FR2747399A1 - ELECTROLYTE FOR ELECTROPOLISHING, PROCESS FOR ELECTROPOLISHING A STAINLESS STEEL OR A NICKEL ALLOY USING THIS ELECTROLYTE, AND ITS APPLICATION TO DECONTAMINATION - Google Patents

Abstract

The invention concerns an electropolishing electrolyte composed of a mixture of glycolic acide, nitric acid and water in the following concentrations: glycolic acid: 152-538 g/kg, nitric acid: 170-568 g/kg, water: 280-678 g/kg. The invention also features a method for the electropolishing of a stainless steel or of a nickel alloy such as an Iconel utilizing this electrolyte, which is particularly useful in the decontamination of surfaces contaminated by radioelements, especially in the nuclear industry.

Description

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

 Electropolishing is a process known since 1930 and in particular by the document FR-A-707526.

This process consists of anodic dissolution of the metal part to be treated. This treatment leads to either leveling or brightening of the surface in the absence of crystallographic corrosion. The dissolution current density is determined from the intensity-potential curve. This voltammogram has a diffusion plateau during which polishing is possible.

 Electropolishing can be carried out on various metals, but is particularly suitable for stainless steel or alloys such as Inconel.

 Therefore, the electropolishing treatment can be used in particular on the various constituents of the steam generators of the PWR type nuclear power plants (PWR) where these metals (for example Inconel 600 and AISI steel 308L) are implemented; such a treatment can be carried out either as a surface preparation mode to reduce the susceptibility of the material to be contaminated, or as a method of decontamination by dissolving the contaminated oxides.

 The vast majority of known polishing electrolytes have a high viscosity - which makes them difficult to implement - and contain very little water: namely substantially less than 20% by weight.

Electrolytic electrolytes, in fact, are generally very concentrated acidic media that can be classified into two broad categories
- A first category includes electrolytes formed of a mixture based on sulfuric acid, phosphoric acid and water with optional addition of chromic acid or other organic agents. These electrolytes are the most common in the field of electropolishing.

 Thus, the document FEDOT'EV "Electropolishing, anodizing and electrolytic pickling of metals", Moscow, Robert Draper Ltd. Does Teddington, 1959, describe an electropolishing process in which reference is made to the electropolishing of alloy steel (Table 9, page 76) by ternary mixtures of sulfuric, phosphoric, and small amounts of water? , ie less than 20% by weight. These mixtures are optionally supplemented with chromic acid to increase the gloss of the treated surface.

CL FAUST in "Electropolishing", Metal
Finishing, September 1992, 9, pp. 89-91 indicates that the electropolishing is an ideal treatment for stainless steels among which are cited stainless steels type 302, 410 and 430. Various appearances, from a satin appearance to a glossy "mirror-like" can be obtained with baths of phosphoric acid and sulfuric acid which all contain very little water. The preferred baths which make it possible to obtain the "mirror" type gloss are those in which the proportion of water is the smallest, that is to say generally less than 20% by weight, as indicated by 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 the electropolishing of stainless steel in concentrated solutions of phosphoric acid and sulfuric acid. A low water concentration appears to be rather favorable to brightening.

For example, the electropolishing of Fe 13Cr steel by mixtures containing 65% phosphoric acid, 20% sulfuric acid and 15% water is studied.

 In an article by HOAR T.P., MEARS D.C.

ROTHWELL GP "The relationship between anodic passivity, brightening and pitting" - Corrosion
Science, 5, 1965, pp. 279-289, it is stated that the anodic passivation is produced by the formation on the surface of the anode, a very thin oxide film of low ionic conductivity, while the anodic gloss is, in turn, produced by the passage of cations in and through very thin films, solid and compact, of high anionic conductivity, which are not composed of pure oxide, but oxide contaminated with foreign anions from the brightening solution.

 The authors indicate, indeed, that the compositions of the solutions which give, on the one hand anodic passivation, or on the other hand a gloss, are very different; in particular, the brightening and polishing solutions have a water concentration of zero or very low, 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 essential knowledge about 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 the reasons for this given in the literature seem somewhat contradictory, although various theoretical explanations can be used to explain that low water concentrations are favorable for electropolishing.

