WO2014155339A1

WO2014155339A1 - Method for treating in continuous the surface of a laminate made of stainless steel in a solution based on hydrochloric acid

Info

Publication number: WO2014155339A1
Application number: PCT/IB2014/060224
Authority: WO
Inventors: Stefano Martines; Giovanni ASTENGO; Luca Lattanzi; Armando Giannetti
Original assignee: Tenova S.P.A.
Priority date: 2013-03-29
Filing date: 2014-03-27
Publication date: 2014-10-02
Also published as: EP2978878A1; ITMI20130494A1; EP2978878B1

Abstract

The present invention refers to a method for treating in continuous the surface of a laminate made of stainless steel that comprises at least the steps of: (a) subjecting said laminate to an electrolytic pickling treatment in alternating current, in an aqueous electrolytic solution comprising at least HCl and Fe³⁺ ions; (b) subjecting said electrolytically pickled laminate to a chemical pickling treatment in an aqueous chemical pickling solution comprising at least HCl and Fe³⁺ ions; (c) subjecting said chemically pickled laminate to a surface finishing and surface passivation treatment.

Description

METHOD FOR TREATING IN CONTINUOUS THE SURFACE OF A LAMINATE MADE OF STAINLESS STEEL IN A SOLUTION BASED ON HYDROCHLORIC ACID The present invention concerns a method for treating in continuous the surface of a laminate made of stainless steel in a solution based on hydrochloric acid .

The method of the present invention is particularly suitable for treating a laminate made of stainless steel in order to remove the surface oxide layer (so-called "scale") which inevitably forms during the various heat treatments to which the laminate is subjected, including the lamination process itself.

As known, the removal of scale from a laminate made of stainless steel is necessary to restore to the surface thereof the base chemical composition of the steel, so as to give the laminate the well-known properties of resistance to corrosion.

The process for removing scale, known as

"pickling", aims to remove both the outermost surface layer of metal oxide (richer in chrome with respect to the basic composition of steel) and the layer of alloy beneath it that, on the other hand, has a lower chrome content with respect to the base chemical composition of steel (dechromatized layer) .

The pickling processes of the prior art generally provide three distinct steps. In the first step (descaling) a chemical-physical modification of the scale is induced in order to promote detachment from the laminate. This step is carried out, for example, by immersing the manufactured product in baths of molten oxidizing salts, such as Kolene baths (mixtures of NaOH, NaN0₃ and NaCl) at temperatures around 500 °C ( thermochemical descaling) , or electrolytically (electrolytic descaling) in neutral or acidic aqueous solutions (for example, aqueous solutions of sodium sulfate or sulfuric acid) . As a function of the type of steel and the composition of the scale, the descaling step can also be preceded by mechanical scale-removal treatments (so-called "scale-breaking" treatments), like sand-blasting, shotblasting and abrasive brushing.

In the second step (the actual pickling) , the residual scale is removed from the surface of the laminate together with the dechromatized layer beneath. This step is generally carried out by immersing the laminates in acid baths with high oxidizing capability, such as the baths of mixtures of mineral acids (e.g. mixtures of HNO₃ and HF, mixtures of H₂SO₄, HC1 and H₃PO₄, etc.) in the presence of oxidizing compounds, like for example permanganates, persulfates or hydrogen peroxide.

In the third step (finishing and passivation), the protective layer of chromium oxide forms on the surface of the laminate. This step is generally carried out immersing the manufactured product in baths containing nitric acid or mixtures of mineral acids (generally at lower concentrations with respect to the pickling step) . In certain cases, in the pickling step, as well as the removal of the scale and of the dechromatized layer, there is also surface oxidation of the laminate that thus makes the finishing/passivation step superfluous .

The pickling processes of the prior art have various drawbacks concerning their potential environmental impact, safety of the work spaces, operative control of the process as well as the equipment costs and the consumption of raw materials.

The use of pickling baths of nitric acid and hydrofluoric acid, for example, leads to the formation of large quantities of nitrogen oxides (NOx) and waste water contaminated by nitrates and fluorinated compounds, which make it necessary to adopt specific measures to keep down gaseous emissions and to purify the waste water. The quantity of contaminating compounds to be treated or disposed of depends on the quantity of material removed from the laminate in the baths containing nitric acid and/or hydrofluoric acid, a quantity that in turn depends - all other conditions being the same - on the duration of the treatment in these baths.

The efficiency of pickling processes of the prior art is also such that an adequate removal of the scaling can be obtained only with prolonged contact of the laminate with the pickling baths. This means that the processes are rather slow and necessarily carried out in large sized plants.

Processes for pickling laminates made of stainless steel having improved efficiency and less environmental impact are described, for example, in WO 02/12596 A2 and WO 00/15880 Al .

The process described in WO 02/12596 comprises an electrolytic pickling step in alternating current carried out in an aqueous solution of sulfuric acid in the presence of Fe³⁺ ions or in the presence of at least one second acid selected from hydrofluoric acid and phosphoric acid.

The process described in WO 00/15880 comprises an electrolytic pickling step in continuous current carried out in an aqueous solution of sulfuric acid in the presence of ions Fe³⁺ ions. The process can also comprise steps of descaling (in baths of molten salts), chemical pickling (in solutions of H₂SO and HF in the presence of Fe²⁺ and Fe³⁺ ions) and/or final passivation (in H2SO4 and H₂0₂) .

In the state of the art, for example in US

5851304, it is also known to use aqueous solutions of HC1 to pickle the laminates made from stainless steel. The use of HC1, however, has not been widely applied in the technical field of the present invention, mainly due to the high aggressivity of this acid against stainless steel. Such aggressivity, indeed, causes high erosion of the laminate during pickling as well as problems of corrosion of the apparatuses. For these reasons, in the state of the art the use of hydrochloric acid is currently limited to the pickling of less valuable laminates made from stainless steel, such as steels with low content of alloying elements (for example, AISI409 steels (12% Cr) ) .

In the state of the art it is also known (DE 19624436 Al ) to use solutions of hydrochloric acid containing Fe³⁺ ions for the electrolytic pickling (in continuous current) of laminates made from stainless steel .

