EP0236354A1 - Method for the acid etching of stainless steel products. - Google Patents

Abstract

Pickling process for stainless steel products, in which a pickling bath of initial composition is used: HF 10 to 50 g / l; ferric iron (Fe3 +) dissolved> = 15 g / l; water: the balance; at a temperature between 15 and 70oC, characterized in that, during the pickling operation or operations, the ferric iron content of the bath is kept at least 15 g / l by oxidation of the bath comprising at least one or more injections of air of total flow greater than or equal to 1 Nm3 per m2 of pickled stainless steel and per hour of pickling of each pickled surface element. The method of the invention is particularly applicable to the industrial pickling of stainless steel sheets and strips, in which it makes it possible to avoid the use of nitric acid and the pollution which results therefrom.

Description

 PROCESS FOR ACID STRIPPING OF STAINLESS STEEL PRODUCTS

The field of the present invention is that of surface treatments and more precisely the acid pickling of stainless steel products.

EXPOSURE OF THE PROBLEM

The acid pickling of stainless steels is usually carried out with fluonitric baths, in which the use of nitric acid has the disadvantage of causing the formation of nitrous vapors polluting the atmosphere and soluble nitrates polluting the liquid effluents.

In the context of continuous acid pickling of stainless steel sheets, the applicant has sought to develop a modified pickling process which, while remaining industrially economical, makes it possible to limit or better avoid such pollution.

KNOWN STATE OF THE ART

In his book "STAINLESS IRON AND STEEL" (CHAPMAN & HALL LTD, London 1951), J.H.G. MONYPENNY indicates pp. 183-184 that, to minimize the problem of vapors from fluonitric pickling baths, baths containing 6 to 12% of 90% solution of ferric sulfate and 1.5 to 3% were used for pickling stainless steel sheets hydrofluoric acid and this for example at 70-80 ° C for descaling a hot rolled sheet. The initial concentration of ferric iron in previous baths is therefore about 16.5 to 33 g / l. The Applicant's tests have shown that, when successive samples of stainless steel sheet are stripped in such baths, the speed and quality of the stripping deteriorate rapidly. These acid pickling baths are therefore not satisfactory as such for series or continuous pickling of stainless steel products.

The use is also known for pickling baths containing hydrofluoric acid and hydrogen peroxide. Industrial stainless steel strip stripping tests have been carried out by the Applicant, who observed runaway temperatures in the baths as well as a high consumption of hydrogen peroxide making the process very expensive compared to the fluonitric pickling process of stainless steels. In this process, the replacement of nitric acid by hydrogen peroxide therefore does not seem to be suitable for industrial exploitation.

STATEMENT OF THE INVENTION

The subject of the invention is a process for pickling stainless steel products in which, as is known, a pickling bath of initial composition is used:

HF 10 to 50 g / l

Ferric iron (Fe ³⁺ ) dissolved ≥ 15 g / l

Water: the balance

at a temperature between 15 and 70 ° C, and in which, in a new way, during the pickling operation or operations, the ferric iron content of the bath is maintained at at least 15 g / l by oxidation of the bath comprising at least one or more air injections with a total flow rate greater than or equal to 1 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element, or else equivalent ventilation by circulation in the open air.

For industrial practice, and in particular for repetitive or continuous pickling of stainless steel products in at least one large tank, typically one or more pickling baths are used, initially containing HF 10 at 35 g / l and Fe ³⁺ ≥ 20 g / l, and during the pickling operation (s) the Fe ³⁺ content of this bath or these baths is kept at least 20 g / l thanks to oxidation of the bath or baths comprising one or more air injections total flow between 1 and 8 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element. Air injections with a higher total flow rate have proved to be of no interest, the saturation of the oxygen bath of the air being undoubtedly reached and the additional air flow rates no longer apparently serving only to agitate. the bath, and this in a possibly excessive way.

The oxygen of the introduced air seems to intervene in the process of the invention as oxidizing agent regenerating Fe ²⁺ in Fe ³⁺ , whereas Fe ³⁺ constitutes an oxidizing agent acting on the base metal to dissolve it. The main reactions could be:

- dissolution reaction:

2 Fe ³⁺ + Fe 3 Fe ²⁺ (A)

balance almost completely displaced in direction 1 under normal stripping conditions;

- other dissolution reaction:

Fe + 2 HF H ₂ + Fe F ₂ (B) also possible in an oxidizing medium, which is the case;

- oxidation of Fe ²⁺ by aeration of the pickling solution, possibly supplemented by another means of oxidation:

4 Fe ²⁺ + O ₂ + 4 H + 4 Fe ³⁺ + 2 H ₂ O (C)

balance strongly displaced in direction 3 if the solution is correctly oxidized and in the case of the pH of the pickling bath which is between 1 and 3 approximately.

