EP1040211B1 - Method for pickling and passivating special steel - Google Patents

Description

The invention relates to a method for pickling and / or passivating Stainless steel (also known as "stainless steel"). As rustproof or rustproof are generally used in steel, in which under usual Environmental conditions such as B. the presence of atmospheric oxygen and Moisture and rust formation in aqueous solutions is prevented. tougher Resist corrosion conditions such as acids and saline solutions the mostly higher alloyed so-called corrosion-resistant or acid-resistant Steels. In summary, these steels are called stainless steels. In Ullmann's Encyclopedia of Technical Chemistry, 4th edition, volume 22, pp. 106-112 and in the German industrial standard DIN 17440, July 1985, is a list of the technically most important stainless steels with their material numbers, designations and Alloy components as well as mechanical and chemical properties contain. Stainless steels are iron-based alloys that contain at least 10% chromium contain. The formation of chromium oxide on the material surface gives the Stainless steels have a corrosion-resistant character.

Stainless steels can be divided into families: austenitic steels, ferritic ones Steels, martensitic steels, precipitation hardened steels and duplex steels. These groups differ in their physical and mechanical properties as well as in their corrosion resistance, which is due to the different Alloy components are caused. Austenitic stainless steels are called Stainless steels of the 200 and 300 series listed. They are the most common Stainless steels and represent 65 to 85% of the stainless steel market. They are chemical characterized in that it has a chromium content> 17% and a nickel content Have> 8%. They have a face-centered cubic structure and are excellent malleable and weldable. The most common is probably the type UNS S 30400 (type 304), or "18/8". Modifications of this are S 32100 (stabilized with titanium) and S 34700 (stabilized with niobium). For increased corrosion resistance are alloys with higher levels of chromium, nickel or Molybdenum available. Examples are S 31600, S 31700, S 30900 and S 31000. In contrast, the 200 series of austenitic stainless steels has a reduced one Contains nickel and contains manganese instead.

During the annealing or hot rolling etc. of stainless steel, the surface forms a layer of scale, which gives the steel surface the desired metallic gloss Appearance takes. After this production step, this must Surface layer can therefore be removed. This can be done by the invention Pickling processes take place. The oxide-containing surface layer to be removed differs fundamentally different from the oxide layer on low-alloy steels or on Carbon steels. In addition to iron oxides, the surface layer contains oxides of Alloy elements such as chrome, nickel, aluminum, titanium or Niobium. The surface layer is enriched with chromium oxide, especially during hot rolling because chromium is thermodynamically less noble than iron. This turns chrome enriched against iron in the oxide layer. Conversely, this leads to the fact that the steel layer immediately below the oxide layer is depleted of chromium. On Pickling process with suitable acid pickling solutions preferably solves them Chromium-depleted layer below the oxide layer, so that the oxide layer is blown up.

After pickling, the surface is chemically activated so that it is in the air again covered with an optically disruptive surface layer. This can can be prevented that the freshly pickled surfaces after or passivated during pickling. This can be similar to treatment solutions Pickling solutions are used, but one for the passivation is a higher one Redox potential sets as for the pickling process. Through the targeted Passivation step forms an optically invisible on the metal surface Passivation layer. In this way, the steel surface retains its metallic shine Appearance. Whether a treatment solution against stainless steel or has a passivating effect mainly depends on the solutions according to the invention from the set redox potential. Acidic solutions with pH values below about 2.5 act pickling when due to the presence of oxidizing agents a redox potential compared to a silver / silver chloride electrode in the Have a range from about 200 to about 350 mV. If you increase the redox potential to values above about 350 mV, the treatment solution has a passivating effect.

Pickling processes for stainless steel are well known in the art. Older procedures use pickling baths containing nitric acid. These often contain additional Hydrofluoric acid, which has a complexing effect on iron ions Pickling process promotes. Such pickling baths are economically efficient and technically satisfactory, but have the major ecological disadvantage, that they emit significant amounts of nitrogen oxides and that high Nitrate concentrations get into the waste water. The necessary Suction devices make the process and ultimately into the atmosphere more expensive Amounts of nitrogen oxide have a considerable environmental impact Potential.

