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EP3559321B1 - Chemical wall-treatment method that reduces the formation of coke - Google Patents

Chemical wall-treatment method that reduces the formation of coke Download PDF

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Publication number
EP3559321B1
EP3559321B1 EP17821602.4A EP17821602A EP3559321B1 EP 3559321 B1 EP3559321 B1 EP 3559321B1 EP 17821602 A EP17821602 A EP 17821602A EP 3559321 B1 EP3559321 B1 EP 3559321B1
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EP
European Patent Office
Prior art keywords
carbides
treatment
treatment step
chemical
treatment method
Prior art date
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Application number
EP17821602.4A
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German (de)
French (fr)
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EP3559321A1 (en
Inventor
Christel AUGUSTIN
Sophie CAZOTTES
Nicolas VACHÉ
Philippe STEYER
Claude DURET THUAL
François Dupoiron
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centre National de la Recherche Scientifique CNRS
Institut National des Sciences Appliquees de Lyon
TotalEnergies Raffinage Chimie SAS
Universite Claude Bernard Lyon 1
Original Assignee
Centre National de la Recherche Scientifique CNRS
Institut National des Sciences Appliquees de Lyon
Total Raffinage Chimie SAS
Universite Claude Bernard Lyon 1
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25FPROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F3/00Electrolytic etching or polishing
    • C25F3/02Etching
    • C25F3/06Etching of iron or steel
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B24GRINDING; POLISHING
    • B24CABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
    • B24C1/00Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
    • B24C1/04Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for treating only selected parts of a surface, e.g. for carving stone or glass
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02Pretreatment of the material to be coated

