EP0922787A1 - Process for pickling stainless steel - Google Patents

Abstract

Stainless steel is pickled and decarurized in an iron (II) and iron (III) ion-containing acid solution containing added potassium permanganate. Stainless steel is pickled in an acid solution which contains Fe<2+> and Fe<3+> ions, potassium permanganate being used such that MnO4<-> ions are reduced in solution to the Mn<2+> ion state.

Description

The invention relates to a pickling and descaling process. of stainless steel.

Japanese patent No. JP S 56-171,638 discloses a process for descaling steel wire in which the wire is etched in a hydrochloric acid bath, the bath having a hydrochloric acid concentration of between 10 and 20 % in weight. To accelerate the pickling, the acid concentration of the bath is modified. However, it is taught that an excessively concentrated solution gives off smoke and obliges to oversize the size of the installations at the expense of cost. According to the technique proposed in this document, an amount of FeCl _{2 is added} at a concentration which can go up to saturation as well as an amount of FeCl ₃ so that the oxidation reduction potential of Fe ³⁺ / Fe ^{2 +} be regulated during descaling. The potential is adjusted by adding FeCl ₃ via an injection nozzle. The pickling process consists of the HCl dissolution of the ferrous substrate, which gives off hydrogen.

It is also known, for example from patent JP H2-205692, a pickling bath for stainless steels in which hot-rolled or annealed products are pickled in a sulfuric acid solution containing Fe ³⁺ and Fe ions ²⁺ , said solution containing at least 10 g / liter of iron, pickling being carried out by aerating the solution with blown air at a flow rate of at least 100 ml / min and per liter of solution. In a bath based on sulfuric acid, the blown air aims to stir the bath.

It is also known from utility certificate No. FR 2 551 465, associated with EP patent n ° 0236 354, a continuous pickling process a strip of stainless steel in which a pickling bath is used mainly composed of hydrofluoric acid containing ferric ions, pickling of the steel strip being carried out while maintaining the iron content ferric at least 15 g / l by oxidation of the bath by means of an injection air or the additional introduction of another oxidant, peroxide of hydrogen, while maintaining the REDOX potential of said bath in a range of values between 0 and 800 mV.

It is taught in these documents the oxidizing role of air which oxidizes Fe ²⁺ ions to Fe ³⁺ ions, the pickling reaction being carried out without release of hydrogen.

We also find in the field of pickling of steels a considerable number of documents which describe pickling processes, the baths of which consist of a mixture of two or three of the acids mentioned above to avoid the use classic nitric acid which has the disadvantage of generating derivatives pollutants of the NOx type of this acid.

The pickling of particularly austenitic stainless steels has been improved very significantly with the generation of baths whose compound oxidant is hydrogen peroxide.

The use of hydrogen peroxide, in particular, with acid hydrofluoric acid, the most dangerous for humans, has eliminated acid nitric harmful to the environment. However, all of this progress has not not limit the operating risks. So the use of acid hydrofluoric acid increases the potential human risk and risks for the environment, during operation and during treatment baths. Finally, the use of peroxide leads to risks important in terms of security, for example the risk of sudden release of oxygen which can lead to explosions and fire, the risk of chemical burns. In addition, storage and handling lead to high investment costs. It is necessary also specify that pickling baths based on hydrofluoric acid are usually neutralized with lime, which generates great amount of sludge.

Another category of oxidizing pickling baths exists. It's about of oxidizing alkaline pickling baths containing permanganate potassium which causes operating difficulties such as for example the breakdown into manganese oxide sludge and problems environment related to dripping potassium permanganate, intense purple color. These reasons led to the gradual elimination of these pickling baths and more generally, the elimination of the oxidant potassium permanganate which is known to generate effluents intense purple in color.

The object of the invention is to develop a pickling solution which reduces operational risks from a safety perspective while keeping the acquired environmental point of view.

The object of the invention is a process for pickling and / or descaling a stainless steel, in an acid treatment solution containing Fe ²⁺ and Fe ³⁺ ions and in which potassium permanganate is used so that in the solution, the permanganate ion MnO ₄ ^- is reduced to the state of ion Mn ²⁺ .

