WO2002086199A2

WO2002086199A2 - Continuous electrolytic descaling of stainless steel

Info

Publication number: WO2002086199A2
Application number: PCT/IT2002/000270
Authority: WO
Inventors: Alessandro Dulcetti; Susanna Ramundo
Original assignee: Centro Sviluppo Materiali S.P.A.
Priority date: 2001-04-24
Filing date: 2002-04-24
Publication date: 2002-10-31
Also published as: WO2002086199A3; ITRM20010223A1; ITRM20010223A0; AU2002304290A1

Abstract

A continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz characterised 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 changing anodic reactions; and k is a time constant for the calculation of the fraction of electric charge quantity, proportional to the current density I (kI), outputted for the indirect anodic reaction linked to the oxidation of the hydroxyl ions of the water, that on the one hand produces and maintains the acidification at the steel/electrolytic solution interface, whereas on the other hand it substracts electric charges to the oxidation reactions of the scale and of the underlying metal.

Description

CONTINUOUS ELECTROLYTIC METHtTD f» ftN GΪ& SOLUTION FOR

THE DESCALING OF STAINLESS STEELS, IN THE PRESENCE OF

CURRENT FLOW INDIRECT EFFECTS

DESCRIPTION The present invention refers to the field of the continuous descaling, in an acid solution, of stainless steel strip for the removal of surface oxides formed by effect of thermal treatments, comprising hot-rolling and annealing. 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 are used, i.e. HNO₃/HF solutions, or, more recently, H₂SO/HF/H₂0₂ solutions. However, prior to the chemical pickling, in order to speed up the entire surface scale removal process in stainless steel making, descaling pre-treat ents 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 bath salt 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. The main anodic reactions of oxide scale change leading to the electrolytic descaling can be schematized as follows :

H₂0 →2H⁺+^0₂+2e^" (1)

Cr₂0₃+5H₂0-»2CrO₄ ^2~+10H⁺+6e^~ (2) In order to make the electrolytic circuit auxiliary electrodes (or counterelectrodes) are employed, onto which the cathodic reactions enabling the electric neutrality of the solution to be preserved occur. Both the abovementioned anodic reactions produce and maintain acidification at the scale/solution interface. Said acidification determines the further dissolving of the scale according to the following reactions: Fe₃0₄+8H⁺→3Fe³⁺ + 4H₂0 + e^~ (3) (NiO+2H⁺→Ni²⁺+H₂0) (4) Of course, reaction (4) merely applies to austenitic stainless steels, as ferritic stainless steels do not contain appreciable quantities of nickel as an alloy element . As a side effect of the dissolving of the iron and nickel oxides, a larger quantity of Cr₂0₃ that is gradually exposed becomes available for the anodic reaction. Therefore, the electrolytic descaling mechanism involves the anodic oxidation of the chromium, the oxidation of the hydroxyl ions of the water with the production of hydrogen ions at the interface, which determines the chemical dissolving of the iron oxides and, when present, of the nickel oxides.

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 (at this point, only the reactions (1) and (5) take place, the latter however at a much slower rate than the former) : Me+nH₂0-»MeO_n+2nH⁺ + 2ne^" (5)

In industrial practice, the electrolytic descaling treatment only partially removes the scale of the cold- rolled austenitic stainless steels, the remaining scale being removed in the subsequent chemical pickling step. On the contrary, in the case of the ferritic and martensitic stainless steels, the electrolytic descaling treatment totally removes the surface scale, so that the subsequent chemical treatment, when applied, merely has a finishing/passivating function.

Though seldom used, in practice the electrolytic descaling treatment can also be applied to the stainless steels in the hot-rolled state.

During the electrolytic descaling process all the abovedescribed reactions take place under diffusion control. This means that the reaction rates 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, since from one side it removes the reaction product, but from the other side it increases the flow of H⁺ which leaves the interface (reaction 2) . 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 substance converted 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 change a given quantity of substance is constant (e.g. for one equivalent of any substance one Faraday, i.e. 96500 Coulomb, is required). Hence, for the electrolytic conversion 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 Coulomb, C) , I_ot is ^tne 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 descaling process the 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 descaling, i.e., the complete conversion of the scale, to 1 cm² of oxidised steel surface at least 10 A for 10 s of anodic treatment should be applied. Now, in case it be desirable to apply another value of current density (e.g., 40 A/dm² to speed up the process) in order to descale this same material, the new treatment time would be incalculable by the classical equation of electrolysis, as operating at a higher current density the resulting value would prove too short to ensure an effective descaling.

