EP0892862B1 - Elctrolyte for electropolishing, method of electropolishing a stainless steel or a nickel alloy utilizing the said electrolyte, and its application to decontamination - Google Patents

Description

The present invention relates to a electrolyte for electropolishing, a process electropolishing of stainless steel or nickel alloy using this electrolyte, and its application to decontamination, this process can be used in particular for electropolishing and decontamination of stainless steels austenitic, ferritic and austenitic-ferritic and nickel alloys.

Electropolishing is a known process since 1930 and in particular by document FR-A-707526. This process consists in carrying out a dissolution anodic of the metal part to be treated. This treatment leads to either leveling or surface gloss in the absence of corrosion crystallographic. The current density of dissolution is determined from the curve intensity-potential. This voltammogram presents a diffusion stage during which the polishing is possible.

Electropolishing can be implemented on various metals, but particularly suitable for stainless steel or for alloys such as the Inconel.

Therefore, the treatment electropolishing can be used especially on the various constituents of the steam generators water reactor type nuclear power plants pressurized (REP) where these metals (e.g. Inconel 600 and AISI 308L steel) are used; such treatment can be performed either as a mode of surface preparation to reduce susceptibility of the material to be contaminated, either as a method of decontamination by dissolution of contaminated oxides.

The vast majority of electrolytes known polishes have a high viscosity - which makes it difficult to implement - and contain very little water: i.e. significantly less than 20% in mass.

• une première catégorie comprend les électrolytes formés d'un mélange à base d'acide sulfurique, d'acide phosphorique et d'eau avec addition éventuelle d'acide chromique ou d'autres agents organiques. Ces électrolytes sont les plus courants dans le domaine de l'électropolissage.

Electropolishing electrolytes, in fact, are generally very concentrated acid media which can be classified into two main categories:

• a first category includes electrolytes formed from a mixture based on sulfuric acid, phosphoric acid and water with the optional addition of chromic acid or other organic agents. These electrolytes are the most common in the field of electropolishing.

Thus, the FEDOT'EV document "Electropolishing, anodizing and electrolytic pickling of metals ", Moscow, Robert Draper Ltd. Teddington, 1959 describes an electropolishing process in which it refers to electropolishing of alloy steel (Table 9, page 76) by ternary mixtures of acids sulfuric, phosphoric, and water in small quantities, namely less than 20% by weight. These mixtures are possibly added chromic acid to increase the gloss of the treated surface.

C.L. FAUST in "Electropolishing", Metal Finishing, September 1992, 9, pp. 89-91 indicates that electropolishing is an ideal treatment for steels stainless steel among which are mentioned steels stainless steel type 302, 410 and 430. Various appearances, from a satin aspect to an aspect shiny "like a mirror" can be obtained with phosphoric acid and sulfuric acid baths all of which contain very little water. The baths preferred, which allow to obtain the type gloss "mirror" are those in which the proportion of water is the most reduced, that is to say generally less than 20% by weight, as shown in Figure 1 of this document.

MAGAINO S., MATLOSZ M., LANDOLT D., in "An impedance study of stainless steel electropolishing ".- J. Electrochem. Soc., 140, 5, 1993 study electropolishing of stainless steel in concentrated solutions of phosphoric acid and sulfuric acid. Low water concentration appears to be rather favorable to glossing. For example, electropolishing of Fe 13Cr steel with mixtures containing 65% phosphoric acid, 20% sulfuric acid and 15% water is studied.

In an article by HOAR T.P., MEARS D.C. ROTHWELL G.P. "The relationships between anodic passivity, brightening and pitting "- Corrosion Science, 5, 1965, pp. 279-289, it is stated that the anodic passivation is produced by training on the surface of the anode, a very thin oxide film of low ionic conductivity, while the shiny anodic is produced by the passage of cations in and through very thin films, solid and compact, with high anionic conductivity, which are not composed of pure oxide, but of oxide contaminated with foreign anions from the solution of shine.