This is why the technology became intensive after alternative pickling and passivation processes wanted, which get by without using nitric acid. A possible replacement Fe (III) ions are responsible for the oxidizing effect of nitric acid. Your concentration is by hydrogen peroxide, which is the treatment baths continuously or is added discontinuously, maintained. Such pickling or Passivation baths contain about 15 to about 65 g / l of trivalent iron ions. During the pickling process, trivalent iron ions become a bivalent stage reduced. At the same time, the stained surface becomes more divalent Iron ions released. The pickling bath is therefore depleted during operation trivalent iron ions, while divalent iron ions accumulate. This shifts the redox potential of the treatment solution so that it eventually loses its pickling effect.

By continuous or discontinuous addition of oxidizing agents such as for example hydrogen peroxide or other oxidizing agents such as perborates, Peracids or organic peroxides are oxidized to divalent iron ions back to the trivalent level. As a result, this remains for the pickling or Passivation effect required redox potential.

For example, EP-B-505 606 describes a nitric acid-free process for Pickling and passivation of stainless steel, in which you can treat the Material in contact with a bath that has a temperature between 30 and 70 ° C and at least at the beginning of the pickling process at least 150 g / l Contains sulfuric acid, at least 15 g / l Fe (III) ions and at least 40 g / l HF. This bath also contains up to about 1 g / l additives such as nonionic surfactants and pickling inhibitors. The bath is given continuously or discontinuously Amounts of hydrogen peroxide increase the redox potential to the desired range is held. The other bath components are replenished in such a way that their concentration remains in the optimal working range. By blowing in air the pickling bath is kept in motion. There is a movement of the pickling bath required to achieve a uniform pickling result. A similar Process that differs essentially only from the set Differences in redox potential is described in EP-A-582 121.

The above-mentioned pickling processes are technically satisfactory and have the ecological advantage of not emitting nitrogen oxides into the environment. However, the use of hydrogen peroxide is economically disadvantageous Oxidant. On the one hand, the consumption of hydrogen peroxide increases the Cost of the pickling process, on the other hand, the production of hydrogen peroxide energy-intensive.

It would be desirable to oxidize the divalent iron in the pickling or Passivation baths for the trivalent level directly containing oxygen or containing oxygen To be able to use gases like ideally air. The ecological and This would avoid the economic disadvantages of known pickling processes. The However, experience in circulating pickling baths by blowing air shows that in the usual pickling baths the oxidation of the divalent iron is not in sufficient measure. There is therefore a need for improved pickling or Passivation processes in which the oxidation of the divalent iron is accelerated by oxygen.

EP-A-795 628 describes a method for pickling stainless steel, in which the resulting divalent iron catalytically in an external fixed bed reactor trivalent stage is oxidized. Pure oxygen or serves as the oxidizing agent an oxygen-containing gas. In this process, part of the pickling bath is in transferred an oxidation reactor containing a solid catalyst. As Precious metals such as platinum in particular are used as catalysts. Farther can use palladium, ruthenium, rhodium, gold and their alloys become. The catalytic oxidation of the divalent iron is therefore carried out with a heterogeneous catalyst.

It is known from technical processes for the production of iron (III) sulfate that divalent iron in sulfuric acid solution by oxygen in the presence of oxidize catalytic amounts of copper ions to the trivalent stage. For example, US-A-4,707,349 describes a process for making Iron (III) sulfate, in which a solution of iron (II) sulfate in the presence of about 200 ppm copper ions oxidized to iron (III) sulfate. An excess of Sulfuric acid should be avoided here.

