ITRM960849A1 - METHOD FOR PICKLING METAL ALLOY PRODUCTS CONTAINING IRON AND TITANIUM AND ITS ALLOYS - Google Patents

Abstract

A method for pickling products made of metal alloys containing iron, and products made of titanium and alloys thereof, in the absence of nitric acid as an oxidising agent, characterised by the fact that the product to be pickled is submerged in the anolyte (as the pickling solution) of an electrolytic cell in which the anolyte is made up of an aqueous solution of sulphuric acid, of hydrofluoric acid and optionally of phosphoric acid and hydrochloric acid, and with the catholyte made up of an aqueous solution of sulphuric acid and the oxidising agent in the pickling solution being the ferric ion, or the ions titanium(III) and titanium(IV).

Description

Description of the industrial invention entitled: "METHOD FOR PICKLING PRODUCTS IN METALLIC ALLOY CONTAINING IRON AND TITANIUM AND ITS ALLOYS"

The present invention relates to a method for pickling iron-containing metal alloy products and, more precisely, to a stainless steel pickling process characterized by avoiding the use of nitric acid as an oxidizing agent.

The present invention is also applicable for the pickling of titanium and its alloys.

The use of pickling processes for steel industry products is known in order to eliminate the scale of thermal oxides formed during high temperature heat treatments, to eliminate the depleted chromium layer (decromized layer) below the scale itself and to allow an efficient final passivation of the surface.

In order to carry out an efficient pickling process for stainless steels and titanium, a mixture of nitric (HN03) and hydrofluoric (HF) acids is normally used, at a temperature generally ranging between 60 and 75 ° C.

However, the use of nitric acid entails significant environmental control problems, due to the following reasons:

- considerable presence of nitrogen oxides (NOx) in the vapors overlying the pickling bath that develop from the bath itself;

formation of exhausted solutions that generate sludge rich in nitrates that go to landfills. - difficulties in complying with current regulations in the field of waste disposal containing nitrates.

In order to overcome these difficulties, various methodologies have been developed which envisage reducing or eliminating the use of nitric acid in chemical pickling processes.

These methods are based on the use of various oxidants added as a reactive to the bath, among which, for example, permanganates, persulfates, ferric chloride, hydrogen peroxide (hydrogen peroxide , H202).

Currently, the use of hydrogen peroxide is the one that has given the best results on an industrial level. In the pickling bath, however, hydrofluoric acid is used, in combination with different mineral acids (generally mixtures of acids) including: sulfuric, hydrochloric, phosphoric acid.

In particular, a first type of method provides for the reduction of the quantity of nitrogen oxides N0X that develop from the bath. The above method is generally based on the use of hydrogen peroxide, which allows not only to possibly reduce the total amount of nitric acid in the bath necessary for the process, but also to oxidize the nitrogen oxides to the highest oxidation state, which therefore they tend not to leave the bathroom, but to remain there as nitric acid.

However, this method has the drawback of only partially solving the problems described above, as it only allows a reduction (and not elimination) of the NOX content in the vapors and leaves the problem of nitrates in the exhausted baths unchanged. In view of these only partial improvements, more or less complex variations of the basic pickling process (mixtures of HF / HN03) are required.

There is a second method which aims to completely eliminate the presence of nitric acid, using, in addition to hydrofluoric acid whose presence remains practically unchanged in the processes examined, ferric sulphate, hydrogen peroxide, sulfuric acid and planning to control the intake hydrogen peroxide by means of redox potential measurements of the solution However, this method also has the following drawbacks:

a) complexity of management, derived from the complexity of the analytical control of hydrogen peroxide in the baths (this reagent is in fact unstable and different stabilizers are used) and of the different reagents (both hydrogen peroxide and hydrofluoric acid interact with metals , essentially chromium and iron which enter into solution during the pickling process) with loss of efficiency;

b) complexity to keep the dissolution kinetics (these strongly dependent on the redox potential of the medium) in the due values; and c) high management rates, also deriving from the high costs of the reagents, in particular stabilized hydrogen peroxide.

