aqueous pickling solution containing hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of the aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe3+ ions at a value of between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during the pickling, the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference Ag/AgCl electrode which are placed in the solution. Accordingly, the present invention provides a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in the solution. Other features of the invention are: the reoxidation of the Fe2+ ions by oxygenation is carried out by aeration of the pickling solution, the aeration of the pickling solution is carried out by means selected from the group comprising: pumping and discharge of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen element, spraying of the pickling solution in an enclosure containing air, the reoxidation is completed by the addition of compounds selected from the group comprising: peroxides and/or persalts and preferably hydrogen peroxide (H2O2) and/or potassium permanganate (KMn04), the treating solution preferably has a concentration of hydrochloric acid of between 35 g/litre and 250 g/litre, the treating solution is applied at a temperature of between 10°C and 95°C and preferably between 65°C and 85°C, the piece to be treated is heated prior to the application of the solution, the period of application of the solution on the strip is less than 2 min. the REDOX potential is adjusted between 0 mV and 800 mV and preferably between 400 mV and 600 mV by the addition of a compound and/or an oxidizing gas. The invention also relates to the use of the pickling process according to the invention in the field of the accelerated pickling of a piece of steel and in particular a strip of stainless steel in an installation for the continuous line production of the steel strip. The invention also concerns a continuous production line for producing a strip of rolled sheet of steel and in particular stainless steel in which the strip is subjected to in succession: a mechanical treatment, for example a levelling under tension and/or shot blasting, a primary pickling, a transforming operation such as for example rolling, annealing operations, a final pickling, a finishing operation such as for example a cold rolling of the "skin pass" type at least one of the picklings comprising applying an aqueous pickling solution comprising hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of the aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe^+ ions at a value of between 1 g/litre and 300 g/litre by reoxidation by means of an oxygenation of the Fe2+ ions produced during the pickling, the REDOX potential being maintained at a value of between 0 and 800 mV, the potential being measured between a platinum electrode and a reference Ag/AgCl electrode which are placed in the solution. The following description will explain the invention. Among the various known pickling processes, the f pickling baths based on hydrochloric acid are considered to produce a corrosion of the surface of the pickled pieces, this corrosion being of the pinhole type. Non-polluting pickling processes employed preferably solutions comprising, alone or in combination, at different concentrations, sulphuric and hydrofluoric acids. Various pickling solutions have been tested in order to obtain a rapid pickling in particular on a strip of stainless steel so that this strip may be used, in a continuous manner, after pickling, in an installation comprising a production line producing a sheet strip, the pickling installation being inserted in said line, between ievices transforming the steel, such as for example a Leveller employing tension and a rolling mill. Preferably, pickling tests have been carried out with a single acid so as to avoid the formation of insoluble complex compounds, the solution being intended to be recycled, i.e. re-used for pickling. In a comparative test of pickling solutions containing a sulphuric, hydrofluoric or hydrochloric acid, it was Eound that the hydrochloric acid in a concentrated solution lad a surprising effectiveness as concerns the pickling rate. The following were compared: an aqueous pickling solution A of sulphuric acid laving a normality F N, corresponding to a concentration of L96 g/litre of acid and containing 60 g/litre of total iron. an aqueous pickling solution B of hydrofluoric acid having a normality 4N, corresponding to a concentration of 80 g/litre acid and containing 60 g/litre of total iron. an aqueous pickling solution C of hydrochloric acid having a normality 4N, corresponding to a concentration of 146 g/litre acid and containing 60 g/litre of total iron. These pickling solutions were tested in respect of effectiveness of the pickling, by taking into account their maximum effectiveness by the adjustment of the REDOX potential corresponding to various concentrations of acid employed. The period of stay in the solutions is adapted to the composition of the steel and to the type of oxide to be removed. In the pickling test with the aqueous solution of hydrochloric acid, the REDOX potential is fixed at 460 mV relative to a reference Ag/AgCl electrode, the potential being maintained constant by the injection of air and an additional supply of hydrogen peroxide. The following table shows the various pickling rates measured as loss of material in grams per square metre and per second for various grades of steel: Steel Solution A Solution B Solution C (invention) AISI 304 0.41;0.37 0.42;0.37 0.59 0.28;0.27 0.31;0.27 0.63;0.56 AISI 316L 0.31 0.40 0.53 AISI 430 0.46;0.46 0.92;0.90 1.6;1.1 AISI 430 Ti 0.65;0.67 1.01;0.99 1.5;1.8 AISI 409 0.55;0.58 0.93;0.96 1.3;1.6 Consequently, the pickling in a hydrochloric solution is remarkably effective as concerns the pickling rate. Further, it is found that, for a given pickling duration, the traces of oxide are distinctly less after pickling in a hydrochloric solution. Further, the surface of the strip