EP1358367B1

EP1358367B1 - Continuous electrolytic pickling and descaling of carbon steel and stainless steel

Info

Publication number: EP1358367B1
Application number: EP01271468A
Authority: EP
Inventors: Susanna Centro Sviluppo Materiali S.p.A. RAMUNDO; A. Centro Sviluppo Materiali S.p.A. DULCETTI
Original assignee: Centro Sviluppo Materiali SpA
Current assignee: Centro Sviluppo Materiali SpA
Priority date: 2000-12-18
Filing date: 2001-12-18
Publication date: 2004-09-15
Anticipated expiration: 2021-12-18
Also published as: DE60105653D1; ATE276386T1; EP1358367A1; AU2002217449A1; CN1231615C; DE60105653T2; KR20030076589A; WO2002050344A1; CN1486373A; ES2232564T3

Abstract

Continuos electrolytic method in a neutral solution for the pickling and the descaling of carbon steels and stainless steels, in the presence of electrolysis current flow indirect effects, said current being AC or DC and having a frequency lower than 3 Hz, characterized in that the anodic treatment times and the cell currents are selected according to the formula: It=c+kI where: I is the current density crossing the cell; t is the anodic treatment time; c is the constant fraction of electric charge density outputted for the direct oxide change anodic reactions; and k is a time constant for the calculation of the fraction of electric charge density, proportional to the current density I (kI), outputted for the indirect anodic reactions linked to Oxygen development and to the consequent acidification at the steel/electrolytic solution interface (Carbon steels) or at the scale/electrolytic solution interface (stainless steels).

