FR3087209A1 - COMPOSITION FOR CHROMING A SUBSTRATE AND METHOD OF CHROMING USING SUCH A COMPOSITION - Google Patents

Abstract

The invention relates to an aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate, of pH between 0 and 1 and which contains a trivalent chromium salt, glycine, an alkali metal salt , an aluminum salt and, optionally, an ammonium salt. The chroming process according to the invention comprises immersing the substrate to be treated in this composition and applying a current between this substrate and an anode. This process makes it possible to form a high quality chrome coating over the entire surface of the substrate.

Description

The present invention is in the field of electrochemical deposition of chromium on the surface of a substrate, with a view to improving its resistance to wear, such deposition being commonly qualified by the term "chromium plating".

More particularly, the present invention relates to an aqueous composition for the electrolytic deposition of a chromium coating on the surface of a substrate, as well as to a method of electrolytic deposition of a chromium coating on the surface of a substrate using using such a composition. The invention also relates to a substrate covered on the surface with a chromium coating obtained by such a process.

The electroplating of a chromium surface coating on a substrate, also called chromium plating, is used in many fields, in particular in the aeronautical, automotive, mechanical, etc. fields, to improve the wear resistance of the parts ( this is called hard chromium plating) or to make decorative deposits (this is then called decorative chromium plating), the thickness of the surface coating varying from a few tenths of a micron for decorative chromium plating to a few hundred microns for hard chromium plating. Hard chrome plating, in which the present invention is more particularly concerned, makes it possible in particular to reduce the wear of the parts during relative movements as well as their coefficient of friction.

To obtain a coating of chromium on the surface of a substrate, chromium trioxide (CrOg) is generally used, a substance based on hexavalent chromium which, in aqueous solution, generates chromic acid. Hard chromium is obtained by electrolytic reduction of hexavalent chromium to metallic chromium on the surface of the part to be treated. However, since substances based on hexavalent chromium are harmful to living organisms, it has been sought for several years to limit their use, if not completely eliminate them.

Several technologies have been proposed in the prior art to replace the current hard chromium plating processes based on hexavalent chromium.

Among these technologies, one can cite the thermal projection of the HVOF type (for English High Velocity Oxygen Fuel - projection by supersonic flame), which is interesting but which does not seem to be sufficiently flexible to apply, and which in particular is not suitable for processing parts with complex geometry. This technology is also proving to be very expensive: its application would be two to ten times more expensive than hard chrome plating with hexavalent chromium.

So-called dry processes, such as physical vapor deposition or chemical vapor deposition, have also been proposed and are used industrially. However, the operational constraints linked to these technologies, and the consequences which result from them, limit their application on a large scale. In addition, these methods also do not make it possible to produce deposits whose properties are satisfactory over the entire surface of the part when they are applied to parts of complex geometry.

Electrolytic or chemical deposition methods, making it possible in particular to solve the difficulties introduced by complex geometric surfaces, have otherwise been proposed by the prior art. These methods generally use aqueous solutions of nickel or cobalt salts to produce metallic deposits, with or without inclusions of particles such as hard, self-lubricating particles, etc., on the surface of the part to be treated. However, the use of cobalt salts, like that of nickel salts, is undesirable due to the risk of toxicity to living organisms.

Seeking to develop an alternative to hard chromium plating processes based on hexavalent chromium, the present inventors are more particularly interested in electrolytic processes using trivalent chromium in aqueous solution.

Such methods have been proposed by the prior art for chrome plating metal parts. By way of illustration of such a prior art, mention may in particular be made of documents US 4,142,948, US 2009/0211914 or also WO 2015/110627, which all describe chromium-plating methods using an aqueous solution based on trivalent chromium .

However, these prior art methods all have drawbacks. In particular, either they are more decorative chrome plating than hard chrome plating, that is to say that the wear resistance properties of the chromium coatings which they make it possible to form on the parts are not sufficiently high to the applications targeted by hard chromium plating, either they use harmful substances, or they operate over a range of current density that is too limited, or even they combine several of these drawbacks.

The range of current density in which the chromium plating process can operate is particularly important, especially when the parts to be treated have a complex geometry. Indeed, during an electrolytic deposition, it is observed that for a current density imposed in the electrolytic bath, the actual current density on the surface of the treated part can vary locally very significantly. In order to guarantee a homogeneous chromium deposit on an entire part, it is therefore important to have a process that can operate with the widest possible current density range, this being all the more important. that the piece is more complex in shape.