 A second class of electrolyte for electropolishing includes electrolytes based on 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 contraindicated.

 The solvent is therefore of organic type: it may be for example acetic acid or acetic anhydride methanol or ethylene glycol monobutyl ether: these electrolytes are gradually abandoned because they are very hazardous and have explosive properties as described in the work of WJ TEGART "Electrolytic and chemical polishing of metals". Dunod Editions, Paris, 1960. Therefore, during the manufacture and use of these baths, many precautions must be taken.

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

This type of electrolyte needs to be replaced for three essential reasons
The first reason is that, if these electrolytes give good results on Inconel 600 or austenitic stainless steels (316L, 304 ...) this is not the case for austenoferritic steels. The two-phase structure of these steels gives them an original behavior in that it is common practice to observe a preferential dissolution of one of the phases, generally ferrite, with respect to the other.

 Even if the electropolishing of such surfaces makes it possible to improve the Ra or arithmetic roughness by about 20% after an erosion of 20 μm, the sample is matt and an attack of the ferrite is observed as shown in FIG. represents the scanning electron microscope visualization of the surface of a sample of 308L steel having undergone an electropolishing treatment in a sulfuric acid and phosphoric acid medium with an erosion of 40 μm.

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

 The third reason is that the phosphoric acid added to this electrolyte has the disadvantage, as well as the phosphates which are derived from it, to pose significant problems in the treatment of effluents loaded with these compounds from the electropolishing treatment.

 It has been suggested to use nitric acid in electropolishing electrolytes, but nitric acid has the same troublesome properties as perchloric acid already mentioned above, since it is oxidizing.

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

However, electropolishing and decontamination tests have already been carried out on austenitic stainless steels (304L, 316L) in nitric acid (9 mol / l), for example in the documents of TURNER AD, JUNKISON AR, POTTINGER
JS, LAIN MJ - 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 tests have been carried out on 308L austenitic-ferritic steel. In addition, this electrolyte has disadvantages because its viscosity is too low. It does not provide polishing of good quality because of ripple defects related to the hydrodynamic regime.

 There is therefore a need for an electrolyte having a reduced viscosity without being too low, in order to allow easy operation, which can be used both on Inconels such as Inconel 600 and austenitic, ferritic stainless steels than on the austenitic-ferritic steels, and giving excellent results in all cases: that is to say essentially a glossy (and not matte) surface of the sample without crystallographic attack, ie no pitting or intergranular attack or selective corrosion of one phase with respect to the other.

 The electrolyte must be safe and safe to use and inexpensive.

 On the other hand, the electrolyte must be free of fluoride, chloride and sulfur ions. Its operating temperature should preferably be less than 60 ° C.

 It must also have good storage conditions over time at room temperature.

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

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

 The volume of solid and liquid waste generated, which can be stored, must finally be reduced to a minimum.

 The object of the present invention is therefore to provide an electrolyte for electropolishing which satisfies, inter alia, all the requirements mentioned 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 of stainless steel, as well as on nickel alloys such as Inconels used in particular in the nuclear industry.

These and other objects are achieved, according to the invention, by an electrolyte consisting of a ternary mixture 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 surprisingly goes against all prejudices existing in the literature since despite all the contraindications that are 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 the prior art (less than 20% by weight).

 Similarly, the use of nitric acid as the main constituent of the mixture, corresponds to an approach that departs fundamentally from the steps followed so far in this area of the art.

 Finally, the incorporation of glycolic acid in the mixture while maintaining a high water concentration also goes against another prejudice in this area of the art that is reflected in many documents and that wanted to does not mix in an electrolyte for electropolishing an oxidizing acid such as nitric acid with organic compounds such as glycolic acid in the presence of water.

 It is therefore at least three technical prejudices widely used in the prior art, as reflected in particular by the documents cited and discussed above, which have been overcome by the inventors of the present application, since it has On the one hand, it was chosen to use water as a solvent in a high proportion, on the other hand to use nitric acid as the basic constituent, and finally to incorporate glycolic acid into the mixture.