Description

The present invention refers to the continuous pickling of hot-rolled carbon steels strips by an electrolytic process in a neutral solution (pH ranging from 6.0 to 8.0).
The present invention further refers to the field of the continuous descaling of stainless steels strip for the removal of surface oxides formed by effect of thermal treatments, comprising hot-rolling and annealing.
The advantages of the neutral electrolytic pickling and descaling processes with respect to the conventional processes in acid baths are substantially the following: adoption of non-dangerous, non-harmful and non-polluting pickling baths; easy treating and reclaiming of the residues; and elevated surface quality of the pickled materials.
As it is known, the main anodic reactions which in theory can take place in a solution onto the oxidised surface of hot-rolled Carbon steels can be schematized as follows: Fe + 2H2O → Fe(OH)2 + 2H+ + 2e - H2O → 2H+ + ½O2 + 2e- Fe3O4 + 8H+ → 3Fe3+ + 4H2O + e- Fe → Fe3+ + 3e-
The first reaction (0) is merely viable at a low electrode potential and it is marginal as a pickling reaction; it becomes virtually negligible for current density (I) values exceeding a predetermined threshold value (I₀).
Therefore, for I>I₀ values, onto the surface of the oxide-covered steel the second (1) and the third (2) reactions take place. Reaction (1) acidifies the metal-scale interface, whereas reaction (2) changes the scale into a soluble compound, by virtue of the presence of the acidified interface. Hence, these two reactions (1) and (2) constitute the essential mechanism of the electrolytic pickling in a neutral solution.
At near-complete dissolution of the surface scale, the anodic oxidation of the underlying metal, according to the fourth reaction (3), begins to rise until reaching its equilibrium rate at the passive state, when all the scale has been removed from the surface (end of the pickling treatment). Therefore, under the electrolytic pickling mechanism defined by the second (1) and by the third (2) reaction the fourth reaction (3) is marginal.
Of course, in order to make the electric circuit, suitable auxiliary electrodes (also called counterelectrodes) are employed, onto which the cathodic reactions enabling the electric neutrality of the solution to be preserved take place.
During the electrolytic pickling process in a neutral solution, the abovereported anodic reactions take place under diffusion control. This means that the reaction ratios depend on the diffusion of the reactants and of the reaction products through the boundary layer (flow), which in turn is determined by the fluid dynamics onto the steel surface. Apparently, the increase in the burble of the solution at the interface can cause contrasting effects onto the attack rate of the scale, as it also increases the flow of Hydrogen ions (H⁺) which leaves the interface acidified [reaction (1)].
Moreover, it is known from Faraday's laws of electrolysis, pertaining to anodic dissolution as well as to cathodic deposition processes, that the quantity of a substance obtained (changed) at the electrodes is proportional to the quantity of electric charge passed through the electrolytic circuit. More particularly, the quantity of electric charge required to obtain (change) a given quantity of substance is constant: e.g., for one equivalent of any one substance one Faraday, i.e. 96.500 Coulomb, is required. The above is expressed by the following equation Q = Itot * t = constant where Q is the quantity of electric charge (in Coulombs, C), I_tot is the electric current applied (in Amperes, A) and t is the electrolysis time (in seconds, s).
This equation applies to any selection of I_tot or t, hence the same effect is attainable applying several different values of current I_tot for the corresponding electrolysis times.
As it is known, due to the high susceptibility to oxidation of Chromium, the surface scale of thermally treated stainless steels is remarkably enriched with Chromium oxide, which is very difficult to remove during the subsequent pickling treatment in an acid solution.
Usually, due to ecological reasons, in the pickling of stainless steels mixtures of strong inorganic acids, i.e. HNO₃/HF solutions, or, more recently, H₂SO₄/HF/H₂O₂ solutions, are used.
However, prior to the chemical pickling, in order to speed up the entire surface scale removal process in stainless steelmaking, descaling pre-treatments are carried out. The function of the descaling is that of modifying the scale in order to facilitate the subsequent removal thereof. The scale conditioning methods for hot-rolled stainless steel strips mainly use molten salt baths (thermochemical descaling) or electrolytic treatments.
A type of thermochemical process currently used for descaling provides the immersion in an oxidising molten salt bath which is capable of converting the Chromium oxides (or the mixed Chromium/Iron oxides) into soluble hexavalent Chromium compounds.
Electrolytic descaling is a common industrial process which can be carried out in acid electrolytes as well as in neutral electrolytes, the anion usually being the sulfate ion. Particularly attractive is the process of electrolytic descaling in a neutral solution. In fact, this type of descaling is effective in dissolving the scale and the removed scale is directly separated from the solution by precipitation, with no need of a residue treatment (e.g., by neutralizing). Moreover, for the construction of the plant no material particularly resistant to corrosion is required.