The authors indicate, in fact, that the compositions of solutions which give, on the one hand an anodic passivation, on the other hand a shine, are very different; in particular, the polishing and polishing solutions present zero or very low water concentration, the authors conclude that the brightening conditions are obtained by an anion ratio (such as sulfate, phosphate) on sufficiently high water.

In an article summarizing the knowledge essentials on electropolishing, D. LANDOLT "Fundamental aspect of electropolishing" in Electrochem. Acta, 32, 1, 1987 pp. 1-11 indicates that many electropolishing electrolytes of this first category, implemented in practice, contain a small amount of water and that reasons given in the literature for this seem somewhat contradictory, although diverse theoretical explanations can help explain that low water concentrations are favorable electropolishing.

A second category of electrolyte for electropolishing, includes electrolytes based perchloric acid.

In this type of electrolyte, containing a very aggressive and oxidizing anion, such as perchlorate, the use of water as a solvent is against indicated.

The solvent is therefore of the organic type: it can be for example acetic acid or anhydride methanol acetic acid or monobutyl ether ethylene glycol: these electrolytes are gradually abandoned because they are very dangerous and have explosive properties as described in the work of W.J. TEGART "Electropolishing and chemical of metals ". Editions Dunod, Paris, 1960. De this fact, during the manufacture and use of these baths, many precautions should be taken.

So far, the electrolyte used to electropolishing of austenoferritic steels is the same as that used for the Inconel 600 and belongs to the first category mentioned above: it it is a ternary mixture of sulfuric acid, acid phosphoric and water in very small quantity, to which can add certain additives such as acid oxalic, chromium or aluminum oxides.

This type of electrolyte must be replaced for three main reasons:

The first reason is that, if these electrolytes work well on Inconel 600 or on austenitic stainless steels (316L, 304 ...) this is not the case for steels austenoferritics. The two-phase structure of these steels indeed gives them an original behavior in the extent it is common to observe dissolution preferential of one of the phases, generally the ferrite, relative to each other.

Even if the electropolishing of such surfaces improves the Ra or roughness arithmetic of about 20% after an erosion of 20 µm, the sample is mat and a ferrite attack is observed as shown in Figure 1 which represents the visualization under the electron microscope scanning the surface of a 308L steel sample having undergone an electropolishing treatment in the middle sulfuric acid and phosphoric acid with erosion 40 µm.

The second reason leading to replacement of this type of electrolyte is the fact that in certain electrolyte applications electropolishing such as in circuits pressurized water reactors (PWR), the electrolyte must not contain sulfuric acid, because sulfur is prohibited from these primary circuits of fact that it is likely to cause corrosion.

The third reason is that the acid phosphoric added in this electrolyte present the downside, as are the phosphates that are derivatives, to pose significant problems in terms of treatment of effluents loaded with these compounds from electropolishing treatment.

It was suggested to implement nitric acid in electrolytes electropolishing, but nitric acid has the same annoying properties as perchloric acid already mentioned above, insofar as it is oxidant.

If nitric acid, like acid perchloric is mixed with a body or a compound organic which can be oxidized in particular by presence of water, the oxidation reaction can cause an explosion.

Electropolishing and decontamination have however already been carried out on austenitic stainless steels (304L, 316L) in nitric acid (9 mol / l), for example in documents from TURNER A.D., JUNKISON A.R., POTTINGER J.S., LAIN M.J. - Nuclear science and technology - "Development of remote electrochemical decontamination for hot cell applications "Commission of the European Communities - Final report - EUR 14192 - 1993. However, no test has been performed on the steel. austeno-ferritic 308L. In addition, this electrolyte has disadvantages because its viscosity is too low. It does not allow to obtain a polishing of good quality due to ripple defects related to hydrodynamic regime.

There is therefore a need for a electrolyte with reduced viscosity without however, be too low to allow a bet easy to use, can be used as well on Inconels such as the Inconel 600 and steels austenitic, ferritic stainless only on austeno-ferritic steels, and giving excellent results in all cases: i.e. essentially a shiny (not a matte) surface of the sample without crystallographic attack, at know no stings or intergranular attack or selective corrosion of a phase compared to the other.

