FR2587369A1 - ACID PICKLING PROCESS 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 (Fe<3+>) >= 15 g/l; water: the remainder, at a temperature comprised between 15 and 70<o>C, 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 Nm<3> per m<2> 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

ACID PICKLING PROCESS OF STAINLESS STEEL PRODUCTS
The field of the present invention is that of surface treatments and more specifically of acid pickling of stainless steel products.

STATEMENT OF THE PROBLEM
The acid pickling of stainless steels is usually carried out with fluoronitric baths, in which the use of nitric acid has the drawback of causing the formation of nitrous vapors polluting the atmosphere and of soluble nitrates polluting the liquid effluents.

In the context of the 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 to avoid such pollution.

KNOWN STATE OF THE ART
In his work 'tSTAINLESS IRON AND STEEL' (CHAMAN & HALL LTD, London 1951) JHG MONYPENNY indicates pp. 183-184 that, to minimize the problem of vapors from fluoronitric pickling baths, it has been used for the pickling of steel sheets stainless baths containing 6 to 12 z of 90 z solution of ferric sulfate and 1.5 to 3 of hydrofluoric acid and eg at 70-800C for descaling a hot rolled sheet.

The initial ferric iron concentration of the previous baths is thus approximately 16.5 to 33 g / l. The Applicant's tests have shown that, when successive samples of stainless steel sheet are pickled in such baths, the speed and the quality of the pickling deteriorate rapidly. These acid pickling baths are therefore not satisfactory as such for the serial or continuous pickling of stainless steel products.

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

DISCLOSURE 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) dissolved) 15 g / l
Water: the balance at a temperature between 15 and 700C, and in which, again, during the pickling operation (s), the ferric iron content of the bath is maintained at at least 15 g / l by oxidation of the bath comprising at least its ventilation by circulation in the open air or by air injection.

For industrial practice, and in particular for the repetitive or continuous pickling of stainless steel products in at least one large tank, one or more pickling baths initially containing HF 10 to 35 g / l and Fe3 + 20 g / l are typically used. , and during the pickling operation (s), the Fe content of this bath or baths is maintained at at least 20 g / l by oxidizing the bath (s) comprising one or more injections of air with a higher total flow rate or equal to 2 Nm3 / h per m2 / h of pickled stainless steel and per hour of pickling of each pickled surface element.

equilibre 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 Fe 2+ par aération de la solution décapante, éventuellement complétee 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 of the air introduced seems to intervene in the process of in 2+ as a regenerating oxidant Fe2 + 3+ 3+, in addition as an oxidizing agent which regenerates Fe to Fe, while Fe constitutes an oxidant acting on the base metal for the dissolve. The essential reactions could be the following - dissolution reaction

balance almost completely shifted in direction 1 under normal pickling conditions; - other dissolution reaction

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

equilibrium 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 ferric iron concentration of the bath can be calculated as the difference between the total iron concentration, determined for example by atomic absorption, and the Fe concentration determined by its oxidation to Fie3 + in the presence of permanganate KMnO4. A suitable aeration of the pickling bath, typically by air injection, allows the quality of the pickling to be maintained during successive pickling or continuous pickling of stainless steel products by regenerating Fe3 +.

The total volume of air injected into the pickling bath depends essentially on the quantity of pickled stainless steel, an amount itself proportional to the pickled surface and to the pickling time of this surface. For the pickling thus considered, and according to the tests and industrial adjustments already carried out, the total flow rate of air injected into the pickling bath of the invention is typically between 2 and 5 Nm3 / h per m2 / h of pickled stainless steel and per hour of pickling of each pickled surface element. In order for the pickling bath to be properly aerated, it is then necessary to inject a good part of this volume of air, that is typically at least half of this volume, with nozzles directed towards the bottom of the bath at half lower part of this bath.

For industrial operation of the pickling bath, the HF refills are carried out as usual, and, rather than determining the Fie3 + concentration of the bath, it is practical to determine the REDOX potential of the bath and 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 minimum value depends on the grades of stainless steel, and has been found between 0 and +100 mV for certain ferritic stainless steels with a Cr content of less than 18%.

