EP0188975B1 - Process for the acid pickling of steels, in particular stainless steels - Google Patents

Abstract

Process for pickling, for example after hot-rolling operations. The pickling is performed by means of a hydrofluoric acid solution in conditions which now allow only a small amount of hydrogen to be discharged to the atmosphere. Application: elimination of pollution by effluents and especially by nitric acid.

Description

The present invention relates to a new method for carrying out the pickling of steel parts, and it relates more particularly, although not exclusively, to pickling by acid of stainless steels.

It is known that during the manufacture of steel products, in particular steel slabs or ingots, subjected to hot rolling operations, or semi-products undergoing heat treatments such as, for example, annealing, pieces are covered with a layer of oxidized calamines. Also, given the need to obtain a good surface quality on finished products, remove all of these oxidized layers. The removal is ensured by a pickling operation.

According to the methods known to date, the ultimate operation for carrying out the stripping consists in applying to the steel parts, mixtures composed of nitric acid HN0 ₃ at approximately 150 g per liter and hydrofluoric acid HF at a concentration between 10 and 30 g per liter. Processing temperatures are often in the range of 40 to 60 _° C.

One of the drawbacks of these pickling methods of known type is the production of intense pollution, both in atmospheric air (nitrous vapors NO _x ) and in liquid form (soluble nitrates).

The present invention aims to avoid these drawbacks by eliminating the use of nitric acid as a pickling oxidant.

To solve this problem, it has already been proposed in US Pat. No. 2,474,526 to use a combination of ferric fluoride and a non-oxidizing mineral acid. This document in fact recommends using baths containing ferric ions, fluoride ions and sulphate ions.

In GB-A-2 000 196, it has also been described compositions of pickling baths containing ferric sulfate and hydrofluoric acid, the redox potential of which is regulated.

The subject of the present invention is a process for pickling steel parts, and in particular stainless steel, which consists in applying an aqueous pickling solution consisting of hydrofluoric acid, fluoride ions and ferric pickling ions in solution in l water and, in order to maintain a constant pickling power in the aqueous hydrofluoric acid solution having a pH of 0 to 3, to maintain the concentration of Fe ³⁺ ions, present in the form of fluorinated complexes, at an Fe value ³⁺ between 1 and 150 g per liter, by reoxidation of the Fe2 + ions formed during the etching by oxygenation, maintaining the REDOX potential at a value between -200 and +800 mV, potential measured between a platinum electrode and an electrode Ag / AgCl standards placed in the solution.

According to another characteristic of the invention, the solution has a concentration of hydrofluoric acid of between 5 and 50 g per liter, for a pH of between 0 and 3.

According to another characteristic of the invention, the treating solution is applied at a temperature between 10 and 70 _° C, for a residence time between 30 seconds and 5 minutes.

It is recalled that the REDOX potential is the difference in potential measured between a non-corrodable electrode (for example in platinum) and a reference electrode (for example Ag / AgCl or saturated calomel), these two electrodes being immersed in the bath to be studied. The value thus measured makes it possible to characterize the oxidizing power of the chemical solution.

By way of nonlimiting illustration, we will now give a few examples allowing a better understanding of the invention and the advantages which it is likely to provide.

Example 1

The new process consists in replacing the oxidant HN0 ₃ by the oxidant Fe ³⁺ introduced in the form of fluorinated complexes.

• - La réaction de dissolution se fait selon le mécanisme suivant :
  
  Cet équilibre est presque totalement déplacé dans le sens 1, c'est-à-dire vers la formation de Fe2+ avec dissolution du fer métallique, dans les conditions normales du traitement.
• - La réaction :
  
  est également possible si le milieu est oxydant, ce qui est le cas; mais cette deuxième réaction n'est pas favorable si elle devient prépondérante par rapport à la réaction (1), car elle conduit à un déplacement de pH vers le moins acide, ce qui n'est pas favorable pour la suite du traitement.
• - On oxyde ensuite Fe2+ par une aération convenable de la solution décapante . La réaction d'oxydation se fait selon la réaction:
  