Definitions

  • the invention relates to a method of treating the surface of a metal wall having the effect of reducing the formation of coke on the surface of this wall. More specifically, the invention relates to a process for the surface removal of carbides from a metal alloy wall, in particular by chemical treatment. The invention also relates to the use of a metal wall treated by the treatment process in a hydrocarbon treatment process.
  • the walls of the reactors of certain units in the petrochemical or chemical industry are sometimes subjected to very severe operating conditions which can cause coking phenomena.
  • the manufacture of alkenes, monomers valued in the polymer industry is obtained by cracking hydrocarbons derived from petroleum at temperatures of the order of 800 to 900 ° C.
  • a mixture of hydrocarbons and water vapor is circulated at high speed in reactors, generally made up of metal tubes, often made of alloys rich in nickel and chromium.
  • the reactors are thus subjected to high temperatures and complex aggressive atmospheres and the formation of carbon (coke) is observed on the surface of the walls of the tubes, this formation being catalyzed by the iron and nickel present in the alloy. metal constituting the walls.
  • This coke deposit can result in clogging of the tubes, leading to a pressure drop, a degradation of the conductivity of the walls and a reduction in yields. It is then necessary to stop the unit in order to remove the coke formed, an operation which is detrimental to the productivity of the unit.
  • a protective oxide layer can in particular be obtained by using suitable alloys, for example rich in chromium or in aluminum, or by oxidation pretreatments.
  • a process is proposed for treating a wall made of Fe-Ni-Cr metal alloy of an industrial reactor reducing the formation of coke on said surface when it is subjected to operational conditions favorable to coking, l metal alloy comprising in particular within its structure carbides some of which may be flush with the surface.
  • the metal alloy contains at least 5% by mass of iron, at least 18% by mass of chromium, at least 25% by mass of nickel and at least 0.05% by mass of carbon.
  • operation conditions favorable to coking is understood to mean conditions liable to generate the formation of coke on the surface.
  • the parameters influencing coking include, for example, the temperature, the nature of the liquid or gaseous fluids circulating inside the reactor and in contact with the surface, the fluid flow regime (turbulence).
  • the method comprises a chemical surface treatment step, during which at least part of the carbides initially present in the alloy, in particular at the surface, is removed by electrolytic dissolution.
  • Such a surface treatment step has the advantage of being easy to implement and relatively inexpensive.
  • the method according to the invention can advantageously be carried out to treat a wall of a reactor after the latter has been manufactured and before the reactor is put into service.
  • a step of oxidizing the wall can be envisaged, which makes it possible to further reduce the formation of coke.
  • the surface treatment seems to promote the formation of a homogeneous oxidized layer and thus reduce the formation of coke.
  • the invention is more particularly suitable for treating a wall of a steam cracking reactor (furnace), or of any other installation in which the formation of coke catalyzed by iron, nickel and optionally by other metal catalyst elements is observed. present in the metal alloy constituting the reactor.
  • the invention thus also relates to a process for treating hydrocarbons under conditions capable of causing the formation of coke, characterized in that the hydrocarbons are brought into contact with a surface of a wall made of a Fe-Ni-Cr metal alloy. , said surface of the metal wall being treated beforehand by a treatment method according to the invention so as to reduce the formation of a coke deposit.
  • the metal alloy is preferably a metal alloy containing at least 5% by mass of iron, at least 18% by mass of chromium, at least 25% by mass of nickel and at least 0.05% by mass of carbon .
  • the hydrocarbon treatment process can be a cracking process, in which the hydrocarbons are contacted with the wall mixed with water vapor.
  • a treatment is, for example, implemented in a steam cracking reactor.
  • the hydrocarbons can be brought into contact with the surface of the metal wall at a temperature of 800 to 900 ° C., in particular mixed with water vapor.
  • the treatment process according to the invention is intended for the treatment of Fe-Ni-Cr metal alloys, in particular containing carbides within their structure. These carbides may be flush with the surface, in other words be in contact with the gaseous medium surrounding the alloy, and / or may be located in close proximity to the surface, for example from a depth of 1 ⁇ m or more.
  • Such carbides are formed by precipitation during the manufacture of the wall. They may also appear partly in service.
  • the carbides which are particularly chemically stable, are formed from the carbon present in the metal alloy. These carbides can in particular be observed for a carbon content of the metal alloy of at least 0.05% by mass.
  • This type of metal alloy is particularly suitable for use at high temperature (“heat resistant alloys”).
  • the alloys treated are alloys having an Fe-Ni-Cr matrix, optionally austenitic, within which chromium carbides (Cr x C y ) precipitate, or even niobium carbides (NbC) when this element is present and / or carbonitrides when the alloy contains nitrogen, and / or other carbides optionally.
  • these alloys comprise carbon, in particular from 0.05 to 1% by mass of carbon, preferably from 0.08 to 0.6% by mass.
  • nickel or iron can be the predominant element.
  • the iron content is 100% complement of the content of other elements present in the alloy.
  • the metal alloy used can preferably be suitable for centrifugal molding.
  • it can comply with standard EN 10295 relating to refractory cast steels.
  • This technique consists in pouring the liquid metal in a mold animated by a rotational movement around its main axis.
  • the mold rotates at such a speed that it creates an average acceleration of the order of several hundred and up to 1000m / s 2 or more, in some cases.
  • the molds can be in sand or in metal shell, mounted on machines with horizontal, vertical or oblique axis. The parts obtained by centrifugation have very good physical and mechanical characteristics.
  • the treated wall can thus advantageously be produced by centrifugal molding.
  • this step is an electrochemical step, in particular an electrochemical step of selective dissolution.
  • the carbides initially present in the alloy are removed by electrolytic dissolution, in particular on the surface, namely the carbides flush with the surface and / or located in the immediate vicinity of the surface.
  • this step is advantageously carried out under conditions suitable for dissolving at least part of these carbides to a depth of at least 10 ⁇ m (from the treated surface), preferably of at least 20 ⁇ m, more preferably of at least 30 ⁇ m, or even at least 40 ⁇ m.
  • the electrolytic dissolution conditions can advantageously be adapted to dissolve one or more carbides chosen from chromium carbides, niobium carbides when the alloy contains niobium, carbonitrides when the alloy contains nitrogen or even other carbides, preferably chromium carbides.
  • the method according to the invention can comprise at least one other chemical treatment step, during which at least some of the carbides initially present in the alloy, in particular at the surface, and not dissolved during a previous step treatment chemical, is removed by electrolytic dissolution.
  • a chemical treatment step which is a step of electrolytic dissolution of chromium carbides and another chemical treatment step which is a step of electrolytic dissolution of niobium carbides when the alloy contains niobium, for example example in the quantities mentioned above.
  • a washing step can be provided between two successive chemical treatment steps under conditions suitable for removing traces of electrolytes from the treated surface. It may be one or more steps of rinsing the wall with water, preferably distilled water, optionally followed by one or more steps of rinsing with an alcohol, for example ethanol. This washing can be followed by drying under conditions allowing the rinsing fluid (s) to be removed from the wall to be treated.
  • the electrochemical dissolution of the chromium carbides is carried out followed by the electrochemical dissolution of the niobium carbides.
  • the chemical step is implemented by placing the wall to be treated at the anode of an electrolysis cell, the cathode being formed from a conductive part (for example metallic or graphite) and by applying an electric potential to the through the electrolysis cell.
  • a conductive part for example metallic or graphite
  • the chemical treatment step can be carried out in an electrolysis cell comprising an aqueous solution of an alkali metal hydroxide or an aqueous solution of sulfuric acid.
  • the electrolyte solution can thus comprise an aqueous solution of a soluble metal hydroxide.
  • This metal can be an alkali metal such as Na, K, Li, for example Na.
  • the electrolyte solution can comprise from 100 to 200 g / L of alkali metal hydroxide, preferably from 120 to 150 g / L.
  • the chloride content of the solution is less than 10 ppm by mass.
  • a procedure for dissolving niobium carbides is for example described in " Anode dissolution characteristics of titanium, niobium and chromium carbides ", 1971. V.Cihal, A. Desestret, M. Froment and GH Wagner .
  • the electrolytic solution can thus be an aqueous solution of sulfuric acid, the sulfuric acid concentration of which can be from 1 to 10 mol.L -1 , preferably from 2 to 9 mol.L -1 .
  • the invention is not however limited to these particular conditions: a person skilled in the art is able to determine other suitable concentrations of sulfuric acid, or even to use other suitable electrolytic solutions.
  • the difference in electric potential applied to the electrolysis cell can be 4 to 8 volts or 3 to volts, or even 3 to 5 volts. It may be preferable to avoid larger potential differences so as not to generate too much heat.
  • the current flow passing through the electrolysis cell is variable depending on the surface to be treated.
  • the current density can typically be from 5A / in 2 (7750A / m 2 ) to 10A / in 2 (15500A / m 2 ) of wall surface to be treated.
  • the duration of the treatment can be variable, for example from 4 to 50 hours or from 2 to 50 hours, for example from 2 to 30 hours, in depending on the amount of carbides and / or the wall depth that is to be treated.
  • the temperature of the electrolyte solution can vary from room temperature up to approximately 85 ° C. However, it is preferable that the temperature of the solution is kept below 60 ° C.
  • this step is preferably carried out after the chemical treatment step described above.
  • This surface treatment by impact is obtained by hammering the surface by spraying particles under conditions adapted to obtain a permanent plastic deformation of the surface, in particular under conditions adapted to obtain a covering of the carbides initially present on the surface by permanent plastic deformation of the surface.
  • the carbides initially present at the surface may be flush with the surface and / or be located in the immediate vicinity of the surface, in particular located at a depth of 1 ⁇ m and more from the surface.
  • this type of impact surface treatment is to compress the material below the impacted surface: this compressed material tends to regain its initial volume, resulting in strong residual compressive stresses. This makes it possible to significantly increase the life of an alloy part because almost all of the fatigue and stress corrosion failures are initiated on the surface of such parts.
  • the impacts caused by the projectiles will cover this surface with a uniform layer in compression.
  • the chemical treatment step generates the formation of cavities
  • the permanent plastic deformation obtained by the implementation of the mechanical treatment step makes it possible to fill these cavities at least in part.
  • an overlap of the carbides initially present on the surface is also observed - in other words a covering of the carbides flush with the surface and / or located in the immediate vicinity of the surface before the treatment. mechanical - which have not been dissolved by the chemical treatment step.
  • this modification of the surface limiting access to the carbides trapped inside the metal alloy also makes it possible to reduce the formation of coke.
  • such a surface treatment can be designated by the terms “microbanding” (use of balls), “sandblasting”, “corundum” (use of corundum particles), “shot blasting”.
  • the particles can be of various nature (mineral, metallic, etc.) of shapes (spherical or angular) and of various dimensions.
  • the particles can thus be chosen from particles of aluminum oxide (for example white or brown corundum), metal particles, balls of material inert under the operational conditions of use of the metal alloy wall, for example in glass or aluminum oxide, nesosilicate particles.
  • particles of aluminum oxide for example white or brown corundum
  • metal particles balls of material inert under the operational conditions of use of the metal alloy wall, for example in glass or aluminum oxide, nesosilicate particles.
  • the particles of nesosilicate have a general formula A m B n (SiO 4 ) t , where A is a transition metal or an alkaline earth and B is a transition metal or a rare earth.
  • A can be chosen from Mg, Ca and Mn and B can be chosen from Y, Ce, La.
  • the particles can have an average diameter of 100 to 500 ⁇ m.
  • glass beads with an average diameter of 100 to 200 ⁇ m, aluminum oxide particles with an average diameter of 250 to 500 ⁇ m can be used.
  • the particles can be projected by a gaseous fluid, for example air, argon or the like, under a pressure of 200 to 400kPa (2 to 4 bars), preferably of 250 to 350kPa (2.5 to 3 , 5 bars).
  • a gaseous fluid for example air, argon or the like
  • a pressure of 300 to 350 kPa can be used.
  • pressures of 270 to 320kPa can be used.
  • the projection distance can be 5 to 25cm, for example 10 to 20 cm.
  • the projection duration can be from 0.2 to 3 minutes, preferably from 0.5 to 2 minutes (in particular for an area of a few cm 2 ).
  • the step of surface treatment by impact can be implemented under suitable conditions to obtain a covering of the carbides and / or a closing of the cavities to a depth of at least 20 ⁇ m, preferably to a depth of at least less 30 ⁇ m.
  • This mechanical step is preferably carried out “cold”, in other words at ambient temperature, namely a temperature ranging from 18 to 35 ° C.
  • This step is carried out after the chemical treatment step, optionally after the mechanical surface treatment step. It is carried out under conditions which make it possible to form an oxide layer (s) on the treated surface of the wall, in particular a layer containing one or more chromium oxides.
  • the oxidation conditions can be those usually used to form an oxide layer (s) on this type of alloy and known from the prior art.
  • the oxidation can be carried out at a temperature of 800 to 1100 ° C., under a partial pressure of oxygen of 10 -6 atm to 0.2atm, for a period of 30min to 5h.
  • the figure 1 schematically represents an electrolytic cell 1. An electric potential difference is applied between two electrodes 2, 3 immersed in an electrolytic solution 4. The positive terminal is the anode 2, site of oxidation and the negative terminal is the cathode 3, seat of a reduction. A direct current generator 5 connected to the anode 2 and to the cathode 3 supplies the current.
  • the material to be dissolved must be located on the anode 2 (terminal +).
  • the distance between the two electrodes 2, 3 is for example around 1 cm.
  • a simple metal plate can be used for the cathode (the - terminal).
  • the electrolyte 4 will for example be a sodium hydroxide solution.
  • HP modified 25-35 type and 35-45 type metal alloy samples were tested. These alloys consist of an austenitic Fe-Ni-Cr matrix within which niobium (NbC) and chromium (Cr 7 C 3 ) carbides precipitate.
  • NbC niobium
  • Cr 7 C 3 chromium
  • Table 1 Typical chemical composition (% by mass) of the materials used Cr Or Fe VS Yes Mn Nb HP 25-35 25 35 26 0.5 1.4 1.6 0.5 HP 35-45 35 45 15 0.5 2.5 1.6 0.4
  • the samples used are plates of dimensions 8 x 30mm (samples C1 to C5) and 8 x 25mm (samples C6 to C9) and 2mm thick obtained by electroerosion in the heart of 5cm portions of new steam cracking tubes, of thickness initial 8mm.
  • the initial surface condition is a rough machined condition.
  • the tubes from which the tested samples were obtained were manufactured by centrifugal molding.
  • SEM Scanning electron microscope
  • Ionic cutting the transverse cuts are made by ionic cutting by defocused ion beam. This technique uses accelerated argon ions to tear off material, allowing a very fine and pollution-free surface polishing.
  • the samples are glued onto titanium masks using a “silver lacquer” made up of fine silver platelets suspended in a solvent.
  • Example 1 electrochemical treatment of the surface
  • the sample is subjected to a chemical treatment of electrolytic dissolution.
  • the sample to be tested is placed at the anode of an electrolysis cell as described in figure 1 , the cathode being a metallic plate of stainless steel or graphite, of similar or larger dimensions than the sample.
  • the anode and cathode are spaced a distance of about 1 cm, the plates being substantially parallel inside the electrolytic cell.
  • An electrolytic solution is prepared by dissolving, with mechanical stirring, 135 g of NaOH (in the form of pellets) in 1 L of distilled water, then the electrolysis cell is filled with the solution obtained.
  • the chloride content of the solution is less than 10 ppm by mass.
  • a potential difference is applied between the anode (sample) and the cathode.
  • the figures 2 and 3 are photographs of the C4 sample dissolved for 15h ( fig.2 ) and sample C5 dissolved 20h ( fig. 3 ).
  • the acceleration voltage applied for the measurement is 15kV
  • the magnification is 619x ( fig.2 ) and 629x ( fig.3 ) and the 10 ⁇ m scale.
  • sample C4 cavities are observed to a depth of about 40 ⁇ m, which seems to indicate the existence of interconnected carbide networks.
  • the cavities extend to a depth of 80 ⁇ m. Between 50 and 80 ⁇ m there are still chromium carbides, which seems to indicate that the carbide network is not completely interconnected.
  • Table 2 indicates for samples C6 to C9 the maximum depth to which a dissolution of chromium carbides was observed.
  • the figures 4 and 5 schematically represent observations typical of a section of an untreated sample ( fig. 4 ) and a chemically treated sample ( fig. 5 ).
  • the black parts correspond to chromium carbides
  • the gray parts correspond to niobium carbides.
  • Niobium carbides are observed in the cavities.
  • the solution could propagate by dissolving the chromium carbides resulting from the interconnected networks but preserving the niobium carbides (NbC).
  • NbC niobium carbides
  • the cavities are not completely empty.
  • SEM / EDX Energy Dispersive X-ray Spectrometry
  • Example 2 mechanical surface treatment / microbeading
  • a sample M1 is obtained.
  • Example 3 mechanical surface treatment / sandblasting (corundum)
  • Example 4 Chemical treatment + mechanical treatment / microbeading
  • Sample C4 of Example 1 is subjected to the same microbeading treatment as that described in Example 2. A sample CM4 is obtained.
  • Example 5 Chemical treatment + mechanical treatment / corundum
  • Sample C4 of Example 1 is subjected to the same microbeading treatment as that described in Example 3.
  • a sample CM5 is obtained.
  • the figure 6 schematically represents the typical observation of a section of a sample of alloy having undergone a chemical and mechanical treatment. It is noted that the chromium carbides are no longer in direct contact with the surface and that the cavities formed by the electrochemical dissolution have been at least partly closed for most of them.
  • the samples were observed by SEM.
  • the figures 7a and 7b are photographs (35x and 150x magnifications respectively) of the surface of the reference sample which has not undergone any particular treatment apart from the initial polishing. The formation of coke is observed on the surface.
  • the figures 8a and 8b are photographs of sample C4 having undergone the electrochemical treatment (35x and 150x magnifications respectively), the figures 9a and 9b are photographs of the CM5 sample (35x and 150x magnifications respectively).
  • the samples which have undergone a chemical treatment show less coke overall than the reference sample. Coke is still observed on about 10% of the surface of the sample.
  • Example 7 electrochemical treatment of the surface
  • the sample undergoes a chemical electrolytic dissolution treatment to remove niobium carbides.
  • the sample to be tested is placed at the anode of an electrolysis cell of the same type as that shown figure 1 and described in Example 1.
  • a potential difference of the order of 0.8 V is applied between the anode (sample) and the cathode for 2 hours.
  • the sample is then rinsed with distilled water and then with ethanol, dried and stored in a case protected from scratches and air in a desiccator.
  • a second test was carried out under the same electrolysis conditions on a sample of the same dimensions and of the same alloy previously subjected to electrolytic dissolution of the chromium carbides. This is carried out with a current density of 5A.in -2 (0.775A.cm -2 ) for 2 hours in a solution of NaOH (135g in the form of pellets in 1L of water). The sample obtained is then rinsed with distilled water and then with ethanol and dried before being introduced into the sulfuric acid solution for dissolving the niobium carbides.
  • the successive electrolytic decomposition of chromium carbides and niobium carbides therefore dissolves the NbCs on the surface.
  • the electrolytic dissolution of M 23 C 6 / M 7 C 3 could come in part "loosen” the NbCs and increase the free surface in contact with the electrolyte of the second dissolution.

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Description

DOMAINE TECHNIQUETECHNICAL AREA

L'invention concerne un procédé de traitement de surface d'une paroi métallique ayant pour effet de réduire la formation de coke à la surface de cette paroi. Plus précisément, l'invention concerne un procédé d'élimination surfacique de carbures d'une paroi en alliage métallique, notamment par traitement chimique. L'invention concerne également l'utilisation d'une paroi métallique traitée par le procédé de traitement dans un procédé de traitement d'hydrocarbures.The invention relates to a method of treating the surface of a metal wall having the effect of reducing the formation of coke on the surface of this wall. More specifically, the invention relates to a process for the surface removal of carbides from a metal alloy wall, in particular by chemical treatment. The invention also relates to the use of a metal wall treated by the treatment process in a hydrocarbon treatment process.