pickled with a hydrochloric solution has a white appearance, without pinholes, while the surface of the strip pickled with an aqueous solution of sulphuric acid has a blackish appearance. After this remarkable comparison, various tests were carried out for determining the various parameters for optimising the pickling solution: -Variation of the hydrochloric acid concentration: Pickling tests were carried out with aqueous solutions C of hydrochloric acid with a normality between 3N and 5N, corresponding to a concentration of between 108 g/litre and 180 g/litre of acid and containing 60 g/litre of total iron. in these pickling tests, the REDOX potential is fixed at 460 mV relative to a reference AG/AGC1 electrode, the potential being maintained constant by injection of air and an additional supply of hydrogen peroxide, the temperature of the solution being maintained at 80°C. Under these conditions, the pickling rate increases with concentration in acid and may reach values up to two to three times higher than the values with the baths of reference A or B. In order to achieve such a pickling effectiveness, the oxidoreduction potential of the bath according to the invention is controlled, this control being effected by addition in particular of hydrogen peroxide in addition to the injection of air, which permits maintaining a high level of attack and treating continuously, in particular on a rolling line, the sheet strips on which the solution is applied. -Variation of the concentration of total iron: Pickling tests were carried out with aqueous pickling solutions C of hydrochloric acid having a normality 4N, corresponding to a concentration of 146 g/litre of acid and containing 30 g/litre to 250 g/litre of total iron. With a concentration of iron of about 250 g/litre, the iron is at the limit of solubility in the acid solution. In these pickling tests with the hydrochloric solution, the REDOX potential is fixed at 460 mV rel -ive to a reference AG/AGC1 electrode, the potential being maintained constant by injection of air and an additional supply of hydrogen peroxide, the temperature of the solution being maintained at 80°C. Under these conditions, the pickling rate increases with concentration of iron and reaches values up to three times higher than the values obtained with the conventional baths A or B. -Variation of the temperature: The temperatures of the pickling solution may vary from 10°C to 95°C and preferably between 65°C and 85°C. The pickling rate increases with the temperature. Below 70°C, the pickling rate increases moderately with the temperature, when the temperature reaches 70°C to 85°C, the pickling rate increases by nearly 20%. Although above a certain temperature a slight evaporation may occur, it is possible to contemplate using in an installatoin a closed enclosure which permits the use of the solution at a temperature higher than 85°C, associated with a device for condensating the solution for its return it for re-use. In order to benefit from the effect of the temperature, the piece or the sheet strip may be heated to compensate for the thermal inertia effects. -Variation of the REDOX potential: The measure of the REDOX potential is a means for controlling the pickling quality of the solution in an industrial installaton for an optimization of the pickling rate and the maintenance of constant quality with constant pickling effectiveness. The pickling rate may increase from 30% to 50% when the REDOX potential is brought from 400mv to 600mV. The pickling rate may be modulated by adding to the hydrochloric acid bath according to the invention an amount of another acid, such as for example hydrofluoric acid, in a proportion lower than 40 g/litre. WE CLAIM: 1. A process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in the solution. 2. The process as claimed in claim 1, wherein the reoxidation of the Fe2+ ions by oxygenation is effected by aeration of the descaling solution in a known manner. 3. The process as claimed in claim 1 or 2, wherein the aeration of the descaling solution is effected by means selected from pumping and discharge of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen or spraying of said solution in an enclosure containing air. 4. The process as claimed in any one of the claims 1 to 3, wherein the reoxidation is completed by addition of compounds selected from the group comprising known peroxides and/or persalts and preferably hydrogen peroxides H202 and potassium permanganate KMn04. 5. The process as claimed in any one of the claims 1 to 4, wherein the solution preferably has a concentraton of hydrochloric acid of between 35 g/litre and 250 g/litre. 6. The process as claimed in any one of the claims 1 to 5, wherein the said descaling solution is applied at a temperature between 10°C and 95°C and - preferably between 65°C and 85°C. 7. The process as claimed in any one of the claims 1 to 6, wherein the steel piece to be treated is heated prior to the application of the solution. 8. The process as claimed in any one of the claims 1 to 7, wherein the period of application of the solution on said strip is less than 2 min. 9. The process as claimed in any one of the claims 1 to 8, wherein the RADOX potential is adjusted between 400 mVand 600 mV. 10. A process for effecting an accelerated pickling of a piece of steel and in particular a strip of stainless steel in a continuous production line producing said strip, comprising: applying an aqueous pickling solution containing hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of said aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre by reoxidation by means of an oxygenation of the Fe2+ ions produced during said pickling, the REDOX potential being maintained at a value between 400 and 600 mV, said potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in said solution. 11. A process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel substantially as herein described with reference to the accompanying drawings.