The present invention aims to remedy the drawbacks of the methods proposed by the prior art for hard chrome plating without hexavalent chromium, in particular the drawbacks set out above, by proposing a method, and a composition for its implementation, which allow, without use of substance toxic to living organisms, and in particular hexavalent chromium, to form on a substrate a coating of good quality chromium, having in particular a wear resistance which is at least equivalent to that obtained by the chromium plating processes hard of the prior art using hexavalent chromium, this coating being formed over the entire surface of the latter, regardless of the shape of the substrate, including for substrates of complex geometry.

An additional objective of the invention is that the cost of this composition and of the implementation of this process is as low as possible, this implementation being moreover easy.

It has been discovered by the present inventors that this objective can be achieved by using, as an electrolytic bath, an aqueous composition based on trivalent chromium of particular constitution, as regards the choice of its constituents and that of their respective concentrations, and whose pH is specifically between 0 and 1.

Thus, according to a first aspect, there is proposed according to the present invention an aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate, this electrolytic deposition also being designated, in the present description, by term "chrome plating".

The aqueous composition according to the invention contains:

- 0.4 to 0.9 mol / l of a trivalent chromium salt,

- 0.6 to 2.8 mol / l of glycine,

- an alkali metal salt,

- aluminum salt,

- and optionally, an ammonium salt,

Its pH is between 0 and 1.

The aqueous composition according to the invention is also preferably essentially devoid of hexavalent chromium. By this is meant that the composition does not contain hexavalent chromium, or only trace amounts.

In the present description is meant, conventionally in itself, by trivalent chromium, chromium in the +3 oxidation state, and, by hexavalent chromium, chromium in the +6 oxidation state.

The glycine concentration in the composition can in particular be between 0.63 and 2.80 mol / l.

The aqueous composition according to the invention, used as an electrolytic bath in a chromium-plating process, makes it possible to obtain, on the surface of the treated substrate, a coating of high-quality chromium, and this over all of this. surface, including when the substrate has a complex shape.

Such an advantageous result is in particular due to the fact that this aqueous composition allows efficient operation of the chromium plating process which implements it over a very wide range of current density, of amplitude as high as 72 A / dm ² , and which can even be greater than 84 A / dm ² . The chromium plating process using the composition according to the invention can thus function, to form a high quality chromium coating on the surface of the treated substrate, in a current density range up to more than 100 A / dm ² , in particular as wide as ranging from 13 A / dm ² to more than 100 A / dm ² .

The chromium coating which can be formed on the surface of the treated substrate thanks to the aqueous composition according to the invention advantageously has the following properties:

- It has a Vickers hardness greater than 800 Hv, measured conventionally in itself, for an applied load of 100 g. ;

- It has good adhesion to the substrate. In particular, no detachment of the coating is observed when the treated substrate is subjected to the grinding wheel test according to standard ASTM B571; nor is there any detachment of the coating following a thermal shock, in particular after the treated substrate has been subjected to 3 cycles each comprising the heating of the coated substrate at 300 ° C. for one to two hours, then its rapid cooling at 20 ° C by immersing in cold water.

Such a property advantageously allows the coating to withstand the high stresses to which it is subjected when using the chrome substrate under severe conditions, for example when this coating is formed on the surface of actuating jacks, brake pistons, etc. ;

- it has a high resistance to grinding. In particular, no peeling of the coating is observed after a correction of thickness greater than 100 μm;

- it has a high resistance to wear. In this regard, its performance, measured by the Taber test, according to standard ASTM D4060, is in particular equivalent to that obtained for the coatings obtained by the processes using hexavalent chromium of the prior art;

- It does not degrade, or very little, the mechanical properties of the treated substrates. For example, subjected to the hydrogen embrittlement test, the treated substrates being under traction, according to standard ASTMF519, no rupture is observed after 200 hours at 75% of the breaking limit;

- It has good adhesion to the sub-layers, in particular to the sub-layers with anticorrosion properties, in particular the nickel sub-layers, which may have been previously formed on the substrate.

As indicated above, these properties are advantageously uniform over the entire surface of the substrate, even when it has a complex shape.

It has also been discovered by the present inventors that, when it is used for the electrolytic deposition of chromium on the surface of a substrate, quite surprisingly, the lifetime of the aqueous composition according to The invention, having a pH between 0 and 1, is significantly more important than for equivalent aqueous compositions, that is to say containing the same components in the same concentrations, but whose pH is greater than 1. The composition aqueous according to the invention can thus advantageously be used, for the electrolytic deposition of chromium on the surface of a substrate, after having imposed on it the equivalent of 22 Ah / L, and even more than 50 Ah / L for certain modes of production, without significant loss of quality of the deposit produced, without having added one or more of its constituents in the composition. Such an advantageous result suggests, for industrial use, in which readjustments of the concentrations of the constituents of the aqueous composition used will be regularly carried out, a lifetime of the composition considerably higher and particularly important for the field of hard chrome plating.