The electrolyte must be of use safe and secure, and inexpensive.

The electrolyte must also be free fluoride, chloride and sulfur ions. Its temperature implementation should preferably be less than 60 ° C.

It must also present good storage conditions over time at temperature ambient.

This electrolyte must also not contain phosphoric acid and / or phosphates and be fully recyclable or destructible by current effluent treatment techniques.

In particular, the use of the electrolyte must be compatible with the possibilities of the sites nuclear and in particular with the possibilities of effluent treatment in nuclear power plants.

The volume of solid and liquid waste generated, capable of being stored must be finally minimized.

The object of the present invention is therefore to provide an electrolyte for electropolishing which meets, among other things, all of the above requirements above.

Another object of the present invention is an electropolishing and / or decontamination process using the above electrolyte which can give satisfactory results on all types stainless steel, as well as on alloys of nickel such as the Inconels used in particular in the nuclear industry.

• acide glycolique : de 152 à 538 g/kg
• acide nitrique de 170 à 568 g/kg
• eau de 280 à 678 g/kg.

These and other objects are achieved, in accordance with the invention, by an electrolyte consisting of a ternary mixture of glycolic acid, nitric acid and water in the following concentrations:

• glycolic acid: from 152 to 538 g / kg
• nitric acid from 170 to 568 g / kg
• water from 280 to 678 g / kg.

The formulation of this electrolyte ranges from surprisingly against all prejudices existing in the literature since despite all the contraindications found in the prior art and in particular in the documents mentioned above; the electrolyte according to the present invention uses water as a solvent at a high concentration, higher than the concentrations recommended in art anterior (less than 20% by mass).

Likewise, the use of nitric acid as the main basic constituent of the mixture, corresponds to an approach which differs fundamentally steps previously taken in this area of technical.

Finally, the incorporation of glycolic acid in the mixture while maintaining a concentration of high water also goes against another prejudice in this area of technology which is reflected by many documents and who wanted us not to mix in an electrolyte for electropolishing an acid oxidant like nitric acid with compounds organic such as glycolic acid in the presence of water.

So these are at least three prejudices techniques widely used in the prior art, such that it is particularly reflected in the documents cited and discussed above, which have been overcome by the inventors of this application, since it was on the one hand chose to use water as a solvent at a high proportion on the other hand to use acid nitric as the basic constituent, and finally incorporate glycolic acid into the mixture.

Glycolic acid, incorporated in the electrolyte allows in particular to increase the viscosity while maintaining a high water concentration, at know greater than or equal to 28-29% by mass.

Recall that glycolic acid or acid hydroxyacetic still designated by the letters HOAC is part of the series of hydroxycarboxylic acids.

It comes from sugarcane juice, beetroot or cob. It is a product readily available and inexpensive.

Its formula is:

Glycolic acid is an organic acid, relatively strong, soluble in water in all proportions and combines the acid and alcohol functions.

The alcohol function can be used in particularly for its solvent power; HOAC owns also bactericidal and descaling properties and is commonly used as the basis for formulation of cleaners for household use and collective.

Glycolic acid is also used, as mentioned in document US-A-4 137 132 in the chromating baths.

US-A-4,137,132 describes baths for the electrolytic deposition of gold-nickel alloys and chromium-nickel which contain glycolic acid.

The document JP-A-55047399 describes a electrolyte for electropolishing Fe-Al-Si alloys which includes a binary mixture of glycolic acid (40 60% by volume) and sulfuric acid. This mixture does contains neither water nor nitric acid.

HILL E.F.'s document: "Development of the glycolic-citric acids (GCA) process for decontamination of LMFBR components ", Trans. Am. Nucl. Soc., 30, 1978 relates to the use of a mixture glycolic acid (2.5% by weight) and citric acid (2.5% by weight) in aqueous solution, called mixture CGA, at 70-90 ° C, for chemical decontamination and without current from the components weakly contaminated in stainless steel 304 and 316 from LMFBR. This document therefore does not describe a electrolyte for electropolishing and further concerns a mixture of two organic acids.