The main anodic reactions of oxide scale change leading to the electrolytic descaling in a neutral solution can be schematized as follows: H2O → 2H++ ½O2 + 2e- Cr203 + 5H20 → 2CrO4 2- + 10H+ + 6e-
In order to make the electrolytic circuit auxiliary electrodes (or counterelectrodes) are employed, onto which the cathodic reactions enabling the electrical neutrality of the solution to be preserved develop.
Both the abovementioned anodic reactions produce acidification at the scale/solution interface. Said acidification determines the further dissolving of the scale according to the following reactions: Fe3O4 + 8H+ → 3Fe3+ + 4H2O + e- (NiO +2H+ → Ni2+ + H2O)
Of course, reaction (7) merely applies to austenitic stainless steels, as ferritic stainless steels do not contain appreciable quantities of Nickel as an alloy metal.
As a side effect of the dissolving of the Iron and Nickel oxides, a larger quantity of Cr₂O₃ becomes available for the anodic change.
Therefore, the resulting electrolytic descaling mechanism in a neutral solution involves the anodic oxidation of the Chromium and the interface acidification, which determines the dissolving of the Iron oxide, and, when present, of the Nickel oxide.
At near-complete dissolution of the surface scale, the anodic oxidation of the underlying metal begins to increase, until reaching its equilibrium rate at the passive state, according to the following schematic reaction: Me + nH2O → MeOn+ 2nH+ + 2ne- where Me indicates the Fe-Cr-Ni alloy; then, only the reactions (1) and (5) take place, the latter however at a much slower rate with respect to the former.
During the electrolytic descaling process in a neutral solution all the abovedescribed reactions take place under diffusion control. This means that the reaction ratios depend on the diffusion of the reactants and of the reaction products through the boundary layer, which in turn is determined by the fluid dynamics onto the steel surface. Apparently, the increase in the burble of the solution at the interface can cause contrasting effects onto the attack rate of the scale, as it also increases the flow of H⁺ which leaves the interface acidified [reaction (1)].
It is also known from Faraday's laws of electrolysis, pertaining to anodic dissolution as well as to cathodic deposition processes, that the quantity of a substance obtained (changed) at the electrodes is proportional to the quantity of electric charge passed through the electrolytic circuit. More particularly, the quantity of electric charge required to obtain (change) a given quantity of substance is constant (e.g., for one equivalent of any substance one Faraday, i.e. 96.500 Coulomb, is required). Hence, for the electrolytic change of a given quantity of substance, the associated current density is constant. Q = Itot·t = constant where Q is the quantity of electric charge (in Coulombs, C), I_tot is the electric current applied (in Amperes, A) and t is the electrolysis time (in seconds, s). This equation applies to any selection of I_tot or t, so that the same effect is attainable applying several different values of current I_tot for the corresponding electrolysis times t.
It has now been found that for the electrolytic pickling or descaling processes in a neutral solution the abovementioned classical equation of electrolysis could lead to erroneous results. In fact, for a given quantity of surface scale (assuming the composition and the structure of the scale oxides to be constant) and for a given process configuration, it can be observed that in order to attain a satisfactory pickling or a satisfactory descaling, i.e., the complete change of the scale, to 1 dm² of oxidised steel surface at least 15 A for 40 s (Carbon steels) and at least 10 A for 10 s (stainless steels) of anodic treatment should be applied. Now, in case it be desirable to apply another value of current density I (e.g., of 60 A/dm², to speed up the process) in order to process (pickle/descale) this same material, the new treatment time would be incalculable by the classical equation on electrolysis, as operating at the highest current density the resulting value would prove too short to ensure an effective process.
Therefore, the information deducible by the classical equations of electrolysis is unsuitable to the calculation of the quantity of electric charge to be applied in the cells of a neutral electrolytic process.
Hence, in the specific field there is a demand for methods which, in the presence of current flow indirect effects, provide the correct selection of the anodic treatment times and of the cell currents, as well as the calculation of the dimensions of the related descaling line and plant.
The present invention meets this demand, further providing additional advantages which will hereinafter be made apparent.
In fact, an object of the present invention is a continuous electrolytic method in a neutral solution for the pickling and the descaling of carbon steels and stainless steels according to claim 1.
The neutral saline solution preferably consists of sodium sulfate, in a concentration from 0,5 to 2,5 M, at a temperature ranging from 30 to 100°C.
In particular, it was observed that in the case of DC electrolysis satisfactory results are attainable when the minimum quantity of electric charge c ranges from 200 to 1250 C/dm² (Carbon steels) and from 40 to 200 C/dm² (stainless steels) and the time constant k ranges from 2 s to 25 s, preferably from 2 to 11 s for Carbon steels and from 2 to 25 s for stainless steels.