The REDOX potential is measured between a platinum electrode and an Ag / AgCl or saturated calomel reference electrode. A device for measuring this REDOX potential can be suitably sealed so as to allow continuous measurements in the bath.

Depending on the Fe3 + 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 to the concentration more quickly. in desired Fe or at the setpoint REDOX potential, so as to recover a good pickling. At least one addition of a 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 injection of oxygen or to increase the air flow.

For the preparation of the pickling bath, ferric fluoride or ferric sulphate or ferric chloride are 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 concentrations of HF and the following pickling temperatures - ferritic stainless steels: HF 10 to 25 g / l, 35 to 500C.

- austenitic stainless steels: HF 20 to 35 g / l, 40 to 600C.

In addition to solving the pollution problem posed, the pickling process of the invention provides significant savings in industrial operation.

TESTS AND EXAMPLES Test series no.1
Its aim 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 hot-rolled 17% Cr ferritic stainless steel AISI 430 type, shot peened and electrolytically pickled, in the form 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 dissolved iron concentration (ferric fluoride) varying from O
at 60 g / l H202 concentration from 0 to 5 g / l injection of air or not into the solution.

temperature: 450C.

This air injection was here of the order of 1 1 / mon, that is to say very much in excess with respect to the useful flow rate.

For each condition, 3 to 5 samples were successively etched.

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 from "O" to "5".
. "O": no stripping
"1": start of stripping, irregular
"3": acceptable stripping, fairly regular
. "5": very good quality stripping.

The main scores obtained, corresponding to the 30 samples of the various conditions, are summarized in Table I below.

<tb> Concentration <SEP> without <SEP> H2O2 <SEP> with <SEP> 2 <SEP> g / 1 <SEP> H22
<tb><SEP> in <SEP> iron
<tb><SEP> without <SEP> with <SEP> without <SEP> with
<tb><SEP> (Fe3 +) <SEP> injection <SEP> injection <SEP> injection <SEP> injectian
<tb><SEP> g / l <SEP> air <SEP> air <SEP><SEP> air <SEP><SEP> air
<tb><SEP> O <SEP> - <SEP><SEP> 1 <SEP> 1 <SEP>
<tb><SEP> 5 <SEP> 1 <SEP> 2 <SEP> 1 <SEP> 3
<tb><SEP> 10 <SEP> l <SEP> 2 <SEP> 2 <SEP> 5
<tb><SEP> 20 <SEP> 2 <SEP> 3 <SEP> 3 <SEP> 5
<tb><SEP> 30 <SEP> 2 <SEP> 4 <SEP> 3 <SEP> 5
<tb><SEP> 60 <SEP> 5 <SEP> 5 <SEP> 5 <SEP> 5
<tb>
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 Fie3 + and that the quality of the pickling is then "acceptable" from 15 at 20 g / l of Fe3 + and "good" from 25 to 30 gil of Fie3 +. Associated with a small addition of 2 g / l of hydrogen peroxide, the injection of air here makes it possible to obtain very good stripping from 10 g / l of Fe. At the level of 60 g / l of
Fe3 + 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 no.2
Consecutive pickling tests were carried out in the laboratory of several hundred samples similar to the samples of test series no.1, always in the same pickling solution of initial composition HF 20 g / l, with periodic recharges of '' on the one hand in HF to keep HF 20 g / l and on the other hand in H202 to keep the H202 concentration to the minimum necessary given the iron concentration in the solution, this with injection of air into the pickling bath .

The total concentration of dissolved iron, the cumulative consumption of HF and the cumulative consumption of hydrogen peroxide H202 as a function of the number of pickled samples were followed, 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 HF and H202 consumptions are quite high and roughly proportional to the number of pickled samples, and that at- beyond this, the consumption of HF and H202 becomes very low. Thus, when the dissolved iron concentration becomes greater than 25 g / l, the consumption of HF concentrated at 70% surprisingly drops 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 (Fe3) according to the equilibrium reaction (C) already indicated, by shifting this equilibrium in the
3+ sense 3 of the formation of Fe, the pH of the solution being favorable and of the order of 2 as a result of the HF concentration. If we adjust this reaction (C) so that it allows a regeneration of Fe to Fe fast enough to always have Fe3> 20 to 25 g / l, there is no longer any need for H2O2. And the HF consumption is surprisingly much lower than for the lower concentrations of iron and therefore in
Fe3 +.