ETAT DE LA TECHNIQUESTATE OF THE ART

Les parois des réacteurs de certaines unités de l'industrie pétrochimique ou chimique sont parfois soumises à des conditions de service très sévères qui peuvent engendrer des phénomènes de cokage. Par exemple, la fabrication d'alcènes, monomères valorisés dans l'industrie des polymères, est obtenue par craquage des hydrocarbures dérivés du pétrole à des températures de l'ordre de 800 à 900°C. Dans ce type de procédé, un mélange d'hydrocarbures et de vapeur d'eau est mis en circulation à vitesse élevée dans des réacteurs, généralement constitués de tubes métalliques, souvent en alliages riches en nickel et en chrome. Les réacteurs sont ainsi soumis à des températures élevées et à des atmosphères agressives complexes et l'on observe la formation de carbone (coke) à la surface des parois des tubes, cette formation étant catalysée par le fer et le nickel présents dans l'alliage métallique constituant les parois. Ce dépôt de coke peut résulter en un encrassement des tubes, entrainant une perte de charge, une dégradation de la conductivité des parois et une diminution des rendements. Il est alors nécessaire d'arrêter l'unité afin d'éliminer le coke formé, opération préjudiciable à la productivité de l'unité.The walls of the reactors of certain units in the petrochemical or chemical industry are sometimes subjected to very severe operating conditions which can cause coking phenomena. For example, the manufacture of alkenes, monomers valued in the polymer industry, is obtained by cracking hydrocarbons derived from petroleum at temperatures of the order of 800 to 900 ° C. In this type of process, a mixture of hydrocarbons and water vapor is circulated at high speed in reactors, generally made up of metal tubes, often made of alloys rich in nickel and chromium. The reactors are thus subjected to high temperatures and complex aggressive atmospheres and the formation of carbon (coke) is observed on the surface of the walls of the tubes, this formation being catalyzed by the iron and nickel present in the alloy. metal constituting the walls. This coke deposit can result in clogging of the tubes, leading to a pressure drop, a degradation of the conductivity of the walls and a reduction in yields. It is then necessary to stop the unit in order to remove the coke formed, an operation which is detrimental to the productivity of the unit.

C'est la raison pour laquelle de nombreuses recherches ont été conduites par les industriels afin de limiter la formation de coke.This is the reason why much research has been carried out by manufacturers in order to limit the formation of coke.

Ces recherches ont permis la mise en œuvre de solutions comprenant notamment :

  • la formation d'une couche protectrice en surface, en particulier une couche d'oxydes,
  • l'utilisation de revêtements limitant la formation de cokage, voire augmentant les rendements,
  • l'ajout d'espèces soufrées dans la charge ce qui conduit à la formation de sulfures métalliques ayant un rôle protecteur en surface,
  • la réalisation d'un design particulier des réacteurs.
This research has enabled the implementation of solutions including in particular:
  • the formation of a protective layer on the surface, in particular a layer of oxides,
  • the use of coatings limiting the formation of coking, or even increasing yields,
  • the addition of sulfur species in the feed which leads to the formation of metal sulphides having a protective role on the surface,
  • the realization of a particular design of the reactors.

La formation d'une couche d'oxyde protectrice peut notamment être obtenue par utilisation d'alliages appropriés, par exemple riches en chrome ou en aluminium, ou par des prétraitements d'oxydation.The formation of a protective oxide layer can in particular be obtained by using suitable alloys, for example rich in chromium or in aluminum, or by oxidation pretreatments.

Malgré l'existence de ces solutions, il existe toujours un besoin pour un traitement simple et peu couteux permettant de réduire la formation de coke.Despite the existence of these solutions, there is still a need for a simple and inexpensive treatment making it possible to reduce the formation of coke.

RESUME DE L'INVENTIONSUMMARY OF THE INVENTION

A cet effet, il est proposé un procédé de traitement d'une paroi en alliage métallique Fe-Ni-Cr d'un réacteur industriel réduisant la formation de coke sur ladite surface lorsqu'elle est soumise à des conditions opérationnelles favorables au cokage, l'alliage métallique comprenant notamment au sein de sa structure des carbures dont certains peuvent affleurer la surface. Par exemple, l'alliage métallique contient au moins 5% en masse de fer, au moins 18% en masse de chrome, au moins 25% en masse de nickel et au moins 0,05% en masse de carbone.To this end, a process is proposed for treating a wall made of Fe-Ni-Cr metal alloy of an industrial reactor reducing the formation of coke on said surface when it is subjected to operational conditions favorable to coking, l metal alloy comprising in particular within its structure carbides some of which may be flush with the surface. For example, the metal alloy contains at least 5% by mass of iron, at least 18% by mass of chromium, at least 25% by mass of nickel and at least 0.05% by mass of carbon.

Par « conditions opérationnelles favorables au cokage », on entend des conditions susceptibles d'engendrer la formation de coke sur la surface. Les paramètres influant le cokage comprennent par exemple la température, la nature des fluides liquides ou gazeux circulant à l'intérieur du réacteur et en contact avec la surface, le régime d'écoulement des fluides (turbulences).The term “operational conditions favorable to coking” is understood to mean conditions liable to generate the formation of coke on the surface. The parameters influencing coking include, for example, the temperature, the nature of the liquid or gaseous fluids circulating inside the reactor and in contact with the surface, the fluid flow regime (turbulence).

Selon l'invention, le procédé comprend une étape chimique de traitement de surface, au cours de laquelle on élimine par dissolution électrolytique au moins une partie des carbures initialement présents dans l'alliage, notamment en surface.According to the invention, the method comprises a chemical surface treatment step, during which at least part of the carbides initially present in the alloy, in particular at the surface, is removed by electrolytic dissolution.

De manière surprenante, alors que la formation de coke est catalysée essentiellement par la présence de fer et de nickel, on observe une amélioration de la résistance au cokage d'une surface en alliage métallique en utilisant le traitement selon l'invention. Autrement dit, la formation de coke est réduite par rapport à une surface non traitée.Surprisingly, while the formation of coke is catalyzed primarily by the presence of iron and nickel, an improvement in the resistance to coking of an alloy surface is observed. metallic using the treatment according to the invention. In other words, the formation of coke is reduced compared to an untreated surface.

Sans vouloir être lié par une théorie, l'élimination de la surface d'au moins une partie des carbures par dissolution électrolytique permet de réduire la formation de coke.Without wishing to be bound by theory, the removal of at least part of the carbides from the surface by electrolytic dissolution makes it possible to reduce the formation of coke.

Une telle étape de traitement de surface présente l'avantage d'être facile à mettre en œuvre et relativement peu coûteuse.Such a surface treatment step has the advantage of being easy to implement and relatively inexpensive.

Avantageusement, plusieurs étapes chimiques de traitement par dissolution électrolytique peuvent être prévues.Advantageously, several chemical steps of treatment by electrolytic dissolution can be provided.

Le procédé selon l'invention peut avantageusement être réalisé pour traiter une paroi d'un réacteur après la fabrication de celui-ci et avant la mise en service du réacteur.The method according to the invention can advantageously be carried out to treat a wall of a reactor after the latter has been manufactured and before the reactor is put into service.

Avantageusement, après l'étape chimique de traitement, une étape d'oxydation de la paroi peut être envisagée, ce qui permet de réduire davantage la formation de coke. Sans vouloir être lié par une théorie, le traitement de surface semble favoriser la formation d'une couche oxydée homogène et réduire ainsi la formation de coke.Advantageously, after the chemical treatment step, a step of oxidizing the wall can be envisaged, which makes it possible to further reduce the formation of coke. Without wishing to be bound by theory, the surface treatment seems to promote the formation of a homogeneous oxidized layer and thus reduce the formation of coke.

L'invention est plus particulièrement adaptée pour traiter une paroi d'un réacteur de vapocraquage (four), ou de toute autre installation dans laquelle on observe la formation de coke catalysée par le fer, le nickel et éventuellement par d'autres éléments métalliques catalyseurs présents dans l'alliage métallique constituant le réacteur.The invention is more particularly suitable for treating a wall of a steam cracking reactor (furnace), or of any other installation in which the formation of coke catalyzed by iron, nickel and optionally by other metal catalyst elements is observed. present in the metal alloy constituting the reactor.

L'invention concerne ainsi également un procédé de traitement d'hydrocarbures dans des conditions aptes à provoquer la formation de coke, caractérisé en que l'on met les hydrocarbures en contact avec une surface d'une paroi en alliage métallique Fe-Ni-Cr, ladite surface de la paroi métallique étant préalablement traitée par un procédé de traitement selon l'invention de manière à réduire la formation d'un dépôt de coke. Notamment, l'alliage métallique est de préférence un alliage métallique contenant au moins 5% en masse de fer, au moins 18% en masse de chrome, au moins 25% en masse de nickel et au moins 0,05% en masse de carbone.The invention thus also relates to a process for treating hydrocarbons under conditions capable of causing the formation of coke, characterized in that the hydrocarbons are brought into contact with a surface of a wall made of a Fe-Ni-Cr metal alloy. , said surface of the metal wall being treated beforehand by a treatment method according to the invention so as to reduce the formation of a coke deposit. In particular, the metal alloy is preferably a metal alloy containing at least 5% by mass of iron, at least 18% by mass of chromium, at least 25% by mass of nickel and at least 0.05% by mass of carbon .

Le procédé de traitement des hydrocarbures peut être un procédé de craquage, dans lequel les hydrocarbures sont mis en contact avec la paroi en mélange avec de la vapeur d'eau. Un tel traitement est par exemple mis en œuvre dans un réacteur de vapocraquage.The hydrocarbon treatment process can be a cracking process, in which the hydrocarbons are contacted with the wall mixed with water vapor. Such a treatment is, for example, implemented in a steam cracking reactor.

Avantageusement, les hydrocarbures peuvent être mis en contact avec la surface de la paroi métallique à une température de 800 à 900°C, notamment en mélange avec de la vapeur d'eau.Advantageously, the hydrocarbons can be brought into contact with the surface of the metal wall at a temperature of 800 to 900 ° C., in particular mixed with water vapor.

DESCRIPTION DETAILLEE DE L'INVENTIONDETAILED DESCRIPTION OF THE INVENTION Alliages métalliquesMetal alloys

Le procédé de traitement selon l'invention est destiné aux traitements des alliages métalliques Fe-Ni-Cr, contenant notamment des carbures au sein de leur structure. Ces carbures peuvent affleurer la surface, autrement dit être en contact avec le milieu gazeux environnant l'alliage, et/ou peuvent être situés à proximité immédiate de la surface, par exemple à partir d'une profondeur de 1µm ou plus.The treatment process according to the invention is intended for the treatment of Fe-Ni-Cr metal alloys, in particular containing carbides within their structure. These carbides may be flush with the surface, in other words be in contact with the gaseous medium surrounding the alloy, and / or may be located in close proximity to the surface, for example from a depth of 1 μm or more.

La présence de ces carbures peut être observée par des observations au microscope à balayage électronique de la surface de la paroi et/ou de coupes de la paroi.The presence of these carbides can be observed by scanning electron microscopic observations of the surface of the wall and / or sections of the wall.