The main purpose of the present invention is to avoid the drawbacks of the pickling processes of laminates made of stainless steel of the prior art.

In such a general purpose, a purpose of the present invention is to provide a method for pickling a laminate made of stainless steel that makes it possible to obtain a high-quality treated product, but with low environmental impact and lower consumption of raw materials (e.g. mineral acids and additives) .

Another purpose of the present invention is to provide a method for pickling a laminate made of stainless steel with improved efficiency, so as to reduce the duration of the pickling treatments and the bulk of the plants necessary to carry it out.

The Applicant has found that these and other purposes are accomplished by the method for treating in continuous the surface of a laminate made of stainless steel according to the present invention, which comprises at least the following steps:

(a) subjecting said laminate to an electrolytic pickling treatment in alternating current in an aqueous electrolytic solution of HC1 comprising at least Fe³⁺ ions, the concentration of HC1 being comprised in the range 0.5-200 g/1 and the concentration of Fe³⁺ ions being comprised in the range 5-60 g/1;

(b) subjecting said electrolytically pickled laminate to a chemical pickling treatment in an aqueous solution of HC1 comprising at least Fe³⁺ ions (chemical pickling solution) , the concentration of HC1 being comprised in the range 0.5-200 g/1 and the concentration of Fe³⁺ ions being comprised in the range 5-60 g/1;

(c) subjecting said chemically pickled laminate to a surface finishing and a surface passivation treatment .

For the purposes of the present invention, by laminate made of stainless steel (hereafter also "laminate") it is meant a semi-worked product made of stainless steel of the type like a plate, a strip, a wire, a rod iron, a wire rod, a section bar or similar, of indefinite length. These semi-worked products are generally obtained through lamination processes, either hot or cold, possibly accompanied by annealing heat treatments .

In the description of the present invention the electrolytic solutions, the pickling solutions and the descaling solutions are also indicated, without distinction, as "pickling baths".

The method according to the present invention can be applied to effectively remove the layer of metal oxide (scale) from the surface of the laminate and the dechromatized layer beneath, forming a layer of chrome oxide in its place that protects the laminate from corrosion .

Advantageously, the method according to the invention can be used to pickle various types of steel, in particular austenite, martensitic and ferritic steels, whether they are obtained through hot and cold lamination processes (hereafter respectively indicated as "hot rolled laminates" and "cold rolled laminates") .

The method according to the invention comprises at least one electrolytic pickling step of the laminate carried out in alternating current (AC) in an aqueous electrolytic solution comprising at least HC1 and Fe³⁺ ions .

The concentration of HC1 in the electrolytic solution is selected in the range 0.5 g/1 - 200 g/1 according to the type of steel and the type of treatments to which the laminate has been subjected. For example, in the case of cold rolled laminates, the concentration of HC1 is more preferably comprised in the range 0.5-150 g/1. In the case of austenitic steels, the concentration of HC1 is more preferably comprised in the range 0.5-120 g/1. In the case of ferritic steels (stabilized and not), the concentration of HC1 is more preferably comprised in the range 0.5- 100 g/1.

In the present description, the concentration values of HC1 are meant to refer to the concentration of the free acid in the aqueous solution, as measurable, for example, through an acid-base titration or conductometric analysis. The concentration values of HC1 do not comprise the fraction of chloride ions that forms complexes with the metals in solution.

The concentration of Fe³⁺ ions is also selected as a function of the type of steel and the type of treatments to which the laminate has been subjected. Typically, the concentration of Fe³⁺ ions in the electrolytic solution is selected between 5 g/1 and 60 g/1. In the case of cold-rolled laminates, the concentration of Fe³⁺ ions is more preferably comprised in the range 5-40 g/1. In the case of austenitic steels, the concentration is more preferably comprised in the range 20-45 g/1.

As the electrolytic treatment proceeds, through the effect of the dissolution reactions of the scale, of the dechromatized layer and of the steel as well as the reactions between species in solution, the concentrations of free acid and of ferric Fe³⁺ ions in the solutions progressively decrease, whereas that of ferrous Fe ions increases. In particular, during the course of the electrolytic pickling, the Fe³⁺ ions react with the metallic iron of the alloy that forms the laminate - solubilizing the latter in the form of Fe²⁺ ions - reducing in turn to Fe²⁺ ions .

In order to have an effective pickling action it is thus preferable to restore the lost hydrochloric acid and keep the molar ratio Fe³⁺/Fe²⁺ within predetermined values. Preferably, the molar ratio Fe³⁺/Fe²⁺ in the electrolytic solution is equal to or greater than 0.1 and, preferably, less than or equal to 1.

The concentration of HC1 can be kept at desired levels through periodic addition of fresh acid or by recycling the aqueous solutions of the acid used in other steps of the method, provided that they still have adequate pickling capability.

Preferably, the concentration of ferric ions is regulated by oxidising the Fe²⁺ ions present in solution to Fe³⁺ ions according to the methods known in the state of the art. For this purpose, for example, it is possible to add oxidising compounds such as hydrogen peroxide, organic and/or inorganic peracids and/or their salts to the pickling baths. The yield of the oxidation reaction through hydrogen peroxide can be improved through the use of specific stabilizers for this compound, like for example phenacetin, secondary or tertiary aliphatic alcohols, glycols, glycol ethers.

Alternatively, the oxidation of the ferrous ions to ferric ions can be obtained by blowing air or other gases and/or mixtures containing oxygen, possibly in the presence of catalysts. The Fe ions can be added to the electrolytic solution in the form of soluble salts, for example halides, preferably FeCl3. It is also possible to add Fe²⁺ salts, for example FeCl₂, and then oxidise the Fe²⁺ ions to Fe³⁺ according to the methods described above.

Typically, the electrolytic solution also contains other metals that derive from the dissolution of steel during pickling (Cr, Ni, Mn, etc.) . The total content of metals in solution (dissolved metals), including the Fe³⁺ and Fe²⁺ ions, is preferably comprised in the range 30 g/1 - 150 g/1. In the case of cold-rolled laminates, the concentration of dissolved metals is more preferably comprised in the range 30-140 g/1, whereas in the case of cold-rolled and annealed austenitic steels and of stabilized ferritic steels said concentration is more preferably comprised in the range 60-120 g/1.