The kinetics of the oxidation of Fe (II) in hydrochloric or sulfuric acid solution by molecular oxygen in the presence of a copper catalyst was described in a conference report (Y. Awakura, M. Iwai, H. Majima "Oxidation of Fe (II) in HCl and H ₂ SO ₄ solutions with molecular oxygen in the presence and absence of a cupric catalyst ", Iron Control Hydrometall. [Int.-Symp.] (1986), pp. 202-222; edited by J. Dutrizac and A. Monhemius, Horwood Publishing house, Chichester, England).

In "Proceedings of Australasian Institute of Mining and Metallurgy", No. 242 (June 1972), Pages 47-56, C.T. Mathews, R.G. Robius "The Oxidation of Aqueous Ferrous Sulphate Solutions by Molecular Oxygen "is the oxidation of Fe (II) sulfate to Fe (III) sulfate in aqueous solution with air and oxygen at temperatures in the range of 20 to 80 ° C. and pH values in the range from 0 to 2 were examined. The rate of oxidation will increased by Cu (II) ions, with increasing concentration of the Cu (II) ions this effect is reinforced.

In "Journal of the American Chemical Society", Vol. 77 (1955), pages 793 to 798, M. Cher, N. Davidson "The Kinetic of the Oxygenation of Ferrous Iron in Phosphoric Acid Solution "is the kinetics of the oxidation of Fe (II) ions with oxygen in phosphoric acid solution examined. Cu (II) ions show a catalytic effect.

In "Patent Abstract of Japan", volume 14, volume 497 (C-774), 1990, abstract of JP-A-02205692 is a process for pickling stainless steel in sulfuric acid solution described. The surface covering of the material is obtained by pickling in a sulfuric acid bath, containing Fe (III) and Fe (II) ions, and blowing in air, whereby Fe (II) - are oxidized to Fe (III) ions.

However, it was not previously known to use divalent iron at the specific temperature and Concentration conditions of pickling or passivation baths for stainless steel with the required speed by oxygen in the presence of a homogeneous Oxidize copper catalyst to the trivalent stage. The invention turns the Task, an economically and ecologically improved pickling process for stainless steel To provide molecular oxygen for the oxidation of divalent Iron ions is used without affecting the Surface quality comes through the catalyst.

The object is achieved by a method for pickling and / or passivating Stainless steel, taking the stainless steel with an aqueous treatment solution with a pH less than or equal to 2.5 with a temperature in the range of 30 to 70 ° C, the 15 to 100 g / l Contains iron ions, characterized in that during the pickling process Fe (II) formed is oxidized to the trivalent stage using a treatment solution, the at least 10 g / l Fe (III) ions and hydrogen fluoride as essential Contains component in a concentration of 1 to 60 g / l and free of nitric acid is, the treatment solution for pickling austenitic stainless steel in Presence of 50 to 600 mg / l copper (II) ions, for pickling martensitic, ferritic or precipitation hardened stainless steel or duplex steel in the presence from 50 to 300 mg / l copper ions in contact with oxygen.

The catalytic oxidation is therefore homogeneous. Cu (II) can be used as a water-soluble salt like for example chloride, acetate or preferably sulfate. With less Copper concentrations will slow the oxidation reaction. higher Copper concentrations further accelerate the oxidation but lead increasingly at risk of unwanted copper redeposition the stained surfaces. Attempts by patent applicants have shown that austenitic stainless steels in contrast to martensitic and ferritic Stainless steels have a particularly low tendency to redeposit copper. thats why the method according to the invention in particular for austenitic stainless steel suitable. In the case of austenitic stainless steel, a wider range is available Copper concentrations possible than with other types of stainless steel. With these others Stainless steel grades are worked with copper concentrations that are in the range of 50 to 300 mg / l. Depending on the aggressiveness of the The pickling solution also involves the accumulation of copper, this can be separated Copper in a subsequent treatment step by oxidation for example, nitric acid or hydrogen peroxide can be removed again. There however, such post-treatment should preferably be dispensed with, that is Methods according to the invention, in particular for pickling and / or passivating austenitic stainless steel.