The pH of the aqueous composition according to the invention can be adjusted to a value between 0 and 1 according to any conventional method in itself for the skilled person, in particular by adding acid, for example hydrochloric acid, in said composition.

Thus, the aqueous composition according to the invention may in particular contain one or more acids, for example hydrochloric acid, in an amount sufficient to give said aqueous composition a pH of between 0 and 1.

Preferably, the pH of the aqueous composition according to the invention is substantially equal to 0.5.

The aqueous composition according to the invention is easy to prepare, by simple mixing of its constituents in water, and it has a low cost price.

The aqueous composition according to the invention can also meet one or more of the characteristics described below, used in isolation or in each of their technically operative combinations.

For all of the components of the composition, the preferred concentration values indicated below are all associated with even better performance of the chromium plating process using the composition according to the invention, in particular in terms of current density range in which this process works effectively. The tightened concentration ranges indicated below are thus associated with current density ranges, in which the chromium plating process operates, which are wider.

The glycine concentration in the composition is preferably between 0.6 and 0.9 mol / l, for example between 0.67 and 0.83 mol / l. Optimally, the concentration of glycine in the aqueous composition according to the invention can be approximately equal to 0.75 mol / l.

In the present description, the term “trivalent chromium salt” means a single trivalent chromium salt or a mixture of different trivalent chromium salts.

The trivalent chromium salt contained in the aqueous composition according to the invention may contain, in addition to the Cr ³⁺ ion, any conventional counter-ion in itself for chromium-plating treatments, or any mixture of such counter-ions.

The trivalent chromium salt can in particular be chosen from the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulfates, sulfites, sulfamates, which may be organic or inorganic, sulfonates, which may be organic or inorganic, thiocyanates, or any of their mixtures.

More generally, in the composition according to the invention, at least one, preferably several, and preferably all, of the trivalent chromium salt, the alkali metal salt, the aluminum salt and, where appropriate, the salt of ammonium, is / are chosen from chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulfates, sulfites, sulfamates, which can be organic or inorganic, sulfonates, which can be organic or inorganic, thiocyanates, or any of their mixtures.

Each of the salts forming part of the aqueous composition according to the invention can comprise a single counterion, or a mixture of several counterions. Preferably each salt is formed with a single counterion.

In preferred embodiments of the invention, in the aqueous composition, the counterion (s) of the trivalent chromium salt, the counterion (s) of the alkali metal salt, the counter (s) -ion (s) of the aluminum salt, and where appropriate, the counterion (s) of the ammonium salt, are identical.

In its particular embodiments above, the aqueous composition according to the invention is then advantageously more stable over time.

The trivalent chromium salt can for example be chosen from the following trivalent chromium salts: CrCl3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr, xH ₂ O, (CH ₃ CO ₂ ) ₇ Cr ₃ (OH) ₂ , xH ₂ O, CrF ₃ , xH ₂ O, etc.

Preferably, the trivalent chromium salt is a chromium chloride, for example CrCI ₃ , 6H ₂ O.

The concentration of trivalent chromium salt in the composition is for example between 0.41 and 0.86 mol / l.

It is preferably between 0.7 and 0.9 mol / l, for example between 0.71 and 0.86 mol / l. Optimally, the concentration of trivalent chromium salt in the aqueous composition according to the invention can be approximately equal to 0.79 mol / l.

In the present description, the term “aluminum salt” means a single aluminum salt or a mixture of different aluminum salts.

The aluminum salt contained in the aqueous composition according to the invention may contain, in addition to the aluminum ion, any conventional counter-ion in itself for chromium-plating treatments, or any mixture of such counter-ions, in particular one or more of the counterions listed above.

For example, the aluminum salt is an AICI ₃ aluminum chloride.

The concentration of aluminum salt in the aqueous composition according to the invention is preferably between 0.06 and 0.7 mol / l, for example between 0.06 and 0.62 mol / l.

It is preferably between 0.2 and 0.3 mol / l, for example between 0.23 and 0.29 mol / l. Optimally, the concentration of aluminum salt in the aqueous composition according to the invention can be approximately equal to 0.26 mol / l.

In the present description, the term “alkali metal salt” is understood to mean a single alkali metal salt or a mixture of different alkali metal salts.

The alkali metal is preferably sodium or potassium, or a mixture thereof.

The alkali metal salt contained in the aqueous composition according to the invention may contain, in addition to the ion of the alkali metal, any conventional counter-ion in itself for chromium-plating treatments, or any mixture of such counter-ions, in in particular one or more of the counterions listed above.