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 descaling cells. Hence, in the specific field there is a demand for a continuous electrolytic method for the descaling 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 plant.

The present invention meets this demand, further providing additional advantages which will hereinafter be made apparent. In fact, the subject of the present invention is a continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz, characterised in that the anodic treatment times and the cell currents are selected according to the formula

It = c + kl 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; and k is a time constant for the calculation of the fraction of electric charge quantity, proportional to the current density I (kl), outputted for the indirect anodic reaction linked to the oxidation of the hydroxyl ions of the water, that on the one hand produces and maintains the acidification at the steel/electrolytic solution interface, whereas on the other hand it subtracts electric charges to the oxidation reactions of the scale and of the underlying metal. The acid solution for the electrolytic descaling can be obtained from strong acids (H₂S0₄, HC1, HN0₃, HF, etc.) or mixtures thereof. Preferably, H₂S0₄-based solutions are employed, given the higher electrochemical and thermal stability of H₂S0₄, in a concentration from 0,2 to 3 M, at a temperature ranging from 30 to 100 °C.

An important improvement of the H₂S0-based electrolytic solution is attained with the chemical control of the redox level thereof (in practice, Fe³⁺ concentration) by adding suitable oxidising agents, like hydrogen peroxide. Thus, there is attained the two-fold advantage of preventing the deterioration of the surface appearance of the stainless steel strip in the cell zones wherein it transits in the absence of electrical field (e.g., interelectrode zones, driving rolls, etc.) and of improving the environmental compatibility of the process. The practical consequence of the results of the invention is that the electrolytic process for the descaling 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 oxidation of the hydroxyl ions of the water) . In fact, in the case of the electrolytic descaling 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.

It has been observed that in a wide range of industrial conditions the annealing scale of the cold-rolled stainless steel strips requires, for the electrolytic descaling carried out with DC current, a minimum charge quantity (c value in the equation) ranging from 30 to 120 C/dm² and a time constant k ranging from 1.5 to 15 s, for anodic treatment times ranging from 2 to 30 s and current densities ranging from 5 to 150 A/dm². Another subject of the present invention is the specific use of the abovedescribed method for the continuous electrolytic descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz. In fact, setting the width and the flow rate of the strip to be descaled, the total anodic electrode length, and therefore the length of the related continuous electrolytic descaling line, are defined according to the outputted current, selected according to the previously described electrolytic descaling method. Therefore, the law governing the electrolytic descaling should also be observed in the design of the related industrial plants, which should ensure the functionality of the descaling process at various flow rates of the strip, particularly in the combined lines of annealing and pickling of the cold-rolled stainless steel strips.

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 descaled. Therefore, the equation according to the present invention may be rewritten as

I = c/(L/v-k) The electrolytic descaling of stainless 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 descaling cell may have a vertical or a horizontal development, according to plant convenience criteria. An explanation of the unexpected results of the present invention could be found considering the electrolytic descaling mechanism, its heterogeneous nature and the different effect of the electric current flow on the individual electrolytic descaling reactions. Considering annealed strips under predetermined industrial conditions (with constant thermal cycles) the scale obtained exhibits a near-constant composition and morphology, requiring in order to be electrolytically descaled, when a DC current in a neutral solution is adopted, a minimum charge quantity of c = 70 C/dm² and k = 3 s . The same typology of scale, in order to be electrolytically descaled in an acid solution with DC current, requires a minimum charge quantity c = 68 C/dm² and k = 2.6 s . Therefore, the electrolytic descaling equation according to 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 an acid solution for stainless steel strips it should be taken into account that the anodic treatment time (t) be t = L/v > k In practice, the electrolytic descaling process being fractionated in a sequence of anodic and cathodic pulses, with

L = n^• L_a

(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/L_a< ^-n/k where the factor is introduced in order to take into account the total descaling time (assuming symmetrical cathodic current pulses) . For kmin = 2 s and n_max = 12 , fmax < 3hz is attained. This frequency limit value for the electrolytic descaling is compatible with the reaction mechanisms as advanced. So far, the present invention has merely been disclosed in general. Hereinafter, with the aid of the following examples, embodiments thereof aimed at making apparent objects, features, advantages and operation modes thereof will be detailed.