In order to achieve a sufficient pickling effect, the aqueous treatment solution have a pH ≤2.5, preferably <2.0. The setting of the The acidic pH of the treatment solution can be combined with any acids if this does not lead to the formation of deposits on the stainless steel surface to lead. For example, those in the prior art are suitable for pickling solutions common acids hydrochloric acid, hydrofluoric acid, sulfuric acid or also strong organic acids. These can also be used in a mixture with one another become. Hydrofluoric acid in particular is used together with other acids because it promotes the pickling process due to the complexing effect on iron ions.

For example, a treatment solution can be used, the 30 to 180 g / l Contains sulfuric acid. A treatment solution is also used which contains 1 to 60 g / l of hydrofluoric acid. A treatment solution is preferably used, which contains both sulfuric and hydrofluoric acid.

Because of the ecological and economic Disadvantages of nitric acid are used in the process according to the invention a treatment solution that is free of nitric acid.

A proportion of the iron ions in the treatment solution must be in the trivalent form available. These iron (III) ions act as oxidizing agents in a reduction-oxidation reaction, where they impoverished the metallic iron of the chrome Oxidize metallic surface layer to the bivalent stage and thereby dissolve the layer. The treatment solution should therefore be at least 10 g / l, preferably contain more than 20 g / l iron (III) ions.

In the redox reaction of the iron (III) ions with the components of Iron (II) ions form on the steel surface. These iron (II) ions will According to the invention back oxidized to the trivalent stage in that the Treatment solution in the presence of catalytically active copper (II) ions in the brings the entire concentration range into contact with oxygen. It can pure oxygen or an oxygen-containing gas can be used. Out For economic reasons, air is particularly suitable as an oxygen-containing gas.

Whether an oxidizing acid treatment solution is rather staining or rather passivating acts, depends on their reduction-oxidation potential (= "redox potential"), which is related, among other things, to the ratio Fe (III) to Fe (II) will be discussed in more detail below). From the polarization curve of the stainless steel grade can be found in which Potential areas of acids attack the metal and thus act as a pickling agent which potential areas passivation occurs.

Pickling action requires a certain minimum oxidizing power, which arises if the ratio Fe (III) to Fe (II) under the other conditions of Claim 1 is greater than about 0.3. Accordingly, you bring in one Embodiment of the invention the treatment solution with so much oxygen in Contact that the ratio of Fe (III) to Fe (II) is at least 0.3. At higher Redox potentials, for example those in which the ratio of Fe (III) to Fe (II)> 1, the treatment solution can vary depending on the polarization curve of the material have an acidic or passivating effect. Accordingly, one brings in another Embodiment in contact with the treatment solution with so much oxygen that the Ratio of Fe (III) to Fe (II) is at least 1.

The inventive method can also be operated so that instead of of the ratio Fe (III) to Fe (II) the redox potential of the solution for assessment uses whether the solution has sufficient pickling and / or passivation capacity having. In order to act as a stain, the treatment solution is said to have a redox potential relative to a silver / silver chloride electrode of at least 200 mV. The redox potential is preferably at least 220 mV and in particular at least 250 mV. The upper limit of the potential range to be set can be at about 800 mV can be selected. Treatment solutions with redox potential are effective below about 350 mV, especially during treatment solutions with a redox potential of 350 mV and above usually have a passivating effect.

Except for the concentration ratio of Fe (III) to Fe (II) according to the Nemst 's equation, the redox potential depends both on the concentration of the Sulfuric acid and hydrofluoric acid. This leads to otherwise the same Conditions an increase in the concentration of sulfuric acid to an increase in potential and an increase in the concentration of hydrofluoric acid to a potential decrease.

The process can be carried out so that the whole Treatment solution in the pickling or passivation bath in contact with oxygen brings. This is also the preferred procedure, since it is plant-technical is easy to implement. Passing an oxygen-containing gas through the Treatment solution has the additional advantage that it keeps it moving becomes. This will be discussed in more detail below.