 Glycolic acid, incorporated into the electrolyte makes it possible in particular to increase the viscosity while maintaining a high water concentration, namely greater than or equal to 28-29 E by mass.

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

 It comes from the juice of sugar cane, beetroot or grape vine. It is a product easily available and low price.

His formula is

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

 The alcohol function can be used in particular for its solvent power; HOAC also has bactericidal and descaling properties and is commonly used as a base for the formulation of household and community cleaners.

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

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

 JP-A-55047399 discloses an electrolyte for the electropolishing of Fe-Al-Si alloys which comprises a binary mixture of glycolic acid (40 to 60% by volume) and sulfuric acid. This mixture contains neither water nor nitric acid.

 HILL E.F .: "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 of glycolic acid (2.5% by weight) and citric acid (2.5% by weight) in aqueous solution, called mixture
CGA, at 70-900C, to chemically decontaminate, with no current, weakly contaminated 304 and 316 stainless steel components from the LMFBR. This document therefore does not describe an electrolyte for electropolishing and moreover relates to a mixture of two organic acids.

 The formulation of the electrolytes is a completely unpredictable field, and the properties of a given mixture can not be inferred from the properties of the known mixtures which differ from it in any of their constituents or properties of each of the constituents of the given mixture taken individually.

 Nothing could therefore suggest that the incorporation of glycolic acid into an electrolyte for electropolishing would lead to all of the surprising and advantageous properties characteristic of the electrolyte according to the invention.

 In addition, the choice of glycolic acid, among the many organic acids existing to incorporate 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, makes it possible to obtain a glossy surface of the samples of treated metals.

 The smoothing and brightening of the surface, for example, of austenitic stainless steels such as 316L and 304L steels, as well as of ferritic steels such as steel 430, is achieved as well as austenitic-ferritic steels such as 308L steel, or Inconels such as Inconel 600, 690 or 800.

 Since 308L steel and Inconel are the essential constituents of steam generators, the electrolyte according to the invention is therefore particularly suitable for treating the surfaces of such apparatus.

 The electrolyte according to the invention is therefore characterized by great versatility. In fact, unlike the electrolytes of the prior art which 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 the austenitic-ferritic steel samples, for example of the 308L type, without any preferential attack of the ferritic network, which constitutes a decisive advantage of the electrolytes according to the invention over the electrolytes of the prior art especially sulfophosphoric electrolytes.

 The electrolyte according to the invention also has the obvious advantage over the electrolytes of the prior art of being free of fluoride, chloride and sulphate ions which are corrosive with respect to stainless steels and nickel alloys.

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

 This electrolyte according to the invention does not require heating to be implemented, it is indeed usable generally at room temperature, that is to say generally from 15 "C to 45" C, preferably from 20 to 30 ° C, more preferably from 20 to 250 ° C and in all cases at a temperature below 450 ° 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 also has good storage properties over time at ambient temperature, and it can be stored over a period of time for example from one to two months without its essential characteristics being affected.

 The electrolyte does not contain reagents that can give rise to very aggressive, oxidizing or explosive reactions. It can be prepared and used without taking special precautions and without danger to the user.

 The spent electrolyte can be easily and completely reprocessed by distillation since the mixture contains a high proportion by weight of nitric acid and water and the glycolic acid is converted into oxalic acid.

 It can also be mineralized in a calciner, which allows the use of this electrolyte to decontaminate components of reprocessing plants.

 The nitric acid recovered in 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, because of the presence of nitric acid, is compatible with the possibilities of reprocessing effluents from nuclear power plants, as well as from other nuclear sites, which constitute one of the preferred fields of application of the electrolyte according to the present invention.

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

A preferred composition will include
glycolic acid from 152 to 538 g / kg
- nitric acid: from 170 to 240 g / kg
water: 292 to 678 g / kg.

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

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

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

This process can be used for the electropolishing of all types of stainless steels - both austenitic, for example 316L, 304L, and ferritic, for example 430 or austenoferritic, for example 308L - but also nickel alloys such as
Inconels, for example Inconel 600 or Inconel 640 or Inconel 800.