De tels carbures se forment par précipitation lors de la fabrication de la paroi. Ils peuvent également apparaître en partie en service. De manière connue, les carbures, particulièrement stables chimiquement, se forment à partir du carbone présent dans l'alliage métallique. Ces carbures peuvent notamment être observés pour une teneur en carbone de l'alliage métallique d'au moins 0,05% en masse.Such carbides are formed by precipitation during the manufacture of the wall. They may also appear partly in service. In a known manner, the carbides, which are particularly chemically stable, are formed from the carbon present in the metal alloy. These carbides can in particular be observed for a carbon content of the metal alloy of at least 0.05% by mass.

Ce type d'alliage métallique est notamment adapté à une utilisation à haute température (« heat resistant alloys »)This type of metal alloy is particularly suitable for use at high temperature ("heat resistant alloys")

Avantageusement, les alliages traités sont des alliages présentant une matrice Fe-Ni-Cr, éventuellement austénitique, au sein de laquelle précipitent des carbures de chrome (CrxCy), voire des carbures de niobium (NbC) lorsque cet élément est présent et/ou des carbonitrures lorsque l'alliage contient de l'azote, et/ou d'autres carbures éventuellement.Advantageously, the alloys treated are alloys having an Fe-Ni-Cr matrix, optionally austenitic, within which chromium carbides (Cr x C y ) precipitate, or even niobium carbides (NbC) when this element is present and / or carbonitrides when the alloy contains nitrogen, and / or other carbides optionally.

De tels alliages comprennent ainsi :

  • au moins 5% en masse de fer, de préférence de 10% à 50%, de manière préférée de 12 à 48% en masse,
  • au moins 18% en masse de chrome, de préférence de 19% à 42% en masse,
  • au moins 25% en masse de nickel, de préférence de 31% à 46% en masse.
Such alloys thus include:
  • at least 5% by weight of iron, preferably 10% to 50%, more preferably 12 to 48% by weight,
  • at least 18% by mass of chromium, preferably from 19% to 42% by mass,
  • at least 25% by mass of nickel, preferably from 31% to 46% by mass.

En outre, ces alliages comprennent du carbone, notamment de 0,05 à 1% en masse de carbone, de préférence de 0,08 à 0,6% en masse.In addition, these alloys comprise carbon, in particular from 0.05 to 1% by mass of carbon, preferably from 0.08 to 0.6% by mass.

Avantageusement, dans ces alliages, le nickel ou le fer peuvent être l'élément prépondérant. En général, la teneur en fer est le complément à 100% des teneurs des autres éléments présents dans l'alliage.Advantageously, in these alloys, nickel or iron can be the predominant element. In general, the iron content is 100% complement of the content of other elements present in the alloy.

Les alliages métalliques traités peuvent comprendre d'autres éléments. Ils peuvent notamment comprendre un ou plusieurs des éléments suivants :

  • du niobium, notamment en une teneur de 0,3 à 2,5% en masse, de préférence de 0,5 à 2% en masse,
  • du manganèse, notamment en une teneur de 0,01 à 2% en masse, de préférence de 0,5 à 1,7% en masse,
  • du silicium, notamment en une teneur de 0,5 à 3% en masse, de préférence de 1 à 2,5% en masse,
  • de l'azote, notamment en une teneur d'au plus 1% en masse, par exemple de 0,01 à 0,5% en masse.
The treated metal alloys can include other elements. They may in particular include one or more of the following elements:
  • niobium, in particular in a content of 0.3 to 2.5% by mass, preferably 0.5 to 2% by mass,
  • manganese, in particular in a content of 0.01 to 2% by mass, preferably 0.5 to 1.7% by mass,
  • silicon, in particular in a content of 0.5 to 3% by mass, preferably 1 to 2.5% by mass,
  • nitrogen, in particular in a content of at most 1% by mass, for example from 0.01 to 0.5% by mass.

L'alliage métallique utilisé peut de préférence être adapté au moulage par centrifugation. Il peut notamment respecter la norme EN 10295 relative aux aciers moulés réfractaires. Cette technique consiste à couler le métal liquide dans un moule animé d'un mouvement de rotation autour de son axe principal. En général, le moule tourne à une vitesse telle qu'il crée une accélération moyenne de l'ordre de plusieurs centaines et jusqu'à 1000m/s2 ou plus, dans certains cas. Les moules peuvent être en sable ou en coquille métallique, montés sur des machines à axe horizontal, vertical ou oblique. Les pièces obtenues par centrifugation possèdent de très bonnes caractéristiques physiques et mécaniques.The metal alloy used can preferably be suitable for centrifugal molding. In particular, it can comply with standard EN 10295 relating to refractory cast steels. This technique consists in pouring the liquid metal in a mold animated by a rotational movement around its main axis. In general, the mold rotates at such a speed that it creates an average acceleration of the order of several hundred and up to 1000m / s 2 or more, in some cases. The molds can be in sand or in metal shell, mounted on machines with horizontal, vertical or oblique axis. The parts obtained by centrifugation have very good physical and mechanical characteristics.

La paroi traitée peut ainsi avantageusement être réalisée par moulage par centrifugation.The treated wall can thus advantageously be produced by centrifugal molding.

Etape chimique de traitement de surfaceChemical surface treatment step

Plus précisément, cette étape est une étape électrochimique, notamment une étape électrochimique de dissolution sélective.More precisely, this step is an electrochemical step, in particular an electrochemical step of selective dissolution.

Au cours de cette étape, on élimine par dissolution électrolytique au moins une partie des carbures initialement présents dans l'alliage, notamment en surface, à savoir les carbures affleurant la surface et/ou situés à proximité immédiate de la surface.During this step, at least some of the carbides initially present in the alloy are removed by electrolytic dissolution, in particular on the surface, namely the carbides flush with the surface and / or located in the immediate vicinity of the surface.

Notamment, cette étape est avantageusement mise en œuvre dans des conditions adaptées pour dissoudre au moins une partie de ces carbures sur une profondeur d'au moins 10µm (depuis la surface traitée), de préférence d'au moins 20µm, davantage de préférence d'au moins 30µm, voire d'au moins 40µm.In particular, this step is advantageously carried out under conditions suitable for dissolving at least part of these carbides to a depth of at least 10 μm (from the treated surface), preferably of at least 20 μm, more preferably of at least 30 μm, or even at least 40 μm.

Notamment, les conditions de dissolution électrolytique peuvent avantageusement être adaptées pour dissoudre un ou plusieurs carbures choisis parmi les carbures de chrome, les carbures de niobium lorsque l'alliage contient du niobium, les carbonitrures lorsque l'alliage contient de l'azote voire d'autres carbures, de préférence les carbures de chrome.In particular, the electrolytic dissolution conditions can advantageously be adapted to dissolve one or more carbides chosen from chromium carbides, niobium carbides when the alloy contains niobium, carbonitrides when the alloy contains nitrogen or even other carbides, preferably chromium carbides.

En variante ou en combinaison, plusieurs étapes chimiques de traitement de surface peuvent être prévues. Ainsi, le procédé selon l'invention peut comprendre au moins une autre étape chimique de traitement, au cours de laquelle au moins une partie des carbures initialement présents dans l'alliage, notamment en surface, et non dissous au cours d'une précédente étape chimique de traitement, est éliminée par dissolution électrolytique.As a variant or in combination, several chemical surface treatment steps can be provided. Thus, the method according to the invention can comprise at least one other chemical treatment step, during which at least some of the carbides initially present in the alloy, in particular at the surface, and not dissolved during a previous step treatment chemical, is removed by electrolytic dissolution.

Avantageusement, on peut mettre en œuvre une étape de traitement chimique qui est une étape de dissolution électrolytique de carbures de chrome et une autre étape de traitement chimique qui est une étape de dissolution électrolytique de carbures de niobium lorsque l'alliage contient du niobium, par exemple dans les quantités mentionnées plus haut.Advantageously, it is possible to implement a chemical treatment step which is a step of electrolytic dissolution of chromium carbides and another chemical treatment step which is a step of electrolytic dissolution of niobium carbides when the alloy contains niobium, for example example in the quantities mentioned above.

Une étape de lavage peut être prévue entre deux étapes successives de traitement chimique dans des conditions aptes à éliminer les traces d'électrolytes de la surface traiter. Il peut s'agir d'une ou plusieurs étapes de rinçage de la paroi avec de l'eau, de préférence de l'eau distillée, optionnellement suivie d'une ou plusieurs étapes de rinçage avec un alcool, par exemple l'éthanol. Ce lavage peut être suivi d'un séchage dans des conditions permettant d'éliminer le ou les fluides de rinçage de la paroi à traiter.A washing step can be provided between two successive chemical treatment steps under conditions suitable for removing traces of electrolytes from the treated surface. It may be one or more steps of rinsing the wall with water, preferably distilled water, optionally followed by one or more steps of rinsing with an alcohol, for example ethanol. This washing can be followed by drying under conditions allowing the rinsing fluid (s) to be removed from the wall to be treated.

Dans un mode de réalisation particulier, on procède à la dissolution électrochimique des carbures de chrome puis à la dissolution électrochimique des carbures de niobium.In a particular embodiment, the electrochemical dissolution of the chromium carbides is carried out followed by the electrochemical dissolution of the niobium carbides.

L'étape chimique est mise en œuvre en plaçant la paroi à traiter à l'anode d'une cellule d'électrolyse, la cathode étant formée d'une pièce conductrice (par exemple métallique ou en graphite) et en appliquant un potentiel électrique au travers de la cellule d'électrolyse.The chemical step is implemented by placing the wall to be treated at the anode of an electrolysis cell, the cathode being formed from a conductive part (for example metallic or graphite) and by applying an electric potential to the through the electrolysis cell.

L'étape chimique de traitement peut être réalisée dans une cellule d'électrolyse comprenant une solution aqueuse d'un hydroxyde de métal alcalin ou une solution aqueuse d'acide sulfurique.The chemical treatment step can be carried out in an electrolysis cell comprising an aqueous solution of an alkali metal hydroxide or an aqueous solution of sulfuric acid.

Une procédure de dissolution des carbures de chrome est par exemple décrite dans le brevet US 4851093 A .A procedure for dissolving chromium carbides is for example described in the patent US 4,851,093 A .

La solution électrolytique peut ainsi comprendre une solution aqueuse d'un hydroxyde de métal soluble. Ce métal peut être un métal alcalin tel que Na, K, Li, par exemple Na.The electrolyte solution can thus comprise an aqueous solution of a soluble metal hydroxide. This metal can be an alkali metal such as Na, K, Li, for example Na.

La solution électrolytique peut comprendre de 100 à 200g/L d'hydroxyde de métal alcalin, de préférence de 120 à 150g/L.The electrolyte solution can comprise from 100 to 200 g / L of alkali metal hydroxide, preferably from 120 to 150 g / L.

De préférence, la teneur en chlorure de la solution est inférieure à 10ppm en masse.Preferably, the chloride content of the solution is less than 10 ppm by mass.