The aim, therefore, of the present invention is to solve the aforementioned drawbacks by providing a method for pickling products in metal alloy containing iron, and products in titanium and its alloys, in the absence of nitric acid as an oxidizing agent, characterized by the fact of immersing the product to be pickled in the anolyte (as a pickling solution) of an electrolytic cell with the anolyte consisting of an aqueous solution of sulfuric acid, hydrofluoric acid and possibly phosphoric acid and hydrochloric acid, and with the catholyte consisting of an aqueous solution of sulfuric acid, the oxidizing agent of the pickling solution being the ferric ion, or the titanium (III) and titanium (IV) ions formed at the anode by oxidation of the ferrous ion or titanium ion (II), coming from the dissolution of the layers surface of the product to be pickled.

In the case of titanium and its alloys, the oxidizing agent is the Ti <3+> and Ti <4+> ions.

The electrode potential is preferably between 771 and 1229 mV SHE (standard hydrogen electrode) in the case of alloys containing iron.

The electrode potential is preferably comprised between -368 and 1200 mV SHE in the case of titanium and its alloys.

The catholyte consists of an aqueous solution of sulfuric acid. The catholyte is also continuously introduced into the pickling solution, as a replenishment of the H2S04 consumed during the pickling reaction.

According to the invention, the method provides that the anodic reaction in the cell is controlled potentiostatically and / or galvanostatically.

Furthermore, the method according to the invention provides that the pickling operations are carried out in a continuous way, by means of circulation of the anolyte, or in a discontinuous way.

The pickling bath has a preferred temperature between 45 and 85 ° C.

The anolyte is an aqueous solution of sulfuric acid, hydrofluoric acid and possibly hydrochloric and phosphoric acid, with the following composition expressed in percentages by weight:

- Free HC1 from 0 to 50 g / 1

- H3P04 free from 0 to 200 g / 1

- H2S04 free from 50 to 200 g / 1

- Free HF from 5 to 50 g / 1

- Fetot in solution ≥ 50 g / 1

Furthermore, the iron-containing products to which the method according to the present invention is applicable are selected from the group comprising:

- stainless steels, rolled or in any case hot and / or cold worked, in particular, austenitic, ferritic, duplex and super stainless steels, -Superalloys with Ni base.

Furthermore, the titanium-containing products to which the method according to the present invention is applicable are selected from the group comprising:

- CP Titanium (commercial purity) of the different grades;

- Titanium alloys.

The present invention will be better illustrated hereinafter by a detailed description of a preferred embodiment thereof, given by way of non-limiting example, with reference to the attached drawings, in which:

Figure 1 schematically shows a preferred embodiment of the present invention; and Figure 2 shows a diagram of the variation in weight of the metal product as a function of the concentrations of acids and ferric ions of the pickling bath of the present invention.

The method object of the present invention, for the replacement of nitric acid is based on the concept that this acid performs two fundamental actions:

a) increase in the total acidity of the bath; and

b) raising of the redox potential of the medium, in relation to its oxidizing properties.

The overall action of nitric acid can therefore be obtained with different reagents capable of guaranteeing, even separately, these two specific actions.

Finally, as regards the use of oxidants, the method of the present invention does not resort to direct additions of reagents in the bath, but uses an electrochemical treatment of the solution which generates the desired oxidant directly in the pickling bath.

Specifically, the method object of the present invention is based on the use of a nitric acid-free pickling bath and through which only the ferric ion Fe <3+> (or, in the case of titanium, the Ti <3+> and Ti <4+> ions). The ferric ion is not added in the form of a reagent, but is directly obtained in the pickling solution by anodic oxidation in the electrolytic cell of the Fe <2+> ions that are generated during the pickling process itself (dissolution of the steel). In the case of titanium, on the other hand, the anodic oxidation of Ti <2+> ions to Ti <3+> and Ti <4+> ions is carried out in the cell.