Treatment times range from 7 to 50 s for Carbon steels and from 2 to 45 s for stainless steels. Current density ranges from 10 to 80 A/dm² (Carbon steels) and from 5 to 150 A/dm² (stainless steels).
In light of theoretical considerations, not reported here, the unforeseen results according to the invention may be explained considering the mechanism of electrolytic pickling, the heterogeneous nature thereof and the different effect of the electric current flow on the individual electrolytic pickling reactions. Thus, the rate of the electrochemical change reaction is found to increase less than proportionally with respect to the increase of the total current applied to the cell.
The practical consequence of this fact, as hereto mentioned, is that the electrolytic process in a neutral solution cannot be controlled according to the constancy of the quantity of electric charge outputted, as is usually the case in the electrolytic processes not involving current flow indirect effects (in this case, the interface acidification). The selection of the anodic treatment times and of the cell currents should be carried out taking into account that at the increase of the applied current also the electric charge quantity should increase.
The conditions governing the neutral electrolytic process should also be observed in the design of the related pickling lines, which should ensure the functionality of the process at various flow rates of the strip to be processed.
The anodic treatment time depends on the line speed (v) and on the total length of the electrodes (L) which give the anodic polarization to the strip to be processed.
Therefore, the preceding equation describing the quantity of electric charge outputted during the electrolytic process in a neutral solution may be rewritten as follows I = c/(L/v - k)
Hence, another object of the present invention is the use of the abovedescribed electrolytic method, characterised in that, setting the width and the flow rate of the strip, the total anodic electrode length, and therefore the length of the related continuous neutral electrolytic treatment line, the current to be outputted, selected according to the abovedescribed method and defined according to claims 4 and 8.
The electrolytic treatment of steel strips is usually carried out in cells consisting of a set of electrodes connected to opposite poles of the power supplies, which determine alternatively anodic and cathodic polarization sequences onto the strip to be descaled. Though the descaling process merely requires the anodic polarization, the addition of the cathodic stage entails the advantage of having the electrochemical reactions take place directly onto the strip, with no direct connection of the latter to the power supplies; thus, the employ of costly current carrier rolls can be avoided.
Therefore, the total length (L) of the electrodes imposing the anodic polarization onto the strip is given by the sum of the unitary lengths (L_a) of the individual electrode units.
The cell may have a vertical or a horizontal development, according to plant convenience criteria.
Moreover, the neutral electrolytic treatment equation disclosed in the present invention shows that there is an electrolysis time (k) which is inactive for the treatment. This means that in the design of a plant for an electrolytic treatment in a neutral solution for steel strips it should be taken into account that the total anodic treatment time (t) be greater than k t = L/v > k
In practice, the electrolytic descaling process is fractionated in a sequence of anodic and cathodic current pulses, with L = n * La (where L_a is the length of the individual anodic current pulse and n is the number of current pulses), the frequency (f) of each anodic current pulse should be f = v/La < ½ * n/k where the factor ½ is introduced in order to take into account the total treatment time (therefore assuming symmetrical cathodic current pulses).
For k_min = 2 s and n_max = 12,
f_max < 3 hz is attained.
This frequency limit value for the electrolytic pickling in a neutral solution is compatible with the reaction mechanisms advanced for the treatment, implying the acidification of the electrified interface in order to foster the dissolving of the oxides.
Adopting, e.g., hot-rolled Carbon steels strips under predetemined industrial conditions (with constant post-rolling cooling modes), the scale obtained exhibits a near-constant composition and morphology, requiring in order to be electrolytically pickled a minimum charge quantity c, depending on the pre-pickling scale-breaking mechanical treatment.
The electrolytic method in a neutral solution according to the present invention may also be carried out by AC current having a frequency lower than 3 Hz, with total treatment times and applied currents selected, for suitable values of c' and k', according to the formula: I.t = c' + k' X t
Hereto, the present invention has merely been outlined. Hereinafter, with the aid of the following examples and of the attached figures, embodiments thereof aimed at making apparent purposes, features, advantages and application modes thereof will be disclosed.
Fig. 1 shows patterns of the descaling, as scale fraction P(t)/P_T changed as a function of time, for initial scales P_T1 and P_T2 > P_T1. The patterns were obtained integrating a descaling equation obtained in light of the experimental observation of the loss of mass during the descaling process.
Fig. 2 relates to the case wherein c = 70 C/dm² and it shows the four hyperbola branches having parameter k, bottom to top, equal to 1, 2, 3 and 4 seconds, respectively, with asymptotes I = 0 A/dm² and t = k seconds.