The ferric iron concentration in the bath can be calculated as the difference between the total iron concentration, determined for example by atomic absorption, and the concentration of Fe ²⁺ measured by its oxidation to Fe ³⁺ in the presence of KMnO ₄ permanganate. Adequate aeration of the pickling bath, typically by air injection, allows the quality of pickling to be maintained during successive pickling or continuous pickling of stainless steel products by regenerating Fe ³⁺ .

The total volume of air injected into the pickling bath essentially depends on the amount of pickled stainless steel, which quantity itself is proportional to the pickled surface and the duration of pickling of this surface. For the pickling thus considered, and according to the tests and the industrial developments already carried out, the total flow rate of air injected into the pickling bath of the invention is typically between 2 and 5 Nm ³ per m ² d '' pickled stainless steel and per hour of pickling of each pickled surface element. So that the pickling bath is adequately aerated, it is then advisable to inject a good part of this volume of air, typically typically at least half of this volume, with nozzles directed towards the bottom of the bath at the lower half. of this bath. The injected air is preferably preheated to a temperature close to that of the bath, ie typically between 35 and 60 ° C.

For the industrial control of the pickling bath, HF recharges are carried out as usual, and, rather than determining the concentration of Fe ³⁺ in the bath, it is practical to determine the REDOX potential of the bath and to adjust it between 0 and +800 mV and preferably between +100 and +300 mV by acting if necessary on the oxidation of the bath. The reference REDOX potential is chosen according to the shade and surface condition of the strip and readjusted, if necessary, based on surface condition observations after pickling.

The REDOX potential is measured between a platinum electrode and an Ag / AgCl reference electrode or with fixed, reproducible potential and with zero irreversibility power. A device for measuring this REDOX potential can be suitably sealed so as to allow continuous measurements in the bath.

Depending on the Fe ³⁺ concentration observed, or more conveniently depending on the value of the REDOX potential, we may need an oxidation means temporarily and / or locally supplementing the action of the air to return more quickly to the desired Fe ³⁺ concentration or at the set REDOX potential, so as to find a good pickling. At least one addition of strong oxidant, for example hydrogen peroxide or potassium permanganate, is then used as a complementary means of oxidizing the bath. It is still possible in certain cases to introduce an oxygen injection or to increase the air flow.

In the frequent industrial case where large quantities of stainless products are pickled with the same bath, preferably added to the bath in the form of constant or repetitive additions of small quantities of hydrogen peroxide, additions typically representing on average 0.1 to 0.4 1 of H ₂ O ₂ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element. Another oxidant such as the potassium permanganate already mentioned can be used in an equivalent manner. In the process of the invention, the oxygen of the injected air is the main oxidant and typically produces 90% of the oxidation action.

The Applicant has found that it was then possible to modify the solubility of the sludge, or precipitated from the spent bath, by adjusting the REDOX potential of the bath during pickling. The "sludge" is not very soluble when the bath has been adjusted below +100 mV or above +300 to 350 mV, and their solubility is greatly improved between +100 mV and +300 mV, and more particularly between +190 mV and +260 mV, an optimal bath control being 220 + 20 mV.

For a spent bath having thus stripped strips of stainless steel with a REDOX potential of between 200 and 240 mV, and containing approximately 60 g / 1 of iron in the form of "slurries" of precipitated fluorides, the recycling of these sludges in a bath new can be done as follows: the liquid from the used bath is drawn off, then hot water (40 to 60 ° C) is sent to the sludge to dissolve it, then the HF content is adjusted by adding free HF ( 15 to 20 g / l) and stirred. Then inject a little hydrogen peroxide to adjust the potential to about +220 mV and you get a new bath. This possibility of recycling sludge is particularly interesting from an industrial point of view. As will be shown in Examples 3 to 5, it seems that this favorable dissolution of the sludge either related to the precipitation of a mixed iron fluoride, formed mainly between +100 mV and +300 mV and more especially between + 190 mV and +260 mV.