Alternatively, the process is carried out by batchwise or preferably continuously removes a portion of the treatment solution for which to set the target redox potential or the target Weight ratio of Fe (III): Fe (II) required time with oxygen in Contact and then again with the rest of the treatment solution united.

During the contact with oxygen, the treatment solution shows regardless of the procedure, preferably a temperature in the range from about 40 to about 60 ° C. The optimal working temperature of the treatment solution, which also essentially the temperature when passing oxygen depends on the geometry of the parts to be pickled. For compact, elongated Products such as rods, tubes or bars are the preferred Working temperature usually between 40 and 60 ° C. For flat products like Tapes and plates, preferably in a continuous process shorter pickling times are treated, a temperature is preferably selected in the range of 50 to 70 ° C. Under otherwise identical conditions, the pickling rate increases with the temperature too.

The simplest procedure is carried out by oxygen-containing gas mixture such as air in the form as fine as possible Gas bubbles pass through the treatment solution. You can do this yourself either tubes or plates, frits or holes which are as fine as possible operate gas-permeable membranes. The filler can be inserted Improve the contact of the gas with the liquid. This will cause the oxidation of the Fe (II) accelerated to Fe (III). A vertical elongated form of the Reaction vessel extends the contact of the treatment solution with the Oxygen and thus promotes the oxidation of Fe (II) to Fe (III).

The efficiency of the oxidation reaction can be increased if instead of air oxygen enriched air or pure oxygen through the Treatment solution leads. By applying pressure the Accelerate oxidation reaction further. The amount of time to contact with Oxygen or the amount of oxygen used is preferably controlled the measurement of the redox potential, for example with a redox electrode. This makes it possible to see whether the desired value of the redox potential or the desired degree of oxidation of the iron ions has been reached.

The procedure described therefore makes it possible to inexpensive Use oxygen as an oxidizing agent in the pickling process and on others To largely avoid oxidizing agents such as hydrogen peroxide. For special reasons, the solution should have a particularly high redox potential be desired, it is of course possible to do this by adding additional Oxidizing agents such as hydrogen peroxide, persulfates or the like acting reagents. Preferably and particularly economically However, the process according to the invention is advantageously carried out in such a way that Oxygen is used as the only oxidizing agent to convert Fe (II) to Fe (III) oxidize.

An essential component of the pickling bath are hydrogen fluoride or fluoride ions formed therefrom. These particularly complex the trivalent iron ions and keep them in solution. Therefore, the Treatment solution contain the more hydrogen fluoride or fluoride ions, each the concentration of the trivalent iron ions is set higher. The Adaptation of the fluoride ion concentration to the iron content is particularly important Treatment solutions important that are used as pickling solutions. By the during the pickling process divalent iron and its catalytic iron Oxidation to the trivalent stage increases the concentration of trivalent Iron during the duration of the procedure. To stabilize this Solutions are therefore fluoride ion concentrations in the upper half of the claimed range, for example from about 25 to about 40 g / l. If the treatment solution is used as a passivation bath, the Metal surface no more iron loosened, so that it Passivation bath concentration not increased. Accordingly, there is a concentration of Fluoride ions in the lower half of the claimed range, e.g. from 1 to 25 g / l is sufficient.

For example, pickling solutions of the following composition can be used, which for the catalytic oxidation of Fe (II) according to the invention can additionally preferably contain 200 to 600 mg / l Cu (II) ions (concentrations in g / l): Solution 1 Solution 2 Solution 3 Solution 4 H ₂ SO ₄ 110 110 110 110 HF 30 10 10 30 Fe (III) 20 40 30 30 Fe (II) 40 20 30 30

Treatment solutions are preferably used for the process according to the invention a, in addition to the above-mentioned components, a total of about Contain 0.1 to about 2 g / l nonionic surfactants and / or pickling inhibitors. This results in particularly uniform surfaces of the pickling reaction optically attractive appearance.