Example 1

A common low-Carbon steel having hot-rolling scale mechanically pre-conditioned by roll-induced squashing (about 2.5% lengthening) is subjected to the continuous electrolytic pickling method in a neutral solution according to the present invention.
For this type of scale, it was found that c is equal to 490 C/dm² and k is equal to 3.7 s. Moreover, for the scale changing reactions to take place, a current density I>I₀, with I₀ = 10 A/dm² should be applied.
For a continuous neutral electrolytic pickling line for 1.5 m wide strips, operating at 90 m/min at steady state and at 20 m/min at roll start and roll end, the total anodic electrode length (L), and therefore the length of the pickling plant, are set with regard to the current applied in the cell according to the provisions of the equation of the neutral electrolytic pickling according to the present invention.

In light of the indications from the equation at issue, the current density (I) to be applied to the electrolytic pickling cell as a function of the anodic electrode length (L) and of the varying of the line speed (v) is reported in column 3 of Table 1; in column 4 the electric charge density (Q), and in column 5 the total current (I_tot) to be outputted, calculated multiplying the current density for the anodic electrode surface, are indicated. Apparently, at the steady state speed of 90 m/min the electric charge density (Q) to be outputted for the electrolytic pickling increases when the anodic electrode length decreases. Likewise, the total current (I_tot) applied increases.

v, (m/min) line speed	L, (m) anode length	I, (A/dm²) I = 490/(L/v-3,7)	Q, (C/dm²) Q = I * t	I_tot (kA) I_tot=I * S	I_tot, ^o (kA)
90	32	28	593	267	221
90	28	33	611	275	221
90	24	40	637	287	221
90	20	51	678	305	221
90	16	70	750	338	221
20	16	11	531	53	49
20	8	24	579	58	49

In column 6, the current I_tot ^o, calculated according to the classical law of electrolysis (I_tot ^o = I^o * S = 490 * v/L * S) is reported. Apparently, disregarding the method according to the invention, the dimensioning of the current of the power supplies, line speeds being equal, would have been markedly insufficient to ensure the complete pickling at the decreasing of the plant length.
Moreover, from Table 1 it may be inferred that the slow-speed (20 m/min) operation implies the employ of a total anodic length not exceeding 16 m, lest the condition that I>I₀ be not met. This is attained by sectioning the electrodes and power-supplying only a section thereof, regardless of the total length of the anodes installed in the cells.

Example 2

A Silicon (3% Si) steel for magnetic employs with hot-rolling scale, mechanically pre-conditioned by in-line peening, is subjected to the continuous neutral electrolytic pickling method according to the invention. As the peening machine causes a partial removal of the scale, whose entity is inversely proportional to the line speed, it was found that for this material c₁ = 525 C/dm² when v = 20 m/min, c₂ = 680 C/dm² when v = 40 m/min; in both cases, k = 3.1 s; moreover, for the scale changing reactions to take place, a current density I>15 A/dm² should be applied.
For a continuous neutral electrolytic pickling line for 1.2 m wide strips, operating at 40 and 60 m/min, the total anodic electrode length (L), and therefore the length of the pickling plant, are set with regard to the current applied in the cell, according to the provisions of the equation of continuous neutral electrolytic pickling according to the invention.

In light of the indications from the equation at issue, the current density (I) to be applied to the electrolytic pickling cell as a function of the anodic electrode length (L) and of the line speed (v) is reported in Table 2, as well as all the other related quantities.

v, (m/min) line speed	L, (m) Anode length	I, (A/dm²) I = c/(L/v-3,1)	Q, (C/dm²) Q = I * t	I_tot (kA) Itot=I * S	I_tot,^o (kA)
20	12	16	574	46	42
20	10	20	586	47	42
20	8	25	603	48	42
20	6	35	634	51	42
20	4	59	708	57	42
40	24	21	744	119	109
40	20	25	758	121	109
40	16	33	781	125	109
40	14	38	798	128	109
40	12	46	821	131	109
40	10	57	857	137	109
40	8	76	917	147	109

Thus, it is confirmed that, speeds being equal, when the anodic electrode length decreases the electric charge density (Q) to be outputted for the electrolytic pickling increases. Likewise, the total current (I_tot) applied increases. At the speed of 20 m/min the actual anodic length should not exceed 12 m, lest the current density be too low (I < I₀). It may be inferred therefrom that the pickling plant for the material at issue could conveniently be dimensioned with an anodic electrode length of 10-14 m.
However, also in this case, the design of the electrolytic pickling plant according to the classical laws of electrolysis would have led to an underrating of the power supplies.