  L'équilibre (3) est très déplacé dans le sens 1, si la solution est correctement aérée et le pH maintenu entre 0 et 3, ce qui est le cas du mélange décapant concerné.
• - Une aération convenable est obtenue:
  • ...soit naturellement (surface du liquide au contact de l'air);
  • ...soit par pompage et refoulement à l'air libre du mélange de décapage;
  • ...soit par une injection convenable d'oxygène ou d'air ou d'un autre gaz oxydant.
• - Une oxydation complémentaire de Fe2+ peut également être effectuée avec des liquides oxydants tels que le peroxyde d'hydrogène H₂0₂ ou le permanganate de potassium K Mn 0₄.
• - La réaction (3) est suivie par mesure du potentiel REDOX de la solution du décapage, potentiel mesuré entre une électrode de platine et une électrode de référence Ag/AgCl à la température de la solution décapante.
• - De bons résultats ont été obtenus sur aciers inoxydables ferritiques et austénitiques, par exemple avec les bains suivants:
  • . HF = 5 à 50 g/I
  • . pH compris entre 0 et 3
  • . F_e ³⁺> 1 g/I
  • . potentiel REDOX mesuré par rapport à Ag/AgCl compris entre -200 et +800 mV selon l'état chimique et thermodynamique du bain
  • . température comprise entre 20 et 60 °C pour des temps de séjour de l'ordre de 30 secondes à 5 minutes
  • . oxygénation et aération du bain par pompage et refeulement à l'air libre et/ou injection convenable d'oxygène ou d'air dans le bain, de façon que le débit injecté soit compris entre 1 et 150 Nm³/h sous 0,01 à 5 bars de pression pour un bac de traitement de 30 m³ et/ou injection d'un liquide oxydant tel que H₂0₂ ou K Mn 0₄.
• - Le procédé ne donne évidemment pas de vapeurs nitreuses NO_X et ne rejette aucun nitrate soluble puisqu'il n'utilise pas d'acide nitrique . Il s'agit donc d'un procédé particulièrement non polluant.
• - Le procédé est autorégulé par le produit de solubilité de Fe F₃ et de Cr F₃ ( pour une moindre part ) à la température du bain. Ces produits de solubilité limitent la charge de fer et de chrome dissous dans le bain.
• - Le rôle de l'acide fluorhydrique est quintuple, à savoir:
  • . maintenir un pH acide de l'ordre de 0 à 3;
  • . solubiliser les oxydes très difficilement solubles dans les autres acides ( H CI ou H₂ S0₄ ) tels que Si 0₂, Cr₂ O₃, Ti O₂, Al₂ O₃, Mo O₃, Nb₂O₃, W O₃, V₂ O₅, etc...
  • . complexer Fe³⁺ et Cr³⁺ le plus fortement possible;
  • . boucler le bilan en charge de la solution décapante;
  • . dépassiver le métal pour permettre la dissolution (traverser la couche passive ).
• - Le nouveau procédé peut également s'appliquer ( en modifiant ou ajustant certains paramètres de réglage ) aux aciers ordinaires au carbone, aux aciers de construction, aux aciers d'outillage, et aux aciers au silicium pour usage électrique.

Hydrofluoric acid HF is also the other necessary constituent of the stripper mixture.

• - The dissolution reaction is carried out according to the following mechanism:
  
  This equilibrium is almost completely displaced in direction 1, that is to say towards the formation of Fe2 + with dissolution of metallic iron, under normal conditions of treatment.
• - The reaction :
  
  is also possible if the medium is oxidizing, which is the case; but this second reaction is not favorable if it becomes preponderant compared to reaction (1), because it leads to a shift in pH towards the least acidic, which is not favorable for the rest of the treatment.
• - Fe2 + is then oxidized by suitable aeration of the pickling solution. The oxidation reaction takes place according to the reaction:
  