Une procédure de dissolution des carbures de niobium est par exemple décrite dans " Anode dissolution characteristics of titanium, niobium and chromium carbides", 1971. V.Cihal, A. Desestret, M. Froment et G.H. Wagner .A procedure for dissolving niobium carbides is for example described in " Anode dissolution characteristics of titanium, niobium and chromium carbides ", 1971. V.Cihal, A. Desestret, M. Froment and GH Wagner .

La solution électrolytique peut ainsi être une solution aqueuse d'acide sulfurique dont la concentration en acide sulfurique peut être de 1 à 10 mol.L-1, de préférence de 2 à 9 mol.L-1. L'invention n'est toutefois pas limitée à ces conditions particulières : l'homme du métier est à même de déterminer d'autres concentrations d'acide sulfurique adaptées, voire d'utiliser d'autres solutions électrolytiques adaptées.The electrolytic solution can thus be an aqueous solution of sulfuric acid, the sulfuric acid concentration of which can be from 1 to 10 mol.L -1 , preferably from 2 to 9 mol.L -1 . The invention is not however limited to these particular conditions: a person skilled in the art is able to determine other suitable concentrations of sulfuric acid, or even to use other suitable electrolytic solutions.

La différence de potentiel électrique appliquée à la cellule d'électrolyse peut être de 4 à 8 Volts ou de 3 à Volts, voire de 3 à 5 Volts. Il peut être préférable d'éviter des différences de potentiels supérieures afin de ne pas générer trop de chaleur.The difference in electric potential applied to the electrolysis cell can be 4 to 8 volts or 3 to volts, or even 3 to 5 volts. It may be preferable to avoid larger potential differences so as not to generate too much heat.

Le flux de courant traversant la cellule d'électrolyse est variable en fonction de la surface à traiter. La densité de courant peut typiquement être de 5A/in2 (7750A/m2) à 10A/in2 (15500A/m2) de surface de paroi à traiter.The current flow passing through the electrolysis cell is variable depending on the surface to be treated. The current density can typically be from 5A / in 2 (7750A / m 2 ) to 10A / in 2 (15500A / m 2 ) of wall surface to be treated.

La durée du traitement peut être variable, par exemple de 4 à 50 heures ou de 2 à 50 heures, par exemple de 2 à 30 heures, en fonction de la quantité de carbures et/ou de la profondeur de paroi que l'on souhaite traiter.The duration of the treatment can be variable, for example from 4 to 50 hours or from 2 to 50 hours, for example from 2 to 30 hours, in depending on the amount of carbides and / or the wall depth that is to be treated.

La température de la solution électrolytique peut varier de la température ambiante jusqu'à 85°C environ. Il est toutefois préférable que la température de la solution soit maintenue inférieure à 60°C.The temperature of the electrolyte solution can vary from room temperature up to approximately 85 ° C. However, it is preferable that the temperature of the solution is kept below 60 ° C.

Etape mécanique de traitement de surface par impactMechanical step of surface treatment by impact

Lorsqu'elle est présente, cette étape est de préférence mise en œuvre après l'étape chimique de traitement précédemment décrite. Ce traitement de surface par impact est obtenu en martelant la surface par projection de particules dans des conditions adaptées pour obtenir une déformation plastique permanente de la surface, notamment dans des conditions adaptées pour obtenir un recouvrement des carbures initialement présents en surface par déformation plastique permanente de la surface.When it is present, this step is preferably carried out after the chemical treatment step described above. This surface treatment by impact is obtained by hammering the surface by spraying particles under conditions adapted to obtain a permanent plastic deformation of the surface, in particular under conditions adapted to obtain a covering of the carbides initially present on the surface by permanent plastic deformation of the surface.

Les carbures initialement présents en surface peuvent affleurer la surface et/ou être situés à proximité immédiate de la surface, notamment situés à une profondeur de 1µm et plus de la surface.The carbides initially present at the surface may be flush with the surface and / or be located in the immediate vicinity of the surface, in particular located at a depth of 1 μm and more from the surface.

Ces projections de particules créent en surface une petite empreinte ou cratère. Cette surface est ainsi déformée plastiquement en traction ou en compression lors de l'impact.These projections of particles create a small imprint or crater on the surface. This surface is thus plastically deformed in traction or in compression upon impact.

Habituellement, l'objectif de ce type de traitement de surface par impact est de comprimer la matière sous la surface impactée : cette matière comprimée tend à retrouver son volume initial, entraînant de fortes contraintes résiduelles en compression. Ceci permet d'augmenter de manière notable la durée de vie d'une pièce en alliage car la quasi-totalité des ruptures en fatigue et en corrosion sous tension s'initient en surface de telles pièces.Usually, the objective of this type of impact surface treatment is to compress the material below the impacted surface: this compressed material tends to regain its initial volume, resulting in strong residual compressive stresses. This makes it possible to significantly increase the life of an alloy part because almost all of the fatigue and stress corrosion failures are initiated on the surface of such parts.

Dans la présente invention, les impacts causés par les projectiles vont recouvrir cette surface d'une couche uniforme en compression.In the present invention, the impacts caused by the projectiles will cover this surface with a uniform layer in compression.

Alors que l'étape de traitement chimique engendre la formation de cavités, la déformation plastique permanente obtenue par la mise en œuvre de l'étape de traitement mécanique permet de reboucher au moins en partie ces cavités. En outre, on observe également un recouvrement des carbures initialement présents en surface - autrement dit un recouvrement des carbures affleurant la surface et/ou situés à proximité immédiate de la surface avant le traitement mécanique - qui n'ont pas été dissous par l'étape de traitement chimique. De manière surprenante, cette modification de la surface limitant l'accès aux carbures piégés à l'intérieur de l'alliage métallique permet également de réduire la formation de coke.While the chemical treatment step generates the formation of cavities, the permanent plastic deformation obtained by the implementation of the mechanical treatment step makes it possible to fill these cavities at least in part. In addition, an overlap of the carbides initially present on the surface is also observed - in other words a covering of the carbides flush with the surface and / or located in the immediate vicinity of the surface before the treatment. mechanical - which have not been dissolved by the chemical treatment step. Surprisingly, this modification of the surface limiting access to the carbides trapped inside the metal alloy also makes it possible to reduce the formation of coke.

Selon la forme des particules et la puissance de projection, un tel traitement de surface peut être désigné par les termes « microbillage » (utilisation de billes), « sablage », « corindonnage » (utilisation de particules de corindon), « grenaillage ».Depending on the shape of the particles and the power of the projection, such a surface treatment can be designated by the terms “microbanding” (use of balls), “sandblasting”, “corundum” (use of corundum particles), “shot blasting”.

Les particules peuvent être de nature diverse (minérale, métallique,...) de formes (sphériques ou anguleuses) et de dimensions variées.The particles can be of various nature (mineral, metallic, etc.) of shapes (spherical or angular) and of various dimensions.

Les particules peuvent ainsi être choisies parmi des particules d'oxyde d'aluminium (par exemple corindon blanc ou brun), des particules métalliques, des billes en matériau inerte dans les conditions opérationnelles d'utilisation de la paroi en alliage métallique, par exemple en verre ou en oxyde d'aluminium, des particules de nesosilicates.The particles can thus be chosen from particles of aluminum oxide (for example white or brown corundum), metal particles, balls of material inert under the operational conditions of use of the metal alloy wall, for example in glass or aluminum oxide, nesosilicate particles.

Notamment, les particules de nesosilicate (grenat), présentent une formule générale AmBn(SiO4)t, où A est un métal de transition ou un alcalino-terreux et B est un métal de transition ou une terre rare. Notamment A peut être choisi parmi Mg, Ca et Mn et B peut être choisi parmi Y, Ce, La.In particular, the particles of nesosilicate (garnet) have a general formula A m B n (SiO 4 ) t , where A is a transition metal or an alkaline earth and B is a transition metal or a rare earth. In particular, A can be chosen from Mg, Ca and Mn and B can be chosen from Y, Ce, La.

Les particules peuvent présenter un diamètre moyen de 100 à 500µm.The particles can have an average diameter of 100 to 500 µm.

A titre d'exemple, on peut utiliser des billes de verre d'un diamètre moyen de 100 à 200µm, des particules d'oxyde d'aluminium de diamètre moyen de 250 à 500µm.By way of example, glass beads with an average diameter of 100 to 200 μm, aluminum oxide particles with an average diameter of 250 to 500 μm can be used.

Les particules peuvent être projetées par un fluide gazeux, par exemple de l'air, de l'argon ou autre, sous une pression de 200 à 400kPa (2 à 4 bars), de préférence de 250 à 350kPa (2,5 à 3,5 bars).The particles can be projected by a gaseous fluid, for example air, argon or the like, under a pressure of 200 to 400kPa (2 to 4 bars), preferably of 250 to 350kPa (2.5 to 3 , 5 bars).

Pour des billes de verre, une pression de 300 à 350kPa est utilisable. Pour des particules d'oxyde d'aluminium, on peut utiliser des pressions de 270 à 320kPa.For glass beads, a pressure of 300 to 350 kPa can be used. For aluminum oxide particles, pressures of 270 to 320kPa can be used.

La distance de projection peut être de 5 à 25cm, par exemple de 10 à 20 cm.The projection distance can be 5 to 25cm, for example 10 to 20 cm.

La durée de projection peut être de 0,2 à 3 minutes, de préférence de 0,5 à 2 minutes (notamment pour une surface de quelques cm2).The projection duration can be from 0.2 to 3 minutes, preferably from 0.5 to 2 minutes (in particular for an area of a few cm 2 ).

A titre d'exemple, on peut utiliser une installation pouvant contenir environ 40 litres de particules, projetées à l'aide d'une buse de 7 à 8 mm de diamètre, avec de l'air comprimé sous pression de 2,5 à 3,5 bars.By way of example, one can use an installation which can contain approximately 40 liters of particles, projected using a nozzle of 7 to 8 mm in diameter, with compressed air under pressure of 2.5 to 3 , 5 bars.

D'autres conditions de projection peuvent néanmoins être envisagées en fonction des particules utilisées afin d'obtenir la déformation plastique de la surface telle que décrite dans l'objet de la revendication 5.Other spraying conditions can nevertheless be envisaged depending on the particles used in order to obtain the plastic deformation of the surface as described in the subject of claim 5.

Avantageusement, l'étape de traitement de surface par impact peut être mise en œuvre dans des conditions adaptées pour obtenir un recouvrement des carbures et/ou une fermeture des cavités sur une profondeur d'au moins 20µm de préférence, sur une profondeur d'au moins 30µm.Advantageously, the step of surface treatment by impact can be implemented under suitable conditions to obtain a covering of the carbides and / or a closing of the cavities to a depth of at least 20 μm, preferably to a depth of at least less 30µm.

Cette étape mécanique est de préférence réalisée « à froid », autrement dit à température ambiante, à savoir une température allant de 18 à 35°C.This mechanical step is preferably carried out “cold”, in other words at ambient temperature, namely a temperature ranging from 18 to 35 ° C.