The success of the pickling process depends on the fact that there is sufficient per unit of time Number of Fe (III) ions in contact with the surface of the material to be treated come to trigger the redox reaction with the metallic iron. Therefore it is advisable to treat the material to be treated or, preferably, the treatment solution keep moving. This becomes the boundary layer of the solution quickly renewed on the surface of the material to be treated, so that the formed Fe (II) ions are removed and new Fe (III) ions are brought to the surface become. Too little mass transfer on the surface of the material to be treated leads however not only to a slow pickling reaction, but can also result in the inventive methods have the undesirable consequence that elementary Copper precipitates on the metal surface. This is a particular risk for complex shaped objects such as wire bundles, where near adjacent wire surfaces only a limited possibility of movement the solution is given. At an economically reasonable pickling speed to achieve and reduce the risk of copper deposition therefore recommended for a strong movement towards the treatment solution the surface of the material to be treated, for example by strong Mix the treatment solution. For this it can be in the course of method according to the invention are already sufficient that amount of air through the To lead solution that is required for the desired oxidation of Fe (II). Leads However, the oxygen required for this reaction is not in the form of a strong air flow, but for example in the form of fine gas bubbles or through Membranes too, it may be necessary by blowing in additional air or, if necessary, also the treatment solution by stirring or pumping devices Keep moving.

The implementation of the invention is illustrated by the following examples: embodiments analytics:

The content of divalent iron in the treatment solution can be in sulfuric acid Solution can be determined manganometrically. To determine the trivalent Iron is added to the treatment solution calcium chloride or lanthanum nitrate, to precipitate fluoride ions and thereby destroy the fluoroe iron complexes. Potassium iodide is then added in the presence of hydrochloric acid Iodide is oxidized to elemental iodine by the trivalent iron. This will conventionally determined by titration with thiosulfate.

120 g/l H₂SO₄

25 g/l Fe(III)

25 g/l HF

Fe(II) in g/l gemäß Tabellen

Cu(II) in mg/l gemäß Tabellen (zugesetzt als Kupfersulfat).

A pickling solution of the following composition was used for the oxidation tests:

120 g / l H ₂ SO ₄

25 g / l Fe (III)

25 g / l HF

Fe (II) in g / l according to tables

Cu (II) in mg / l according to tables (added as copper sulfate).

In each 100 ml of the solution air or Oxygen introduced through a glass frit. Unless otherwise stated, was the gas flow is one liter per minute.

example 1

The pickling solution contained 40 g / l Fe (II) and 200 ppm copper ions. At a solution temperature of 23 ° C., air was passed through 100 ml of solution and the decrease in the content of Fe (II) was determined. Result: Baseline: 40 g / l after 1 hour 39 g / l after 2 hours 38.5 g / l After 3 hours 38 g / l

From these data it can be calculated that the conversion rate from Fe (II) to Fe (III) (hereinafter referred to as σ) is 13.7 kg per m ³ pickling solution and day.

Example 2:

The conversion rate σ (in kg / m ³ x day) was determined for different Cu (II) concentrations analogously to Example 1 (bath temperature: 23 ° C.). Result: Cu (II) concentration (ppm) σ (kg / m ³ x day) 200 13.7 400 37.3 600 40

Example 3:

Example 2 was repeated, with technical grade oxygen gas being passed through the pickling solution instead of air. Result: Cu (II) concentration (ppm) σ (kg / m ³ x day) 200 41.1 400 70.7 600 81.0

Example 4:

At a constant Cu (II) concentration of 200 ppm, the conversion rate σ when air was passed through the pickling solution was determined as a function of the temperature of the pickling solution. Result: Temperature (° C) σ (kg / m ³ x day) 23 13.8 40 53.9 60 73.1

Example 5:

Example 4 was repeated with oxygen instead of air. Result: Temperature (° C) σ (kg / m ³ x day) 23 41.1 40 126.5 60 205.3

Example 6:

At a Cu (II) concentration of 600 ppm and a bath temperature of 23 ° C the effect of the amount of air passed on the conversion rate σ was checked. Result: Air flow (liters / minute) σ (kg / m ³ x day) 1 40 0.7 37.6

Example 7:

At a bath temperature of 23 ° C, a Cu (II) concentration of 250 ppm and a flow rate of air of 1 liter / minute, the influence of packing elements (hollow cylinders made of plastic) in the gasified pickling solution on the conversion rate σ was examined. Result: Without packing σ = 32.7 kg / m ³ x day With filling σ = 53.2 kg / m ³ x day.