  The equilibrium (3) is very displaced in direction 1, if the solution is correctly aerated and the pH maintained between 0 and 3, which is the case of the stripper mixture concerned.
• - Adequate ventilation is obtained:
  • ... or naturally (surface of the liquid in contact with air);
  • ... either by pumping and discharging the pickling mixture into the open air;
  • ... or by a suitable injection of oxygen or air or another oxidizing gas.
• - An additional oxidation of Fe2 + can also be carried out with oxidizing liquids such as hydrogen peroxide H ₂ 0 ₂ or potassium permanganate K Mn 0 ₄ .
• - Reaction (3) is followed by measuring the REDOX potential of the pickling solution, potential measured between a platinum electrode and an Ag / AgCl reference electrode at the temperature of the pickling solution.
• - Good results have been obtained on ferritic and austenitic stainless steels, for example with the following baths:
  • . HF = 5 to 50 g / I
  • . pH between 0 and 3
  • . F _e ³⁺ > 1 g / I
  • . REDOX potential measured with respect to Ag / AgCl between -200 and +800 mV depending on the chemical and thermodynamic state of the bath
  • . temperature between 20 and 60 ° C for residence times of the order of 30 seconds to 5 minutes
  • . oxygenation and aeration of the bath by pumping and reflow in the open air and / or suitable injection of oxygen or air into the bath, so that the flow injected is between 1 and 150 Nm ³ / h under 0.01 at 5 bar pressure for a 30 m ³ treatment tank and / or injection of an oxidizing liquid such as H ₂ 0 ₂ or K Mn 0 ₄ .
• - The process obviously does not give nitrous vapors NO _X and does not reject any soluble nitrate since it does not use nitric acid. It is therefore a particularly non-polluting process.
• - The process is self-regulated by the solubility product of Fe F ₃ and Cr F ₃ (to a lesser extent) at bath temperature. These solubility products limit the charge of iron and chromium dissolved in the bath.
• - The role of hydrofluoric acid is fivefold, namely:
  • . maintain an acid pH of the order of 0 to 3;
  • . solubilize the oxides which are hardly soluble in other acids (H CI or H ₂ S0 ₄ ) such as Si 0 ₂ , Cr ₂ O ₃ , Ti O ₂ , Al ₂ O ₃ , Mo O ₃ , Nb ₂ O ₃ , WO ₃ , V ₂ O ₅ , etc ...
  • . complex Fe ³⁺ and Cr ³⁺ as strongly as possible;
  • . complete the balance in charge of the stripping solution;
  • . passivate the metal to allow dissolution (pass through the passive layer).
• - The new process can also be applied (by modifying or adjusting certain adjustment parameters) to ordinary carbon steels, structural steels, tool steels, and silicon steels for electrical use.

Example 2.

• . aciers A.i.S.i. 409 et Type 410 (références normalisées );
• . HF = 15 à 18 g/I;
• . pH = 1,5 à 1,8;
• . Fe³⁺ = 30 g/l;
• . potentiel REDOX: + 100 à + 150 mV par rapport à Ag/Ag CI;
• . température : 38 à 40 °C,
• . temps de traitement : 30 secondes;
• . injection d'air:80 Nm³/h par bain de 25 à 30 m³.

Positive results have been obtained under the following conditions, for hot-rolled and annealed steels:

• . AiSi 409 and Type 410 steels (standard references);
• . HF = 15 to 18 g / I;
• . pH = 1.5 to 1.8;
• . Fe ³⁺ = 30 g / l;
• . REDOX potential: + 100 to + 150 mV compared to Ag / Ag CI;
• . temperature: 38 to 40 ° C,
• . processing time: 30 seconds;
• . air injection: 80 Nm ³ / h per bath of 25 to 30 m ³ .

Example 3.

• . A.i.S.i. 430 (standard references);
• . HF: 18 to 20 g / l;
• . pH = 1.5 to 1.8;
• . Fe ³⁺ : 30 g / I;
• . REDOX potential: + 100 to + 250 mV compared to Ag / Ag CI;
• . temperature: 44 to 46 ° C;
• . processing time: 1.5 to 2 minutes;
• . air injection: 80 Nm ³ / h per bath of 25 to 30 m ³ .

Example 4.

• . aciers A.i.S.i. 304 ( références normalisées );
• . HF : 18 à 20 g/l;
• . pH = 1,5 à 1,8;
• . Fe³⁺:30g/l;
• . potentiel REDOX: + 100 à + 250 mV par rapport à Ag/Ag CI;
• . température:46 à 48 °C;
• . temps de traitement : 1,5 à 2 minutes;
• . injection d'air: 80 Nm³/h par bain de 25 à 30 m³.

Good results have been obtained under the following conditions, for hot-rolled and annealed steels:

• . AiSi 304 steels (standard references);
• . HF: 18 to 20 g / l;
• . pH = 1.5 to 1.8;
• . Fe ³⁺ : 30g / l;
• . REDOX potential: + 100 to + 250 mV compared to Ag / Ag CI;
• . temperature: 46 to 48 ° C;
• . processing time: 1.5 to 2 minutes;
• . air injection: 80 Nm ³ / h per bath of 25 to 30 m ³ .

Example 5.

• . aciers A.i.S.i. 304 ( références normalisées );
• . HF: 20 à 25 g/I;
• . pH = 1 à 1,5;
• . Fe³⁺: 30 g/I;
• . potentiel REDOX : + 100 à + 250 mV par rapport à Ag/Ag CI;
• . température: 50 à 55 °C;
• . temps de traitement: 1 à 2 minutes:
• . injection d'air: 80 Nm³/h par bain de 25 à 30 m³.