• le décapage d'acier, notamment d'acier inoxydable austénitique consiste, à appliquer une solution traitante aqueuse oxydante de décapage contenant notamment, l'acide sulfurique en concentration comprise entre 1 g/litre et 300 g/litre, des ions ferriques et ferreux en solution, et en vue de conserver un pouvoir décapant constant dans la solution aqueuse d'acide sulfurique, à maintenir le taux en ions Fe³⁺, par oxydation des ions Fe²⁺ générés par le décapage, par introduction dans la solution traitante de permanganate de potassium assurant à la fois, la présence en continu d'une teneur minimale d'ions Fe³⁺ et la présence d'ions Mn²⁺, le potentiel Redox étant maintenu à une valeur comprise entre 300 et 1200 mV potentiel mesuré entre une électrode de platine et une électrode de référence Ag/AgCl placées dans la solution.
• l'aération de la solution traitante est réalisée par des moyens choisis parmi : pompage et refoulement de la solution à l'air libre, bullage, brassage, injection d'un gaz contenant l'élément oxygène, aspersion de la solution traitante dans une enceinte contenant de l'air.
• la solution présente, de préférence, une concentration en acide sulfurique comprise entre 100 g/litre et 200 g/litre.
• la solution traitante est appliquée dans un intervalle de température compris entre 10°C et 95°C et de préférence entre 80°C et 95°C.
• on ajuste le potentiel Redox entre 600 mV et 1200 mV.

The other characteristics of the invention are:

• pickling of steel, in particular austenitic stainless steel, consists in applying an aqueous oxidizing pickling treatment solution containing in particular, sulfuric acid in concentration between 1 g / liter and 300 g / liter, ferric and ferrous ions in solution, and in order to maintain a constant pickling power in the aqueous sulfuric acid solution, to maintain the level of Fe ³⁺ ions, by oxidation of Fe ²⁺ ions generated by pickling, by introduction into the permanganate treatment solution of potassium ensuring both the continuous presence of a minimum content of Fe ³⁺ ions and the presence of Mn ²⁺ ions, the Redox potential being maintained at a value between 300 and 1200 mV potential measured between a platinum electrode and an Ag / AgCl reference electrode placed in the solution.
• the aeration of the treatment solution is carried out by means chosen from: pumping and delivery of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen element, spraying of the treatment solution into an enclosure containing air.
• the solution preferably has a sulfuric acid concentration of between 100 g / liter and 200 g / liter.
• the treating solution is applied in a temperature range between 10 ° C and 95 ° C and preferably between 80 ° C and 95 ° C.
• the Redox potential is adjusted between 600 mV and 1200 mV.

The following description will make the invention clear.

During the production of steel parts, which can be from slabs of ingots or billets subjected to operations of hot rolling, the parts are covered with scale. In the case of the development of semi-finished products undergoing heat treatments, there are also formation on the surface of an oxidized layer. It is necessary to eliminate all of the oxidized layers to ensure a quality of surface on finished products. The elimination of these oxidized layers is generally provided by stripping. Pickling conditions quality of the final surface of the part.

Current pickling methods preferably use solutions comprising alone or in combination, in different concentrations, sulfuric and or hydrofluoric acids and or hydrochloric and are optionally oxidized using peroxide hydrogen.

Hydrochloric acid generates very corrosive gas emissions and the stripping installations require to be watertight to avoid corrosion of surrounding installations by hydrochloric vapors.

Hydrofluoric acid is a dangerous compound, particularly in the area of its handling, because it causes contact with the skin dramatic, even fatal burns.

Thus, the invention is aimed at reducing the human risks, the risks environmental degradation, operating costs.

Preferably, the pickling solution according to the invention is produced with, in particular, sulfuric acid, so as to avoid the risks inherent in other acids, human risk, corrosion of installations and pollution.

The acid preferably used, sulfuric acid, is then the main acid constituting the pickling solution. Introducing into the solution treating another acid in reduced concentration, will, by example, harmful from an environmental point of view but will not modify or very little the pickling process itself according to the invention.

The oxidant used according to the invention is potassium permanganate easy to handle industrially because it is a solid compound, and it does not leads to no coloring under the recommended conditions of use according to the invention. Depending on the nature of the pickled steel, the aggressiveness of the treating solution is controlled by adjusting the acidity, the temperature, and REDOX potential.

Contrary to the widespread prejudice concerning the coloring of baths containing potassium permanganate, the acid pickling solution according to the invention, under its conditions of use, does not lead to any coloring. Indeed, and according to the invention, it was found that in the presence of Fe ²⁺ ions, the bath did not exhibit the intense purple coloration.

In this medium, the MnO ₄ ^- ion which gives the intense purple appearance is reduced to the Mn ²⁺ state which does not color the solution. The pickling treatment solution then retains the usual green / brown coloration of pickling solutions.

In addition, the oxidant KMnO ₄ makes it possible to achieve high levels of Fe ³⁺ ions due, in particular, to its high chemical stability.

