EP0541448A1 - Method for continuously pickling steel material in a processing line - Google Patents

Abstract

Process for pickling steel materials, especially continuously on a processing line, comprising a shot-blasting operation, a preparing operation, a pickling operation and a passivating operation, in which the preparing operation is carried out by placing the materials in contact with at least one 2-10N sulphuric acid solution, and in which the pickling operation is carried out by placing the materials in contact with at least one solution containing 1 to 10 % by weight of hydrofluoric acid, Fe<2+> and Fe<3+> ions.

Description

The invention relates to a method for treating metallic materials, and more specifically, for pickling steel materials continuously on a processing line, which may successively comprise a blasting operation, a preparation operation, a pickling operation and an operation. passivation.

The pickling process can be implemented on metallic steel materials in an industrial environment, before leaving the factory, for example for descaling but also by non-professionals in metallurgy for cleaning steel elements.

Such a pickling process is known. In a continuous treatment line for a steel product, the treatment process used successively comprises: a shot-blasting operation, an operation of preparation by electrolytic pickling in sodium sulfate, or chemical in a bath of molten salts of composition determined, for example: NaOH 90%, NaNo₃ 9% NaCl 1%, then a pickling operation and a passivation operation.

The preparation operation conditions the layers of scale for the pickling operation which dissolves said layers. The pickling baths are based on hydrofluoric acid with an oxidant which is most often nitric acid but it is advantageous to use hydrogen peroxide as an oxidant. This is followed by a passivation operation in an acid bath, the acid bath generally used being nitric acid.

This process has the disadvantage of being polluting, the preparation operation generating chromium compounds with the degree of oxidation + VI, (CrVI), which are particularly toxic.

Indeed, for example in a preparation of a strip of stainless steel, it is moved in a bath composed of sodium sulphate in a proportion of 180 g / l at 80 ° C under a current density of 2A / dm².

It has been found that during this operation the electrolysis generates the formation of CrVI, the compounds of which are known for their toxicity.

On the other hand, the pickling operation making use of a mixture of nitric acid and HF also produces releases of particularly toxic oxide or nitrogen dioxide vapors, and a release of nitro compounds such as nitrites and nitrates in effluents. If the maximum authorized content of nitrates is relatively high, that relating to nitrites is much lower because nitrites lead to the formation of harmful nitrosamines.

The object of the invention is to eliminate nuisances such as the formation of CrVI compounds, the release of nitrous vapors or the release of nitro compounds described above, and also to reduce the processing time of the materials.

The subject of the invention is a method for pickling steel materials, in particular continuously on a treatment line, comprising a blasting operation, a preparation operation, then at least one pickling operation and at least one passivation operation, characterized in that the preparation operation is carried out by bringing the materials into contact with at least one solution of sulfuric acid of 2 to 10 N at a temperature above 50 ° C, the metal content being below saturation, and in that the pickling operation is carried out by bringing the materials into contact with at least one solution whose temperature is between 40 and 95 ° C, containing 1 to 10% by weight of hydrofluoric acid, and Fe² ions ⁺ and Fe³⁺ in such a way that the concentration of Fe³⁺ ions is between 1 and 190g / l, and that the redox potential or between - 200 and + 800 mV, this potential being measured with a platinum electrode relative to an Ag / Ag Cl electrode.

$$\text{2 Cr + 3H₂SO₄ → 3H₂ + Cr (SO₄)₃}$$

According to the invention, the chemical dissolution of steel in H₂SO₄ does not allow the oxidation of Cr III to Cr VI, during the preparation operation. Indeed, the following reactions are observed: $$\text{Fe + H₂SO₄ → H₂ + Fe SO₄}$$

$$\text{2 Cr + 3H₂SO₄ → 3H₂ + Cr (SO₄) ₃}$$

On the other hand, the absence of nitric acid in the pickling solution makes it possible to suppress any release of oxide or nitrogen dioxide as well as any salting out of nitro compounds such as nitrites and nitrates in the effluents.

$$\text{Fe + 2HF → H₂ + Fe F₂}$$

During this pickling operation, the base metal, Fe, is dissolved by oxidants, mainly Fe³⁺ but also HF, depending on the reactions: $$\text{2 Fe³⁺ + Fe → 3 Fe²⁺}$$

$$\text{Fe + 2HF → H₂ + Fe F₂}$$

The redox potential or REDOX is the potential difference measured between a non-corrodable electrode (for example made of platinum) and a reference electrode (Ag / AgCl), these two electrodes being immersed in the pickling solution. The measured value makes it possible to characterize the oxidizing power of the pickling solution, and to deduce from the values of the concentration of Fe³⁺ and of the temperature of the pickling solution, that of the concentration of Fe²⁺.