The electrolytic pickling is carried out keeping the electrolytic solution at a temperature comprised in the range 30-95°C. In the case of hot-rolled laminates, the temperature is more preferably comprised in the range 50-95°C, whereas for cold-rolled laminates, the temperature is more preferably comprised in the range 30-80°C. In particular, for cold-rolled laminates steels of the austenitic type the temperature is preferably comprised in the range 35-70°C, for stabilized and non-stabilized ferritic ones in the range 30-60°C. Preferably, the cold-rolled steels are subjected to an annealing treatment before being subjected to the electrolytic pickling.

The alternating current (AC) used in the electrolytic pickling has a density (referring to the unit of surface of the laminate) comprised in the range 5-50 A/dm² (both for hot-rolled and cold-rolled laminates) .

For cold-rolled laminates, the current density can preferably be selected as a function of the type of steels, for example, as follows:

- austenitic steels 8-50 A/dm²;

- stabilized ferritic steels 8-30 A/dm²;

- non-stabilized ferritic steels 5-15 A/dm². The frequency of the AC current varies in the range 5-150 Hz. Typically, the frequency of the current is kept constant during the treatment. However, it can also be varied as a function of the process requirements, for example to increase the quality of the treatment or to control the pickling speed as a function of the feeding speed of the laminate to the pickling plant.

Typically, the electrolytic pickling has a duration that can vary from 2 to 60 seconds, both for hot-rolled and for cold-rolled laminates.

For cold-rolled laminates, the duration of the electrolytic pickling in AC current can preferably be selected as a function of the type of steels, for example, as follows:

- austenitic steels 3-15 s;

- stabilized and non-stabilized ferritic steels

2-14 s.

The chemical pickling step is carried out on the laminate after it has been subjected at least to the electrolytic treatment in AC current. The chemical pickling is carried out by placing the laminate in contact with an aqueous electrolytic solution comprising at least HC1 and Fe ions (chemical pickling solution) . The concentration of the species present in the pickling solution (HC1, Fe³⁺, Fe²⁺ dissolved metals, ratio Fe³⁺/Fe²⁺ ions) varies in the ranges indicated above for the composition of the electrolytic solution.

Preferably, the chemical pickling solution comprises 50-250 g/1 of HC1 and 10-60 g/1 of Fe³⁺ ions. Preferably, the Fe²⁺ ions are present in a concentration such as to have a molar ratio between Fe³⁺/Fe²⁺ ions that is greater than 0.1, preferably greater/less than 0.5. Preferably, such a ratio is less than or equal to 1.

Preferably, the concentration of dissolved metals varies in the range 20-140 g/1. In the case of cold- rolled laminates, the concentration of dissolved metals is more preferably comprised in the range 20-100 g/1. In particular, for cold-rolled steels of the ferritic type (stabilized and non-stabilized) and for those of the austenitic type the concentration of the dissolved metals is preferably selected in the range 45-110 g/1.

The duration of the chemical pickling treatment typically varies in the range 10-50 seconds. In the case of cold-rolled laminates, the duration of the treatment is more preferably comprised in the range 5- 40 seconds. For cold-rolled laminates of the stabilized and non-stabilized ferritic type the duration is more preferably comprised in the range 10-40 seconds, whereas for cold-rolled laminates of the austenitic type and then annealed it is more preferably comprised in the range 20-50 seconds.

The temperature of the chemical pickling solution is typically kept within the range 30-85°C. In the case of cold-rolled laminates, the temperature is more preferably comprised in the range 30-70°C.

The method according to the invention also comprises a finishing and final passivation step (hereafter also just "finishing") . This step, which is carried out after having subjected the laminate to careful washing with water to eliminate the residues of HC1 and of oxides possibly still present on its surface, has the purpose of oxidising the surface of the laminate so as to form a protective passivation layer .

In general, the finishing step can be carried out according to the prior art. Preferably, according to the method of the present invention the finishing step is carried out by placing the surface of the laminate in contact with an aqueous solution of at least one mineral acid and having an oxidation-reduction chemical potential (measured with respect to a reference electrode Ag/AgCl) comprised between 100 and 800 mV, preferably between 200 mV and 600 mV (finishing solution) . Such an oxidation-reduction potential can be obtained, for example, with aqueous solutions comprising one or more acids selected among nitric acid, sulfuric acid, in the possible presence of hydrofluoric acid, and Fe³⁺ ions.

The concentration of Fe³⁺ ions in the finishing solution varies in the range 5-40 g/1.

In a first preferred embodiment, the mixture of mineral acids used in the finishing step is a mixture of nitric acid and hydrofluoric acid (nitric- hydrofluoric finishing) . The concentration of nitric acid in the finishing solution is preferably comprised in the range 20-150 g/1, whereas that of the free hydrofluoric acid is preferably comprised in the range 3-35 g/1. The concentration of Fe³⁺ ions is preferably comprised in the range 5-35 g/1.

In a second preferred embodiment, the mixture of mineral acids used in the finishing step is an aqueous solution of sulfuric acid containing, possibly, also hydrofluoric acid (sulfuric or sulfuric-hydrofluoric finishing) .

The concentration of sulfuric acid in the finishing solution is preferably comprised in the range 20-150 g/1, whereas that of the free hydrofluoric acid is preferably comprised in the range 0-35 g/1. The concentration of the Fe³⁺ ions is preferably comprised in the range 5-40 g/1, whereas the molar ratio Fe³⁺/Fe²⁺ ions in solution is preferably kept above 1.

In a further preferred embodiment, the finishing step comprises an aqueous solution of nitric acid also comprising Fe³⁺ ions. The concentration of nitric acid in the finishing solution is preferably comprised in the range 20-150 g/1 (preferably in the range 20-100 g/1, if it is a cold-rolled laminate; preferably in the range 40-150 g/1, if it is a hot-rolled laminate) , whereas that of the Fe³⁺ ions is on the other hand comprised in the range 5-40 g/1.