For the preparation of the pickling bath, ferric fluoride or ferric sulphate or ferric chloride is generally used, with a ferric iron concentration of between 20 and 40 g / l, with a preference for ferric fluoride, so as to have only one acid radical in the bath.

The pickling process of the invention is applied to stainless steel sheets or strips with typically the initial HF concentrations and the following pickling temperatures:

- ferritic stainless steels: HF 10 to 25 g / l, 35 to 50 ° C.

- austenitic stainless steels: HF 20 to 35 g / l, 40 to 60 ° C.

In addition to solving the pollution problem posed, the pickling process of the invention provides industrial advantages with significant advantages:

- the adjustment of the quality of the bath is all the more convenient and precise that the major part of the oxidation is produced by the air injection (s);

- adjusting the level of the reduction oxidation potential makes it possible to obtain reusable "sludge" directly in the form of a new bath.

TESTS AND EXAMPLES

. Test series n ° 1

Its purpose was to qualitatively test the effect of an air injection associated or not with a complementary injection of hydrogen peroxide.

The pickling tests were carried out on samples of ferritic stainless steel 17% Cr type AISI 430 hot rolled, shot and electroplated, having the shape of rectangular test pieces 50 x 25 x 3 mm. The pickling conditions for these samples were as follows:

- HF concentration: 20 g / l

. bath volume: 250 ml

. sample immersion time in the bath: 2 minutes

. initial concentration of dissolved iron (ferric fluoride) varying from 0 to 60 g / l

. H ₂ O ₂ concentration from 0 to 5 g / l

. air injection or not in the solution

. temperature: 45 ° C.

This air injection here was of the order of 1 1 / min, that is to say very in excess relative to the useful flow rate.

For each condition, 3 to 5 samples were successively pickled. The assessment of the quality of the pickling obtained was carried out qualitatively according to an examination with a binocular magnifying glass at 25 magnification, giving a score of "0" to "5":

. "0": no stripping

. "1": start of pickling, irregular

. "3": pickling acceptable, fairly regular

. "5": pickling of very good quality.

The main scores obtained, corresponding to the 3rd samples of various conditions, are summarized in TABLE I below:

Initial iron concentration (Fe ³ +) g / l without with without with injection injiaction injection air injection air air air

0 1 1 5 1 2 1 3 10 1 2 2 5 20 2 3 3 5 30 2 4 3 5 60 5 5 5 5 These tests show that, without the addition of hydrogen peroxide, the injection of air here improves the quality of the pickling between 5 and 30 g / l of dissolved Fe ³⁺ and that, the quality of the pickling is then "acceptable" from from 15 to 20 g / l of Fe ³⁺ and "good" from 25 to 30 g / l of Fe ³⁺ . Associated with a small addition of 2 g / l of hydrogen peroxide, the air injection makes it possible to obtain a very good pickling here from 10 g / l of Fe ³⁺ . At the level of 60 g / l of Fe ³⁺ the brevity of the tests does not make it possible to observe an effect of wear of the baths, and the uniformity of the note "5" in the various cases does not allow any practical conclusion other than that of a satisfactory initial condition.

. Test series n ° 2

Consistent pickling tests were carried out in the laboratory of several hundred samples similar to the samples in test series No. 1, always in the same pickling solution of initial composition HF 20 g / l, with periodic recharging on the one hand in HF to keep HF 20 g / l and on the other hand in H ₂ O ₂ to the minimum necessary taking into account the concentration of iron in the solution, this with injection of air into the pickling bath.

The total dissolved iron concentration, the cumulative consumption of HF and the cumulative consumption of hydrogen peroxide H ₂ O ₂ were monitored respectively according to the number of pickled samples, each for 2 min. It has been observed that up to 275 to 300 pickled samples, corresponding to 25 to 27 g / l of dissolved iron, the consumption of HF and H ₂ O ₂ are quite high and roughly proportional to the number of pickled samples, and that beyond that consumption of HF and H ₂ O ₂ becomes very low. Thus, when the dissolved iron concentration becomes greater than 25 g / l, the consumption of concentrated HF at 70% surprisingly goes from 7 ml per 100 pickled samples to 0.3 ml per 100 pickled samples.