The formulation of electrolytes is a completely unpredictable domain, and the properties of a given mixture absolutely cannot be deduced from the properties of known mixtures which differ by one of their constituents or properties of each of the constituents of the given mixture taken individually.

Nothing could therefore suggest that incorporation of glycolic acid into an electrolyte for electropolishing was going to lead to the whole surprising and advantageous properties, characteristics of the electrolyte according to the invention.

In addition, the choice of glycolic acid, among the many existing organic acids, in order to incorporating it into such a mixture, was absolutely not obvious and is in itself surprising and unexpected.

The electrolyte according to the invention based on a specific ternary mixture from the point of view of its constituents and their proportions, allows to obtain a shiny surface of metal samples treated.

We end up with a smoothing and a surface shine as well for example austenitic stainless steels such as steels 316L and 304L, as ferritic steels such as 430 steel, as austenitic-ferritic steels such as than 308L steel, or Inconels such as the Inconel 600, 690 or 800.

Because 308L steel and Inconel are the essential constituents of steam generators, the electrolyte according to the invention will therefore be suitable particularly for treating the surfaces of such appliances.

The electrolyte according to the invention is therefore characterized by great versatility. Indeed, unlike prior art electrolytes which do not gave good electropolishing results only for Inconel 600 or austenitic stainless steels (316, 304, etc.), the electrolyte according to the invention also allows the smoothing and shining of samples of austenitic-ferritic steel, for example type 308L without an attack occurring preferential of the ferritic network, which constitutes a decisive advantage of the electrolytes according to the invention on prior art electrolytes, in particular sulfophosphoric electrolytes.

The electrolyte according to the invention has also the obvious advantage over the electrolytes of the prior art to be free of fluoride ions, chlorides and sulfates which are corrosive to stainless steels and nickel alloys.

The electrolyte according to the invention has the added benefit of not containing phosphates and / or phosphoric acid, which reduces the volume of waste generated and facilitates treatment.

This electrolyte according to the invention does not does not require heating to be able to be work, it is in fact generally usable at ambient temperature, i.e. generally 15 ° C at 45 ° C, preferably 20 to 30 ° C, preferably still from 20 to 25 ° C and in any case at a temperature below 45 ° C.

The electrolyte according to the invention has a favorable viscosity, generally between 1.3 and 3 mm ² / s.

The electrolyte according to the invention presence also good conservation properties in the time at room temperature, and it can be stored over a period of one to two months, for example, without its essential characteristics are not affected.

The electrolyte does not contain reagents likely to give rise to very aggressive, oxidizing, or explosive. He can be prepared and used without taking precautions particular and safe for the user.

The spent electrolyte can be easily and totally reprocessed by distillation, since the mixture contains a high proportion by mass of acid nitric acid and water and that glycolic acid transforms into oxalic acid.

It can also be mineralized in a calciner, which allows this electrolyte to be used to decontaminate components of factories reprocessing.

Nitric acid recovered from the distillate can be recycled, which from an economic point of view is particularly advantageous.

Finally, the use of the electrolyte according to the invention, due to the presence of nitric acid, is compatible with reprocessing possibilities effluents from nuclear power plants, as well as other nuclear sites, which constitute one of the preferred areas of application of the electrolyte according to the present invention.

The electrolyte has the composition expressed in mass fractions mentioned above.

• acide glycolique : de 152 à 538 g/kg
• acide nitrique : de 170 à 240 g/kg
• eau : de 292 à 678 g/kg.

A composition will include:

• glycolic acid: from 152 to 538 g / kg
• nitric acid: from 170 to 240 g / kg
• water: from 292 to 678 g / kg.

Such an electrolyte is more stable chemically, the instability characterized by the production of nitrous acid, nitrous vapors and the formation of oxalic acid beyond its limit of solubility.

The invention also relates to a method electropolishing of stainless steel or nickel alloy in which said stainless steel or said nickel alloy is contacted with the electrolyte described above.

