FI81126C - Procedure for pickling stainless steel products - Google Patents

Description

1 81126

Method for pickling stainless steel products

The present invention relates to surface treatments and more particularly to the acid coating of stainless steel products.

Acid pickling of stainless steel is usually performed in fluorine nitrogen baths, where the use of nitric acid causes the formation of harmful nitric acid vapors that pollute the air and the formation of soluble nitrates that contaminate the effluent.

In connection with the continuous pickling of stainless steel sheets with an acid, attempts have been made to develop a modified, yet industrially advantageous pickling method which can limit or, preferably, avoid such contamination.

In "Stainless Iron and Steel" (Chapman & Hall Ltd, London 1951) J.H.G. On pages 183-184, Monypenny states that stainless steel plate pickling baths containing 6-12% of a 90% ferric sulfate solution and 1.5-3% of hydrofluoric acid, e.g. The concentration of ferric iron in the initial baths above is thus about 16.5-33 g / l. Studies have shown that when stainless steel specimens are successively pickled in such baths, the pickling speed and quality deteriorate rapidly. Thus, these pickling acid baths are not satisfactory as such to pickle stainless steel products in series or continuously.

It is also known to use baths containing hydrofluoric acid and hydrogen peroxide for pickling. 2 81126 experiments have been carried out in industry on the coating of stainless steel strips and sudden increases in the temperature of the solutions as well as considerable consumption of hydrogen peroxide have been observed, which makes the method very expensive compared to the stainless steel fluorine nitrogen cleaning method. Thus, the replacement of nitric acid with hydrogen peroxide in this process does not appear to be suitable for industrial use.

The invention relates to a method for pickling stainless steel products, in which, as is known, a cover bath is used, the composition of which is initially as follows: HF 10-50 g / l

Dissolved ferric iron (FeP *) £ 15 g / l

Water, if necessary, at a temperature between 15 and 70 ° C and in a completely new way during the pickling operation or operations, the ferric iron content of the bath is kept at least 15 g / l by oxidation with at least one or two air jets with a total flow greater than or equal to 1 Nm ^ per m ^ of pickled stainless steel and per hour of pickling for each pickled surface element, or by conducting an equivalent amount of air by free air circulation.

For industrial use, and especially when repeatedly or continuously pickling stainless steel products in at least one large tank, one or more pickling baths initially containing HF 10-35 g / l and Fe 2+ δ 20 g / l are typically used, and a pickling operation or During these operations, the Fe 2+ content of this bath or baths is maintained at a value of at least 20 g / l by oxidizing the bath or baths so that one or more air jets are passed at a total flow of 1-8 Nm ^ per m ^ of pickled stainless steel and per hour of pickling for each pickled per surface element. Air sprays with a higher total flow have proved to be unnecessary, since the saturation of the bath without Hapel has undoubtedly already been achieved and the additional flows apparently help nothing more than to stir the bath, possibly too much.

The oxygen contained in the supplied air appears to participate in the process of the invention as an oxidizing agent, regenerating Fe 2+ to Fe 3+, while Fe 3+ is an oxidizing agent that acts on the parent metal by dissolving it. The main reactions could be as follows: - dissolution reaction: 2 Fe3 + + Fe -> 3 Fe2 + (A) <- 2 the equilibrium shifts almost completely in direction 1 under normal pickling conditions; - second dissolution reaction:

Fe + 2 HF -> H2 + Fe F2 (B) <- is also possible in the oxidizing medium, as is the case - oxidation of Fe2 + by air introduced into the pickling solution, possibly supplemented by another oxidation means: 3 4 Fe2 + + 02 + 4 H + -> 4 Fe3 + + 2 H2O (C) <- 4 The equilibrium shifts strongly in direction 3 if the solution is oxidized correctly and in the case where the pH of the pickling bath is between approximately 1-3.