The explanatory hypotheses are as follows: the oxygen in the air injected into the bath acts as an ion regenerator (Fe ³⁺ ) according to the equilibrium reaction (C) already indicated, by moving this equilibrium in the direction 3 of the formation of Fe ³⁺ , the pH of the solution being favorable and of the order of 2 as a result of the HF concentration. Yes this reaction (C) is regulated so that it allows a regeneration of Fe ²⁺ to Fe ³⁺ fast enough to always have Fe ³⁺ > 20 to 25 g / l, there is almost no need for H ₂ O ₂ . And the consumption of HF is surprisingly much lower than for the lower concentrations of iron and therefore Fe ³⁺ .

Example 1 of pickling according to the invention

The following conditions have been found satisfactory for the continuous pickling of 1 m wide 17% Cr ferritic stainless steel strips. The strips were pickled in a tank 16 m long x 2 m wide containing about 30,000 l of acid pickling bath, they passed through this bath at a speed of 20 m / min and were then brushed under water.

The bath contained 20 g / l of HF and initially 25 g / l of Fe ³⁺ , coming from ferric fluoride dissolved in the bath. Air was injected into the bath mainly with nozzles spaced 2 to 3 m apart and directed downwards with an inclination of 15 ° with respect to the vertical, the air being released at the end of these nozzles towards the bottom of the tray and 15 cm from this bottom. The total flow rate of air injected into the bath was 100 Nm ³ / h, 2/3 of which towards the bottom and in the vicinity of this bottom with the nozzles which have just been described. The bath temperature was 40 to 45 ° C. The bath was controlled by measuring and adjusting its REDOX potential above +150 mV. Additions of hydrogen peroxide were planned to quickly correct this potential if it became too low. In practice, we have been able to operate for 3 days in a row with a REDOX potential which has remained satisfactory without adding H ₂ O ₂ . In addition, it was noted that the pickling was still satisfactory at the level of a REDOX potential of +100 mV.

In 1 hour, the total surface of stripped strip is: 20x2x1x60 = 2400 m ² / h and the stripping time of each surface element is 16/20 = 0.8 min = 0.8 / 60 h. The total flow of injected air is therefore:

100 Nm ³ per 32 m ² xh or 3.1 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element. Example 2 pickling according to the invention

It concerns the continuous pickling of austenitic stainless steel strips 1.25 m wide, 0.8 mm thick. After treatment in electrolytic baths, the strips were pickled in two successive tanks of the same dimensions as that of Example 1 containing approximately 30,000 l of pickling bath, they passed through these baths at 40 m / min giving a residence time in each 0.4 min bath.

The baths contained 25 g / l of HF and initially 20 g / l of Fe ³⁺ . Air was injected with nozzles with a layout similar to that of Example 1 with a total flow rate for each tank of 80 m ³ / h and a pressure of 0.2 MPa, ie a flow rate of approximately 160 Nm ³ / h. The bath temperature was 50 to 55 ° C.

The bath was controlled by measuring and adjusting its REDOX potential above +200 mV. Additions of hydrogen peroxide were planned as an additional means of oxidation to readjust the REDOX potential when it had become too low. We were able to operate for periods of several days without using this additional oxidation means and while retaining a REDOX potential of +200 to +300 mV with good pickling quality.

The injected air flow here is 4 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element.

Example 3 pickling according to the invention

Strips of austenitic stainless steel were pickled with the following modifications with respect to Example 2:

HF 35 g / l

REDOX potential: +350 to +400 mV

Dissolved iron: 60 g / l, of which approximately 80% Fe ³ .

The complex formed is of the FeF3, 3H ₂ O type. It has been found that this compound was soluble neither in water at 20 ° C. nor in an aqueous solution of 20 g of HF per liter at 20 ° C. hydrolyzes). On the other hand, at 50 ° C, it is moderately soluble: at 31 g / l in water and at 38 g / l in HF 20 g / l. This dissolution, unstable on cooling, is not satisfactory.

Example 4 of pickling according to the inventoon Same pickling conditions except REDOX potential: +50 to +80 mV.

Fe ²⁺ represents approximately 80% of the dissolved iron, and the complex formed is of the FeF ₂ , nH ₂ O type. The same dissolution tests as in Example 3 were carried out. This compound is sparingly soluble, the only dissolution noted is 13 g / l in the case of HF 20 g / l at 50 ° C.

Example 5 of pickling according to the invention

The pickling conditions correspond to those of Example 2, with the exception of the REDOX potential set to +220 mV + -20 mV (measured between a platinum electrode and an Ag / AgCl reference electrode).