Full Text

The invention relates to a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel.
The Japanese patent N° JP S 56-171638 discloses a process for descaling a steel wire in which the wire is pickled in a bath of hydrochloric acid for 20 to 40 minutes, the bath having a concentration of hydrochloric acid of between 10 and 20% by weight. In order to accelerate
the pickling, the acid concentration of the bath is modified. However, it is taught that an excessively concentrated solution results in fumes and requires increasing the size of the installations with increased cost. According to the technique proposed in this document, an amount of FeCl2 at a concentration which might be as much as to cause saturation is added together with an amount of FeCl3 so that the oxidoreduction potentials of Fe^+/ Fe2+ are measured in the course of the descal-ing.
The potential is adjusted by addition of FeC13 through an injection nozzle. The pickling procedure consists in the
dissolution by HC1 of the ferrous substrate which gives off
hydrogen.
It is also mentioned in this document that, generally, pickling baths based on hydrochloric acid result in a corrosion of the steel of the pinhole type.
A bath is also known, for example from the patent JP

H2 205692 for pickling stainless steels in which products hot rolled or subjected to an annealing are pickled in a
solution of sulphuric acid containing Fe3+ and Fe2+ ions, said solution containing at least 10 g/litre of iron, the pickling being carried out by aerating the solution with air blown at a rate of at least 100 ml/min and per litre of solution.
In this type of bath, the blowing of air has for sole function to stir the solution since it is well known that, in a sulphuric medium, the potential of oxidation of the iron is such that a simple stirring with air cannot oxidize the Fe2+ ions into Fe3+ ions.
Further, the Utility Certificate No FR 2551465 associated with the patent EP N° 0236354 disclose a process for continuously pickling a strip of stainless steel in which there is employed a pickling bath comprising principally hydrofluoric acid containing ferric ions, the pickling of the steel strip being carried out by maintaining the ferric iron content at at least 15 g/1 by oxidation of the bath by an injection of air or the addition of another oxidizer while maintaining the REDOX potential of said bath within a range of between 0 and 800 mV.
These documents teach the oxidizing function of the air which oxidizes the Fe2+ions into Fe3+ions, the pickling reaction occurring without giving off hydrogen.

arranged in succession on the strip production line.
A large number of options in the pickling processes have been envisaged in taking into account the most important criterion of the pickling which becomes the pickling rate or the minimum time during which the strip must be in contact with the pickling solution.
An object of the invention is to propose a pickling process which effects the pickling of a piece of steel and in particular a strip of stainless steel, within a very short period of time, the pickling time being compatible with the imposed periods on a production line producing the piece or strip of sheet.
The invention therefore provides a process for pickling a piece of steel and in particular a sheet strip of stainless steel, comprising applying an aqueous pickling solution containing hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of the aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe3+ ions at a value of between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during the pickling, the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference Ag/AgCl electrode which are placed in the solution.