 It is therefore found that this process can be used to treat the various alloys used in particular in the nuclear industry. It is 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 of electrodecontamination of stainless steels or nickel alloys contaminated in particular at their surface by radioelements, for example the
Cobalt 60, wherein the contaminated metal alloy is contacted 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 nickel alloys are for example constitutive of a component of a nuclear power plant or a reprocessing plant and meet especially in the primary circuit of nuclear power plants such as pipes, steam generators. ...

 The method notably allows a dissolution of the contaminating metallic elements, contained essentially in the oxides of the surface layer, and therefore a significant reduction of the contamination to allow intervention of the maintenance personnel.

 The operating conditions of the electropolishing and decontamination process can be easily determined by the skilled person in this field of the art depending in particular on the material to be treated. They will usually be as follows.

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: 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 1 A / cm 2; indeed, a certain risk of heating may cause punctures above 1.7 A / cm2.

 The temperature is generally ambient, ie 15 to 450 ° C, preferably 20 to 30 ° C, more preferably 20 to 25 ° C and the hydrodynamic regime is preferably a laminar regime.

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

 The operating conditions of electropolishing and electrodecontamination processes are generally identical.

 Used electrolysis is thus treated for disposal, recycling and / or disposal.

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

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

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 recycling can therefore comprise
- A wet oxidation of glycolic acid to oxalic acid, then CO2, the final term of oxidation. In this case, a nitric effluent is obtained which can be sent to the effluent treatment station; or
a distillation of the spent electrolyte, followed for example by calcination of the concentrates whose residues may for example be vitrified, as is the case for certain waste from reprocessing plants.

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

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 (scanning electron microscope) display of the surface of a sample of stainless steel 308L having undergone an electropolishing treatment in a 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 FIG. invention (30.7 μm erosion),
FIGS. 4 to 9 represent two and three-dimensional anamorphic roughness profiles of the Inconel 600 surface.

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

 - Figures 6 and 7 show all the peaks of the micro-roughness, while in Figures 8 and 9 (with filter smoothing of 0.8 mm), these peaks have disappeared to show a planing (macroroughness).

 FIG. 10 represents the intensity curves (ordinate: current density in mA / cm 2) potential (abscissa: potential in volt / saturated calomel electrode: SCE) for 316L austenitic stainless steels (solid curve) and 304L (curve in dashes), for austeno-ferritic steel 308L (mixed line curve), and for Inconel (dotted line).

 FIGS. 2 to 9 represent and therefore illustrate the process according to the invention using the specific electrolyte according to the invention in the particular case of an electropolishing treatment of austeno-ferritic steel 308L.

 FIG. 1 makes it possible to visualize the results obtained with an electrolyte of the prior art (sulfuric and phosphoric acid) and to compare the results obtained with the electrolyte of the invention.

 The following examples describe and illustrate the invention.

Example 1: Electropolishing results on Inconel 600.

This electropolishing treatment is carried out with an electrolyte whose composition is as follows
glycolic acid: 213 g / kg
- nitric acid: 239 g / kg
water: 548 g / kg.

 The treatment is performed on Inconel 600 samples.

The operating conditions are as follows
Electrolyte flow rate: 20 l / h
Temperature 200C
Current density 1 A / cm2
Anode-Cathode distance: 2 cm
Surface treated: 30 cm2.

 The purpose of such treatment is to smooth and brighten the surface of Inconel 600 samples.

To measure the state of the surface, two types of measurements are used
1 ") Roughness profile measurements
These measurements are mainly reflected by two parameters: on the one hand, the arithmetic roughness (Ra) and on the other hand the maximum roughness (Rmax) both expressed in micrometers.

 These roughness measurements were performed with a "SURFASCAN" device from SOMICRONIC Company.

The evaluation length is 20 mm; and measurements are made according to NF-E05-015 and
NF E05-052.

 The filter possibly used was 0.8 mm.

Inconel 600 starting samples are sanded and the starting parameter values are as follows: 0.9 <Ra <1.35 μm
The results obtained are as follows and are presented in Table 1.