Particularly preferred finishing solutions are those that do not comprise nitric acid, which make it possible to further reduce the environmental impact of the process in terms of emissions of pollutant gases (NOx) and nitrate compounds in the waste water. In the finishing step, the finishing solution is kept at a temperature that can vary from 30°C to 80°C.

In the aforementioned conditions, generally the duration of the finishing treatment varies from 5 seconds to 40 seconds.

The composition of the scale depends strictly on the type of rolling undergone - hot or cold - and on possible annealing heat treatments to which the laminate has been subjected. As a function of the chemical composition of the steel that forms the laminate and that of the scale, the method can comprise one or more further treatment steps. For example, in the case of hot-rolled laminates, the treatment method according to the invention can advantageously also comprises a chemical descaling step to promote the removal of scale in the subsequent electrolytic pickling step.

The chemical descaling step is carried out by placing the laminate in contact with an aqueous solution comprising HC1 and Fe³⁺ ions (descaling solution) .

The concentration of the compounds present in the descaling solution (Fe³⁺, dissolved metals, ratio Fe³⁺/Fe²⁺ ions) varies in the ranges indicated earlier for the composition of the electrolytic solution. Preferably, the descaling solution comprises 30-250 g/1 of HC1 (more preferably 50-200 g/1) . Preferably, the descaling solution comprises 5-60 g/1 of Fe³⁺ ions. Preferably, the ratio Fe³⁺/Fe²⁺ ions in solution is kept above 0.1, more preferably above 0.5.

The concentration of dissolved metals can vary in the range 10-140 g/1, preferably 50-110 g/1. The duration of the chemical descaling treatment is typically comprised in the range 15-60 seconds, preferably 15-50 seconds. The temperature of the descaling solution is preferably kept within the range 30-95°C.

In the case of cold-rolled laminates, the chemical descaling step, although possible, is generally left out, since it is preferable to treat the surface electrolytically for quality reasons.

In the case of cold-rolled laminates it has been found that the quality of the pickled surfaces can be improved by also subjecting the laminate to at least one electrolytic treatment in continuous current (DC) , using an electrolytic solution having the same composition as the solution used in the treatment in AC current or similar composition.

The electrolytic treatment in DC current is carried out by applying a DC current of variable density in the range 4-20 A/dm².

For cold-rolled laminates, the DC current density can vary as a function of the type of steels in the following way:

- austenitic steels 5-20 A/dm²;

- stabilized ferritic steels 5-18 A/dm².

The duration of the treatment in DC current varies from 2 to 20 seconds, such a duration referring to the period in which a given portion of surface of the laminate is anodically polarized.

For cold-rolled laminates, the duration of the electrolytic pickling in DC current can vary as a function of the type of steels in the following way:

- austenitic steels 4-15 s; stabilized and non-stabilized ferritic steels 6-35 s.

The treatment in DC current can be carried out before or after the treatment in AC current, preferably before. The treatment stages in AC and DC current can be carried out in the same tank or in distinct tanks, arranged in series in the pickling plant.

The method according to the invention can advantageously also comprise at least one recovery step of the hydrochloric acid and/or of the metals from the aqueous solutions used in the various steps of the method. The recovery of hydrochloric acid can be carried out according to the techniques of the prior art. In particular, the recovery can be carried out through a roasting process of the spent hydrochloride solutions, which possibly also provides a preliminary treatment of concentration of the solutions through evaporation of part of the water contained in them. The roasting process leads to the transformation of the metallic chlorides into hydrochloric acid and corresponding metal oxides, which can be separated and recovered from the concentrated solution. The roasting process has a recovery yield, referring to the initial content of chloride ions of the aqueous solution subjected to treatment, of up to 90% by weight.

In a preferred embodiment of the method according to the invention, the chemical pickling solution used in the chemical pickling step is at least partly recirculated to the electrolytic pickling step, where it is used as electrolytic solution. Preferably, the chemical pickling solution is at least partly recirculated to the descaling step, where it is used as descaling solution.

In a particularly preferred embodiment, in order to promote the subsequent recovery of hydrochloric acid and of the metals from the spent pickling baths, the method according to the invention is carried out using a chemical pickling solution in which the concentrations of the ferric Fe³⁺ ions and of the dissolved metals are less than the corresponding concentrations in the descaling solution; in turn, the concentrations of the ferric Fe³⁺ ions and of the dissolved metals in the descaling solution are less than the corresponding concentrations in the electrolytic solution. In this way, in the treatment process of the laminate, the spent chemical pickling solution can be added (at least partially) to that used in the descaling step that, when in turn it will be spent, can be added (at least partially) to the solution used for the electrolytic pickling. The spent electrolytic solution, finally, will be sent to the recovery step of the hydrochloric acid and of the metals .

The solutions based on hydrochloric acid used in the different steps of the method according to the invention, as well as the mineral acids and the metallic ions mentioned above, can also comprise the additives typically used in pickling baths, such as compounds having the functions of improving the wettability of the surface of the laminate, accelerant compounds or inhibitors of the pickling reactions. Among inhibitor compounds, those that are particularly preferred are inhibitors belonging to the following families : - aromatic aldehydes or their acetals, possibly containing ethylene insaturations , having an overall number of carbon atoms from 7 to 12, like for example benzoic aldehyde and benzaldehyde-diethyl-acetal ;

- alkyl- , aryl- (e.g. aniline, toluidine) and alkyl-aryl- amines, primary or secondary, possibly ethoxylated, with alkyl chains from 6 to 22 atoms of C, such as phenyl-amine and benzyl-amine ;

primary, secondary or tertiary acetylene alcohols (alkynols), in which the alkyn chain has from 3 to 12 carbon atoms, for example l-octin-3-ol , propargyl alcohol, l-hexin-3-ol ;

- alkyl or alkyl-aryl quaternary ammonium halides, with linear alkyl chains from 5 to 16 atoms of C and having an overall number of carbon atoms from 8 to 30;

salts of aromatic heterocyclic compounds containing N, in particular halides, such as salts of alkyl-, aryl-, alkyl-aryl-pyridinium and quinolinium with alkyl chains from 6 to 18 atoms of C (for example, N-dodecyl-pyridinium chloride and N-benzyl-quinolinium chloride) ;

- halides of alkyl or alkyl-aryl methyl-sulfonium with linear alkyl chains having from 1 to 16 atoms of C and alkyl-aryl chains from 7 to 20 atoms of C.