Likewise, interesting results have been obtained under the following conditions, for cold-rolled and annealed steels:

• . AiSi 304 steels (standard references);
• . HF: 20 to 25 g / I;
• . pH = 1 to 1.5;
• . Fe ³⁺ : 30 g / I;
• . REDOX potential: + 100 to + 250 mV compared to Ag / Ag CI;
• . temperature: 50 to 55 ° C;
• . processing time: 1 to 2 minutes:
• . air injection: 80 Nm ³ / h per bath of 25 to 30 m ³ .

In general, the flow of air injected is obviously largely in excess compared to the stoichiometry; but the air injection also contributed to a certain agitation which is positive from the point of view of the stripping action.

• - un faible débit d'hydrogène;
• - une faible quantité d'acide fluorhydrique FH à sa tension de vapeur ( ce qui correspond donc à un débit très faible, qu'on peut facilement récupérer pour le recycler );
• - quelques oxydes chimiquement inertes;
• - enfin, si l'on est à basse température: quelques fluorures de fer et de chrome (mais ceux-ci se trouvant en phase liquide ou solide, ils sont faciles à éliminer: on peut aisément en extraire l'acide fluorhydrique FH qui est ensuite recyclé ).

Note that, in the previous examples, the pickling function is ensured by producing effluents only consisting of:

• - low hydrogen flow;
• - a small quantity of hydrofluoric acid FH at its vapor pressure (which therefore corresponds to a very low flow rate, which can easily be recovered for recycling);
• - some chemically inert oxides;
• - finally, if one is at low temperature: some iron and chromium fluorides (but these being in liquid or solid phase, they are easy to eliminate: one can easily extract hydrofluoric acid FH which is then recycled).

Ultimately, no polluting effluent is no longer rejected by the implementation of the method according to the invention.

This process can be used not only for pickling stainless steels, but also for most mild steels, in particular silicon steels (which are currently often pickled with hydrochloric acid or sulfuric acid, acids requiring very important regeneration or treatment facilities).

Claims (11)

1. Process for scouring steel parts, notably stainless steel parts, consisting of applying an aqueous scouring solution made up of hydrofluoric acid, fluoride ions and ferric ions in solution in water and, with a view to preserving a constant scouring power in the aqueous hydrofluoric acid solution having a pH of from 0 to 3, maintaining the concentration of Fe³⁺ ions present in the form of fluoro complexes at an Fe³⁺ level of between 1 and 150 g per litre, by reoxidation of the Fe²⁺ ions formed during the scouring operation by oxygenation, whilst maintaining the REDOX potential at a level between -200 and +800 mV, this potential being measured between a platinum electrode and an Ag/AgCI reference electrode placed in the solution.

2. Process according to claim 1, characterised in that the reoxidation of the Fe²⁺ ions by oxygenation is carried out by suitable aeration of the scouring solution.

3. Process according to claim 1, characterised in that the aeration of the solution is achieved by contact of the surface of the solution with air.

4. Process according to claim 2, characterised in that the aeration of the scouring solution is achieved by pumping and forcing the treatment solution back into the open air.

5. Process according to claim 2, characterised in that the aeration of the solution is achieved by injecting a gas containing oxygen.

6. Process according to claim 5, characterised in that the injection of the gas is carried out at a pressure of between 0.1 and 5 bar.

7. Process according to any one of the preceding claims, characterised in that the reoxidation is completed by the addition of H₂0₂.

8. Process according to claim 1, characterised in that the solution has a concentration of hydrofluoric acid of between 5 and 50 g per litre for a pH of between 0 and 3.

9. Process according to any one of the preceding claims, characterised in that the treatment solution is applied at a temperature of between 10 and 70 °C for a retention time of between 30 seconds and 5 minutes.

10. Process according to any one of the preceding claims, characterised in that the REDOX potential is adjusted to the desired level by regulating the flow rate of gas.

11. Process according to any one of the preceding claims, characterised in that, for cold-rolled and annealed steels, it corresponds to the following data:

HF : 20 to 25 g/I

pH : 1 to 1.5

_Fe³⁺ : 30 g/I

REDOX potential : =100 to +250 mV, in relation to Ag/AgCl,

Temperature : 50 to 55 _° C

Treatment time: 1 to 2 minutes

Air injection : 80 Nm³/h per bath of 25 to 30 m³.