TABLE 1

<tb><SEP> 0 <SEP> 1.25 <SEP> 12.62
<tb> 0.246 <SEP> 3.70 <SEP> 1.03 <SEP> 10.12
<tb> 0.507 <SEP> 7.75 <SEP> 0.96 <SEP> 9.59
<tb> 1.26 <SEP> 18.51 <SEP> 0.71 <SEP> 8.25
<tb> 2.07 <SEP> 30.84 <SEP> 0.61 <SEP> 7.78
<Tb>
The two-dimensional and three-dimensional roughness profiles presented in FIGS. 4 to 9 show that a disappearance of the micro-roughnesses, a decrease in the arithmetic roughness and a planing of the sample are observed.

20) Gloss index and optical sail index measurements
Gloss index measurements and optical veil index measurements were performed on the samples.

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

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

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

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

 Table 3 gives the results made parallel to the scratches: this is the most favorable case, since the light is not diffracted by the scratches, but by the surface microdefects.

TABLE 2

Erosion <SEP> Index <SEP> of <SEP> Index <SEP> of <SEP> Index <SEP> of <SEP> Index <SEP> of
<tb> Sail <SEP> Sheen <SEP> Sheen <SEP> Sheen
<tb><SEP> to <SEP> 20 <SEP> to <SEP> 60 <SEP> to <SEP> 85
<tb><SEP> 0 <SEP> 707 <SEP> 24.3 <SEP> 35.7 <SEP> 13.7
<tb><SEP> 3.70 <SEP> 666 <SEP> 18.9 <SEP> 51.3 <SEP> 21.1
<tb><SEP> 18.51 <SEP> 1446 <SEP> 118.7 <SEP> 193.0 <SEP> 44.2
<tb><SEP> 30.84 <SEP> 1670 <SEP> 231.0 <SEP> 315.6 <SEP> 59.1
<Tb>
TABLE 3

Erosion <SEP> Index <SEP> of <SEP> Index <SEP> of <SEP> Index <SEP> of <SEP> Index <SEP> of
<tb> Sail <SEP> Sheen <SEP> Sheen <SEP> Sheen
<tb><SEP> to <SEP> 20 <SEP> to <SEP> 60 <SEP> to <SEP> 85
<tb><SEP> 0 <SEP> 528 <SEP> 33.0 <SEQ> 138.7 <SE> 57.6
<tb><SEP> 3.70 <SEP> 730 <SEP> 27.4 <SEP> 126.7 <SEP> 52.5
<tb><SEP> 18.51 <SEP> 1350 <SEP> 161.8 <SEP> 406.8 <SE> 88.4
<tb><SEP> 30.84 <SEP> 1455 <SEP> 325.7 <SEP> 470.2 <SEP> 94.7
<Tb>
Optical observations complement the measurements made above - the Inconel 600 samples obtained by this electropolishing treatment have a bright appearance to the naked eye, - observation under a scanning electron microscope (SEM) (observation conditions magnification X 500) of the surface of the Inconel 600 sample having undergone the electropolishing treatment (30 μm erosion) clearly show a clean, smooth and clean surface without crystallographic attack (see Figures 2 and 3).

Indeed in the opposite case, the surface would have a matte appearance.

EXAMPLE 2: ELECTROPOLISHING RESULTS on austenitic stainless steels (316L and 304L), austenoferritic 308L, and on Inconel.

The electropolishing treatment is carried out with an electrolyte according to the invention whose composition is as follows
- glycolic acid: 272 g / kg
- nitric acid: 305 g / kg
water: 423 g / kg.

 The intensity-potential curves (Fig. 10) are made using a rotating disk electrode with a surface area of 0.2 mm 2 at 250 ° C. and a rotational speed of 1000 rpm.

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

 The surface obtained is in all cases polished and shiny.

EXAMPLE 3 (COMPARATIVE)
An electropolishing treatment is carried out under the same conditions as in Example 2 on 308L analogous steel samples, 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 have a matt appearance.