For example, the alkali metal salt is sodium chloride NaCI and / or potassium chloride KCI.

The concentration of alkali metal salt in the aqueous composition according to the invention is preferably between 0.2 and 1.9 mol / l, for example between 0.26 and 1.88 mol / l.

In the present description, the term “ammonium salt” means a single ammonium salt or a mixture of different ammonium salts.

The ammonium salt contained in the aqueous composition according to the invention may contain, in addition to the ammonium ion, any conventional counterion in itself for chromium-plating treatments, or any mixture of such counterions, in particular one or more of the counterions listed above.

For example, the ammonium salt is an ammonium chloride NH ₄ CI.

The concentration of ammonium salt in the aqueous composition according to the invention is preferably between 0 and 1.0 mol / l, for example between 0 and 0.93 mol / l.

In variants of the invention, the aqueous composition contains:

- An ammonium salt concentration of between 0.5 and 0.8 mol / l, for example between 0.58 and 0.72 mol / l; and preferably approximately equal to 0.65 mol / l,

- And an alkali metal salt concentration of between 0.2 and 1.3 mol / l, for example between 0.26 and 1.28 mol / l, and preferably between 0.5 and 0.7 mol / l, for example between 0.54 and 0.66 mol / l; and preferably approximately equal to 0.60 mol / l.

In different variants of the invention, the aqueous composition is devoid of ammonium salt, and it contains an alkali metal salt concentration of between 1.5 and 1.9 mol / l, for example between 1.54 and 1.88 mol / l.

It has been found by the present inventors that, surprisingly, each of these variants of the aqueous composition according to the invention, used in a chromium-plating process, allows this process to work, by forming on the surface of the substrate a high quality chrome coating, in a very wide current density range.

A particular aqueous composition according to the invention contains:

- 0.41 to 0.86 mol / l, preferably 0.71 to 0.86 mol / l, of trivalent chromium salt, for example trivalent chromium chloride;

- 0.63 to 2.80 mol / l, preferably 0.67 to 0.83 mol / l, of glycine;

- 0.26 to 1.28 mol / l, preferably 0.54 to 0.66 mol / l, of alkali metal salt, for example sodium chloride;

- 0 to 0.93 mol / l, preferably 0.58 to 0.72 mol / l, of ammonium salt, for example of ammonium chloride;

- and 0.06 to 0.62 mol / l, preferably 0.23 to 0.29 mol / l, of aluminum salt, for example aluminum chloride;

- its pH being between 0 and 1.

Another particular aqueous composition according to the invention contains:

- 0.41 to 0.86 mol / l, preferably 0.71 to 0.86 mol / l, of trivalent chromium salt, for example trivalent chromium chloride;

- 0.63 to 2.80 mol / l, preferably 0.67 to 0.83 mol / l, of glycine;

- 1.54 to 1.88 mol / l of alkali metal salt, for example sodium chloride;

- and 0.06 to 0.62 mol / l, preferably 0.23 to 0.29 mol / l, of aluminum salt, for example aluminum chloride;

- its pH being between 0 and 1;

- And the composition being devoid of ammonium salt.

The aqueous composition according to the invention may, if appropriate, contain substances other than those listed above, with the exclusion of hexavalent chromium, these substances however not having to interfere with the action of the essential constituents of the composition. aqueous, listed above, for the electrolytic deposition of chromium on the surface of the substrate.

By way of nonlimiting example of the invention, the aqueous composition according to the invention can for example contain one or more surfactants.

Preferably, the aqueous composition according to the invention is substantially free from one or more, preferably all, of the following substances: boric acid / borate, quaternary ammonium, oxalate, vanadium, manganese, iron, cobalt, molybdenum, nickel, tungsten , indium.

By substantially free is meant the fact that the aqueous composition does not contain these substances, except in the trace state, that is to say in a non-operating amount.

In another aspect, the present invention relates to a method of electroplating a chromium coating on the surface of a substrate, known as a chromium plating process. This process includes:

the immersion, in a bath, called an electrolytic bath, of an aqueous composition according to the invention, meeting one or more of the above characteristics, of the substrate and of an anode,

- and the application of a current between the substrate, which then constitutes a cathode, and the anode.

In the bath, on the surface of the part to be treated, the electrolytic reduction of trivalent chromium into hard metallic chromium occurs. The chromium coating thus formed by the process according to the invention on the surface of the substrate has the entirely advantageous properties set out above.

The current applied between the substrate and the anode can be of the pulsed type. In preferred embodiments of the invention, a direct current is applied between the substrate and the anode.