The pickling according to the invention is carried out with concentrations sulfuric acid between 1 and 350 g / liter (0.01 N to 7 N) and preferably between 100 and 200 g / liter (2 N to 4 N). Pickling can be performed at temperatures up to 95 ° C. The stability of potassium permanganate, at temperatures preferably comprised between 80 ° C and 95 ° C, guarantees an oxidation yield of 100%, yield which cannot be achieved with, for example, peroxide hydrogen.

The amount of potassium permanganate introduced into the pickling is between 2 and 6 kg / ton of pickled metal and preferably around 4 kg / ton of pickled metal.

The REDOX potential, in this application example, can be between 300 and 1200 V / Ag / Ag Cl and preferably between 600 and 1200 mV.

The best conditions from the point of view of efficiency - cost of production are obtained with a treating solution having an acidity between 2.5 and 3.5 N and a REDOX potential of around 600 mV, at a temperature of around 90 ° C.

AISI 304L steel wire rod rings are pickled by soaking in a pickling bath containing 150 g / l of H ₂ SO ₄ under a Redox potential of around 600 mV Ag / Ag Cl, at a temperature of 85 ° C in a period of 15 mm.

AISI 304CU steel wire rod crowns are pickled in an identical pickling bath and in the same conditions in 8 mm.

Preferably, the pickling installation may include means of air insufflation which allow the oxygenation of the bath and the oxidation of ferrous ions with a low yield, which can ensure a reduction of the quantity of solid oxidant used and also, better pickling in confined parts of pickled parts mainly due to mixing of the blown air.

Other solid oxidants can be used, for example, potassium dichromate, perborates and perchlorates but some have disadvantages from the point of view of pollution or instability chemicals.

The advantages of the invention lie in the elimination of hydrochloric and hydrofluoric acids, in the use of a solid oxidant ensuring an oxidation yield of 100% and allowing an economical handling without risk, in the use of potassium, naturally intense purple in color and used, according to the invention, in an acid pickling solution not containing MnO ₄ ^- ions.

For specific applications, the pickling treatment solution according to the invention can be doped with, for example, acid hydrofluoric, the acid concentration being less than 20g / liter.

Other acids can replace, if necessary, sulfuric acid, it being understood that the essential condition is the presence of ferrous and ferric ions and of Mn ²⁺ under conditions such that the reduction in Mn ²⁺ is obtained. MnO ₄ ^- ions.

Finally, the invention makes it possible to keep the downstream facilities of treatment of effluents as they are. Used baths are treated with soda, this which reduces the amount of sludge compared to used baths containing hydrofluoric acid and heat treated. Any process modification is only necessary when setting up a sulfuric solution whose REDOX potential is guaranteed by the addition of Potassium permanganate.

Claims (7)

Process for pickling and or descaling stainless steel, in an acid treatment solution containing Fe ²⁺ and Fe ³⁺ ions and in which potassium permanganate is used so that in the solution, the permanganate ion MnO ₄ ^- either reduced to the ion state Mn ²⁺ . Process according to Claim 1, characterized in that the pickling of steel, in particular austenitic stainless steel, consists in applying an aqueous oxidizing pickling treatment solution containing in particular sulfuric acid in a concentration of between 1 g / liter and 300 g / liter, ferric and ferrous ions in solution, and in order to maintain a constant pickling power in the aqueous sulfuric acid solution, to maintain the level of Fe ³⁺ ions, by oxidation of Fe ²⁺ ions generated by pickling , by introduction into the treatment solution of potassium permanganate ensuring both the continuous presence of a minimum content of Fe ³⁺ ions and the presence of Mn ²⁺ ions, the Redox potential being maintained at a value understood between 300 and 1200 mV potential measured between a platinum electrode and an Ag / AgCl reference electrode placed in the solution. Method according to claim 2, characterized in that the solution preferably has a concentration of sulfuric acid included between 100 g / liter and 200 g / liter. Method according to one of Claims 2 and 3, characterized in that that the treating solution is applied within a temperature range between 10 ° C and 95 ° C and preferably between 80 ° C and 95 ° C. Method according to one of claims 2 to 4, characterized in that that the Redox potential is adjusted between 600 mV and 1200 mV. Method according to one of claims 2 to 5 characterized in that the amount of potassium permanganate introduced into the pickling bath is between 2 and 6 kg / tonne of pickled metal and preferably about 4 kg / tonne of pickled metal. Method according to claim 6, characterized in that the aeration of the treatment solution is produced by means chosen from: pumping and delivery of the solution in the open air, bubbling, stirring, injection of a gas containing the oxygen element, spraying of the solution treating in an enclosure containing air.