 An SEM scanning electron microscope observation of the surface of the 308L steel sample, carried out under the same conditions as in Example 2, having undergone the electropolishing treatment (erosion of 40 Wm) clearly shows that ferrite attack occurred, confirming the observed dull appearance (Fig. 1).

 Thus, using the method according to the invention, using a specific electrolyte, a gloss of the austenitic-ferritic steel surface without preferential attack of the ferritic network, while the electrolyte of the prior art gives a matt surface. and an attack of ferrite.

EXAMPLE 4: RESULTS IN DECONTAMINATION
An electrolyte according to the invention having the same composition as that of Example 1 is used under the following conditions
- Laminar diet
- Temperature: 250C
- Current density: 1.5 A / cm2
- Treated surface: 28 to 30 cm2 (according to the sample).

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

 For comparative purposes, the following electrolyte of the prior art has also been used under the same conditions and on the same surface.

It is a sulfo-phosphoric electrolyte having the following composition
sulfuric acid: 440 g / kg
- phosphoric acid: 220 g / kg
water: 340 g / kg.

 The results are summarized in Table 4 below.

TABLE 4

Sulfo-phosphoric <SEP> electrolyte <SEP> Glycolic-nitric <SEP> electrolyte
<tb> erosion <SEP> Activity <SEP> Factor <SEP> of <SEP> erosion <SEP> Activity <SEP> Factor <SEP> of
<tb><SEP> m <SEP> (Bq / cm2) <SEP> decontated @ <SEP> m <SEP> (Bq / cm2) <SEP> decontated @
<tb><SEP> 0 <SEP> 79121 <SEP> 0 <SEP> 101852
<tb><SEP> 5.64 <SEP> 9021 <SEP> 8.77 <SEP> 3.51 <SEP> 33880 <SEP> 3.0
<tb><SEP> 10.8 <SEP> 1478 <SEP> 53.5 <SEP> 9.22 <SEP> 1072 <SEP> 95.01
<tb><SEP> 14.6 <SEP> 572 <SEP> - <SEP> 138.1 <SEP> 14.9 <SEP> 494 <SEP> - <SEP> 205, <SEP> 11
<Tb>
It is noted that the electrolyte according to the invention allows a significant reduction in activity, greater than the comparison electrolyte, without the disadvantages.

 The electrolyte of the invention can be used in particular for decontaminating the components of PWR pressurized water reactors and makes it possible to reduce the "hot spots" sufficiently to allow the operators to intervene in maintenance.

Claims (12)

 Electrolytic 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. A method of electropolishing a stainless steel or a nickel alloy, characterized in that said stainless steel or nickel alloy is brought into contact with the electrolyte according to claim 1.  3. Method according to claim 2, characterized in that the stainless steel is selected from ferritic, austenitic and austenoferritic stainless steels.  4. Method according to claim 2, characterized in that the nickel alloy is selected from Inconel 600, Inconel 690 and Inconel 800.  5. Method according to claim 2, characterized in that the current density is between 0.5 and 1.5 A / cm 2.  6. Method according to claim 2, characterized in that the electropolishing is carried out at ambient temperature.  7. A method of treating the spent electrolyte from the electropolishing process according to claim 2, characterized in that the treatment of the spent electrolyte comprises a wet oxidation of glycolic acid to carbon dioxide, whereby obtains a nitric effluent that can be sent to the effluent treatment plant.  8. A method of treating the spent electrolyte from the electropolishing process according to claim 2, characterized in that the treatment of the spent electrolyte comprises a distillation of said electrolyte, followed by calcination of the concentrates.  9. Process according to claim 8, characterized in that the residues of the calcination of the concentrates resulting from said distillation are vitrified.  10. The method of claim 8, characterized in that the distillate from said distillation is recycled for the reuse of nitric acid in the electropolishing process.  11. The method of claim 2, characterized in that the electrolyte is contacted with the contaminated surface of radioelements of a stainless steel or a nickel alloy.  12. The method of claim 2, characterized in that the stainless steel or the nickel alloy are constituent of a component of a nuclear power plant or a reprocessing plant.