* 81126

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

The total volume of air sprayed into the pickling bath is primarily dependent on the amount of pickling stainless steel, which amount itself is proportional to the pickled surface and the pickling time of this surface. According to the tests carried out in the pickling in question and so far in the industry, the total volume of air sprayed into the pickling bath is typically between 2-5 Nm ^ per m ^ of pickled stainless steel and per pickling hour for each pickled surface element. In order for the pickling bath to be properly aerated, it is advantageous to spray a large part of this air volume, typically at least half of this volume, with nozzles directed towards the bottom of the bath, in the lower half of this bath. The sprayed air is preferably preheated to approximately the bath temperature, i.e. typically between 35-60 ° C.

When the bath is used in industry, the headset is refilled in the usual way and rather than determining the Fe ^ + concentration of the bath, it is practical to determine the REDOX potential of the bath and adjust it between 0 and +800 mV, preferably between +100 and +300 mV. adjusting, if necessary, the oxidation of the bath. The reference value for the REDOX potential is selected according to the quality and condition of the steel strip and is readjusted, if necessary, according to the observations on the condition of the surface after pickling.

The REDOX potential is measured between a platinum electrode and an Ag / AgCl reference electrode or reference electrode with a fixed reproducible potential of 5 81126 and an irreversible force of zero. The measuring device for this REDOX potential can be conveniently sealed so that it is possible to perform measurements from the bath continuously.

Depending on the observed Fe 2+ concentration or, more conveniently, the value of the REDOX potential, it may be necessary to use some additional oxidation means to temporarily and / or topically replenish without effect in order to reach the desired Fe 2+ concentration or the indicated REDOX potential more quickly to achieve good pickling. In this case, the addition of a strong oxidizing agent, for example hydrogen peroxide or potassium permanganate, is used as an additional means of oxidizing the bath. In some cases, it is still possible to bring an oxygen jet or raise the air flow into the bath.

In the frequently used case in the industry, where significant amounts of stainless products are pickled in the same bath, hydrogen peroxide is added to the bath mainly in small continuous or repeated amounts and these additions typically represent on average 0.1-0.4 l H 2 O 2 per m 2 of pickled stainless steel and per pickling hour each per surface element. Another oxidizing agent, such as the potassium permanganate already mentioned, can also be used in an equivalent manner. In the process of the invention, the oxygen of the sprayed air is the main oxidizing agent and typically provides 90% of the oxidizing effect.

It has been found that it is possible to modify the solubility of the slurry, i.e. the precipitate of the spent bath, by controlling the REDOX potential of the bath during pickling. The "slurry" is only slightly soluble when the bath is adjusted below -100 mV or above +300 - 350 mV and its solubility is greatly improved between +100 mV - +300 mV and especially between +190 mV - +260 mV and the optimal control for the bath is 220 ± 20 mV.

6 81126

When the bath has been used in this way to pick up stainless steel strips with a REDOX potential between 200 and 240 mV and containing about 60 g / l of iron in the form of a "slurry" of precipitated fluorides, these slurries can be returned to the cycle in a new pickling bath as follows: , hot water (40-60 ° C) is then passed over the slurry to dissolve it, then the HF content is adjusted by adding free HF (15-20 g / l) and the whole mixture is stirred. A little hydrogen peroxide is then sprayed to adjust the potential to about 220 mV and a fresh bath is complete. This possibility of returning the sludge to the cycle is particularly useful on an industrial scale. As shown in Examples 3-5, this preferred dissolution of the slurry is apparently associated with the precipitation of ferrous mixed fluoride, which is formed mainly between +100 mV and +300 mV and especially between +190 mV and + 260 mV.

Ferric fluoride or ferric sulfate or ferric chloride is usually used to make the pickling bath, and the concentration of ferric iron is between 20 and 40 g / l, and ferrifluoride is preferred so that there is only one acid radical in the bath.

The pickling process of the invention is applied to stainless steel sheets and strips and typically the initial HF concentrations and cleaning temperatures are as follows:

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

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

In addition to solving the problem of contamination, the pickling process according to the invention offers considerable advantages in industrial use: can be reused directly in the form of a new bath.