Preferably, the pickling operation is carried out by bringing the materials into contact with several successive solutions, and even more preferably, with two successive solutions.

On the other hand, the pickling solution preferably has a concentration of Fe³⁺ ions of 30 g / l, a temperature of 50 ° C and a redox potential. between 100 and 250 mV measured with a platinum electrode compared to an Ag / AgCl electrode.

The passivation operation is carried out by bringing the materials into contact with at least one solution chosen from a sulfuric acid solution containing Fe³⁺ ions, a phosphoric acid solution, a hydrogen peroxide solution or a solution of nitric acid or iron nitrates.

According to a preferred embodiment of the invention, the preparation solution has a concentration of 4N in sulfuric acid and a temperature of 80 ° C.

The preparation, pickling and passivation operations can be carried out in different ways, in particular by immersion, spraying or watering.

The following example describes an embodiment of the invention.

Example

On a continuous processing line, a strip of steel is stripped.

This is subjected to a preliminary shot peening, followed by a preparation operation consisting of immersing the steel strip consecutively in two baths of sulfuric acid at a concentration of 4 N, at a temperature of 85 ° C. , the metal content being less than saturation, that is to say less than 100 g / l.

The steel strip is then immersed in two consecutive pickling baths, having a concentration of 2% by weight of hydrofluoric acid, of 30 g / l of Fe³⁺ ions, a temperature of 50 ° C and a potential of oxido- reduction of 180 mV measured with respect to a platinum electrode and a reference Ag / AgCl electrode.

After pickling, the steel strip is treated for passivation in a nitric acid bath.

The production of Cr VI turns out to be less than 0.1 mg per tonne of product treated, and the emission of nitrous vapors (N 0 _x ) is zero.

On the other hand, this highly efficient pickling process reduces the time required to process the products by around 25%.

Claims (7)

Method for pickling steel materials, in particular continuously on a treatment line, comprising a blasting operation, a preparation operation, then at least one pickling operation and at least one passivation operation, characterized in that the operation preparation is carried out by bringing the materials into contact with at least a 2 to 10 N sulfuric acid solution at a temperature above 50 ° C., the metal content being less than saturation, and in that the operation pickling is carried out by bringing the materials into contact with at least one solution whose temperature is between 40 and 95 ° C, containing 1 to 10% by weight of hydrofluoric acid, and Fe²⁺ and Fe³⁺ ions in such a way that the concentration of Fe³⁺ ions is between 1 and 190 g / l and that the redox potential is between -200 and +800 mV, this potential being measured with a platinum electrode with respect to a Ag / AgCl electrode. Method according to claim 1, characterized in that the pickling operation is carried out by bringing the materials into contact with several successive solutions. Method according to claims 1 and 2, characterized in that the pickling operation is carried out by bringing the materials into contact with two successive solutions. Method according to one of claims 1 to 3, characterized in that the pickling solution has a concentration of Fe³⁺ ions of 30g / l, a temperature of 50 ° C and a redox potential between 100 and 250 mV measured with a platinum electrode relative to an Ag / AgCl electrode. Method according to claim 1, characterized in that the passivation operation is carried out by placing in contact with the materials with at least one solution chosen from a sulfuric acid solution containing Fe³⁺ ions, a phosphoric acid solution, a hydrogen peroxide solution or a nitric acid or iron nitrates solution. Process according to Claim 1, characterized in that the preparation solution has a concentration of 4 N in sulfuric acid and a temperature of 80 ° C. Method according to claim 1, characterized in that the preparation, pickling and passivation operations are carried out by immersion, spraying or watering.