Referring now to Figure 1 and according to the method of the invention, an electrochemical cell of the type with a perm-selective porous septum is used and in which one operates by controlling the electrode potential (anode). The method according to the invention can also be applied to galvanostatic control solutions. The anolyte of the cell (in which the reaction of generation of Fe <3+> ions takes place) is made up of the same pickling solution, while a sulfuric acid solution is used as catholyte destined to be sent to the pickling bath.

The mineral acids used are mixtures of hydrofluoric, sulfuric, hydrochloric and phosphoric acids.

According to the invention, the method offers a self-balancing system, ie capable of controlling the kinetics of the process and the final quality of the product. The pickling kinetics are in fact directly controllable by the temporal quantity of Fe <3+> ions generated in the cell.

The constancy and / or control of the concentration of Fe <3+> ions in the pickling bath (easily obtainable by setting and controlling the operating cell parameters) allows a close control of the most critical process parameter (i.e. the value redox potential of the system) with obvious advantages also on the final quality of the product.

The use of anodic oxidation in the cell to generate the oxidant species determines a significant economy of exercises, due to the higher cost of the oxidizing reagents possibly added to the bath, according to normal technologies.

Finally, the choice of the Fe <3+> ion as oxidant does not involve stability problems in the bath that some reagents show (in particular hydrogen peroxide requires the use of expensive stabilizers).

The method object of the present invention is based on the following assumptions.

a) For the replacement of nitric acid it should be borne in mind that this acid performs two fundamental actions:

- increase in the total acidity of the bath;

- raising of the redox potential of the medium, in relation to its oxidizing properties.

b) The overall action of nitric acid can therefore be obtained with different reagents capable of guaranteeing, even separately, the two specific actions of point (a).

The reagents selected for the control of total acidity, which will be specified below, are therefore mineral acids and oxidants.

c) Finally, as regards the use of oxidants, the method of the invention does not resort to direct additions of reagents in the bath, but uses an electrochemical treatment of the solution which generates Fe <3+> ions directly in the pickling bath such as desired oxidizing species (in the case of titanium ions Ti <3+> and Ti <4+>.

For a correct understanding and interpretation of the basic principles that led to the definition of the solutions adopted for the application of the method according to the present invention, the following considerations were made:

a) pickling (both according to traditional processes in HF / HN03 solutions and according to H202-based processes) must necessarily include the presence of an oxidant that maintains the redox potential at the appropriate levels;

b) these oxidants involve the passage of ferrous ions from Fe <2+> to Fe <3+> (or of Ti <2+> to Ti <3+> and Ti <4+>), therefore the pickling baths contain normally only this oxidized species Fe <3+> (or in any case a strong concentration thereof if the oxidizing species is added in default);

c) the same ferric ions Fe <3+> are known to oxidize with respect to the steel that must be pickled (in fact, according to the reversible electrochemical potential scale we have: Erev = -447 mV SHE for the Fe / Fe <2+ pair >; Erev = 771 mV SHE for the Fe <2+> / Fe <3 +> pair); similarly, the high valence ions of titanium (Ti <3+> and Ti <4+>) are oxidizing with respect to titanium from pickling, as we have Erev <=> -1630 mV SHE for the pair Ti / Ti <2+> and Erev = -55 mV SHE and Erev = -368 mV SHE for the pairs, respectively, Ti <4 +> / Ti <3+> and Ti <3 +> / Ti <2+>;

d) from the above points a) b) and c) it emerges the possibility of directly using the species Fe <3+> (or Ti <4+> and Ti <3+>) as an oxidizing reagent in the solution to replace the acid nitric;

e) the total acidity of the solution, in the absence of nitric acid, can be reconstituted with lower cost mineral acids, such as HC1, H2S04 and H3P04, considered both individually and in mixture (as per point f);

f) hydrochloric acid performs in part the same functions as hydrofluoric acid (it is a strong depassivant, or activator) but at lower costs. Furthermore, unlike hydrofluoric acid, it is not complexed by iron ions (both 2+ and 3+) and / or titanium (in particular Ti <4+>) and therefore its efficiency remains constant;