Fe ³⁺ represents 70 to 80% of the dissolved iron, and the majority compound formed seems to be of the type: Fe ₂ F ₅ , 7 H ₂ O. The dissolution tests provided the following results, in dissolved g per liter:

Solubility at 20 ° C Solubility at 50 ° C in water in solution in water in solution HF 20 g / l HF 20 g / l

22.3 26 53 61

This type of "mud" can be recycled in a new bath, according to the method described above.

Claims

1. Process for pickling stainless steel products, in which a pickling bath of initial composition is used:

HF 10 to 50 g / l

Ferric iron (Fe ³⁺ ) dissolved ≥ 15 g / l

Water: the balance at a temperature between 15 and 70 ° C, characterized in that, during the pickling operation or operations, the iron content of the bath is kept at least 15 g / l by oxidation of the bath comprising at minus one or more total air injections greater than or equal to 1 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element.

2. Method according to claim 1, in which a pickling bath is used which initially contains HF 10 at 35 g / l and Fe ³⁺ ≥ 20 g / l, characterized in that during the pickling operation or operations the content is maintained in Fe ³⁺ at at least 20 g / l thanks to an oxidation of the bath comprising one or more air injections of total flow rate between 1 and 8 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element.

3. Method according to claim 2, characterized in that one injects a total of 2 to 5 Nm ³ of air per m ² of pickled stainless steel and per hour of pickling of each pickled surface element and in that, on these

2 to 5 Nm ³ per m2 and per hour, at least half of it is injected towards the bottom of the bath into the lower half of this bath.

4. Method according to any one of claims 1 to 3, characterized in that, to ensure a sufficient concentration of Fe ³⁺ , the REDOX potential of the bath is measured and it is adjusted between 0 and +800 mV by acting if necessary on the oxidation of the bath.

5. Method according to claim 4, characterized in that the REDOX potential of the bath is adjusted between +100 and +300 mV.

6. Method according to claim 5, characterized in that one adjusts the REDOX potential of the bath between +190 and +260 mV.

7. Method according to any one of claims 1 or 2, characterized in that the pickling bath is prepared using ferritic fluoride, with an initial concentration of ferric iron of 20 to 40 g / l.

8. Method according to any one of claims 2, 3, 5 or 6, characterized in that one uses as an additional means of oxidation of the hydrogen peroxide bath or potassium permanganate.

9. Method according to any one of claims 2,3,5 or 6, characterized in that one uses as the only additional means of oxidation of the bath 0.1 to 0.4 1 of hydrogen peroxide H ₂ O ₂ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element.

10. Method according to any one of claims 1 to 3, in the case of pickling of ferritic stainless steel sheets or strips, in which the initial HF concentration of the pickling bath is 10 to 25 g / l and the temperature pickling is between 35 and 50 ° C.

11. Method according to any one of claims 1 to 3, in the case of the pickling of austenitic stainless steel sheets or strips, in which the initial concentration of HF in the pickling bath is from 20 to 35 g / l and the temperature of the pickling bath is between 40 and 60 ° C.

12. Process for pickling stainless steel products, using a pickling bath initially containing HF 10 to 35 g / l and Fe ³⁺ ≥ 20 g / l, and for recycling the precipitated sludge in the spent pickling bath , characterized in that successively: a) during the pickling operation (s), the Fe content is maintained at at least 20 g / l thanks to an oxidation of the bath comprising one or more air injections with total flow rate between 1 at 8 Nm3 per m2 of pickled stainless steel and per hour of pickling of each pickled surface element and possibly one or more additions of strong oxidant, the REDOX potential of the bath being adjusted between + 100 and + 300 mV by acting if necessary on the oxidation of the bath; b) the liquid from the spent bath is drawn off, then hot water is sent to the sludge to dissolve it, then the HF content is adjusted by adding free HF and the mixture is stirred. Then hydrogen peroxide is injected so as to adjust the potential between + 200 and + 240 V, thus obtaining a new pickling bath.

13. Method according to claim 12, characterized in that: a ₁ ) the air injections have a total flow rate of between 2 and 5 Nm ³ per m ² of pickled stainless steel and per hour of pickling of each surface element pickled; a ₂ ) using only the additions of strong oxidant 0.1 to 0.4 1 of hydrogen peroxide H ₂ O ₂ per m ² of pickled stainless steel and per hour of pickling of each pickled surface element; a ₃ ) the REDOX potential of the pickling bath is adjusted between + 190 and + 260 V. b) for recycling the sludge from the spent pickling bath, hot water is used. 40 to 60 ° C.