This process presents all of the advantageous characteristics linked to the electrolyte and already mentioned above; this process according the invention using the specific electrolyte of the invention, also makes it possible to achieve high Faradic dissolution yields, generally greater than 80 or even 85%, the speed of dissolution being in particular 950 µm / h.

This process can be used for electropolishing of all types of stainless steels - as well austenitics, for example 316L, 304L, than ferritics, for example 430 or austenoferritics, for example 308L - but also of nickel alloys such as Inconels, for example the Inconel 600 or the Inconel 640 or again the Inconel 800.

We therefore see that this process can be used to process the various alloys used especially in the nuclear industry. It is well obvious that this process will also be suitable for the treatment of stainless steels and nickel alloys encountered in all types of industries.

The invention also relates to a method electrodecontamination of stainless steels or contaminated nickel alloys including their surface by radioelements, for example the Cobalt 60, in which the contaminated metal alloy is brought into contact with the electrolyte described above.

These stainless steels and / or alloys are for example those encountered in the nuclear industry or in another industry.

Stainless steels and / or alloys of nickel are for example constituent of a component a nuclear power plant or a reprocessing plant and are found in particular in the primary circuit nuclear power plants such as piping, steam generators ....

The process allows in particular a dissolution contaminating metallic elements, contained mainly in the layer oxides superficial, and therefore a noticeable reduction in the contamination to allow intervention of the maintenance staff.

The operating conditions of the process electropolishing and decontamination can be readily determined by those skilled in the art in this technical field depending in particular on the material to be treated. They will generally be the following.

• acide glycolique : de 152 à 538 g/kg,
• acide nitrique : de 170 à 568 g/kg
• eau : de 280 à 678 g/kg.

The composition of the electrolyte for electropolishing and / or decontamination is that already indicated above, namely:

• glycolic acid: from 152 to 538 g / kg,
• nitric acid: from 170 to 568 g / kg
• water: from 280 to 678 g / kg.

The current density is generally greater than or equal to 0.5 A / cm ² , it is preferably between 0.5 A / cm ² and 1.5 A / cm ² , for example 1A / cm ² ; in fact, a certain risk of overheating can cause pitting beyond 1.7 A / cm ² .

The temperature is generally the room temperature, i.e. 15 to 45 ° C, preferably 20 to 30 ° C, more preferably 20 to 25 ° C and the regime hydrodynamics is preferably a laminar regime.

The duration of the process is generally of 60 sec to 80 sec for an erosion, for example of 20 µm.

Process operating conditions electropolishing and electrodecontamination are generally identical.

The spent electrolysis is thus treated with a view to disposal, recycling and / or disposal rejection.

According to a particularly characteristic of the present invention, the electrolyte used contains neither phosphoric acid nor phosphate, and the effluent treatment is thereby greatly facilitated.

Indeed, glycolic acid can oxidize into oxalic acid. On the other hand, it has been shown that the reaction between nitric acid and oxalic acid is a slow reaction and the gases released: oxide of carbon, carbon dioxide and water do not react not with the acid mixture.

• une oxydation en voie humide de l'acide glycolique en acide oxalique, puis en CO₂, terme final de l'oxydation. Dans ce cas, on obtient un effluent nitrique qui peut être envoyé à la station du traitement des effluents ; ou
• une distillation de l'électrolyte usé, suivie par exemple d'une calcination des concentrats dont les résidus peuvent être par exemple vitrifiés, comme c'est le cas pour certains déchets des usines de retraitement.

According to the invention, the treatment of the spent electrolyte at the end of the electropolishing process with a view to its destruction and / or its recycling can therefore comprise:

• wet oxidation of glycolic acid to oxalic acid, then to CO ₂ , the final term for oxidation. In this case, a nitric effluent is obtained which can be sent to the effluent treatment station; or
• distillation of the spent electrolyte, followed for example by calcination of the concentrates, the residues of which may for example be vitrified, as is the case for certain waste from reprocessing plants.

The distillate can optionally be recycled for reuse of nitric acid in the electropolishing process.