Etape d'oxydationOxidation step

Cette étape est réalisée après l'étape chimique de traitement, optionnellement après l'étape mécanique de traitement de surface. Elle est réalisée dans des conditions permettant de former une couche d'oxyde(s) sur la surface traitée de la paroi, notamment une couche contenant un ou plusieurs oxydes de chrome.This step is carried out after the chemical treatment step, optionally after the mechanical surface treatment step. It is carried out under conditions which make it possible to form an oxide layer (s) on the treated surface of the wall, in particular a layer containing one or more chromium oxides.

Les conditions d'oxydation peuvent être celles habituellement utilisées pour former une couche d'oxyde(s) sur ce type d'alliage et connues de l'art antérieur.The oxidation conditions can be those usually used to form an oxide layer (s) on this type of alloy and known from the prior art.

A titre d'exemple, l'oxydation peut être réalisée à une température de 800 à 1100°C, sous une pression partielle de dioxygène de 10-6atm à 0,2atm, pendant une durée de 30min à 5h.By way of example, the oxidation can be carried out at a temperature of 800 to 1100 ° C., under a partial pressure of oxygen of 10 -6 atm to 0.2atm, for a period of 30min to 5h.

BREVE DESCRIPTION DES FIGURESBRIEF DESCRIPTION OF THE FIGURES

L'invention est maintenant décrite au moyen d'exemples et en référence aux dessins annexés, non limitatifs, dans lesquels :

  • la figure 1 représente schématiquement une cellule d'électrolyse utilisable pour l'étape chimique de traitement de surface ;
  • les figures 2 et 3 représentent des photographies MEB de coupes de deux échantillons ayant subi un traitement électrochimique de dissolution sélective ;
  • les figures 4 à 6 sont des représentations schématiques des observations en coupe d'échantillons ayant subi respectivement : uniquement un polissage (fig.4), un traitement de dissolution électrochimique (fig.5), un traitement de dissolution électrochimique suivi d'un traitement mécanique de surface (fig.6) ;
  • les figures 7 à 9 sont des photographies MEB avec un détecteur d'électrons secondaires (tension d'accélération appliquée de 20kV-fig.7, 9 ou 25kV-fig.8) de coupes d'échantillons, selon deux grandissements :
    • ∘ a : grossissement 35x, échelle de 500µm
    • ∘ b : grossissement 150x, échelle de 100µm.
    Les figures 7a, 7b montrent des photographies d'un échantillon de référence, les figures 8a, 8b montrent des photographies d'un échantillon ayant subi un traitement de dissolution électrochimique, les figures 9a, 9b montrent des photographies d'un échantillon ayant subi un traitement de dissolution électrochimique suivi d'un traitement mécanique de corindonnage.
The invention is now described by way of examples and with reference to the appended non-limiting drawings, in which:
  • the figure 1 schematically represents an electrolysis cell which can be used for the chemical stage of surface treatment;
  • the figures 2 and 3 represent SEM photographs of sections of two samples having undergone an electrochemical treatment of selective dissolution;
  • the figures 4 to 6 are schematic representations of cross-sectional observations of samples having undergone respectively: only polishing ( fig.4 ), an electrochemical dissolution treatment ( fig.5 ), an electrochemical dissolution treatment followed by a mechanical surface treatment ( fig.6 );
  • the figures 7 to 9 are SEM photographs with a secondary electron detector (applied acceleration voltage of 20kV- fig.7 , 9 or 25kV- fig.8 ) of sample sections, according to two magnifications:
    • ∘ a: 35x magnification, 500µm scale
    • ∘ b: 150x magnification, 100µm scale.
    The figures 7a, 7b show photographs of a reference sample, figures 8a, 8b show photographs of a sample that has undergone electrochemical dissolution treatment, figures 9a, 9b show photographs of a sample having undergone electrochemical dissolution treatment followed by mechanical corundum treatment.

DESCRIPTION DETAILLEE DES FIGURESDETAILED DESCRIPTION OF THE FIGURES

La figure 1 représente schématiquement une cellule d'électrolyse 1. Une différence de potentiel électrique est appliquée entre deux électrodes 2, 3 immergées dans une solution électrolytique 4. La borne positive est l'anode 2, siège d'une oxydation et la borne négative est la cathode 3, siège d'une réduction. Un générateur de courant continu 5 relié à l'anode 2 et à la cathode 3 fournit le courant.The figure 1 schematically represents an electrolytic cell 1. An electric potential difference is applied between two electrodes 2, 3 immersed in an electrolytic solution 4. The positive terminal is the anode 2, site of oxidation and the negative terminal is the cathode 3, seat of a reduction. A direct current generator 5 connected to the anode 2 and to the cathode 3 supplies the current.

La matière à dissoudre doit être située sur l'anode 2 (borne +). L'écart entre les deux électrodes 2, 3 est par exemple d'environ 1cm. Pour la cathode (la borne -), une simple plaque métallique peut être utilisée. L'électrolyte 4 sera par exemple une solution d'hydroxyde de sodium.The material to be dissolved must be located on the anode 2 (terminal +). The distance between the two electrodes 2, 3 is for example around 1 cm. For the cathode (the - terminal), a simple metal plate can be used. The electrolyte 4 will for example be a sodium hydroxide solution.

EXEMPLESEXAMPLES

Des échantillons en alliage métallique de type HP modifié 25-35 et de type 35-45 ont été testés. Ces alliages sont constitués d'une matrice austénitique Fe-Ni-Cr au sein de laquelle précipitent des carbures de niobium (NbC) et de chrome (Cr7C3). Les caractéristiques des alliages métalliques des échantillons utilisés sont reportées dans le tableau 1 ci-dessous. Tableau 1: Composition chimique typique (% massique) des matériaux utilisés Cr Ni Fe C Si Mn Nb HP 25-35 25 35 26 0,5 1,4 1,6 0,5 HP 35-45 35 45 15 0,5 2,5 1,6 0,4 HP modified 25-35 type and 35-45 type metal alloy samples were tested. These alloys consist of an austenitic Fe-Ni-Cr matrix within which niobium (NbC) and chromium (Cr 7 C 3 ) carbides precipitate. The characteristics of the metal alloys of the samples used are reported in Table 1 below. Table 1: Typical chemical composition (% by mass) of the materials used Cr Or Fe VS Yes Mn Nb HP 25-35 25 35 26 0.5 1.4 1.6 0.5 HP 35-45 35 45 15 0.5 2.5 1.6 0.4

Les échantillons utilisés sont des plaquettes de dimensions 8 x 30mm (échantillons C1 à C5) et 8 x 25mm (échantillons C6 à C9) et d'épaisseur 2mm obtenues par électroérosion au cœur de portions de 5cm de tubes de vapocraquage neufs, d'épaisseur initiale 8mm. L'état de surface initial est un état brut d'usinage.The samples used are plates of dimensions 8 x 30mm (samples C1 to C5) and 8 x 25mm (samples C6 to C9) and 2mm thick obtained by electroerosion in the heart of 5cm portions of new steam cracking tubes, of thickness initial 8mm. The initial surface condition is a rough machined condition.

Les tubes dont sont issus les échantillons testés ont été fabriqués par moulage par centrifugation.The tubes from which the tested samples were obtained were manufactured by centrifugal molding.

Chaque échantillon testé a été poli au moyen de papiers abrasifs à base de SiC dans l'ordre suivant de finesse : 600, 800, 1200, 2400.Each sample tested was polished using SiC-based abrasive papers in the following order of fineness: 600, 800, 1200, 2400.

Techniques de caractérisation utiliséesCharacterization techniques used

Microscope électronique à balayage (MEB) pour l'observation des surfaces et de coupes. Les MEB utilisés sont le MEB PHILIPS XL 20 et MEB Zeiss Supra 55 VP. Scanning electron microscope (SEM) for observation of surfaces and sections. The SEMs used are the PHILIPS XL 20 MEB and Zeiss Supra 55 VP MEB.

Découpe ionique : les coupes transverses sont réalisées par découpe ionique par faisceau d'ions défocalisés. Cette technique utilise des ions argon accélérés pour arracher de la matière, permettant ainsi un polissage de surface très fin et exempt de pollution. Les échantillons sont collés sur des masques en titane à l'aide d'une « laque d'argent » constituée de fines plaquettes d'argent en suspension dans un solvant. Ionic cutting : the transverse cuts are made by ionic cutting by defocused ion beam. This technique uses accelerated argon ions to tear off material, allowing a very fine and pollution-free surface polishing. The samples are glued onto titanium masks using a “silver lacquer” made up of fine silver platelets suspended in a solvent.

Exemple 1 : traitement électrochimique de la surface Example 1 : electrochemical treatment of the surface

Dans cet exemple, l'échantillon subit un traitement chimique de dissolution électrolytique.In this example, the sample is subjected to a chemical treatment of electrolytic dissolution.

L'échantillon à tester est placé à l'anode d'une cellule d'électrolyse telle que décrite en figure 1, la cathode étant une plaque métallique en acier inoxydable ou en graphite, de dimensions similaires ou plus importantes que l'échantillon. L'anode et la cathode sont écartées d'une distance d'environ 1 cm, les plaques étant sensiblement parallèles à l'intérieur de la cellule d'électrolyse.The sample to be tested is placed at the anode of an electrolysis cell as described in figure 1 , the cathode being a metallic plate of stainless steel or graphite, of similar or larger dimensions than the sample. The anode and cathode are spaced a distance of about 1 cm, the plates being substantially parallel inside the electrolytic cell.

On prépare une solution électrolytique en dissolvant sous agitation mécanique 135g de NaOH (sous forme de pastilles) dans 1L d'eau distillé, puis on remplit la cellule d'électrolyse de la solution obtenue. La teneur en chlorure de la solution est inférieure à 10ppm en masse.An electrolytic solution is prepared by dissolving, with mechanical stirring, 135 g of NaOH (in the form of pellets) in 1 L of distilled water, then the electrolysis cell is filled with the solution obtained. The chloride content of the solution is less than 10 ppm by mass.

Une différence de potentiel est appliquée entre l'anode (échantillon) et la cathode.A potential difference is applied between the anode (sample) and the cathode.

Deux séries de 5 et 4 tests ont été effectuées sur des alliages HP 25-35 qui ont tous été polis avant d'être placés dans la solution. Le tableau 2 rassemble les conditions utilisées pour chaque test. Tableau 2 : Paramètres des décompositions électrolytiques Nom d'échantillon Durée (heures) Tension (V) Intensité (A) Remarques C1 20 6 15 Evaporation de la solution avant la fin de la durée C2 2 8 16 3 6 10 C3 15 6,5 12 C4 15 6 10 C5 20 6 10 Nom d'échantillon Durée (heures) Tension (V) Intensité (A) Profondeur de dissolution des carbures de chrome C6 2 3,72 5 environ 30 µm C7 5 3,3 4,14 environ 50 µm 5 4,75 8,34 environ 50 µm C8 15 4,4 8,3 environ 72 µm C9 24 3,45 5 environ 100 µm Two series of 5 and 4 tests were performed on HP 25-35 alloys which were all polished before being placed in solution. Table 2 collates the conditions used for each test. Table 2: Parameters of electrolytic decompositions Sample name Duration (hours) Voltage (V) Intensity (A) Notes C1 20 6 15 Evaporation of the solution before the end of the duration C2 2 8 16 3 6 10 C3 15 6.5 12 C4 15 6 10 C5 20 6 10 Sample name Duration (hours) Voltage (V) Intensity (A) Dissolution depth of chromium carbides C6 2 3.72 5 about 30 µm C7 5 3.3 4.14 about 50 µm 5 4.75 8.34 about 50 µm C8 15 4.4 8.3 about 72 µm C9 24 3.45 5 about 100 µm

Dans les conditions testées, l'observation au MEB de la surface des échantillons C1 à C4 montre une dissolution des carbures de chrome Cr7C3, mais pas de dissolution des carbures de niobium NbC. La matrice austénitique reste intacte.Under the conditions tested, the SEM observation of the surface of samples C1 to C4 shows a dissolution of the chromium carbides Cr 7 C 3 , but no dissolution of the niobium carbides NbC. The austenitic matrix remains intact.