g) total acidity cannot be restored with hydrochloric acid alone, as the Cl <'> ion would be too aggressive towards the surface of the steel, preventing or excessively complicating the normal final passivation treatment; the ion S04 <=> coming from sulfuric acid, on the contrary, has no negative effects on the passivation properties of the surface (indeed it is a corrosion inhibitor) therefore balanced mixtures of the two acids (HC1 and H2S04) help to improve the control of the working electrochemical potentials and dissolution kinetics; when an excessive aggressiveness of the bath is not required (depending on the type of material processed, the characteristics and speed of the selected line, particular treatments or other), a progressively lower concentration of HC1, even zero, can be used with only HF and H2S04, or alternatively mixtures of HF, H2S04 and H3P04. Phosphoric acid acts according to a mechanism similar to sulfuric acid.

On the basis of the above, the pickling bath according to the method of the present invention can conveniently use the following reagents:

acids: HF and mixtures of HC1 / H2S04 / H3P04 with variable concentrations, also providing zero concentration of HCl and / or H3P04;

- oxidants: ferric ion Fe <3+> (for titanium Ti <4+> and Ti <3+>).

By virtue of the particular oxidant chosen (Fe <3+>), the method in question is applied through an electrochemical treatment of the solution, with which the formation in situ and control at the right concentration levels of the same oxidant species is obtained directly.

Below are defined the principles and criteria for making an electrochemical cell through which the method of the present invention can be applied and referring to figure 1 in which, for simplicity, it refers to the case of pickling of metal alloys containing iron. , being similar the case of Titanium and its alloys.

a) Anolyte: the same pickling solution is used, circulated continuously (but a discontinuous treatment can also be provided) from the tank by pumping;

b) Anodic reaction: Fe <2+> => Fe <3+>; by means of this reaction the concentration of the oxidizing species Fe <3+> is kept constant in the pickling solution. The amount of Fe <3+> ions that must be generated over time is equivalent to double the amount of iron that goes into solution during pickling. The pickling mechanism takes place in fact according to the following reactions:

(1) Fe => Fe <2+> + 2e; semi-reaction of primary oxidation of metallic iron:

(2} 2 Fe <3 +> + 2e => 2Fe <2+>; reduction half-reaction.

The resulting reaction (in the bath), that is, the sum of the two half-reactions (1) (2) is:

(3) Fe 2 Fe <3+> => 3 Fe <2+>;

In the electrochemical cell, the reaction is carried out:

(4) 2 Fe <2+> => 2 Fe <3+> + 2e;

The resulting overall reaction (1) (2) (4) [or (3) (4), i.e. reaction in the bath in an electrochemical cell] is:

(5) Fe => Fe <2+> + 2e, reaction equivalent to (1), as expected. In fact, the Fe <3+> ion does not intervene directly in the overall process, except as an intermediate oxidizing species which allows it to unfold at the desired (high) redox potentials. The oxidizing capacity of the system is instead guaranteed by the passage of the anode current in the cell according to reaction (2), of which the Fe <3+> ion is a medium.

Ultimately, the overall pickling reaction (oxidation of the metal), i.e. (5), requires a net passage of anode current that takes place in the cell (anode compartment) since according to the present method no reactants such as oxidants are added.

c) Cathodic reaction: for the reaction to the cathode compartment of the cell it has been found that the most practical solution consists in using a solution of sulfuric acid as the catholyte, in relation to the fact that the pickling process in question foresees the use of sulfuric acid in basin.

The reaction at the cathode is therefore:

(6) H2S04 => 2 H <+> + S04 <=>; dissociation of sulfuric acid

(7) 2H <+> + 2 e => H2; cathodic half-reaction The resulting reaction (6) (7) is:

(8) H2S04 + 2e => S04 <=> + H2.