Such a characteristic advantageously makes it possible to overcome the difficulties of implementing pulsed currents on an industrial scale, and to simplify the implementation of the chroming process according to the invention.

The method according to the invention is in particular entirely suitable for industrial implementation, in a simple manner.

In preferred embodiments of the invention, the direct current density imposed between the substrate and the anode is between 10 and 100 A / dm ² , preferably between 20 and 40 A / dm ² , and preferably approximately equal to 30 A / dm ² .

The temperature of the electrolytic bath is preferably between 20 and 80 ° C, more preferably between 20 and 60 ° C, and preferably between 40 and 60 ° C. Ble is for example around 45 ° C or 50 ° C.

The application of a current between the substrate and the anode is carried out for an adequate time to form on the surface of the substrate a chromium coating with a thickness of between 5 and 500 μm. It is within the skill of a person skilled in the art to know how to choose this duration, depending in particular on the other operating parameters, in particular the temperature of the aqueous composition, its exact composition and the current density applied.

For example, for a set of given operating parameters, a person skilled in the art can for this purpose test several durations of application of the current, then measure, for each sample substrate treated, the thickness of the chromium coating formed on its surface, for example by electron microscopy, and deduce therefrom the appropriate current application time corresponding to these operating parameters and to the thickness of the chromium coating targeted.

The substrate to which the chroming process according to the invention is applied is formed from metallic material or any other material having an electrically conductive surface.

In this regard, it can be formed of any metal. The chromium plating method according to the invention proves to be particularly advantageous for use on steel substrates. This description includes, in the term steel, steel alloys, in particular stainless steels. The chroming process according to the invention can also, for example, be implemented on substrates made of nickel-based superalloys, of cobalt-based superalloys, of bronze, of aluminum alloy, of magnesium alloy, of titanium alloy, etc.

Prior to the implementation of the chroming process according to the invention, the substrate may have been covered by one or more sublayers, for example a nickel sublayer, this by any conventional method per se.

The anode used in the chroming process according to the invention can be formed from any conventional material in itself for the electrolytic deposition of metal, in particular chromium, on a substrate. It can for example be formed from an inert and conductive material such as graphite, iridiated titanium, platinized titanium or titanium coated with a mixture of metal oxide (MMO) or any other conductive material coated with one of these materials.

The method according to the invention may comprise preliminary stages of degreasing, in particular of alkaline degreasing, and / or of pickling, of the substrate.

Such degreasing and pickling steps can be implemented according to any conventional method in itself for the skilled person.

Preferably, the method according to the invention comprises a step of alkaline degreasing of the substrate, by bringing into contact, in particular by immersion, of the substrate in an alkaline composition, such as the composition sold under the name Presol 7045 by the company Coventya. This contacting is for example carried out for a period of 20 minutes, the composition being at a temperature of approximately 60 ° C.

This preliminary degreasing step can be followed by a pickling step.

A step of stripping the substrate may in particular consist of an electrolytic pickling in a composition based on sulfuric acid, for example containing a mixture of sulfuric acid and ethylene glycol. This etching step can be carried out in a conventional manner, for example being carried out at ambient temperature, that is to say at a temperature of approximately 20 ° C., by applying for example, for the anodic phase, a current density of 40 A / dm ² for 45 seconds, and, for the cathode phase, a current density of 30 A / dm ² for 4 minutes.

The method according to the invention can also comprise final stages of:

- rinsing of the substrate, especially with water, and optionally, drying

- And / or degassing, for example at 190 ° C for 3 hours.

The chroming process according to the invention may also include a step of heat treatment of the substrate covered with a coating of chromium obtained, for example at a temperature between 250 ° C. and

700 ° C for a period between 20 and 200 minutes. Such a heat treatment step makes it possible in particular to increase the hardness of the chromium coating covering the substrate, by a phenomenon of structural hardening.

Another aspect of the invention relates to a substrate, in particular a metal substrate or one having an electrically conductive surface, for example a steel substrate, obtained by a chromium-plating process according to the invention.

This substrate is covered on the surface with a chromium coating with a thickness of between 5 and 500 μm, in particular between 15 and 450 μm. This coating has characteristics equivalent to those of chromium coatings formed by the chroming processes of the prior art using hexavalent chromium, in particular in terms of hardness, coefficient of friction and resistance to wear. It has in particular a Vickers hardness, measured for an applied load of 100 g, which is greater than 800 Hv after degassing at 190 ° C for hours, and which is even greater than 1200 Hv after a heat treatment at 300 ° C for 120 minutes.

This substrate can be any mechanical part, including a part of complex geometry.

Its properties, in particular of resistance to wear, make it entirely suitable for use under heavy constraints, in any industrial field, in particular in the aeronautical field.