On notera que l'intensité de courant ne semble pas influencer la profondeur de dissolution des carbures de chrome au contraire de la durée de la dissolution.It will be noted that the current intensity does not seem to influence the depth of dissolution of the chromium carbides, unlike the duration of the dissolution.

Observation MEB des échantillons en coupe transverseSEM observation of samples in cross section

Des coupes des différents échantillons ont été observées au MEB.Sections of the various samples were observed with SEM.

Les figures 2 et 3 sont des photographies de l'échantillon C4 dissous pendant 15h (fig.2) et de l'échantillon C5 dissous 20h (fig .3). La tension d'accélération appliquée pour la mesure est de 15kV, le grossissement de 619x (fig.2) et de 629x (fig.3) et l'échelle de 10µm. Dans l'échantillon C4, on observe des cavités sur une profondeur de 40µm environ, ce qui semble indiquer l'existence de réseaux de carbures interconnectés. Sur l'échantillon C5, les cavités s'étendent sur une profondeur de 80µm. Entre 50 et 80µm il existe encore des carbures de chrome, ce qui semble indiquer que le réseau de carbures n'est pas complètement interconnecté. Le tableau 2 indique pour les échantillons C6 à C9 la profondeur maximale jusqu'à laquelle une dissolution des carbures de chrome a été observée.The figures 2 and 3 are photographs of the C4 sample dissolved for 15h ( fig.2 ) and sample C5 dissolved 20h ( fig. 3 ). The acceleration voltage applied for the measurement is 15kV, the magnification is 619x ( fig.2 ) and 629x ( fig.3 ) and the 10µm scale. In sample C4, cavities are observed to a depth of about 40 µm, which seems to indicate the existence of interconnected carbide networks. On sample C5, the cavities extend to a depth of 80 µm. Between 50 and 80µm there are still chromium carbides, which seems to indicate that the carbide network is not completely interconnected. Table 2 indicates for samples C6 to C9 the maximum depth to which a dissolution of chromium carbides was observed.

Il est ainsi possible d'agir sur la profondeur des carbures atteints en modifiant les conditions d'électrolyse.It is thus possible to act on the depth of the carbides reached by modifying the electrolysis conditions.

Les figures 4 et 5 représentent schématiquement des observations typiques d'une coupe d'un échantillon non traité (fig. 4) et d'un échantillon ayant subi un traitement chimique (fig. 5). Sur ces schémas, les parties noires correspondent aux carbures de chrome, les parties grisées aux carbures de niobium.The figures 4 and 5 schematically represent observations typical of a section of an untreated sample ( fig. 4 ) and a chemically treated sample ( fig. 5 ). In these diagrams, the black parts correspond to chromium carbides, the gray parts to niobium carbides.

On note ainsi sur la figure 5 la présence de carbures de niobium (NbC), de carbures de chrome (Cr7C3) et de cavités. Des carbures de niobium sont observés dans les cavités. Sans vouloir être lié par une théorie, lors de la dissolution électrolytique, la solution pourrait se propager en dissolvant les carbures de chrome issus des réseaux interconnectés mais en préservant les carbures de niobium (NbC). En outre, on observe que les cavités ne sont pas totalement vides. Une analyse chimique par le MEB /EDX (Energy Dispersive X-ray Spectrometry) montre que les carbures de chrome ont été partiellement dissous. On observe également la présence d'oxygène à l'intérieur des cavités, ce qui amène à penser qu'il y a formation d'oxyde ou d'hydroxyde, provenant probablement de la solution électrolytique.We thus note on the figure 5 the presence of niobium carbides (NbC), chromium carbides (Cr 7 C 3 ) and cavities. Niobium carbides are observed in the cavities. Without wishing to be bound by a theory, during the electrolytic dissolution, the solution could propagate by dissolving the chromium carbides resulting from the interconnected networks but preserving the niobium carbides (NbC). In addition, it is observed that the cavities are not completely empty. A chemical analysis by SEM / EDX (Energy Dispersive X-ray Spectrometry) shows that the chromium carbides have been partially dissolved. The presence of oxygen is also observed inside the cavities, which suggests that there is formation of oxide or hydroxide, probably coming from the electrolytic solution.

Exemple 2 : traitement mécanique de surface / microbillage Example 2: mechanical surface treatment / microbeading

Un échantillon d'alliage HP 25-35 poli est soumis au microbillage dans une cabine de sablage à manches. Les paramètres utilisés sont les suivants :

  • Particules: Billes de verre Ø 100 - 200 µm
  • Distance de projection : Environ 15cm
  • Durée de la projection : 15 secondes pour un échantillon de quelques cm2
  • Gaz vecteur : air comprimé sous pression contrôlée de 2,5 à 3,5 bars, diamètre de buse 6 à 8 mm, 40 litres de particules en circuit fermé.
A sample of polished HP 25-35 alloy is subjected to blasting in a sandblasting cabin. The parameters used are as follows:
  • Particles: Glass beads Ø 100 - 200 µm
  • Projection Distance: About 15cm
  • Duration of the projection: 15 seconds for a sample of a few cm 2
  • Carrier gas: compressed air under controlled pressure from 2.5 to 3.5 bars, nozzle diameter 6 to 8 mm, 40 liters of particles in a closed circuit.

On obtient un échantillon M1.A sample M1 is obtained.

Exemple 3 : traitement mécanique de surface / sablage (corindonnage) Example 3: mechanical surface treatment / sandblasting (corundum)

Un échantillon d'alliage HP 25-35 poli est soumis au corindonnage dans une cabine de sablage à manches. Les paramètres utilisés sont les suivants :

  • Particules: Corindon brun (Al2O3) Ø 250 - 400 µm
  • Distance de projection : Environ 15 cm
  • Durée de la projection : 15 secondes pour un échantillon de quelques cm2
  • Gaz vecteur : air comprimé sous pression contrôlée de 2,5 à 3,5 bars, diamètre de buse 6 à 8 mm, 40 litres de particules en circuit fermé.
A sample of polished HP 25-35 alloy is corundated in a bagged blast cabinet. The parameters used are as follows:
  • Particles: Brown corundum (Al 2 O 3 ) Ø 250 - 400 µm
  • Projection Distance: About 15cm
  • Duration of the projection: 15 seconds for a sample of a few cm 2
  • Carrier gas: compressed air under controlled pressure from 2.5 to 3.5 bars, nozzle diameter 6 to 8 mm, 40 liters of particles in a closed circuit.

On obtient un échantillon M3.This gives a sample M3.

Exemple 4 : Traitement chimique + traitement mécanique/ microbillage Example 4: Chemical treatment + mechanical treatment / microbeading

L'échantillon C4 de l'exemple 1 est soumis au même traitement de microbillage que celui décrit dans l'exemple 2. On obtient un échantillon CM4.Sample C4 of Example 1 is subjected to the same microbeading treatment as that described in Example 2. A sample CM4 is obtained.

Exemple 5 : Traitement chimique + traitement mécanique/ corindonnage Example 5: Chemical treatment + mechanical treatment / corundum

L'échantillon C4 de l'exemple 1 est soumis au même traitement de microbillage que celui décrit dans l'exemple 3. On obtient un échantillon CM5.Sample C4 of Example 1 is subjected to the same microbeading treatment as that described in Example 3. A sample CM5 is obtained.

La figure 6 représente schématiquement l'observation typique d'une coupe d'un échantillon d'alliage ayant subi un traitement chimique et mécanique. On note que les carbures de chrome ne sont plus en contact direct avec la surface et que les cavités formées par la dissolution électrochimique ont été au moins en partie fermées pour la plupart d'entre elles.The figure 6 schematically represents the typical observation of a section of a sample of alloy having undergone a chemical and mechanical treatment. It is noted that the chromium carbides are no longer in direct contact with the surface and that the cavities formed by the electrochemical dissolution have been at least partly closed for most of them.

Exemple 6 : cokage Example 6: coking

Des essais de cokage ont été réalisés sur les échantillons C4, CM4, CM5 préparés dans les exemples 1, 4, 5, ainsi que sur un échantillon de référence simplement poli. Les échantillons ont été portés à haute température en présence d'un mélange d'hydrocarbures légers et de vapeur d'eau (proches des conditions industrielles). Ils ont ainsi été soumis à des conditions favorisant la formation de coke.Coking tests were carried out on samples C4, CM4, CM5 prepared in Examples 1, 4, 5, as well as on a simply polished reference sample. The samples were brought to high temperature in the presence of a mixture of light hydrocarbons and water vapor (close to industrial conditions). They were thus subjected to conditions favoring the formation of coke.

Chaque échantillon a été soumis aux conditions suivantes :

  1. 1. Montée en température sous argon sec (impuretés en O2 dans l'Argon d'environ 3ppm en volume) jusqu'à 900°C (5°C/min),
  2. 2. Pré-oxydation des échantillons 900°C, 1h sous air synthétique,
  3. 3. Balayage du four à l'argon sec 30min,
  4. 4. Cokage des échantillons : 45 minutes 860°C : éthane + vapeur d'eau,
  5. 5. Balayage du four sous argon sec 30min,
  6. 6. Arrêt du système de chauffage et refroidissement lent des échantillons.
Each sample was subjected to the following conditions:
  1. 1. Temperature rise under dry argon (O 2 impurities in Argon of about 3 ppm by volume) up to 900 ° C (5 ° C / min),
  2. 2. Pre-oxidation of the samples 900 ° C, 1 hour in synthetic air,
  3. 3. Sweeping of the oven with dry argon 30min,
  4. 4. Coking of samples: 45 minutes 860 ° C: ethane + water vapor,
  5. 5. Sweeping the oven under dry argon 30min,
  6. 6. Shutdown of the heating system and slow cooling of the samples.