The catholyte is sent into the tank (continuously or discontinuously) in an hourly quantity equal to the sulfuric ion content necessary for the combination with the Fe <2+> ion generated by (5), according to the following reaction:

(9) Fe <2+> + S04 <=> => FeS04;

Ultimately, the overall reaction (5) (8) of the process is given by:

(10) Fe H2S04 => Fe <2+> + S04 <=> + H2

d) Anodic control: as regards the control of the passage of current in the electrolytic cell, two alternatives are possible: i) Potentiostatic control.

It operates with a minimum electrode potential that allows the oxidation reaction (4); as far as the maximum value is concerned, there are no precise limits to be respected, on the contrary, operating with high values means increasing the specific currents (i.e. per unit of surface) of the anode, with a reduction in the overall dimensions, however it is preferable to choose a maximum value that does not allow in particular the development of oxygen, according to:

(11) 02 + 4H <+> + 4e => 2H20 (Erev = 1229 mV SHE) The co-presence of reaction (11) with (2) would in fact decrease the efficiency of the cell, carrying out the unwanted reaction of hydrolysis of water with development of oxygen. The chosen potential E lies in the range between 771 <E <1229 mV SHE. At these potential values, however, there is not even the formation of chromium at valence 6 according to the (simplified) reaction:

(12) Cr <3+> => Cr <6+>; (reversible potential Erev =

1350 mV SHE).

In practice, it may be useful to position oneself at values even relatively higher than 1229 mV, exploiting the fact that the oxygen development reaction occurs with a certain overvoltage, and the relative kinetics are in any case negligible if the potential is not sufficiently higher than the indicated limit. ,

ii) Galvanostatic control.

This control is simpler (cheaper) to be carried out in the plant, however the advantages described in point (i) could be lost, in particular there is a risk of the possible development of oxygen. However, an exact experimental knowledge of the characteristics of the cell and an appropriate control of the current allow to obtain the same practical results, in particular to be able to choose suitable currents (and cell geometry) that allow to obtain the electrode (anode) the desired electrochemical potential.

e) Porous septum. Since the anolytic and catholytic solutions are completely different (the first pickling solution, the second sulfuric acid solution) and since in particular, in order not to decrease the efficiency of the cell, the transmigration of ferric ions Fe <3 is not desired +> which form at the anolyte towards the catholyte, the cell is equipped with a suitable perm-selective porous septum. Various commercial baffles can be used, which differ in efficiency, operating temperature, duration, size.

The electrochemical cell in question tested in a pilot plant has provided the following performances, which are reported here purely by way of example (the margins for possible improvements are still wide):

- current efficiency:> 95%

- cell potential (Δν at the terminals) = 3V

- specific power = 6W / dm <2>

- anode current density ≡ 2A / dm <2>

- consumption per mole Fe <3+> produced ≡ 0.081 Kwh

In addition to what has been said, the current value, once the cell characteristics are known, directly expresses the rate of formation of the Fe <3+> ion (reaction (4)). This allows to keep its concentration constant in the bath, knowing the characteristic speed of the pickling process.

The system is self-balancing, since the dissolution reaction (1), i.e. pickling, could not proceed with kinetics higher than the cell reaction (5), as the transfer of electrons for the anodic semi-reaction would be inadequate. of pickling itself. Similarly, if the dissolution according to (5) decreases its kinetics for some reason, a progressive increase in the concentration of Fe <3+> ions would occur in the bath (since the contribution of reaction (5) is constant) with corresponding raising of the redox potential of the solution, therefore of the overall pickling kinetics.

Ultimately, the proposed pickling process overall follows the reaction rate (5), which is easily controlled through the cell parameters.

As an example, Figure 2 shows how the dissolution kinetics of AISI 409 LI stainless steel increases as the Fe <3+> content increases.

It should be noted here that the control of the system can in any case be automated for specific needs if, for example, the tanks receive material with different (or variable) characteristics. For this purpose, the amount of anode current (or anode potential) can be easily adjusted to decrease or accelerate the production of Fe <3+>, based on its concentration actually present in the bath.