The characteristics and advantages of the invention will appear more clearly in the light of the following implementation examples, provided for illustrative purposes only and in no way limitative of the invention, with the support of FIGS. 1 to 3, in which:

- Figure 1 shows a photograph of steel substrates treated by a chroming process according to the invention, the aqueous composition used having a pH of 0.5 and having different stages of aging (expressed in Ah / L), the lower part of the treated substrates having been rubbed with abrasive paper;

- Figure 2 shows a photograph of steel substrates treated by a chroming process according to the invention, the aqueous composition used having a pH of 1 and having different stages of aging (expressed in Ah / L), the lower part treated substrates having been rubbed with abrasive paper;

- and Figure 3 shows a photograph of a substrate partially covered with a chromium coating after a Hull cell test using an aqueous composition according to the invention, the current density values associated varying between 0 A / dm ² (on the right in the figure) and more than 100 A / dm ² (on the left in the figure), a chromium coating being observed for current densities greater than or equal to 13 A / dm ² .

Example 1

XC38 cylindrical steel substrates, 20 mm in diameter and 200 mm long, are subjected to the following stages of a chromium-plating process according to the invention:

1 / Alkaline degreasing, by immersion of the substrate in a Presol 7045 composition from Coventya at a temperature of 60 ° C for 20 min.

2 / Electrolytic pickling in sulfuric medium, by immersion of the substrate in a composition of sulfuric acid and ethylene glycol at room temperature, applying for the anodic phase a current density of 40 A / dm ² for 45 s and for the cathodic phase a current density of 30 A / dm ² for 4 min.

3 / Hard chrome plating

To this end, the substrate is immersed, with an iridized titanium anode, in a bath of an aqueous composition according to the invention, containing, in solution in water:

- 0.79 mo / l CrCI ₃ .6H ₂ O

- 0.75 mol / l glycine

- 0.60 mol / l NaCI

- 0.65 mol / l NH ₄ CI

- 0.26 mol / l AICI ₃

The pH of this aqueous composition has previously been adjusted to a value of 0.5 by adding an appropriate amount of hydrochloric acid to the composition.

The temperature of the aqueous composition is 45 ° C.

Different substrates are treated successively in a bath of this aqueous composition.

For each substrate, a current density of 40 A / dm ² is imposed between the substrate and the anode, for a sufficient period of time to form a chromium coating of thickness 50 μm on the surface of the substrate, which for each coating in this specific case corresponds to an amount of electrical charge imposed per volume of aqueous composition of between 2.2 and 2.3 Ah / L. Several substrates are thus treated successively in this same bath at different stages of aging, until a bath aging of 38.1 Ah / L is reached,

After this treatment, each substrate is subjected to a degassing step for 3 h at 190 ° C.

At the end of this process, for each treated substrate, there is obtained on the surface of the substrate a chrome coating of uniform thickness, of metallic gray color, homogeneous, devoid of black traces which would testify to the presence of chromium oxides. instead of metallic chrome. These observations demonstrate the good quality of the chromium coating formed on the surface of the substrate.

For each treated substrate, the adhesion of the coating is evaluated by rubbing with abrasive paper from the lower part of the substrate.

Figure 1 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah / L associated with each substrate correspond to the stages of aging of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

As can be clearly seen, there has been no change in appearance at the bottom of the rubbed substrates. The metallic coatings are all adherent to the substrate, even when they have been formed in an electrolytic bath in which the equivalent of 38.1 Ah / L has been imposed, without having added one or more constituent in the bath.

This demonstrates that the electrolytic bath according to the invention has a long lifespan, and that the chromium plating process makes it possible to form on the surface of the substrate a metallic chromium coating having good adhesion.

The other properties of this chromium coating are evaluated as follows, for each of the treated substrates.

The thickness of the coating, measured by electron microscopy, is between 5 and 500 µm.

Vickers hardness, measured for an applied load of 100 g, is greater than 800 Hv.

The coating is not peeled off when the substrate is subjected to the grinding wheel test according to standard ASTM B571.

Nor is there any detachment of the coating following a thermal shock, in particular after the substrate has been subjected to 3 heating cycles at 300 ° C. for 1 to 2 h then cooling by immersion in cold water.

The coating is not peeled off after a grinding thickness greater than 100 μm.

The wear resistance of a coating, measured by the Taber test, according to standard ASTM D4060, is equivalent to that obtained for the coatings obtained by the processes using hexavalent chromium of the prior art.

Subjected to the hydrogen embrittlement test, the treated substrate being under traction, according to ASTM F519, no rupture is observed after 200 hours at 75% of the ultimate strength.