Accordingly, the present invention provides a process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in the solution.
Other features of the invention are:

the reoxidation of the Fe2+ ions by oxygenation is carried out by aeration of the pickling solution,
the aeration of the pickling solution is carried out by means selected from the group comprising: pumping and discharge of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen element, spraying of the pickling solution in an enclosure containing air,
the reoxidation is completed by the addition of compounds selected from the group comprising: peroxides and/or persalts and preferably hydrogen peroxide (H2O2) and/or potassium permanganate (KMn04),
the treating solution preferably has a concentration of hydrochloric acid of between 35 g/litre and 250 g/litre,
the treating solution is applied at a temperature of between 10°C and 95°C and preferably between 65°C and 85°C,
the piece to be treated is heated prior to the application of the solution,
the period of application of the solution on the strip is less than 2 min.
the REDOX potential is adjusted between 0 mV and 800 mV and preferably between 400 mV and 600 mV by the addition of a compound and/or an oxidizing gas.
The invention also relates to the use of the pickling process according to the invention in the field of the accelerated pickling of a piece of steel and in particular a strip of stainless steel in an installation for the

continuous line production of the steel strip.
The invention also concerns a continuous production line for producing a strip of rolled sheet of steel and in particular stainless steel in which the strip is subjected to in succession:
a mechanical treatment, for example a levelling under tension and/or shot blasting,
a primary pickling,
a transforming operation such as for example rolling, annealing operations,
a final pickling,
a finishing operation such as for example a cold rolling of the "skin pass" type
at least one of the picklings comprising applying an aqueous pickling solution comprising hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of the aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe^+ ions at a value of between 1 g/litre and 300 g/litre by reoxidation by means of an oxygenation of the Fe2+ ions produced during the pickling, the REDOX potential being maintained at a value of between 0 and 800 mV, the potential being measured between a platinum electrode and a reference Ag/AgCl electrode which are placed in the solution.
The following description will explain the invention.
Among the various known pickling processes, the

f
pickling baths based on hydrochloric acid are considered to produce a corrosion of the surface of the pickled pieces, this corrosion being of the pinhole type.
Non-polluting pickling processes employed preferably solutions comprising, alone or in combination, at different concentrations, sulphuric and hydrofluoric acids.
Various pickling solutions have been tested in order to obtain a rapid pickling in particular on a strip of stainless steel so that this strip may be used, in a continuous manner, after pickling, in an installation comprising a production line producing a sheet strip, the pickling installation being inserted in said line, between ievices transforming the steel, such as for example a Leveller employing tension and a rolling mill.
Preferably, pickling tests have been carried out with a single acid so as to avoid the formation of insoluble complex compounds, the solution being intended to be recycled, i.e. re-used for pickling.
In a comparative test of pickling solutions containing a sulphuric, hydrofluoric or hydrochloric acid, it was Eound that the hydrochloric acid in a concentrated solution lad a surprising effectiveness as concerns the pickling rate.
The following were compared:
an aqueous pickling solution A of sulphuric acid laving a normality F N, corresponding to a concentration of L96 g/litre of acid and containing 60 g/litre of total

iron.
an aqueous pickling solution B of hydrofluoric acid having a normality 4N, corresponding to a concentration of 80 g/litre acid and containing 60 g/litre of total iron.
an aqueous pickling solution C of hydrochloric acid having a normality 4N, corresponding to a concentration of 146 g/litre acid and containing 60 g/litre of total iron.
These pickling solutions were tested in respect of effectiveness of the pickling, by taking into account their maximum effectiveness by the adjustment of the REDOX potential corresponding to various concentrations of acid employed. The period of stay in the solutions is adapted to the composition of the steel and to the type of oxide to be removed.
In the pickling test with the aqueous solution of hydrochloric acid, the REDOX potential is fixed at 460 mV relative to a reference Ag/AgCl electrode, the potential being maintained constant by the injection of air and an additional supply of hydrogen peroxide.
The following table shows the various pickling rates measured as loss of material in grams per square metre and per second for various grades of steel: Steel Solution A Solution B Solution C
(invention) AISI 304 0.41;0.37 0.42;0.37 0.59
0.28;0.27 0.31;0.27 0.63;0.56
AISI 316L 0.31 0.40 0.53