Experiments and examples Test series no

The aim was to qualitatively test the effect of an air jet combined with or without a complementary hydrogen peroxide jet. Pickling tests were performed on stainless ferritic steel specimens containing 17% Cr, type AISI 430, hot rolled, steel sandblasted and electrolytically cleaned and rectangular 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 - immersion time of the samples in the bath: 2 min - initial concentration of dissolved iron (ferrifluoride) ranged from 0 to 60 g / l - Η2θ2: η concentration 0-5 g / l - air was sprayed into the solution or not sprayed - temperature: 45 ° C.

Air spraying was of the order of 1 1 / min, i.e. a large excess calculated from the useful flow.

In all conditions, 3-5 samples were dressed in succession. The quality of the obtained pickling was evaluated qualitatively by examination with a binocular microscope at 25x magnification, giving a rating of 0-5: β 81126 0: no pickling 1: pickling at the beginning, uneven 3: satisfactory pickling, fairly even 5: pickling quality very good.

The main grades obtained, which corresponded to the third samples under different conditions, are summarized in the following Table I:

Iron concentration Without H2O2 2 g / l H2C> 2 at the beginning --------------------------- ------- ------- (Fe ^ +) Without spray- Without spray- Without Without air g / 1 weaving with wetting jet- shower with missing wilting with missing 0 - - 1 1 5 1 2 13 10 1 2 2 5 20 2 3 3 5 30 2 4 3 5 60 5 5 5 5 These experiments show that without the addition of hydrogen peroxide, air spraying improves the quality of the pickling at a concentration of 5-30 g / l dissolved Fe 2+ and the quality of the pickling is then "satisfactory". "from 15-20 g / l Fe 2+ and" good "from 25-30 g / l Fe 2+. When a small addition of 2 g / l of hydrogen peroxide is combined with it, a very good pickling can be obtained from 10 g / l of Fe 2+ by spraying air. Due to the shortness of the experiments, it is not possible to detect any effect of bath wear at a concentration of 60 g / l Fe 2+ and an equal grade in 5 different cases does not justify any practical conclusion other than that the conditions are initially satisfactory.

Test series no. 2

In the laboratory, 9 81126 successive pickling tests were carried out on several hundred samples, similar to those of test series No. 1, always with the same pickling solution initially having a composition of HF 20 g / l and periodically re-batched with HF on one side to HF: The concentration of n would be maintained at 20 g / l and the minimum amount of H 2 O 2, taking into account the concentration of iron in the solution, and air was sprayed into the pickling bath.

The total dissolved iron concentration, the cumulative consumption of HF and the cumulative consumption of hydrogen peroxide H2O2 as a function of the number of pickled samples were monitored for 2 min each. It was found that up to 275-300 pickled samples, corresponding to 25-27 g / l dissolved iron, the consumption of HF and Η2θ2: η is quite high and approximately in relation to the number of pickled samples, and that above this amount HF and Η2θ2: η consumption becomes very small. Thus, when the dissolved iron concentration exceeds 25 g / l, the consumption of concentrated 70% HF changes surprisingly from 7 ml per 100 pickled samples to 0.3 ml per 100 pickled samples.

The explanation is presumably as follows: the air oxygen sprayed into the bath acts as a regenerator of Fe 2+ ions according to the equilibrium reaction C) described above, shifting this equilibrium to Fe 3+ formation direction 3 when the pH of the solution is preferred and about 2 HF. If this reaction C) is adjusted to allow the regeneration of Fe 2+ to Fe 2+ fast enough for Fe 2+ to> 20-25 g / l at all times, then hardly any more H 2 O 2 is needed. The consumption of HF is also surprisingly much lower than for lower concentrations of iron and thus Fe 2+.

Example 1, Pickling according to the invention The following conditions were found suitable for continuous pickling of stainless ferritic steel strips containing 17% chromium and 1 m wide. The strips were washed in a basin 16 m long and 2 m wide containing about 10 81126 30,000 l of acid pickling bath, and passed through this bath at a speed of 20 m / min and then brushed under water.