Example 3

The method according to the present invention is applied to the descaling of common hot-rolled steels, with a mechanical descaling pre-treatment as in example 1 (scale with c = 490 C/dm², k = 3.7 s and I₀ = 10 A/dm²).
The pickling line, comprising a total anodic electrode length L = 24 m (strip width = 1.5 m), should be capable of operating at a speed ranging from 60 to 120 m/min

In Table 3, the current densities (I) to be applied to the electrolytic pickling cell as a function of the anodic electrode length (L) and of the line speed (v) are shown. In Table 3 also all the other related quantities are reported.

v, (m/min) line speed	L, (m) anode length	I, (A/dm²) I = 490/(L/v-2.1)	Q, (C/dm²) Q = I * t	I_tot (kA) I_tot=I * S	I_tot ^o, (kA)
60	24	24	579	174	147
70	24	29	597	209	172
80	24	34	617	247	196
90	24	40	637	287	221
100	24	46	659	330	245
110	24	52	683	376	270
120	24	59	706	425	294

Apparently, disregarding the hereto disclosed invention, the dimensioning of the current of the power supplies would have been markedly insufficient to ensure the descaling at the increase of the line speed.

Example 4

The method according to the present invention is applied to common hot-rolled steels with a mechanical descaling pre-treatment as in example 1 (scale with c = 490 C/dm², k = 3.7 s and I₀ = 10 A/dm²).
The descaling plant consists of 12 cells, each one having an unitary anode length L_a = 2 m, for a total L = 24 m (strip width = 1.5 m); The former should be capable of operating at a speed ranging from 40 to 120 m/min, according to two different process control logics: in the one case (see Table 4a) with the logic of maximising the use of the power from the individual power supplies (i.e., use of a number of cells proportional to the line speed) and, in the other case (see Table 4b), with the constant employ of all the cells (i.e., use of a current density proportional to the line speed).

The indications obtained are reported in the following Tables 4a and 4b.

v, (m/min) line speed	Number of cells n	I, (A/dm²) [I = 490/(L/v-3)]	Q, (C/dm²) [Q = I * t]	I_tot (kA) [I_tot=I * S]	I_tot ^o, (kA)
40	4	59	708	142	98
60	6	59	708	213	147
80	8	59	708	283	196
100	10	59	708	354	245
120	12	59	708	425	294

v, (m/min) line speed	Number of cells n	I, (A/dm²) [I = 490/(L/v-3)]	Q, (C/dm²) [Q = I * t]	I_tot (kA) [I_tot=I * S]	I_tot ^o, (kA)
40	12	15	546	109	98
60	12	24	579	174	147
80	12	34	617	247	196
100	12	46	659	330	245
120	12	59	708	425	294

The equation of continuous neutral electrolytic pickling according to the invention demonstrates that the two process control logics are not equivalent, as overall lesser total pickling currents are required when operation is carried out maximising the number of cells employed.
The classical equation of electrolysis would not have allowed to understand the difference between the two control logics, moreover underrating the current requirement for the pickling.

Example 5

A neutral electrolytic descaling plant, inserted in a combined pickling-annealing line for cold-rolled stainless steel strips, operating with a total anodic electrode length L = 4 m and strip width =1.25 m, capable of varying the treatment speed from 20 to 70 m/min in order to satisfy the demand for thermal cycle constancy on different thicknesses of the stainless steel strip, is considered.

According to the indications from the descaling equation, the direct current density (I) to be applied to the electrolytic pickling cell at the variation of the line speed (v) is reported in column 2 of Table 5; in column 3 the electric charge density (Q), and in column 4 the total current (I_tot) to be outputted, calculated multiplying the current density for the anodic electrode surface, are indicated.