All of these properties testify to a particularly high performance of the chromium plating process according to the invention.

Example 2

In this example, XC38 steel substrates identical to those described in Example 1 are treated, in accordance with the present invention, as indicated in Example 1, except that the pH of the aqueous composition used has been adjusted to a value of 1, by adding hydrochloric acid to the composition.

At the end of this process, a uniform thick chrome coating of metallic gray color, free of black traces, is obtained on the surface of the substrate.

For each treated substrate, the adhesion of the coating is evaluated by rubbing with abrasive paper from the lower part of the substrate.

Figure 2 shows a photograph of the substrates thus obtained. In this figure, the values expressed in Ah / L associated with each substrate correspond to the stages of aging of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.

As can clearly be seen, the metallic coatings are all adherent to the substrate even when they have been formed in an electrolytic bath in which the equivalent of 22.4 Ah / L has been imposed, and this without having performed adding one or more constituents to the bath. A loss of adhesion is then observed, as shown by the white arrows which designate the areas rubbed with abrasive paper. These results, even if they are less good than those obtained with the electrolytic bath of pH 0.5 in Example 1, are still very satisfactory with regard to the lifetime of the bath.

The properties of the coatings formed on the substrates, at least up to 22.4 Ah / L, are similar to those described above for the coatings of Example 1.

Example 3

A method according to the invention is implemented according to the conditions described in Example 1, for a substrate as described in Example 1.

The current density imposed between the substrate and the anode is 40 A / dm ² for 40 min.

At the end of this process, a chromium coating with a thickness of 50 μm is obtained on the surface of the substrate.

This coating has:

- a Vickers hardness (measured with a load of 100 g) of 900 Hv after degassing at 190 ° C for 3 h, and of 1300 Hv after such degassing then a heat treatment of 300 ° C for 2 h;

- a wear index, measured by the Taber test, according to standard ASTM D4060 (grinding wheel: CS-10, load: 1000 g on each arm), equal to 3.

The performances indicated above in Example 1, in terms of adhesion to the substrate and absence of a negative effect on the mechanical properties of the substrate, are also achieved.

Example 4

Different aqueous compositions in accordance with the invention (designated C1 to C13) or not in accordance with the invention (designated C14 to C19) are tested to assess the range of current density in which a chromium plating process using them can operate.

To this end, a study in a Hull cell is carried out so as to define the range of current density corresponding to each composition tested.

The Hull cell has a trapezoidal shape and makes it possible to position the cathode and the anode, constituting opposite walls of the cell, in a manner that is not parallel to one another. The other two walls are parallel and insulating. By making it possible to obtain a wide variation in current density on the cathode surface, this cell makes it possible to evaluate the influence of this on the quality of the chromium deposit.

For these tests, brass plates (substrates) are used at the cathode and an iridized titanium grid is used at the anode. Since the chromium deposit is gray in color and the brass is golden, it is possible to visually estimate the range of current density in which the deposit can form.

For these tests, the temperature of the composition is 45 ° C., and a current intensity of 8 A is applied for 1 min 30 s

The current density applied at a specific point on the cathode can be determined using the following formula:

j = Ix (5.10 - 5.24logd) in which j represents the current density in A / dm ² at the point considered

I represents the current intensity in A passing through the cell d represents the distance in cm between the origin of the cathode and the point considered, the origin of the cathode corresponding to the end of the cathode closest to the anode

For the aqueous composition in accordance with the invention described in Example 1, here called C1, one thus determines for example extreme values of current density, for the range of operating current density associated with the composition, which are equal to 13 A / dm ² for the low value, and greater than 100 A / dm ² for the high value.

Between these extreme values of current density, one obtains on the substrate, as shown in FIG. 3, a chrome coating of quality 5 quite satisfactory, of metallic gray color, homogeneous and devoid of black traces.

The results obtained by the Hull cell test, for the various aqueous compositions tested, are shown in Table 1 below.