Example No. 1 of pickling according to the invention
The following conditions have been found to be satisfactory for the continuous pickling of 1m wide 17% Cr ferritic stainless steel strip. 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, coming from ferric fluoride dissolved in the bath. Air was injected into the bath mainly with nozzles spaced 2 to 3 n and directed downwards with an inclination of 150 with respect to the vertical, the air being released at the end of these nozzles towards the bottom of the tank. and 15 cm from this bottom. The total air flow rate injected into the bath was 100 Nm3 / h, 2/3 of which towards the bottom and in the vicinity of this bottom with the nozzles which have just been described. The temperature of the 6ain was 40 to 450C. The bath was run by measuring and adjusting its REDOX potential above +150 mV.

Additions of hydrogen peroxide were planned for rapid correction of this potential if it became too low. In practice, it was possible to operate for up to 3 days in a row with a REDOX potential that remained satisfactory without adding H202. In addition, it was observed that the pickling was still satisfactory at the level of a REDOX potential of +100 mV.

In 1 hour, the total stripped strip surface is: 20x2x1x60 = 2400 m2 / 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:
2
m bare per 32 m2 xh or 3.1 Nm3 / h per m2 / h of pickled stainless steel and per pickling bde of each pickled surface element.

Exeme n 2 of decaEage according to the invention
It concerns the continuous stripping 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 1 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. Air was injected with nozzles of arrangement similar to that of Example No. 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 temperature of the bath was 50-55 C.

The conduct of the bath was made by measuring and adjusting its potential
REDOX 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 6as. It was possible to operate for periods of several days without using this additional oxidation means and while maintaining a REDOX potential of +200 to + 300 mV with good pickling quality.

The flow rate of injected steel is here 4 Nm3PS per m2 / h of pickled stainless steel and per pickling hour of each pickled surface element.

Claims (10)

1. Process for pickling stainless steel products, in which a pickling bath of the initial composition is used: HF 10 to 50 g / l Dissolved ferric iron (Fe)} 15 g / l Water: the balance at a temperature between 15 and 700C, characterized in that, during the pickling operation (s), the ferric iron content of the bath is maintained at at least 15 g / l by oxidation of the bath comprising at least its ventilation by circulation in the open air or by air injection. 2. Process according to claim 1, in which a pickling bath is used initially containing HF 10 to 35 g / l and Fe3 +> 20 g / l, characterized in that during the pickling operation (s) the content is maintained. in Fe 3+ to at least 20 g / l thanks to an oxidation of the bath comprising one or more injections of air with a total flow rate greater than or equal to 2 Nm3 / h per m2 / h of pickled stainless steel and per h-de stripping of each stripped surface element. 3. Method according to claim 2, characterized in that a total of 2 to 5 Nm3 / h of air is injected per m2 / h of pickled stainless steel and per hour of pickling of each pickled surface element and in that, on these 2 to 5 Nm3 / hz we inject at least half of it towards the bottom of the hain at the upper half of this bath. 4. Method according to any one of claims 1 to 3, characterized 3+ in that, to ensure a sufficient concentration of Fe, 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 any one of claims 1 or 2, characterized in that the pickling bath is prepared using ferric fluoride, with an initial ferric iron concentration of 20 to 40 g / l. 7. Method according to any one of claims 2, 3 or 5, characterized in that one uses as additional means of oxidation of the bath at least one addition of hydrogen peroxide. 8. A method according to any one of claims 2, 3 or 5, characterized in that as additional means of oxidation of the bath at least one addition of potassium permanganate solution is used. 9. A method according to any one of claims I to 3, in the case of pickling ferritic stainless steel sheets or strips, in which the initial HF concentration of the pickling bath is 1D to 25 gil and the pickling temperature. is between 35 and 500C. 10. A method according to any one of claims 1 to 3, in the case of pickling sheets from austenitic stainless steel strips, in which the initial concentration of HF in the pickling bath is 20 to 35 g / l and the temperature. of the pickling bath is between 40 and 600C.