Line speed v, m/min	I, A/dm² I = 70 / (L / v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
20	8	93	8	6
30	14	112	14	8
40	23	140	24	12
50	39	187	38	14
60	70	280	70	18
70	163	560	164	20

In column 5, the current I_tot ⁰ yielded by the classical law of electrolysis (I_tot ^o = I^o·S = 70·v/L·S) is reported. Apparently, disregarding the finding according to the invention, the dimensioning of the current of the power supplies would have been markedly insufficient to ensure the descaling at the increase of the line speed.

Example 6

A neutral electrolytic descaling plant operating within the same range of speed (20-70 m/min) of example 5, and with a greater total electrodic length, equal to L = 5.12 m (strip width = 1.25 m) is considered. The operative parameters of this plant, calculated with the descaling equation according to the invention, are reported in Table 6.

Comparing this case with that of Example 5, the laws of electrolysis would have maintained the same pattern of the total currents (I_tot ^o) at the increase of the line speed. Actually, the descaling equation according to the invention indicates the need of lesser currents, speeds being equal, with respect to the preceding case.

line speed v,m/min	I, A/dm² I = 70 / (L / v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
20	6	88	8	6
30	10	99	12	8
40	15	115	20	12
50	22	137	28	14
60	33	169	42	18
70	50	221	64	20

However, also in this situation, the design according to the known laws of electrolysis would have led to the underrating of the power supplies.

Example 7

A neutral electrolytic descaling plant operating with a total anodic electrode length L = 8 m (strip width = 1.25 m), capable of varying the treatment speed from 60 to 120 m/min is examined. The operative parameters of this plant, calculated by the descaling equation according to the invention are reported in Table 7.

Line speed v, m/min	I, A/dm² I = 70 / (L / v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
60	14	112	28	18
70	18	124	36	20
80	23	140	46	24
90	30	160	60	26
100	39	187	78	30
110	51	224	102	32
120	70	280	140	36

Also this case confirms that the dimensioning of the current of the power supplies according to the classical laws of electrolysis would have been markedly erroneous by defect to the other operative line speeds.

Example 8

The case of a neutral electrolytic descaling plant consisting of four cells, in each one of which the anodic electrode length is L_a = 2 m, for a total L = 8 m (strip width = 1.25m) is examined.

The current distribution as a function of the line speed has already been observed in Example 7. Now, hypothesising that the plant could operate even with mere 3 cells (e.g., for operative reasons, failures, etc.) and therefore with L = 6 m, the descaling currents to be applied are indicated in Table 8.

line speed v,m/min	I, A/dm² I = 70 / (L / v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
50	17	120	26	14
60	23	140	36	18
70	33	168	50	20
80	47	210	70	24
90	70	280	106	26
100	117	420	176	30

Operating with 3 cells, at the increase of the speed a surplus demand of total current ensues, a fact which would have been unpredictable in light of the classical laws of electrolysis.

Example 9

This is the case of a neutral electrolytic descaling plant consisting of n = 6 cells with unitary anodic length L_a = 1 m and L = 6 m (strip width 1,25 m), operating, in the one case (see Table 9a), with a process control logic of maximising the use of the power from the individual power supplies (i.e., use of a number of cells proportional to the line speed) and, in the other case (see Table 9b), with the constant employ of all the cells (i.e., use of a current density proportional to the line speed).

Line speed v, m/min	Number of cells n	I, A/dm² I = 70/(L/v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
10	1	23.0	138	5.7	2.9
20	2	23.0	138	11.5	5.9
30	3	23.0	138	17.2	8.7
40	4	23.0	138	23.0	11.7
50	5	23.0	138	28.7	14.7
60	6	23.0	138	34.5	17.5

Line speed v, m/min	Number of cells n	I, A/dm² I = 70/(L/v - 3)	Q, C/dm² Q = I · t	I_tot, (kA) I_tot = I · S	I_tot ^o, (kA)
10	6	2.1	76	3.1	2.9
20	6	4.7	85	7.0	5.9
30	6	7.7	92	11.5	8.7
40	6	11.7	105	17.2	11.7
50	6	17.0	123	25.5	14.7
60	6	23.0	138	34.5	17.5

The two process control logics are not equivalent, as overall lesser total descaling currents are required when operation is carried out maximising the number of cells employed. In this case as well, the underrating of the descaling current by the classical equation of electrolysis persists.
To the abovedescribed method a person skilled in the art, in order to satisfy further and contingent needs, may effect several further modifications and variants, all however falling within the protective scope of the present invention, as defined by the appended claims.