As regards more specifically the application of the method of the present invention to the pickling of titanium and its alloys, the following is specified:

titanium can be pickled under the same conditions as stainless steel, i.e. classic baths in HN03 / HF solutions or in baths of the type without nitric acid, after a (slight) variation in the composition of the acids and in the temperature, due to the greater reactivity of this element (generally milder concentrations and temperatures around 40-50 ° C are adopted);

the method of the invention described here is conveniently applied also to the pickling of titanium and its alloys, since both the elementary pickling mechanisms involved, the electrochemical cell and the process described are completely similar; in particular, for an exact understanding of what has been said, just keep in mind that in the case of titanium the basic pickling reaction (1) is replaced by:

(13) Ti → Ti <2+> + 2e; (Erev = -1630 mV SHE)

The role assumed by the Fe <3+> ion (reaction 2) is replaced by the Ti <3+> and Ti <4+> ions; (Erev = -55 mV for the pair TÌ3 + / TÌ2 +; Erev = -368 mV SHE for the pair TÌ3 + / TÌ4 +);

Oxidation reactions take place in the electrochemical cell:

Ti <2+> → Ti <3+>

Ti <3+> → Ti <4+>.

The cell anode potential in this case is chosen in the range from -368 mV SHE (oxidation of Ti <2+> to Ti <4+>) to 1229 mV SHE (oxygen development).

A different methodology based on the present invention to pickle titanium and its alloys, uses in practice the same tanks and acid solutions used in the pickling of stainless. This method is very useful because at the major producers the titanium coils are processed in the same lines as the stainless ones.

In this case, in the tanks with solutions containing H2S04 (or other mineral acids) HF and iron ions, the titanium is decapped according to (13) (oxidation reaction), the oxidizing agent being the Fe <3+> ion, which it is reduced according to the reaction (2) Fe <3+> + and -> Fe <2+>, while in the electrochemical cell the concentration of Fe <3+> is restored by means of (4) Fe <2+> → Fe < 3+> + e.

Other embodiment schemes different from that shown in Figure 1 are however possible, among which in particular the direct passage of the strip into the anolyte of the cell (which constitutes the pickling solution). In this case, the pickling tank and the cell would physically coincide or, similarly, the cell would be built inside the tank. Furthermore, as far as the catholyte is concerned, it is not necessary to use sulfuric acid solutions, since it is possible to choose different electrolytes (for example, HCL or HC1 / H2S04 mixtures). Even the introduction of the catholyte into the tank may not be carried out, in particular if electrolytes that cannot be used in the pickling tank are chosen.

All these schemes are alternative to the one presented as preferred in Fig. 1 and are intended as part of the present invention.

PICKLING EXAMPLE

An example of pickling of a hot rolled and shot blasted AISI 304 coil is provided below, according to the method of the present invention.

The starting pickling solution consists of:

Free H2S04 = 150 g / 1

Free HF = 30 g / 1

Total Fe in solution ≥ 50 g / 1.

The solution is started to circulate in the electrolytic cell until the optimal value of Fe <3+> equal to ≥ 30 g / 1 is obtained;

the temperature is adjusted to 65 ° C;

pickling starts (sending the strip to the pickling tank) with the desired line speed (this parameter depends in the continuous annealing and pickling lines essentially on the annealing parameters, depending on the characteristics of the furnace and the thickness of the strip treated) .

The line speed, the thickness and the width of the treated strip obviously define the hourly production, expressed in t / h pickled strip. For example, for a speed of 18 meters / minute, strip thickness 3.2 mm and width 1500 mm, a production (of pickled strip) of about 40 t / h is obtained.

With reference to a 3.2 mm thick strip (average thickness typical of hot rolled products) the pickling is carried out continuously in the following conditions:

- the pickling solution is circulated in an electrochemical cell with a production of Fe <3+> ions ≥ 48 moles / t of pickled strip (according to the reaction Fe <2+> → Fe <3+>);

- consumption (therefore reintegration) of acids equal to:

H2S04> 1.8 kg / t of pickled strip, e

HF ≥ 1.4 kg / t of pickled tape.