EXAMPLE 1

An electrolytic descaling plant in an acid solution (H₂S0₄ = 60 g/1; Fe³⁺ = 35 g/1; temperature = 65 °C) , inserted in a combined pickling-annealing line for cold- rolled stainless steel strips, operating with a total anodic 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 1; 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.

Table 1

In column 5 the current I_tot yielded by the classical law of electrolysis (I_tot°=I⁰- S=68 • v/L- S) is reported. Apparently, disregarding the results according to the invention, the dimensioning of the current of the power supplies would have been in this case as well insufficient to ensure the electrolytic descaling in ah acid solution at the increase of the line speed. Apparently, with respect to the results obtained in an Example of a neutral solution, the request of current surplus in an acid solution is slightly lower. EXAMPLE 2

An acid electrolytic descaling plant (H₂S0₄ 50 g/1, Fe³⁺ 25g/l, temperature 40 °C) operating within the same range of speed (20-70 m/min) of example 1, and with a greater total electrode 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 2. Table 2

Comparing this case with that of Example 1, the laws of electrolysis would have maintained the same pattern of the total currents (Itot)⁰ 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. However, also in this situation, the design according to the known laws of electrolysis would have led to the underestimation of the power supplies. EXAMPLE 3

An acid electrolytic descaling plant (H₂S0₄ 70 g/1, Fe³⁺ 40g/l, temperature 60 °C) operating with a total anodic electrode length L = 14 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 3. Table 3

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 4

The case of an acid electrolytic descaling plant (H₂S0₄ 70 g/1, Fe³⁺ 40g/l, temperature 60 °C) consisting of four cells, in each one of which the anodic electrode length is L_a = 3.5m, for a total L = 14 m (strip width=l .25m) , is examined.

The current distribution as a function of the line speed has already been observed in Example 3. Now, hypothesysing that the plant could operate even with mere 3 cells (e.g., for operative reasons, failures, etc.) and therefore with L = 10.5 m, the descaling currents to be applied are indicated in Table 4.

Table 4

Operating with three cells, at the increase of the speed a surplus demand of total current results, a fact which would have been unpredictable in the light of the classical laws of electrolysis. EXAMPLE 5

This is the case of an acid electrolytic descaling plant (H₂S0₄ 70 g/1, Fe³⁺ 40g/l temperature 60 °C) consisting of n = 6 cells with unitary anodic length La = 2 m and L = 12m (strip width 1.25 m) , operating, in the one case (see Table 5a) , 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 5b), with the employ of all the cells.

Table 5a

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 underestimation of the descaling current by the classical equation of electrolysis persists.

Claims

CLAIMS l. A continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz, characterised in that the anodic treatment times and the cell currents are selected according to the formula

It = c + kl 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; and k is a time constant for the calculation of the fraction of electric charge quantity, proportional to the current density I (kl) , outputted for the indirect anodic reaction linked to outputted for the indirect anodic reaction linked to the oxidation of the hydroxyl ions of the water, that on the one hand produces and maintains the acidification at the steel/electrolytic solution interface, whereas on the other hand it subtracts electric charges to the oxidation reactions of the scale and of the underlying metal. 2. The continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz, according to claim 1, wherein the electrolytic solution contains strong acids and preferably consists of H₂S0₄, in a concentration from 0,

2 to 3M, at a temperature ranging from 30 to 100 °C.

3. The continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz, according to claim 1 or 2, wherein the H₂so₄-based electrolytic solution, preferably in a concentration from 0,2 to 1,5M, is controlled for the Fe³⁺ content thereof, preferably in a concentration from 0.1 to 2M, at a temperature ranging from 30 to 100 °C.

4. The continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, according to claim 1, 2 or 3, wherein the charge quantity c ranges from 30 to 120 C/dm² and the time constant k ranges from 1.5 to 15 s, for total anodic treatment times ranging from 2 to 30 s and current densities ranging from 5 to 150 A/dm².

5. An use of the continuous electrolytic method in an acid solution for the descaling of stainless steels, in the presence of electrolysis current flow indirect effects, said current being DC or AC, with a frequency lower than 3 Hz, according to any one of the preceding claims, characterised in that, setting the width and the flow rate of the strip to be descaled, the total anodic electrode length, and therefore the length of the related continuous electrolytic descaling line, are defined according to the outputted current, selected according to the method of claims 1 to 4.