• la figure 1 représente une visualisation au MEB (microscope électronique à balayage) de la surface d'un échantillon d'acier inoxydable 308L ayant subi un traitement d'électropolissage en milieu acide sulfurique et acide phosphorique (érosion de 40 µm),
• les figures 2 et 3 représentent une visualisation au MEB de la surface d'un échantillon d'Inconel 600 ayant subi un traitement d'électropolissage en milieu acide nitrique et acide glycolique, c'est-à-dire avec un électrolyte conforme à l'invention (érosion de 30,7 µm),
• les figures 4 à 9 représentent des profils de rugosité anamorphosés bi et tridimensionnels de la surface de l'Inconel 600.

The invention will be better understood on reading the following description of a preferred embodiment given by way of illustrative and nonlimiting example, this description being made with reference to the accompanying drawings in which:

• FIG. 1 represents a SEM display (scanning electron microscope) of the surface of a 308L stainless steel sample having undergone an electropolishing treatment in sulfuric acid and phosphoric acid medium (erosion of 40 μm),
• FIGS. 2 and 3 represent a SEM visualization of the surface of a sample of Inconel 600 having undergone an electropolishing treatment in a nitric acid and glycolic acid medium, that is to say with an electrolyte conforming to invention (erosion of 30.7 µm),
• Figures 4 to 9 show anamorphic bi- and three-dimensional roughness profiles of the surface of the Inconel 600.

• les figures 6 et 7 montrent tous les pics de la microrugosité, alors que sur les figures 8 et 9 (avec lissage filtre de 0,8 mm), ces pics ont disparu pour faire apparaítre un planage (macrorugosité).
• la figure 10 représente les courbes intensités (ordonnée : densité de courant en mA/cm²) - potentiel (abscisse : potentiel en volt/électrode au calomel saturé : ECS) pour des aciers inoxydables austénitiques 316L (courbe en trait plein) et 304L (courbe en tirets), pour un acier austéno-ferritique 308L (courbe en trait mixte), et pour de l'Inconel (courbe en pointillés).

The different two and three-dimensional profiles presented are respectively: the initial surface of the steel (Figures 4, 6 and 8), then this same surface after an erosion of 30 µm (Figures 5, 7 and 9).

• Figures 6 and 7 show all the peaks of microroughness, while in Figures 8 and 9 (with filter smoothing of 0.8 mm), these peaks have disappeared to reveal a leveling (macroroughness).
• FIG. 10 represents the intensity (ordinate: current density in mA / cm ² ) - potential (abscissa: volt potential / saturated calomel electrode: DHW) curves for austenitic stainless steels 316L (solid line curve) and 304L ( dashed curve), for an austeno-ferritic steel 308L (dashed line curve), and for Inconel (dashed curve).

Figures 2 to 9 show and therefore illustrate the method according to the invention using uses the specific electrolyte according to the invention in the special case of an electropolishing treatment of a 308L austenitic-ferritic steel.

Figure 1 shows the results obtained with an electrolyte of art anterior (sulfuric and phosphoric acid) and compare the results obtained with the electrolyte of the invention.

The following examples describe and illustrate the invention.

Example 1 : Results in electropolishing on Inconel 600.

• acide glycolique : 213 g/kg
• acide nitrique : 239 g/kg
• eau : 548 g/kg.

This electropolishing treatment is carried out with an electrolyte the composition of which is as follows:

• glycolic acid: 213 g / kg
• nitric acid: 239 g / kg
• water: 548 g / kg.

The treatment is carried out on samples in Inconel 600.

Débit d'électrolyte : 20 1/h

Température 20°C

Densité de courant 1 A/cm²

Distance Anode-Cathode : 2 cm

Surface traitée : 30 cm².

The operating conditions are as follows:

Electrolyte flow: 20 1 / h

Temperature 20 ° C

Current density 1 A / cm ²

Anode-Cathode distance: 2 cm

Treated area: 30 cm ² .

The purpose of such treatment is achieve smoothing and shining of the surface samples in Inconel 600.