The experiment shows that additional surfaces in the pickling solution make contact with the Pickling solution with the gas and thus also improve the conversion rate.

Example 8:

110 g/l H₂SO₄

20 g/l Fe(III)

40 g/l Fe(II)

25 g/l HF

A pickling solution of the following composition was used:

110 g / l H ₂ SO ₄

20 g / l Fe (III)

40 g / l Fe (II)

25 g / l HF

Different amounts of copper according to the table were added to the solution. At a temperature of 55 ° C, one liter of air per minute was passed through the solution. The increase in the redox potential (as a result of the oxidation of Fe (II) to Fe (III)), measured compared to a saturated calomel electrode (SCE), was followed as a function of time. The potential values are entered in the table below. They show that in all three solutions the redox potentials increase over time and that the more copper the solution contains, the more pronounced this increase. Time / Cu concentration 400 ppm Cu 1000 ppm Cu 2000 ppm Cu begin 266 mV 266 mV 266 mV 1 hour 295 mV 303 mV 311 mV 2 hours 310 mV 317 mV 330 mV 3 hours 319 mV 324 mV 339 mV 4 hours 326 mV 336 mV 350 mV 5 hours 330 mV 342 mV 356 mV

Claims (10)

1. Process for pickling and/or passivating stainless steel, the stainless steel being contacted with an aqueous treatment solution having a pH of less than or equal to 2.5 at a temperature in the range from 30 to 70°C and containing 15 to 100 g/l of iron ions, characterized in that Fe(II) formed during the pickling process is oxidized to the trivalent state using a treatment solution which contains at least 10 g/l of Fe(III) ions and hydrogen fluoride as essential component in a concentration of 1 to 60 g/l and is free of nitric acid, the treatment solution being contacted with oxygen in the presence of 50 to 600 mg/l of copper(II) ions for pickling austenitic stainless steel, and being contacted with oxygen in the presence of 50 to 300 mg/l of copper ions for pickling martensitic, ferritic or precipitation-hardened stainless steel or duplex steel.
2. Process according to Claim 1, characterized in that the treatment solution contains 30 to 180 g/l of sulphuric acid.
3. Process according to one or both of Claims 1 and 2, characterized in that the treatment solution is contacted with sufficient oxygen so that the ratio of Fe(III) to Fe(II) is at least 0.3.
4. Process according to Claim 3, characterized in that the treatment solution is contacted with sufficient oxygen so that the ratio of Fe(III) to Fe(II) is at least 1.
5. Process according to one or more of Claims 1 to 4, characterized in that the treatment solution contains at least 20 g/l of Fe(III).
6. Process according to one or both of Claims 1 and 2, characterized in that the treatment solution is contacted with sufficient oxygen so that the redox potential of the treatment solution relative to a silver/silver chloride electrode lies in the range from 200 to 800 mV.
7. Process according to one or more of Claims 1 to 6, characterized in that a proportion of the treatment solution is removed, contacted with oxygen and then recombined with the remainder of the treatment solution.
8. Process according to one or more of Claims 1 to 6, characterized in that the treatment solution is contacted with oxygen by passing oxygen gas or an oxygen-containing gas through the treatment solution.
9. Process according to one or more of Claims 1 to 8, characterized in that oxygen is used as the sole oxidizing agent to oxidize Fe(II) to Fe(III).
10. Process according to one or more of Claims 1 to 9, characterized in that the treatment solution is maintained in motion by blowing in air or by stirrers or pumping equipment.