The concentration range of use of the aforementioned inhibitor compounds goes from 0.05 to 5 ml/1, if liquid, or from 0.05 to 5 g/1, if solid.

Preferably, the inhibitors are added, individually or in mixture, preferably binary and ternary mixtures, to the electrolytic solutions, to the pickling solutions and to the descaling solutions used to treat the cold-rolled laminates. With these additives, indeed, it is possible to obtain pickled surfaces of the desired quality, in particular when the treatment method does not provide for preliminary descaling steps before the electrolytic treatment.

As happens for the electrolytic solutions, also in the other pickling baths the concentration of HC1 and that of the ferric and ferrous ions varies over time as the treatment of the laminates proceeds. Also in these cases, the concentrations of the different species can be restored through the discharge of the spent solutions, the addition (periodic or continuous) of fresh reactants, water, or, in the case of metals (e.g. ferrous and ferric ions), making oxidation reactions occur in solution that bring the molar ratio between the Fe²⁺ and Fe³⁺ ions to the optimal values.

The method of the invention can also comprise the conventional steps (intermediate or preliminary) of washing and degreasing the surface of the laminate with suitable aqueous solutions containing surfactants or other additives. In particular, the washing steps (which can be carried out by immersion, spraying with water jets and possibly with the help of metallic brushes) have the purpose of eliminating the residues of the previous treatments from the surface and avoiding contamination of the solutions used in the subsequent steps of the method.

Further steps that can be comprised in the method according to the invention are the steps of mechanical scale removal, through sandblasting or shotblasting, or tension levelling of the laminate.

In order to carry out the method of the present invention it is possible to use the apparatuses and the devices commonly used in the field of treatment processes of the surfaces of iron and steel manufactured products, in particular of steel pickling processes .

The electrolytic pickling step, in particular, can be carried out in a tank of the type described in patent application WO 2011/039596.

Figure 1 schematically represents a possible embodiment of an apparatus in which it is possible to carry out the electrolytic pickling step according to the method of the invention.

The apparatus, wholly indicated with 10, comprises a treatment tank 12 suitable for containing the electrolytic solution SE and inside which the laminate 11 is made to pass in continuous along a direction and in the advancing sense indicated by the arrow F. The position of the laminate inside the treatment tank 12 is determined by the rollers 20 and 21 and by the pull applied on the strip.

The apparatus 10 also comprises at least two pairs of electrodes 14 that are opposite one another and between which the laminate 11 is made to pass in continuous .

Each pair of electrodes 14 comprises at least one first electrode 15 facing one of the two faces of the laminate 11 and at least one second electrode 17 facing the other of the two faces of the laminate 11. The electrodes 15, 17, are also substantially equidistant from the laminate 11, each of them being at a distance Lc from the laminate in general comprised between 50 and 250 mm. The electrodes 15, 17 are arranged in a direction substantially parallel to the faces of the laminate along the path in the tank 12 and they extend substantially for the entire width La of the laminate (the width La is not shown in the figures) . The two electrodes 15, 17 of a pair of electrodes 14 are at a distance Lg preferably comprised between 100 and 500 mm. Each of the electrodes 15, 17 has a length Le , measured along the movement direction of the laminate, preferably comprised between 200 mm and 3000 mm. Each pair of electrodes 14 is at a distance Lx from the next pair, in the movement direction of the laminate, in general comprised between 100 and 1600 mm according to the type of tank adopted.

The electrodes 15, 17 of each pair of electrodes 14 are immersed in the electrolytic solution SE and can be associated with an electric power supply group (not shown in the figures) capable of delivering a continuous current DC or an alternating current AC at variable frequency. The ways of obtaining an electric power supply group having the aforementioned characteristics are known to the person skilled in the art .

In the case of electrolytic pickling in AC current, a first preferred configuration is that for which the AC current is supplied to the electrodes 15, 17 so that the two electrodes of each pair 14 are with voltage in phase between them, so as to have, at a given moment, on both of the electrodes, a polarization of the same sign. For this purpose, for example, it is possible to connect the electrodes 15 and 17 to a phase of a transformer or of an inverter taking care to arrange a sufficient distance between pairs of electrodes 14 connected to different phases. Preferably, between pairs of adjacent electrodes 14 there is at least one separator element made of an insulating material, for example in the form of an immersing roller coated in plastic material or of a static separator made of plastic material (not shown in the figures) . The separator made of an insulating material allows the formation of dispersed currents between electrodes of adjacent pairs that have opposite polarization to be reduced.

This first configuration is particularly preferred in the case of the electrolytic pickling in DC current.

A second preferred configuration provides that the electrodes 15 and 17 facing each other are connected to two different phases of the alternating current; in this case it is preferable for successive pairs of electrodes 14, if adjacent and not separated by immersing rollers and other insulating separators, to be connected to the same phases; pairs of electrodes separated by appropriate distance and by immersing rollers and insulating separators, on the other hand, will be connected to different phases so as to minimise the dispersed currents and, at the same time, avoid unbalanced loads on the power supply network.

With the aforementioned second configuration of the electrodes, the resulting electric field has current lines that are substantially vertical and orthogonal to the surface of the laminate, which is thus passed through by the current along the thickness.

In the case of electrolytic pickling in AC current, it has been found that it is possible to improve the efficiency of the treatment by suitably selecting the frequency of the alternating current applied, within the range 5-150 Hz, as a function of the position of the electrodes 15, 17 with respect to the laminate 11 subjected to treatment.

In particular, in a preferred embodiment, the Applicant has found that the efficiency of the treatment improves if a given surface portion of the laminate 11 is prevented from being substantially exposed to the same polarization at the pairs of electrodes 14 during the crossing path of the electrolytic bath. Such a circumstance, indeed, would lead to having regions of the laminate treated unevenly. Such circumstances can be encountered when the distance Lx between successive pairs of electrodes 14 is comparable to or greater than the distance Lc between the electrodes 15, 17 and the laminate.