Claims (17)

CLAIMS 1. Method for pickling products in metal alloy containing iron, and products in titanium and its alloys, in the absence of nitric acid as an oxidizing agent, characterized by immersing the product to be pickled in the anolyte (as a pickling solution) of an electrolytic cell with the anolyte consisting of an aqueous solution of sulfuric acid, hydrofluoric acid and possibly phosphoric acid and hydrochloric acid, and with the catholyte consisting of an aqueous solution of sulfuric acid, the oxidizing agent of the pickling solution being the ferric ion, or the titanium (III) and titanium (IV) ions formed at the anode by oxidation of the ferrous ion or titanium (II) ion, deriving from the dissolution of the surface layers of the product to be pickled. 2. Method for pickling iron-containing metal alloy products according to claim 1, wherein the electrochemical working potential of the cell anode is comprised between 771 and 1229 mV SHE. 3. Method for pickling titanium and its alloys products according to claim 1, wherein the electrochemical working potential of the anode of the is comprised between -2000 and 1229 mV SHE. 4. Method for pickling titanium and its alloys products according to claim 3, wherein the electrochemical working potential of the anode of the is comprised between -368 and 1229 mV SHE. 5. Method for pickling iron-containing metal alloy products and titanium and its alloys products according to claims 1, 2 and 3, in which the anodic reaction is controlled potentiostatically. 6. Method for pickling iron-containing metal alloy products and titanium and its alloys products according to claims 1, 2 and 3 in which the anodic reaction is galvanostatically controlled. 7. Method for pickling iron-containing metal alloy products, and titanium and its alloys products according to any one of the preceding claims, in which the pickling operations are carried out continuously by continuous circulation of the anolyte. 8. Method for pickling iron-containing metal alloy products and titanium and its alloys products according to claims 1 to 5, in which the pickling operations are carried out in a discontinuous manner. 9. Method for the pickling of products in metal alloy containing iron, and of products in titanium and its alloys, according to any one of the preceding claims, in which the catholyte of the electrolytic cell is constituted by a solution of sulfuric acid and is sent, in exit from the cathode compartment, into the anolyte. Method according to claim 6 or 7, wherein the catholytic solution is constituted by any electrolyte which is sent or not in the anolyte. Method according to any one of the preceding claims, in which the anolyte (pickling solution) is sent to an external pickling tank where the pickling operation takes place with immersion of the materials to be pickled in the anolyte. 12. Method for the pickling of products in metal alloy containing iron, and of products in titanium and its alloys according to any one of the preceding claims, in which the pickling bath has a temperature between 40 and 90 ° c. Method for pickling iron-containing metal alloy products according to any one of claims 1, 2 or 4-11, wherein the anolyte is an aqueous solution containing the following components: - Free HC1 from 0 to 50 g / 1 - H3P04 free from 0 to 200 g / 1 - H2S04 free from 50 to 250 g / 1 - HF free from 5 to 50 g / 1, e - Fetot in solution => 50 g / 1. Method for pickling titanium and its alloys products according to any one of claims 1 or 3-11, wherein the anolyte is an aqueous solution containing: - Free HC1 from 0 to 50 g / 1 - H3P04 free from 0 to 200 g / 1 - H2S04 free from 50 to 250 g / 1 - free HF from 5 to 50 g / 1; And - Fetot in solution> 50 g / 1 or, alternatively,. Titot in solution> 50 g / 1 Method for pickling iron-containing metal alloy products according to any one of claims 1, 2 or 4-12, wherein said iron-containing metal alloy products are selected from the group comprising: - stainless steels, rolled or in any case hot and / or cold worked, in particular, austenitic, ferritic, duplex and super-stainless steels; - Ni-based superalloys. 16. Method for pickling titanium and its alloys products according to any one of claims 1, 3-11 or 13, wherein said titanium and its alloys products are selected from the group comprising: - CP titanium (commercial purity) of different grades; - Ti-based alloys. 17. Method for the pickling of metal alloy products containing iron, and titanium and its alloys as previously described, exemplified and claimed.