AISI 430 0.46;0.46 0.92;0.90 1.6;1.1 AISI 430 Ti 0.65;0.67 1.01;0.99 1.5;1.8 AISI 409 0.55;0.58 0.93;0.96 1.3;1.6
Consequently, the pickling in a hydrochloric solution is remarkably effective as concerns the pickling rate. Further, it is found that, for a given pickling duration, the traces of oxide are distinctly less after pickling in a hydrochloric solution.
Further, the surface of the strip pickled with a hydrochloric solution has a white appearance, without pinholes, while the surface of the strip pickled with an aqueous solution of sulphuric acid has a blackish appearance.
After this remarkable comparison, various tests were carried out for determining the various parameters for optimising the pickling solution: -Variation of the hydrochloric acid concentration:
Pickling tests were carried out with aqueous solutions C of hydrochloric acid with a normality between 3N and 5N, corresponding to a concentration of between 108 g/litre and 180 g/litre of acid and containing 60 g/litre of total iron.
in these pickling tests, the REDOX potential is fixed at 460 mV relative to a reference AG/AGC1 electrode, the potential being maintained constant by injection of air and an additional supply of hydrogen peroxide, the temperature

of the solution being maintained at 80°C.
Under these conditions, the pickling rate increases with concentration in acid and may reach values up to two to three times higher than the values with the baths of reference A or B.
In order to achieve such a pickling effectiveness, the oxidoreduction potential of the bath according to the invention is controlled, this control being effected by addition in particular of hydrogen peroxide in addition to the injection of air, which permits maintaining a high level of attack and treating continuously, in particular on a rolling line, the sheet strips on which the solution is applied. -Variation of the concentration of total iron:
Pickling tests were carried out with aqueous pickling solutions C of hydrochloric acid having a normality 4N, corresponding to a concentration of 146 g/litre of acid and containing 30 g/litre to 250 g/litre of total iron.
With a concentration of iron of about 250 g/litre, the iron is at the limit of solubility in the acid solution.
In these pickling tests with the hydrochloric solution, the REDOX potential is fixed at 460 mV rel -ive to a reference AG/AGC1 electrode, the potential being maintained constant by injection of air and an additional supply of hydrogen peroxide, the temperature of the solution being maintained at 80°C.
Under these conditions, the pickling rate increases

with concentration of iron and reaches values up to three times higher than the values obtained with the conventional baths A or B. -Variation of the temperature:
The temperatures of the pickling solution may vary from 10°C to 95°C and preferably between 65°C and 85°C. The pickling rate increases with the temperature. Below 70°C, the pickling rate increases moderately with the temperature, when the temperature reaches 70°C to 85°C, the pickling rate increases by nearly 20%. Although above a certain temperature a slight evaporation may occur, it is possible to contemplate using in an installatoin a closed enclosure which permits the use of the solution at a temperature higher than 85°C, associated with a device for condensating the solution for its return it for re-use.
In order to benefit from the effect of the temperature, the piece or the sheet strip may be heated to compensate for the thermal inertia effects. -Variation of the REDOX potential:
The measure of the REDOX potential is a means for controlling the pickling quality of the solution in an industrial installaton for an optimization of the pickling rate and the maintenance of constant quality with constant pickling effectiveness.
The pickling rate may increase from 30% to 50% when the REDOX potential is brought from 400mv to 600mV.
The pickling rate may be modulated by adding to the

hydrochloric acid bath according to the invention an amount of another acid, such as for example hydrofluoric acid, in a proportion lower than 40 g/litre.