In accordance with a preferred embodiment of the method of the present invention, this drawback can be overcome by applying to the electrodes 15, 17 an alternating current of frequency f , variable from 5 Hz to 150 Hz, said frequency f being correlated to the forward velocity v (expressed in m/s) of the laminate and to the length L_e (expressed in metres) of the electrodes 15, 17 by the relation (1)

f(Hz) > (A-v)/L_e (1) where A is a rational number greater than 1, preferably greater than 2. Generally, A does not exceed the value of 300.

This provision makes it possible to take into account the geometry of the cell and the evolution phenomena of the electrolysis gases.

The solution indicated above to improve the efficiency of treatment of the laminate can also be applied to the case in which the apparatus for carrying out the electrolytic treatment comprises electrodes consisting of a plurality of elements (for example plates of the type described in WO 2011/039596) . This configuration, schematically illustrated in figure 2, has the advantage of avoiding the accumulation of electrolysis gases and of solid residues (essentially metal oxides) that detach from the surface during the electrolytic process, thus improving the efficiency of the treatment.

The apparatus of figure 2 comprises pairs of electrodes 140 consisting of two groups 150, 170 of elements 160, 180. The elements 160, 180 extend for the entire width L_a of the laminate and have a length L_b (measured in the forward direction of the laminate) . The elements 160, 180 are arranged substantially parallel with respect to the laminate 11. The distance between two adjacent elements 160, 180 is indicated in figure 2 with reference symbol La . Each element 160, 180, in the apparatus 10, performs the function of electrode in the same way as the electrodes 17, 15 of the apparatus 10 described in figure 1.

In figure 2, the elements indicated with the same reference symbols used for figure 1 correspond to the same elements described for figure 1.

Also for the configuration of the apparatus described in figure 2 it has been found that the electrolytic treatment of the laminate 11 could not be uniform over the entire surface for particular geometric arrangements of the electrodes 150, 170, of the elements 160, 180 and of the laminate 11. In particular, non-uniform treatments can be encountered when the distance La between two adjacent elements is close to or greater than the distance L_c between the element 160, 180 and the laminate 11.

In accordance with the method of the present invention, this drawback can be overcome in the case of an apparatus of the type illustrated in figure 2 by applying to the electrodes an alternating current of frequency f , that can vary from 5 Hz to 150 Hz, said frequency f being correlated to the forward speed v (expressed in m/s) of the laminate and to the length L_b (expressed in metres) of the element 160, 180 by the relation (2)

f (Hz ) > (A' -v) /L_b (2 ) where A' is a rational number greater than 1, preferably comprised between 1 and 25, more preferably comprised between 2 and 10.

In the configuration schematically represented in figure 2, it is suitable for the frequency selected for the AC current of the electrolytic treatment to satisfy one or both of the aforementioned relations (1) and (2) .

In particular, when the ratio between the distance L_g between the opposite electrodes 160, 170 and the distance La of the elements 160, 180 that form said opposite electrodes 150, 170 is greater than 4, it is preferable for the frequency f to respect at least the relation (1) . When, on the other hand, the aforementioned ratio L_g/L_d is equal to or less than 4, it is preferable for the frequency f to respect at least the relation (2) .

The method according to the present invention makes it possible to overcome or at least lessen the drawbacks highlighted by the state of the art in the field of pickling treatments of laminates made of stainless steel. The pickling treatments carried out with the method according to the invention have an improved efficacy, making it possible to obtain high- quality pickled surfaces with an overall duration of the treatment that is short.

Thanks to the particular efficacy of the electrolytic treatment and of the subsequent chemical pickling, the method according to the invention, also contemplating carrying out a final finishing and passivation step of the laminates in mixtures of nitric and hydrofluoric acid, determines a reduction in the environmental impact with respect to that typically observed in the processes of the prior art. The pickling carried out according to the present invention, indeed, is able to remove more than 80% by weight of the overall mass of scale and dechromatized steel, thus making it sufficient to have a nitric- hydrofluoride finishing step that is even very short and, consequently, significantly limiting the formation of pollutant residues to be disposed of (nitrate compounds, fluorinated compounds and Nox emissions) .

The method according to the invention also involves rather low consumption of raw materials, in particular hydrochloric acid. The acid used in the process, indeed, can be recycled between the different steps before being subjected to regeneration and, finally, be put back into the pickling cycle. The regeneration process of the hydrochloric acid also makes it possible to recover the metals (in the form of oxides) present in the spent pickling solutions. The improved efficacy of the treatment method according to the present invention, moreover, makes it possible to treat the laminates in plants of smaller dimensions with respect to the state of the art, being able to provide for shorter residency times of the laminate in contact with the pickling baths.

The following example embodiments are provided merely to illustrate the present invention and should not be taken to limit the scope of protection defined by the attached claims .

EXAMPLES

The method according to the present invention was applied to treat strips of stainless steel of different chemical composition, obtained through hot or cold rolling processes, possibly accompanied by annealing treatments .

HOT ROLLED LAMINATES

In the case of hot rolled laminates, the production and treatment plant comprised the following sections :

continuous annealing section, of overall length equal to 90 m, capable of heating the strip up to the temperature of 1120°C with a maximum productivity equal to about 133 t/h with strips of width up to 1550 mm;

cooling section, of overall length equal to 45 m, equipped with cooling means consisting of air blades combined with jets of nebulised water and jets of just water capable of cooling the laminate up to about 80°C;

scale-breaker roller for tension levelling the strip with elongation up to 1% of its original length;

sandblasting section consisting of 3 cabins equipped with 4 turbines each capable of homogeneously projecting, on both surfaces of the strip, spherical shots of type S110 at speed comprised between 50 m/s and 80 m/s, with flow rate of shots for each turbine comprised between 500 and 1300 kg/min;

chemical descaling section consisting of 2 immersion tanks of length equal to 15 m each;

electrolytic pickling section consisting of 2 tanks of length equal to 16 m each, each tank being equipped with a series of electrodes arranged above and below the strip, in a position substantially parallel to its surface, the overall length of which (taken as the sum of the lengths Le of all of the electrodes arranged on the same side with respect to the laminate) is equal to 15 m; the electrodes are connected to 4 transformers capable of delivering a current up to 110 kA; the electrolytic section is made according to what is described in document WO 2011/039596 and has the following geometric parameters (with reference to figure 2 ) :