The bath contained 20 g / l HF and initially 25 g / l Fe 2+ from the ferric fluoride dissolved in the bath. Air was sprayed into the bath mainly from nozzles 2-3 m away and directed towards the bottom by an angle of inclination of 15 ° with respect to the vertical plane, and air was released from the tip of these nozzles towards the bottom of the basin and 15 cm from this bottom. The total flow of air sprayed into the bath was 100 Nm2 / h, of which 2/3 towards the bottom and the bottom environment using the nozzles described above. The bath temperature was 40-45 ° C. The bath was regulated by measuring its REDOX potential and keeping it above +150 mV. Provision had been made for the addition of hydrogen peroxide to rapidly correct this potential if it became too weak. In practice, the bath has been able to operate for up to 3 consecutive days with the REDOX potential remaining satisfactory without the addition of Η2θ2ΐη. In addition, it could be seen that the pickling result was still satisfactory at the REDOX potential level of +100 mV.

In one hour, the total surface area of the pickled strip is 20 x 2 x 1 x 60 = 2400 m2 / h and the pickling time of each surface element is 16/20 = 0.8 min = 0.8 / 60 h. The total flow of sprayed air is thus: 100 Nm2 per 32 m2 xh, ie 3.1 Nm2 per m2 per pickled stainless steel and per pickling hour for each pickled surface element.

Example 2, pickling according to the invention

This is a continuous pickling of stainless austenitic steel strips, 1.25 m wide and 0.8 mm thick. After the treatment in the electrolytic baths, the strips were pickled in two successive basins, the dimensions of which were the same as those of Example 1 and contained approximately 30,000 l of pickling baths; the strips passed through them at 40 m / min, and the residence time in each pool was 0.4 min.

The baths contained 25 g / l HF and the initial concentration of Fe 2+ was 20 g / l. Air was sprayed from nozzles arranged in the same manner as in Example 1, with a total flow in each pool of 80 m 2 / h and a pressure of 0.2 MPa, i.e. a flow of about 160 Nm / h. The bath temperature was 50-55 ° C.

The bath was controlled by measuring its REDOX potential and adjusting it above +200 mV. Hydrogen peroxide additions had been prepared as an additional oxidation means to readjust the REDOX potential when it was too low.

It has been possible to operate the bath for several days without using this additional oxidation means and maintaining a REDOX potential between +200 and +300 mV to obtain a good level of pickling.

The flow of sprayed air in this case is 4 Nnr * per m ^ for pickled stainless steel and per h for each pickled surface element.

Example 3, pickling according to the invention

The stainless austenitic steel strips were pickled with the following modifications relative to Example 2: HF 35 g / l

REDOX potential: +350 - +400 mV

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

^ Fe: a.

The complex formed is of the type FeF3, 3 H2O. It was found that this compound was not soluble in water at 20 ° C, nor in an aqueous solution of 20 g HF per liter at 20 ° C (it hydrolyzes). In contrast, at 50 ° C it is moderately soluble: 31 g / l in water and 38 g / l in HF solution (20 g / l). This dissolution of 12 81126, which is unstable on cooling, is not satisfactory.

Example 4, pickling according to the invention Same pickling conditions except REDOX potential: +50 to +80 mV. Fe 2+ represents about 80% of the dissolved iron and the complex formed is of the FeF 2, n 2 O type. The same solubility experiments were performed as in Example 3. This compound is sparingly soluble and the only solubility observed is 13 g / l in the case of HF at 20 g / l at 50 ° C.

Example 5, pickling according to the invention

The pickling conditions correspond to Example 2 with the exception that the REDOX potential is adjusted to 220 mV ± 20 mV (measured between the platinum electrode and the Ag / AgCl reference electrode).

Fe 2+ represents 70-80% of the dissolved iron and the main compound formed appears to be of the type: Fe 2 F 5, 7 H 2 O. The following results have been obtained from solubility tests in grams per liter of solute:

Solubility at 20 ° C Solubility at 50 ° C

In a water solution which In a water solution containing HF 20 g / l contains HF 20 g / l 22.3 26 53 61 This type of "slurry" can be taken back into the cycle in a new bath according to the method described above.