Composition Current density (A / dm ² ) No CrCI ₃ (mol / l) Glycine (mol / l) NaCI (mol / l) NH ₄ CI (mol / l) AICI3 (mol / l) PH Max. Min. C1 0.79 0.75 0.60 0.65 0.26 0.5 > 100 13 C2 0.41 0.75 0.60 0.65 0.26 0.5 > 100 16 C3 0.86 0.75 0.60 0.65 0.26 0.5 > 100 16 C4 0.79 0.63 0.60 0.65 0.26 0.5 > 100 20 C5 0.79 2.80 0.60 0.65 0.26 0.5 > 100 28 C6 0.79 0.75 0.26 0.65 0.26 0.5 > 100 24 C7 0.79 0.75 1.28 0.65 0.26 0.5 > 100 15 C8 0.79 0.75 0.60 0.00 0.26 0.5 > 100 18 C9 0.79 0.75 0.60 0.93 0.26 0.5 > 100 15 C10 0.79 0.75 0.60 0.65 0.06 0.5 > 100 15 C11 0.79 0.75 0.60 0.65 0.62 0.5 > 100 18 C12 0.79 0.75 0.60 0.65 0.26 0 > 100 28 C13 0.79 0.75 0.60 0.65 0.26 1.0 > 100 24 C14 0.79 0.75 0.60 0.65 0.26 1.5 35 18 C15 0.79 0.75 0.60 0.65 0.00 0.5 60 15 C16 0.79 0.75 0.00 0.65 0.26 0.5 100 40 C17 0.79 3.33 0.60 0.65 0.26 0.5 100 50 C18 0.34 0.75 0.60 0.65 0.26 0.5 70 16 C19 0.94 0.75 0.60 0.65 0.26 0.5 60 24

Table 1 - results of the Hull cell test for compositions in accordance with the invention (C1 to C13) and other compositions (C14 to C19)

These results clearly show that the compositions in accordance with the present invention (C1 to C13) are all associated with very wide current density ranges, the most effective composition being composition C1.

The current density ranges determined for the compositions not in accordance with the present invention (C14 to C19) are, in comparison, much narrower.

In particular, the results obtained, in terms of amplitude of the current density range, are significantly lower when the pH of the aqueous composition is greater than 1, than when the pH is between 0 and 1 as recommended by the present invention.

A Hull cell test is also carried out, under the operating conditions described above, but with a temperature of the composition of 50 ° C. or 55 ° C., for a composition in accordance with the invention containing, in solution in the water:

- 0.79 mo / l CrCI ₃ .6H ₂ O

- 0.75 mol / l glycine

- 1.71 mol / l NaCI

- and 0.26 mol / l of AICI ₃ .

This solution is devoid of ammonium salt. Its pH has previously been adjusted to a value of 0.5 by adding an appropriate amount of hydrochloric acid to the composition.

The following operating density ranges are obtained:

- for the composition at 50 ° C: 13 to 100 A / drrf

- for composition at 55 ° C: 16 to 100 A / drrf

Again, these results are particularly good.

Claims (14)

1. Aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate, characterized in that it contains: - 0.4 to 0.9 mol / l of a trivalent chromium salt, - 0.6 to 2.8 mol / l of glycine, - an alkali metal salt, - aluminum salt, - and optionally, an ammonium salt, and in that its pH is between 0 and 1. 2. Composition according to claim 1, in which the glycine concentration is between 0.6 and 0.9 mol / l. 3. Composition according to any one of claims 1 to 2, in which the concentration of trivalent chromium salt is between 0.7 and 0.9 mol / l. 4. Composition according to any one of claims 1 to 3, in which at least one, preferably several, of said trivalent chromium salt, of said alkali metal salt, of said aluminum salt and, where appropriate, of said salt ammonium, is chosen from chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulfates, sulfites, sulfamates, sulfonates , thiocyanates, or any of their mixtures. 5. Composition according to any one of claims 1 to 4, in which the aluminum salt concentration is between 0.06 and 0.7 mol / l, preferably between 0.2 and 0.3 mol / l. 6. Composition according to any one of claims 1 to 5, in which the concentration of alkali metal salt is between 0.2 and 1.9 mol / l. 7. Composition according to any one of claims 1 to 6, in which the ammonium salt concentration is between 0 and 1.0 mol / l. 8. Composition according to any one of claims 1 to 7, in which the ammonium salt concentration is between 0.5 and 0.8 mol / l and the alkali metal salt concentration is between 0.2 and 1.3 mol / l. 9. Composition according to any one of claims 1 to 7, in which the composition is devoid of ammonium salt and the concentration of alkali metal salt is between 1.5 and 1.9 mol / l. 10. A method of electroplating a chromium coating on the surface of a substrate, characterized in that it comprises: the immersion, in a bath of an aqueous composition according to any one of claims 1 to 9, of said substrate and of an anode, - and the application of a current between said substrate and said anode. 11. The method of claim 10, comprising applying a direct current between the substrate and the anode, the density of said direct current preferably being between 10 and 100 A / dm ² . 12. Method according to any one of claims 10 to 11, according to which the temperature of said bath is between 20 and 80 ° C. 13. Method according to any one of claims 10 to 12, according to which the application of a current between the substrate and the anode is carried out for an adequate time to form on the surface of the substrate a chromium coating of thickness. between 5 and 500 pm. 14. