EP0236354B1 - Method for the acid etching of stainless steel products - Google Patents

Abstract

Method for etching stainless steel products wherein an etching bath is used which has the following ignition composition: HF 10 to 50 g/l; dissolved ferric iron (Fe3+) >= 15 g/l; water: the remainder, at a temperature comprised between 15 and 70oC, characterized in that during the etching operation or operations, the content of ferric iron of the bath is maintained at at least 15 g/l by oxidation of the bath comprising at least one or a plurality of air injections with a total flow rate higher or equal to 1 Nm3 per m2 of etched stainless steel and per hour (h) of etching of each etched surface element. The method of the invention is particularly applicable to the industrial etching of sheets and bands of stainless steel, and avoids the utilization of nitric acid and the pollutions resulting therefrom.

Description

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

Statement 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, stainless steel sheets were used for pickling baths containing 6 to 12% of 90% solution of ferric sulfate and 1.5 to 3% hydrofluoric acid and this for example at 70 - 80 ° C for descaling a hot rolled sheet. The initial concentration of ferric iron in the preceding baths is thus approximately 16.5 to 33 g / I The tests of the applicant have shown that, when successive samples of stainless steel sheet are scoured in such baths, the speed and the quality of the pickling deteriorate rapidly. These acid pickling baths are therefore not satisfactory as such for serial or continuous pickling of stainless steel products.

Furthermore, the document DE-C-899 890, published in 1953, describes a process for regenerating an HF / Fe ³⁺ pickling bath of steels by adding nitrites, part of these nitrites being able to be replaced by l oxygen introduced into the bath by means of air injection. For its part, document GE-A-2 000 196 describes a process for controlling the composition of a pickling bath for stainless steels containing HF and ferric sulphate, in which the REDOX potential of the bath is maintained within a chosen interval by addition of hydrogen peroxide and sulfuric acid. Industrial tests for pickling stainless steel strips by baths containing hydrofluoric acid and hydrogen peroxide were made by the applicant, who observed runaway temperatures in the baths as well as a significant consumption of hydrogen peroxide making the process very expensive compared to the fluonitric pickling process for 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

• HF 10 à 50 g/l
• Fer ferrique (Fe³+) dissous > 15 g/1
• Eau : le solde

à une température comprise entre 15 et 70°C, et dans lequel, de façon nouvelle, pendant la ou les opérations de décapage, on maintient la teneur en fer ferrique du bain à au moins 15 g/I par oxydation du bain comportant au moins une ou des injections d'air de débit total supérieur ou égal à 1 Nm³ par m² d'acier inoxydable décapé et par h de décapage de chaque élément de surface décapé, ou bien une aération équivalente par circulation à l'air libre.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 / 1
• 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 / I 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 / I and Fe ³⁺ > 20 g / I, and during the pickling operation (s) the Fe ³⁺ content of this bath or these baths is kept at least 20 g / I thanks to an 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 of greater total flow have proved to be of no interest, the saturation of the bath with oxygen in the air being undoubtedly reached and the additional air flows no longer apparently serving only to stir the bath, and this in a way eventually excessive.

• - réaction de dissolution:
  
  équilibre presque totalement déplacé dans le sens 1 dans les conditions normales du décapage;
• - autre réaction de dissolution:
  
  également possible en milieu oxydant, ce qui est le cas;
• - oxydation de Fe2+ par aération de la solution décapante, éventuellement complétée par un autre moyen d'oxydation:
  
  équilibre fortement déplacé dans le sens 3 si la solution est correctement oxydée et dans le cas du pH du bain de décapage qui est compris entre 1 et 3 environ.

The oxygen in the air introduced seems to intervene in the process of the invention as an oxidizing agent regenerating Fe2 + to Fe ³⁺ , while Fe ³ + constitutes an oxidant acting on the base metal to dissolve it. The essential reactions could be as follows

• - dissolution reaction:
  
  balance almost completely displaced in direction 1 under normal stripping conditions;
• - other dissolution reaction:
  
  also possible in an oxidizing medium, which is the case;
• - Fe2 + oxidation by aeration of the pickling solution, possibly supplemented by another means of oxidation:
  
  balance strongly shifted 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 concentration of ferric iron in the bath can be calculated as the difference between the concentration of total iron, determined for example by atomic absorption, and the concentration of Fe2 + measured by its oxidation to Fe ³⁺ in the presence of permanganate KMn0 ₄ . 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. from 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, the refills are carried out as usual, and, rather than determining the concentration of the bath, it is practical to determine the REDOX potential of the bath and to set 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 grade 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 / AgCI reference electrode or with fixed potential, reproducible 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 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 small amounts of hydrogen peroxide are added to the bath in the form of constant or repetitive additions, additions typically representing on average 0.1 at 0.4 1 of H ₂ 0 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, the optimal bath control being 220 ± 20 mV.

For a used bath having thus stripped stainless steel strips 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: draw up the liquid from the used bath, then send hot water (40 to 60 ° C) to the sludge to dissolve it, then adjust the content by adding free HF (15 to 20 g / I) 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 is linked 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, use is generally made of ferric fluoride or ferric sulphate or ferric chloride, 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.

• - aciers inoxydables ferritiques : HF 10 à 25 g/I, 35 à 50° C.
• - aciers inoxydables austenitiques 20 à 35 g/I, 40 à 60° C.

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

• - ferritic stainless steels: HF 10 to 25 g / I, 35 to 50 ° C.
• - austenitic stainless steels 20 to 35 g / I, 40 to 60 ° C.

• - le réglage de la qualité du bain est d'autant plus commode et précis que la majeure partie de l'oxydation est produite par la ou les injections d'air;
• - le réglage du niveau du potentiel d'oxydation réduction permet d'obtenir des "boues" réutilisables directement sous forme de bain neuf.

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);
• - the adjustment of the reduction oxidation potential level 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 an additional injection of hydrogen peroxide. The pickling tests were carried out on samples of 17% Cr ferritic stainless steel type AISI 430 hot rolled, shot-blasted and electroplated, having the shape of rectangular test pieces 50 x 25 x 3 mm.

• - concentration en HF : 20 g/l
• . volume du bain : 250 ml
• . temps d'immersion de l'échantillon dans le bain: 2 minutes
• . concentration initiale en fer dissous (fluorure ferrique) variant de 0 à 60 g/l
• . concentration en H₂O₂ de 0 à 5 g/I
• . injection d'air ou non dans la solution
• température : 45°C.

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 / I
• . 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 with respect to the useful flow rate.

• . "0" : pas de décapage
• . "1" : début de décapage, irrégulier
• . "3" décapage acceptable, assez régulier
• . "5" : décapage de très bonne qualité.

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 using a binocular magnifying glass at 25 magnification, giving a score of "0" to "5":

• . "0": no stripping
• . "1": start of pickling, irregular
• . "3" acceptable pickling, fairly regular
• . "5": pickling of very good quality.

The main scores obtained, corresponding to the 30 samples of various conditions, are summarized in TABLE 1 below:

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 / I of dissolved Fe ³⁺ and that, the quality of the pickling is then "acceptable" from from 15 to 20 g / I of Fe ³ + and "good" from 25 to 30 g / I of Fe ³ +. Associated with a small addition of 2 g / I of hydrogen peroxide, the air injection allows here to obtain a very good pickling from 10 g / I of Fe ³ +. At the level of 60 g / I 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 a 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 / I, with periodic recharging. on the one hand in order to conserve 20 g / I and on the other hand in H ₂ 0 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 and the cumulative consumption of hydrogen peroxide H ₂ 0 _{2 were} respectively monitored as a function of 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 / I of dissolved iron, the consumption of HF and H ₂ 0 ₂ are fairly high and roughly proportional to the number of pickled samples, and beyond that consumption of HF and H ₂ 0 ₂ becomes very low. Thus, when the concentration of dissolved iron becomes greater than 25 g / l, the consumption of concentrated HF at 70% jumps surprisingly 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 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. If this reaction (C) is adjusted so that it allows a regeneration of Fe2 + to Fe ³⁺ fast enough to always have Fe ³⁺ > 20 to 25 g / I, there is almost no need for H ₂ 0 ₂ . And consumption is surprisingly much lower than for 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 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 run 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 up to 3 days in a row with REDOX potential remaining satisfactory without adding H ₂ 0 ₂ . In addition, it was noted that the pickling was still satisfactory at the level of a REDOX potential of +100 mV.

soit 3,1 Nm³ par m² d'acier inoxydable décapé et par h de décapage de chaque élément de surface décapé.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:

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 relates to 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 about 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 / I of HF and initially 20 g / I 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 a complementary 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 potential of +200 to +300 mV with a good quality of pickling.

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

• HF 35 g/I
• Potentiel REDOX +350 à +400 mV
• Fer dissous : 60 g/I dont environ 80 % de Fe³.

Strips of austenitic stainless steel were stripped with the following modifications with respect to example 2:

• HF 35 g / I
• REDOX potential +350 to +400 mV
• Dissolved iron: 60 g / I, of which approximately 80% Fe ³ .

The complex formed is of the FeF ³ , 3H ₂ 0 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 per liter at 20 ° C. hydrolyzes). On the other hand, at 50 ° C, it is moderately soluble: at 31 g / I in water and at 38 g / I in 20 g / I. This dissolution, unstable on cooling, is not satisfactory.

Example 4 of pickling according to the invention

Same pickling conditions except REDOX potential +50 to +80 mV. Fe2 + represents approximately 80% of the dissolved iron, and the complex formed is of the FeF ₂ , nH ₂ 0 type. The same dissolution tests as in Example 3 were carried out. This compound is sparingly soluble, the only dissolution noted is 13 g / I in the case of HF 20 g / I at 50 ° C.

Example 5 of pickling according to the invention

• Solubilité à 20°C Solubilité à 50°C
• dans l'eau dans solution dans l'eau dans solution
• HF 20 g/l HF 20 g/l
• 22,3 26 53 61

The pickling conditions correspond to those of Example 2, with the exception of the REDOX potential set at +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 0. The dissolution tests provided the following results, in g dissolved 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 (13)

1. Process for pickling stainless steel products, which use is made of a pickling bath with the following initial composition:

HF 10 to 50 g/I

dissolve ferric iron (Fe³⁺) ≽ 15 g/l

water: the remainder

at a temperature between 15 and 70°C, characterized in that, the pickling operation or operations, the ferric iron content of the bath is kept at at least 15 g/I by oxidation of the bath involving at least one or more air injecions with a total flow rate equal to or higher than 1 Nm³/m² of pickled stainless steel and per pickling hour of each pickled surface element.

2. Process according to claim 1 which use is of a pickling bath initially containing HF 10 to 35 g/I and Fe³+ >20 g/I, characterized in that during the pickling operation or operations, the Fe³+ content is kept at at least 20 g/I by oxidation of the bath involving one or more air injections with a total flow rate between 1 and 8 Nm³/m² of pickled stainless steel and per pickling hour of each pickled surface element.

3. Process according to claim 2; characterized in that a total of 2 to 5 Nm³ of air are injected per m² of pickled stainless steel per pickling hour each pickled surface element and in that, on said 2 to 5 Nm³/m²/h, at least half is injected towards the bottom of the bath in the lower half thereof.

4. Process according to any one of the claims 1 to 3, characterized in that, in order to ensure an adequate Fe³+ concentration, the redox potential of the bath is measured and regulated at between 0 and +800 mV by, if necessary, acting on the oxidation of the bath.

5. Process according to claim 4, characterized in that the redox potential of the bath is regulated at between +100 +300 mV.

6. Process according to claim 5, characterized in that the redox potential of the bath is regulated at between +190 and +260 mV.

7. Process according to any one of the claims 1 or 2, characterized in that the pickling bath is prepared by using ferriric fluoride wirh an initial ferric iron concentration of 20 to 40 g/I.

8. Process according to any one of the claims 2, 3, 5 or 6, characterized in that as the complimentary oxidation means for the bath use is made of hydrogen peroxide or potassium permanganate.

9. Process according to any one of the claims 2, 3, 5 or 6, characterized in that the sole complementary oxidation means for the bath is 0.1 to 0.4 I of hydrogen peroxide H₂0_2/m² of pickled stainless steel and per pickling hour of each pickled surface element.

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

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

12. Process for pickling steinless steel products by means of a pickling bath initially containing HF 10 to 35 g/I and Fe³⁺ ₀20 g/I and recycling mud precipitated in the spent pickling bath, characterized in that successively:

a) during the pickling operation or operations, the Fe³⁺ content is kept at at least 20 g/I by oxidizing the bath involving an injection or injections of air with a total flow rate between 1 and 8 Nm³/m² of pickled stainless steel and per pickling hour of each pickled surface element and optionally one or more strong oxidizing agent additions, the redox potential of the bath being regulated at between +100 and +300 mV by, if necessary, acting on the oxidation of the bath,

b) the spent bath liquide is sucked up and then hot water is passed onto the mud in order to solubilize it, followed by the adjustment of the HF content by adding free HF and stirring, followed by the injection of hydrogen peroxide so as to adjust the potential to between +200 and +240 V, thereby obtaining a new pickling bath.

13. Process according to claim 12, characterized in that

a,) the air injections have a total flow rate between 2 and 5 Nm³/m² of pickled stainless steel and per pickling hour of each pickled surface element,

a₂) the sole strong oxidizing agent addition used consists of 0.1 to 0.4 I of hydrogen peroxide H₂0_2/m² of pickled stainless steel and per pickling hour of each pickled surface element,

a3) the redox potential of the pickled bath is regulated at between +190 and +260 V,

b) for recycling the mud of the spent pickling bath use is made of hot water at 40 to 60° C.