- Le = 1.5 m;

- Lg = 0.4 m;

- Ld = 0.04 m

- Lb = 0.06 m

- Lx = 0.5 m;

intermediate washing and brushing section equipped with high pressure pumps up to 100 bar;

chemical pickling section in hydrochloric acid consisting of 2 tanks of length equal to 10 m each where the strip is immersed in the solution that is continuously renewed through a recirculation system having an overall flow rate equal to 800 m³/h;

intermediate washing and brushing section;

chemical pickling/passivation finishing section in mixed acids (nitric acid and hydrofluoric acid) consisting of 1 tank of length equal to 10 m;

washing, final brushing and drying section equipped with high pressure pumps and non-abrasive brushes .

The line is equipped with a regeneration system of the acids based on a roasting system that allows the recovery of the hydrochloric acid and of the dissolved metals, in the form of oxides, with yield of over 90% (yield calculated with respect to the original content of chloride ions) .

The strips of different dimensions were processed in the aforementioned plant at the speed given in table 3.

LAMINATES 1-4

Four different laminates made from stainless steel

(AISI 304 (LI), AISI 430 (L2), AISI 441 (L3) and AISI 409 (L4)) obtained through hot-rolling and subsequent annealing were subjected to a pickling treatment according to the present invention comprising the following steps in succession: chemical descaling, electrolytic pickling, chemical pickling and nitric- hydrofluoride finishing.

The concentrations of the different species in the aqueous solutions and the operative conditions adopted in each step of the process together with the dimensional parameters of the laminates L1-L4 are given in Table 3. The efficacy of the method was evaluated by calculating the percentage amount of metal removed from the laminate and/or dissolved during each step with respect to the amount of metal removed and/or dissolved overall. From the data given in Table 1 it can be seen that for all of the laminates a finishing treatment lasting 10 seconds proved sufficient to obtain a uniformly passivated surface. Such a treatment generated a significantly small amount of pollutant products to be disposed of (muds containing nitrate compounds, fluorinated compounds and NO_x emissions), the amount of metal removed and/or dissolved in the finishing step being equal to or less than 15% by weight of the mass of steel removed and/or dissolved overall in the entire treatment (chemical descaling, electrolytic pickling, chemical pickling and nitric- hydrofluoride finishing) . By comparison, in the pickling processes of the prior art, the amount of metal removed and/or dissolved in the nitric and/or hydrofluoric acid baths can reach 80% by weight of the amount pickled overall.

Table 1

descaling

Electrochemical 30 25 25 30 pickling AC

chemical pickling 25 25 25 20

Finishing 5 10 15 15

The method according to the invention, applied to the hot rolled laminates, has resulted in a significant reduction of the polluting substances to be treated.

COLD ROLLED LAMINATES

In the case of cold rolled laminates, the production and treatment plant comprised the following sections :

continuous annealing section, of overall length equal to 120 m, capable of heating the strip up to the temperature of 1120°C with a maximum productivity equal to about 133 t/h with strips of width up to 1550 mm;

cooling section, of overall length equal to 65 m, equipped with cooling means consisting of air blades combined with jets of water that intervene on the strip when it reaches temperatures below 200°C up to a temperature of about 70°C;

electrolytic pickling section comprising 1 tank of length equal to 55 m to carry out the treatment in DC and AC current; for the treatment in DC current the tank is equipped with a series of horizontal electrodes arranged above and below the strip, in a position substantially parallel to its surface; the overall length of the electrodes that anodically polarize the strip for the DC treatment (taken as the sum of the lengths of all of the electrodes arranged on the same side with respect to the laminate that anodically polarize the strip) is equal to 24 m; the electrodes are connected to 8 current rectifiers capable of delivering a DC current up to 10 kA (max 40 V) each; for the AC treatment, the tank is equipped with a series of horizontal electrodes, arranged above and below the strip, in a position substantially parallel to its surface, the overall length of which (taken as the sum of the lengths L_e of the electrodes arranged on the same side with respect to the laminate) is equal to 9 m; the electrodes are connected to 3 monophase transformers capable of delivering an AC current up to 10 kA (max 45 V) each; the electrolytic section for the AC treatment is made according to what is described in document WO 2011/039596 and has the following geometric parameters (with reference to figure 2) :

Le = 1.5 m,

Lg = 0.2 m,

- Ld = 0.05

- Lb = 0.05 m

Lx = 0.5 m;

chemical pickling section consisting of 3 tanks of length equal to 10 m each where the strip is immersed in the solution that is continuously renewed through a recirculation system having an overall flow rate equal to 800 m³/h.

The strips of different dimensions were processed in the aforementioned plant at the speeds given in table 4.

LAMINATES 5-7

Three different laminates made from stainless steel (AISI 304 (L5) , AISI 441 (L6), and AISI 409 (L7)) obtained through cold-rolling and subsequent annealing were subjected to a pickling treatment according to the present invention comprising the following steps in succession: electrolytic pickling, chemical pickling and finishing in nitric acid or in nitric-hydrofluoride mixture .

The concentrations of the different species in the aqueous solutions and the operating conditions adopted in each step of the process together with the dimensional parameters of the laminates L5-L7 are given in Table 4.

The solutions of hydrochloric acid used in each step also contained l-octin-3-ol and N-dodecyl- pyridinium chloride as corrosion inhibitors.

The efficacy of the method was evaluated by calculating the percentage amount of metal removed from the laminate and/or dissolved during each step with respect to the amount of metal removed and/or dissolved overall. For all of the laminates a finishing treatment lasting 10 seconds proved sufficient to obtain an evenly passivated surface. Such a treatment generated a significantly small amount of pollutant products to be disposed of (muds containing nitrate compounds, fluorinated compounds and NOx emissions), since the amount of metal removed and/or dissolved in the finishing step is equal to or less than 25% by weight of the mass of steel removed and/or dissolved overall in the entire treatment. By comparison, in the pickling processes of the prior art, the amount of metal removed and/or dissolved in the nitric and/or hydrofluoric acid baths can reach 80% by weight of the amount pickled overall .