Claims

A continuous electrolytic method in a neutral solution for the pickling and the descaling of carbon steels and stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, having a frequency lower than 3 Hz, wherein the anodic treatment time when the desired cell current is applied, and the cell current, when the desired treatment time is adopted, are selected according to the formula: It = c + kI where:

I is the current density crossing the cell;

t is the anodic treatment time;

c is the constant fraction of electric charge density outputted for the direct oxide changing anodic reactions;

k is a time constant for the calculation of the fraction of electric charge density, proportional to the current density I (kI), outputted for the indirect anodic reactions linked to oxygen development and to the consequent acidification at the steel/electrolytic solution interface, for the carbon steels, and at the scale/solution interface for the stainless steels;

the constants c and k being known for each kind of steel and each kind of scale.
The continuous electrolytic method in a neutral solution, in the presence of electrolysis current flow indirect effects, said current being DC or AC, having a frequency lower than 3 Hz, according to claim 1, wherein the neutral solution preferably consists of sodium sulfate, in a concentration from 0,5 to 2,5 M, at a temperature ranging from 30 to 100°C.
The continuous electrolytic method in a neutral solution for the pickling of carbon steels, in the presence of DC electrolysis current flow indirect effects, according to claim 1 or 2, wherein the quantity of electric charge c ranges from 200 to 1250 C/dm² and the time constant k ranges from 2 to 11 sec, for anodic treatment times ranging from 7 to 50 sec and current density ranging from 10 to 80 A/dm².
The use of the continuous electrolytic method in a neutral solution for the pickling of carbon steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC having a frequency lower than 3 Hz, according to any one of the claims 1 to 3, characterised in that, upon setting the width of the strip to be pickled, the speed v of the strip to be pickled and the cell current I, the total anodic electrode length, and therefore the length L of the related continuous neutral electrolytic pickling line, is defined by the formula I = c / (L / v - k) which is the formula of claim 1 rewritten by replacing the time t by the ratio L/v.
The use according to claim 4, wherein a process control logic providing the employ of a number of cells proportional to the line speed and the use of the maximum power available is adopted.
The use according to claim 4, wherein a process control logic providing the constant employ of all the cells and the use of a current density proportional to the line speed is adopted.
The continuous electrolytic method in a neutral solution for the descaling of stainless steels, in the presence of DC electrolysis current flow indirect effects, according to claim 1 or 2, wherein the quantity of electric charge c ranges from 40 to 200 C/dm² and the time constant K ranges from 2 e 25 sec, for anodic treatment times ranging from 2 to 45 sec and current densities ranging from 5 to 150 A/dm².
The use of the method for the continuous electrolytic descaling of stainless steels in a neutral solution, in the presence of electrolysis current flow indirect effects, said current being DC or AC, having a frequency lower than 3 Hz, according to claims 1, 2 or 7, characterised in that, upon setting the width and the speed v' of the strip to be descaled and the cell current I', the total anodic electrode length, and therefore the length L' of the related continuous electrolytic neutral descaling line, is defined by the formula I' = c'/(L'/v' - k') which is the formula of claim 1 rewritten by replacing the time t' by the ratio L'/v'.
The use of the method according to claim 8, wherein a process control logic providing the employ of a number of cells proportional to the line speed and the use of the maximum power available is adopted.
The use of the method according to claim 9, wherein a process control logic providing the constant employ of all the cells and the use of a current density proportional to the line speed is adopted.