To measure the surface condition, we uses two types of measures:

1 °) Roughness profile measurements

These measures are mainly reflected by two parameters: on the one hand, the roughness arithmetic (Ra) and on the other hand the maximum roughness (Rmax) both expressed in micrometers.

These roughness measurements have been made with a "SURFASCAN" device from SOMICRONIC. The evaluation length is 20 mm; and the measures are carried out according to standards NF-E05-015 and NF-E05-052.

The possible filter was 0.8 mm.

The starting samples in Inconel 600 are sanded and the values of the initial parameters are therefore as follows: 0.9 <Ra <1.35 µm.

The results obtained are as follows and are presented in Table 1.

The roughness profiles bi and three-dimensional shown in Figures 4 to 9 show that we observe a disappearance of micro-roughness, a decrease in roughness arithmetic, as well as a sample leveling.

2) Measurements of gloss indices and of optical haze indices

Gloss index measurements and optical haze index measurements were performed on the samples.

These measurements were carried out according to the following standards: DIN 67530; ISO 2813; ASTM D523 in using a "HAZEGLOSS" gloss meter from the BYK GARDNER Company.

The optical haze index is defined by the measurement of diffuse light in a geometry at 20 degrees and the gloss index by the ratio reflected intensity / emitted intensity, following a specular reflection.

The results are given in the tables 2 and 3 following.

Table 2 gives the results when the measurement is carried out perpendicular to the lines of sanding; this is the worst case.

Table 3 gives the results carried out parallel to the scratches: this is the most favorable case, since the light is not diffracted by the scratches, but by the surface micro-defects.

• les échantillons d'Inconel 600 obtenus par ce traitement d'électropolissage présentent à l'oeil nu un aspect brillant,
• des observation au microscope électronique à balayage (MEB) (conditions d'observation : grandissements X 500) de la surface de l'échantillon d'Inconel 600 ayant subi le traitement d'électropolissage (érosion de 30 µm) montrent clairement une surface nette, lisse et propre, sans attaque cristallographique (voir figures 2 et 3). En effet dans le cas contraire, la surface aurait un aspect mat.

Optical observations complete the above measurements:

• the Inconel 600 samples obtained by this electropolishing treatment present a shiny appearance to the naked eye,
• observations with a scanning electron microscope (SEM) (observation conditions: magnifications X 500) of the surface of the sample of Inconel 600 having undergone the electropolishing treatment (erosion of 30 μm) clearly show a clean surface, smooth and clean, without crystallographic attack (see Figures 2 and 3). Otherwise, the surface would have a matt appearance.

EXAMPLE 2 : RESULTS IN ELECTRO-POLISHING on austenitic stainless steels (316L and 304L), austenoferritic 308 L, and on Inconel.

• acide glycolique : 272 g/kg
• acide nitrique : 305 g/kg
• eau : 423 g/kg.

The electropolishing treatment is carried out with an electrolyte according to the invention, the composition of which is as follows:

• glycolic acid: 272 g / kg
• nitric acid: 305 g / kg
• water: 423 g / kg.

The intensity-potential curves are produced (Fig. 10) using a 0.2 mm ² surface rotating disk electrode at 25 ° C and at a rotation speed of 1000 rpm.

The intensity-potential curves obtained (where the potential expressed in V on the abscissa is plotted relative to the saturated calomel electrode (DHW) and on the ordinate the current density in mA / cm ² ) for 304L steel (in dashes) ) for 316L steel (in solid lines), for 308L steel (in dashed lines) and for Inconel (in dotted lines) show that the electrolyte according to the invention makes it possible to polish all types of stainless steels , whether they are austenitic (304L and 316L), ferritic or austenitic-ferritic (308L as in Example 1), and alloys of the Inconel type.

The surface obtained is in all cases polished and shiny.

EXAMPLE 3 (COMPARATIVE) :

• acide sulfurique : 440 g/kg,
• acide phosphorique : 440 g/kg,
• eau : 120 g/kg.