Les échantillons ont été observés au MEB. Les figures 7a et 7b sont des photographies (grossissements 35x et 150x respectivement) de la surface de l'échantillon de référence n'ayant subi aucun traitement particulier hormis le polissage initial. On observe la formation de coke à la surface. Les figures 8a et 8b sont des photographies de l'échantillon C4 ayant subi le traitement électrochimique (grossissements 35x et 150x respectivement), les figures 9a et 9b sont des photographies de l'échantillon CM5 (grossissements 35x et 150x respectivement).The samples were observed by SEM. The figures 7a and 7b are photographs (35x and 150x magnifications respectively) of the surface of the reference sample which has not undergone any particular treatment apart from the initial polishing. The formation of coke is observed on the surface. The figures 8a and 8b are photographs of sample C4 having undergone the electrochemical treatment (35x and 150x magnifications respectively), the figures 9a and 9b are photographs of the CM5 sample (35x and 150x magnifications respectively).

Les échantillons ayant subi un traitement chimique présentent globalement moins de coke que l'échantillon de référence. On observe encore du coke sur environ 10% de la surface de l'échantillon.The samples which have undergone a chemical treatment show less coke overall than the reference sample. Coke is still observed on about 10% of the surface of the sample.

Une réduction notable de la quantité de coke est également observée pour les échantillons ayant subi un traitement mécanique après le traitement chimique (échantillons CM4 et CM5), comme on peut le distinguer sur les figures 9a et 9b pour l'échantillon CM5.A notable reduction in the quantity of coke is also observed for the samples having undergone a mechanical treatment after the chemical treatment (samples CM4 and CM5), as can be seen on the figures 9a and 9b for sample CM5.

Exemple 7 : traitement électrochimique de la surface Example 7 : electrochemical treatment of the surface

Dans cet exemple, l'échantillon subit un traitement chimique de dissolution électrolytique pour éliminer les carbures de niobium.In this example, the sample undergoes a chemical electrolytic dissolution treatment to remove niobium carbides.

L'échantillon à tester est placé à l'anode d'une cellule d'électrolyse du même type que celle représentée figure 1 et décrite dans l'exemple 1.The sample to be tested is placed at the anode of an electrolysis cell of the same type as that shown figure 1 and described in Example 1.

On prépare une solution électrolytique d'acide sulfurique (H2SO4) à 7,2 mol.L-1 dont on remplit la cellule d'électrolyse.An electrolytic solution of sulfuric acid (H 2 SO 4 ) at 7.2 mol.L -1 is prepared and the electrolysis cell is filled.

Un premier test a été effectué sur un alliage HP 25-35 de dimensions 8 x 25 mm et d'épaisseur 2 mm qui a été poli avant d'être placé dans la solution d'acide sulfurique.A first test was carried out on an HP 25-35 alloy 8 × 25 mm in size and 2 mm thick which was polished before being placed in the sulfuric acid solution.

Une différence de potentiel de l'ordre de 0,8 V est appliquée entre l'anode (échantillon) et la cathode pendant 2 heures. L'échantillon est ensuite rincé à l'eau distillée puis à l'éthanol, séché et conservé dans un étui à l'abri des rayures et de l'air dans un dessiccateur.A potential difference of the order of 0.8 V is applied between the anode (sample) and the cathode for 2 hours. The sample is then rinsed with distilled water and then with ethanol, dried and stored in a case protected from scratches and air in a desiccator.

Un deuxième test a été effectué dans les mêmes conditions d'électrolyse sur un échantillon de mêmes dimensions et du même alliage préalablement soumis à une dissolution électrolytique des carbures de chrome. Celle-ci est réalisée avec une densité de courant de 5A.in-2 (0,775A.cm-2) pendant 2 heures dans une solution de NaOH (135g sous forme de pastilles dans 1L d'eau). L'échantillon obtenu est ensuite rincé à l'eau distillée puis à l'éthanol et séché avant d'être introduit dans la solution d'acide sulfurique pour la dissolution des carbures de niobium.A second test was carried out under the same electrolysis conditions on a sample of the same dimensions and of the same alloy previously subjected to electrolytic dissolution of the chromium carbides. This is carried out with a current density of 5A.in -2 (0.775A.cm -2 ) for 2 hours in a solution of NaOH (135g in the form of pellets in 1L of water). The sample obtained is then rinsed with distilled water and then with ethanol and dried before being introduced into the sulfuric acid solution for dissolving the niobium carbides.

Un examen de l'état de surface par microscopie électronique à balayage en électrons rétrodiffusés (mode d'imagerie sensible au contraste chimique) montre qu'il n'y a pas eu dissolution des carbures de niobium dans le cas du premier test.An examination of the surface state by backscattered electron scanning electron microscopy (imaging mode sensitive to chemical contrast) shows that there was no dissolution of the niobium carbides in the case of the first test.

Au contraire, on observe une dissolution de tous les carbures (chrome et niobium) par examen de l'état de surface de l'échantillon préalablement soumis à une dissolution des carbures de chrome. L'absence de carbures de niobium (NbC) en surface a été confirmée au MEB par une analyse EDX (Energy Dispersive X-ray). La répartition chimique du niobium sur la surface analysée montre quelques points localement riches en Nb mais plus aucun carbure de niobium.On the contrary, a dissolution of all the carbides (chromium and niobium) is observed by examination of the surface state of the sample. previously subjected to a dissolution of chromium carbides. The absence of niobium carbides (NbC) on the surface was confirmed by SEM by EDX (Energy Dispersive X-ray) analysis. The chemical distribution of niobium on the analyzed surface shows a few points locally rich in Nb but no niobium carbide.

Contrairement à une simple dissolution à l'acide sulfurique, la décomposition électrolytique successive des carbures de chrome et des carbures de niobium permet donc de dissoudre les NbC en surface. Sans vouloir être lié par une théorie, la dissolution électrolytique des M23C6/M7C3 pourrait venir en partie «déchausser» les NbC et augmenter la surface libre en contact avec l'électrolyte de la seconde dissolution.Unlike simple dissolution with sulfuric acid, the successive electrolytic decomposition of chromium carbides and niobium carbides therefore dissolves the NbCs on the surface. Without wishing to be bound by a theory, the electrolytic dissolution of M 23 C 6 / M 7 C 3 could come in part "loosen" the NbCs and increase the free surface in contact with the electrolyte of the second dissolution.

Claims (15)

  1. A method for treating an Fe-Ni-Cr metal alloy wall of an industrial reactor, which reduces the formation of coke on said wall when it is subjected to operating conditions conducive to coking, the metal alloy comprising carbides within its structure, some of which may be flush with the surface, the method comprising:
    - a chemical surface treatment step, during which at least a part of the carbides initially present in the alloy, in particular at the surface, is removed by electrolytic dissolution.
  2. The treatment method according to claim 1, wherein the chemical treatment step is carried out under conditions suitable for dissolving at least a part of the carbides to a depth of at least 10 µm, preferably at least 20 µm.
  3. The treatment method according to either of claims 1 and 2, wherein the chemical treatment step is carried out under conditions suitable for dissolving at least a part of the carbides chosen from chromium carbides, niobium carbide when the alloy contains niobium and carbonitrides when the alloy contains nitrogen.
  4. The treatment method according to any one of claims 1 to 3, wherein the chemical treatment step is carried out in an electrolysis cell comprising an aqueous solution selected from a solution of an alkali metal hydroxide and an aqueous solution of sulphuric acid.
  5. The treatment method according to any one of claims 1 to 4, comprising at least one other chemical treatment step, during which at least a part of the carbides initially present in the alloy, in particular at the surface, and not dissolved during a previous chemical treatment step, is removed by electrolytic dissolution.
  6. The treatment method according to any one of claims 1 to 5, wherein:
    - one chemical treatment step is an electrolytic dissolution step of chromium carbides,
    - another chemical treatment step is an electrolytic dissolution step of niobium carbides, the metal alloy containing niobium.
  7. The treatment method according to claim 6, wherein the electrochemical dissolution of chromium carbides is carried out followed by the electrochemical dissolution of the niobium carbides.
  8. The treatment method according to any one of claims 1 to 7, characterised in that it further comprises, after the chemical surface treatment step:
    - a mechanical surface treatment step by impact, during which the surface of the wall which has undergone the chemical treatment is hammered by spraying particles under conditions suitable for obtaining a permanent plastic deformation of the surface.
  9. The treatment method according to claim 8, wherein the particles used during the mechanical treatment step are chosen from particles of aluminium oxide, metal particles, balls of material that is inert under said operating conditions, and particles of nesosilicates.
  10. The treatment method according to either of claims 8 and 9, wherein the particles used in the mechanical treatment step have an average diameter of 100 to 500 µm.
  11. The treatment method according to any one of claims 8 to 10, wherein, during the mechanical treatment step, the particles are projected by a gaseous fluid under a pressure of 200 to 400 kPa.
  12. The treatment method according to any one of claims 1 to 11, wherein the metal alloy contains at least 5 wt% iron, at least 18 wt% chromium, at least 25 wt% nickel and at least 0.05 wt% carbon.
  13. The treatment method according to any one of claims 1 to 12, characterised in that it comprises, after the chemical treatment step, and optionally after the mechanical surface treatment step when it is implemented:
    - an oxidation step carried out under conditions suitable for forming a layer of oxide(s) on the surface which has undergone the chemical treatment, and optionally the mechanical treatment, in particular a layer containing one or more chromium oxides.
  14. A method for the treatment of hydrocarbons under conditions capable of causing the formation of coke, characterised in that the hydrocarbons are brought into contact with a surface of a wall made of an Fe-Ni-Cr metal alloy, said surface of the metal wall being previously treated by a treatment process according to one of claims 1 to 13 so as to reduce the formation of a coke deposit.
  15. The method of treating hydrocarbons according to the preceding claim, wherein the hydrocarbons are brought into contact with the surface of the metal wall at a temperature of 800 to 900°C.
EP17821602.4A 2016-12-20 2017-12-19 Chemical wall-treatment method that reduces the formation of coke Active EP3559321B1 (en)

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FR1662911A FR3060611A1 (en) 2016-12-20 2016-12-20 PROCESS FOR CHEMICAL TREATMENT OF A WALL REDUCING COKE FORMATION
PCT/EP2017/083566 WO2018114963A1 (en) 2016-12-20 2017-12-19 Chemical wall-treatment method that reduces the formation of coke

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SU441355A1 (en) * 1972-11-30 1974-08-30 Уральский Филиал Всесоюзного Ордена Трудового Красного Знамени Теплотехнического Научно-Исследователького Института Имени Ф.Э.Дзержинского Electrolyte to isolate carbides m 23 s 6
US4851093A (en) 1988-06-06 1989-07-25 United Technologies Corporation Selective decomposition of a chromium carbide coating from a chromium carbide coated nickel alloy substrate
JPH02240298A (en) * 1989-03-10 1990-09-25 Komatsu Ltd Porous steel
US5873950A (en) * 1996-06-13 1999-02-23 Inco Alloys International, Inc. Strengthenable ethylene pyrolysis alloy
WO2010108517A1 (en) * 2009-03-26 2010-09-30 Siemens Aktiengesellschaft Electropolishing method and electromagnetic flowmeter having electropolished electrodes

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EP3559321A1 (en) 2019-10-30
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FR3060611A1 (en) 2018-06-22
US20190390362A1 (en) 2019-12-26

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