Claims (13)

1. Process for pickling stainless steel products using a pickling bath with the initial composition: HF 10-50 g / l Dissolved iron 1 ferrite condition (Fe + +) £ 15 g / l Water: the remainder at a temperature setting to between 15 and 70 ° C, characterized in that during the pickling step or steps, the ferrite-type iron in the bath is maintained at at least 15 g / l by oxidation of the bath comprising at least one or more injections of air having a total amount of excess or equal to 1 Nm 2 per stained stainless steel and per hour staining of each stained surface element. Method according to claim 1, wherein a pickling bath containing from the beginning 10-35 g / l HF and Fe 2 +> 20 g / l is used, characterized in that during the pickling step or steps the content of Fe g / l by means of an oxidation of the bath comprising a single or multiple injections of air having a total supply amount of up to between 1 and 8 Nm 2 per m 2 of coated stainless steel and per hour of each coated surface element. Method according to claim 2, characterized in that a total of 2-5 Nnr * of air per bit of stainless steel and per hour of grazing of each grazed surface element is injected and of these 2-5 Nm 2 per m 2 and per hour of at least half are injected. towards the bottom of the bath at the bottom half of this bath. 4. A method according to any of claims 1-3, characterized in that in order to ensure a sufficient concentration of Fe 2+, the REDOX potential of the bath is measured and this tili is adjusted between 0 and +800 mV, if necessary, to effect the oxidation in the bath. 17 81126 Method according to claim 4, characterized in that the REDOX potential of the bath is regulated to between +100 and +300 mV. 6. A method according to claim 5, characterized in that the REDOX potential of the bath is adjusted to between +190 and +260 mV. Process according to claim 1 or 2, characterized in that the pickling bath is prepared using ferrifluoride having an initial concentration of ferric iron in the ferric state of 20-40 g / l. Process according to any one of claims 2, 3, 5 or 6, characterized in that hydrogen peroxide or potassium permanganate is used as a complementary oxidizing agent of the bath. Process according to any one of claims 2, 3, 5 or 6, characterized in that as the only complementary oxidizing agent of the bath 0.1-0.4 l of hydrogen peroxide H2O2 per m 2 of stained stainless steel and per hour of each stained stain is used. surface elements. Method according to any of claims 1-3, characterized in that grazing plates or bands of ferritic stainless steel are used, wherein the initial concentration of the pickling bath of HF is 10-25 g / l and the pickling temperature is between 35 and 50 ° C. Method according to any one of claims 1-3, characterized in that austenitic stainless steel plaques or strips are grazed, wherein the initial concentration of the pickling bath of HF is 20-35 g / l and the temperature of the pickling bath is between 40 and 60 ° C. 12. Method of pickling stainless steel products by means of a pickling bath containing from the beginning 10-35 g / l of ice 81126 HF and Fe₂ +> 20 g / l and with re-cycling the sludge precipitated in the 1 used bath, characterized in that success -sively: a) during the pickling step or steps, the content of Fe 2+ is maintained at at least 20 g / l by oxidation of the bath. Including one or more aerosol sprays having a total supply amount of between 1 and 8 Nm per hour, coating each coated surface element and optionally one or more additives of a strong oxidizing agent, wherein the REDOX potential of the bath is regulated to between +100 and +300 mV by, if necessary, affecting the oxidation of the bath; b) you extract the liquid from the used bath, then send hot water over the sludge to solubilize it, then install the HF content by the addition of free HF and stir, and then introduce hydrogen peroxide to set the potential to between +200 and +240 mV, thereby obtaining a new pickling bath. Method according to claim 12, characterized in that; a ^) the air injections have a total applied amount of applied tili between 2 and 5 Nm 2 per m 2 of worked stainless steel and per hour of each mated surface element; a2 is used as the only strong oxidizing agent 0.1 to 0.4 liters of hydrogen peroxide H2O2 per m 2 of coated stainless steel and per hour of each mashed surface element; 33. regulating the REDOX potential of the pickling bath to between +190 and +260 mV; b) for the cycling of the mud bath used, hot water of 40 to 60 ° C is used. in