An electropolishing treatment is carried out under the same conditions as Example 2 on similar samples of 308L steel, but using a conventional electropolishing electrolyte having the following composition:

• sulfuric acid: 440 g / kg,
• phosphoric acid: 440 g / kg,
• water: 120 g / kg.

The 308L steel samples obtained by this electropolishing treatment has an aspect mast.

Observation by electron microscope SEM scanning the surface of the steel sample 308L, produced under the same conditions as for Example 2, having undergone the treatment electropolishing (40 µm erosion) clearly shows that a ferrite attack has occurred, which confirms the matt aspect observed (Fig. 1).

We thus obtain, thanks to the process according to the invention, using an electrolyte specific, a shine on the steel surface austeno-ferritic without preferential attack of the ferritic network, while the electrolyte of art anterior gives a mat surface and an attack of the ferrite.

EXAMPLE 4: RESULTS IN DECONTAMINATION

• Régime laminaire
• Température : 25°C
• Densité de courant : 1,5 A/cm²
• Surface traitée : 28 à 30 cm² (selon l'échantillon).

An electrolyte according to the invention having the same composition as that of Example 1 is used under the following conditions:

• Laminar regime
• Temperature: 25 ° C
• Current density: 1.5 A / cm ²
• Surface treated: 28 to 30 cm ² (depending on the sample).

The treated surface is a surface contaminated with Cobalt 60 forming part of a cuff thermal in 304L stainless steel taken from the power plant du Blayais (Gironde - France).

For comparison purposes, we also have implemented under the same conditions and on the same surface, the electrolyte according to the prior art.

• acide sulfurique : 440 g/kg
• acide phosphorique : 220 g/kg
• eau : 340 g/kg.

It is a sulfophosphoric electrolyte having the following composition:

• sulfuric acid: 440 g / kg
• phosphoric acid: 220 g / kg
• water: 340 g / kg.

The results are collated in Table 4 below.

It is noted that the electrolyte according to the invention allows a significant reduction in activity, higher than the comparison electrolyte, without present the disadvantages.

The electrolyte of the invention can be used in particular to decontaminate the components of REP pressurized water reactors and reduces enough "hot spots" to allow operator intervention in maintenance.

Claims (12)

1. Electropolishing electrolyte characterized in that it consists of a mixture of glycolic acid, nitric acid and water in the following concentrations:

   glycolic acid, from 152 to 538 g/kg;

   nitric acid, from 170 to 568 g/kg;

   water, from 280 to 678 g/kg.
2. Process for the electropolishing of a stainless steel or of a nickel alloy, characterized in that the said stainless steel or nickel alloy is brought into contact with the electrolyte according to Claim 1.
3. Process according to Claim 2, characterized in that the stainless steel is chosen from among ferritic, austenitic and austenoferritic stainless steels.
4. Process according to Claim 2, characterized in that the nickel alloy is chosen from among Inconel 600, Inconel 690 and Inconel 800.
5. Process according to Claim 2, characterized in that the current density is between 0.5 and 1.5 A/cm².
6. Process according to Claim 2, characterized in that the electropolishing is carried out at room temperature.
7. Process according to Claim 2, characterized in that the treatment of the spent electrolyte comprises an oxidation, by a wet route, of the glycolic acid into carbon dioxide, by means of which a nitric effluent is obtained which may be sent to the effluent treatment station.
8. Process according to Claim 2, characterized in that the treatment of the spent electrolyte comprises a distillation of the said electrolyte, followed by a calcination of the concentrates.
9. Process according to Claim 8, characterized in that the residues of the calcination of the concentrates coming from the said distillation are vitrified.
10. Process according to Claim 8, characterized in that the distillate coming from the said distillation is recycled for the purpose of reutilizing the nitric acid in the electropolishing process.
11. Process according to Claim 2, characterized in that the electrolyte is brought into contact with the radioelement-contaminated surface of a stainless steel or a nickel alloy.
12. Process according to Claim 2, characterized in that the stainless steel or the nickel alloy are constituents of a component of a nuclear power station or of a reprocessing plant.

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