WE CLAIM:
1. A process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel comprising applying an aqueous solution containing hydrochloric acid and ferric and ferrous ions in solution, at a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre, by reoxidation by means of an oxygenation of the Fe2+ ions produced during descaling the REDOX potential being maintained at a value between 0 and 800 mV, the potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in the solution.
2. The process as claimed in claim 1, wherein the reoxidation of the Fe2+ ions by oxygenation is effected by aeration of the descaling solution in a known manner.
3. The process as claimed in claim 1 or 2, wherein the aeration of the descaling solution is effected by means selected from pumping and discharge of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen or spraying of said solution in an enclosure containing air.
4. The process as claimed in any one of the claims 1 to 3, wherein the reoxidation is completed by addition of compounds selected from the group comprising known peroxides and/or persalts and preferably hydrogen peroxides H202 and potassium permanganate KMn04.
5. The process as claimed in any one of the claims 1 to 4, wherein the solution preferably has a concentraton of hydrochloric acid of between 35 g/litre and 250 g/litre.

6. The process as claimed in any one of the claims 1 to 5, wherein the said
descaling solution is applied at a temperature between 10°C and 95°C and
- preferably between 65°C and 85°C.
7. The process as claimed in any one of the claims 1 to 6, wherein the steel piece to be treated is heated prior to the application of the solution.
8. The process as claimed in any one of the claims 1 to 7, wherein the period of application of the solution on said strip is less than 2 min.
9. The process as claimed in any one of the claims 1 to 8, wherein the RADOX potential is adjusted between 400 mVand 600 mV.
10. A process for effecting an accelerated pickling of a piece of steel and in particular a strip of stainless steel in a continuous production line producing said strip, comprising: applying an aqueous pickling solution containing hydrochloric acid and ferric and ferrous pickling ions in solution, and, for the purpose of maintaining a constant pickling power of said aqueous solution of hydrochloric acid having a pH lower than 1, maintaining the concentration of Fe3+ ions at a value between 1 g/litre and 300 g/litre by reoxidation by means of an oxygenation of the Fe2+ ions produced during said pickling, the REDOX potential being maintained at a value between 400 and 600 mV, said potential being measured between a platinum electrode and a reference AG/AGC1 electrode which are placed in said solution.

11. A process for chemical descaling to remove oxide layer from a piece of steel such as a strip of stainless steel substantially as herein described with reference to the accompanying drawings.

Documents:

0370-mas-1997 abstract-duplicate.pdf

0370-mas-1997 abstract.pdf

0370-mas-1997 assignment.pdf

0370-mas-1997 claims-duplicate.pdf

0370-mas-1997 claims.pdf

0370-mas-1997 correspondence-others.pdf

0370-mas-1997 correspondence-po.pdf

0370-mas-1997 description (complete)-duplicate.pdf

0370-mas-1997 description (complete).pdf

0370-mas-1997 form-2.pdf

0370-mas-1997 form-26.pdf

0370-mas-1997 form-4.pdf

0370-mas-1997 form-6.pdf

0370-mas-1997 petition.pdf

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Patent Number

198056

Indian Patent Application Number

370/MAS/1997

PG Journal Number

27/2006

Publication Date

07-Jul-2006

Grant Date

20-Mar-2006

Date of Filing

24-Feb-1997

Name of Patentee

M/S. UGINE S A

Applicant Address

IMMEUBLE "LA PACIFIC", LA DEFENSE 7, 11/13 COURS, VALMY 92800, PUTEAUX

Inventors:

#	Inventor's Name	Inventor's Address
1	GIRAUD HENRI	35 ROUTE DE CHASSY, 71130 GUEUGNON
2	VIALATTE BERNARD	2 RUE TRAVERSIERE, 71120 LE CREUSOT
3	LAUVERGNE MARTINE	16 ROUTE DE CHASSY, 71130 GUEUGNON

PCT International Classification Number

C23G 1/08

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	96 02405	1996-02-27	France