The method according to the invention, applied to cold-rolled laminates, has achieved a significant reduction of the pollutant substances to be treated.

Moreover, the spent pickling solution was regenerated with the auxiliary regeneration plant obtaining a recovery of the hydrochloric acid used equal to 90% thereof.

Table 2

chemical 25 20 20 pickling

Finishing 25 20 20

Table 3

Free HF (g/l) 30 5 10 absent

FeF3 (g/l) 60 60 40 40

Temperature

ro 60 40 45 40

Note: HR = hot rolling

CR = cold rolling

Table 4

Note: HR = hot rolling

CR = cold rolling

Claims

1. Method for treating in continuous the surface of a laminate made of stainless steel comprising at least the following steps:

(a) subjecting said laminate to an electrolytic pickling treatment in alternating current in an aqueous electrolytic solution of HC1 comprising at least Fe³⁺ ions, the concentration of HC1 being comprised in the range 0.5-200 g/l and the concentration of Fe³⁺ ions being comprised in the range 5-60 g/l;

(b) subjecting said electrolytically pickled laminate to a chemical pickling treatment in an aqueous solution of HC1 comprising at least Fe³⁺ ions (chemical pickling solution) , the concentration of HC1 being comprised in the range 0.5-200 g/l and the concentration of Fe³⁺ ions being comprised in the range 5-60 g/l;

(c) subjecting said chemically pickled laminate to a surface finishing and a surface passivation treatment.

2. Method according to the preceding claim wherein, prior to said step (a) , said laminate is subjected to a chemical descaling step (a') in an aqueous solution of HC1 comprising at least Fe³⁺ ions (descaling solution) , the concentration of HC1 being comprised in the range 30-250 g/l and the concentration of Fe³⁺ ions being comprised in the range 5-60 g/l.

3. Method according to claim 1 or 2 wherein said aqueous electrolytic solution, said chemical pickling solution and said descaling solution comprise Fe²⁺ ions in an amount corresponding to a ratio Fe³⁺/Fe²⁺ equal to or greater than 0.1 and preferably less than or equal to 1.

4. Method according to one or more of the preceding claims wherein the chemical pickling solution used in said step (b) is at least partly recirculated to said step (a) , where it is used as electrolytic solution .

5. Method according to one or more of the preceding claims wherein the chemical pickling solution used in said step (b) is at least partly recirculated to said step (a' ) , where it is used as descaling solution .

6. Method according to one or more of the preceding claims wherein the electrolytic solution used in said step (a) is at least partly fed to said step (b) where it is used as pickling solution.

7. Method according to one or more of the preceding claims wherein said electrolytic pickling comprises at least one first stage and one second stage, said first stage being carried out in continuous current and said second stage being carried out in alternating current.

8. Method according to one or more of the preceding claims wherein said electrolytic pickling in alternating current is performed by applying an alternating current having a frequency (F) variable from 5 Hz to 150 Hz, the density of current being variable from 5 A/dm² to 50 A/dm², the duration of the treatment being variable from 2 to 60 s.

9. Method according to claim 7 or 8 wherein said electrolytic pickling in continuous current is carried out by applying a continuous current having a density variable from 4 A/dm² to 14 A/dm², the duration of the anodic polarization treatment of the strip being between 2 and 20 s.

10. Method according to one or more of the preceding claims wherein said step (c) is carried out by putting said laminate into contact with an aqueous solution (finishing and passivation solution) having an oxidation-reduction chemical potential (measured with respect to a reference electrode of the Ag/AgCl type) comprised between 100 and 800 mV, preferably between 200 mV and 600 mV.

11. Method according to the preceding claim wherein said finishing solution is an aqueous solution of nitric acid comprising Fe³⁺ ions and optionally hydrofluoric acid.

12. Method according to one or more of the claims

1-10 wherein said finishing solution is an aqueous solution of sulfuric acid and hydrofluoric acid comprising Fe³⁺ ions and not containing nitric acid.

13. Method according to one or more of the preceding claims wherein one or more of said electrolytic solution, chemical pickling solution and descaling solution is subjected to a treatment for the recovery of hydrochloric acid and/or metal oxides.

14. Method according to one or more of the preceding claims wherein said step (a) is carried out by making said laminate (11) pass in continuous between at least two electrodes (150, 170) arranged in a direction substantially parallel to the movement direction of the laminate (11) and having a length L_e measured along said direction, said electrodes (150, 170) facing the opposite faces of said laminate (11) and being arranged in a position facing each other, and wherein said step (a) is carried out by applying an alternating current having a frequency f variable from 5 Hz to 150 Hz, said frequency f being correlated to the forward velocity v of the laminate and to said length L_e of said electrodes (150, 170) by the relation f ≥ (A *v) /L_e , where A is a rational number greater than 1.

15. Method according to one or more of the preceding claims wherein said step (a) is carried out by making said laminate (11) pass in continuous between at least two electrodes (150, 170), each of said electrodes (150, 170) comprising a plurality of elements (160, 180) arranged side by side to each other in a substantially parallel manner and arranged transversally to the movement direction of the laminate, each of said elements (160, 180) having a length L_b measured along said movement direction of the laminate, said electrodes (150, 170) facing the opposite faces of said laminate (11) and being arranged in a position facing each other, and wherein said step (a) is carried out by applying an alternating current having a frequency f variable from 5 Hz to 150 Hz, said frequency f being correlated to said velocity v and to said length L_b by the relation f ≥ (Α' -v) /L_b, where A' is a rational number greater than 1.

16. Method according to one or more of the preceding claims wherein said laminate (11) is made to pass between at least one first and one second pair (14) of electrodes (15, 17) immersed in said electrolytic solution and arranged in a direction substantially parallel to the movement direction of the laminate (11), between said pairs of electrodes (14) there being interposed at least one separator element made of an insulating material, in the form of an immersing roller or of a static separator, suitable for reducing the formation of dispersed currents between adjacent electrodes having opposite polarization.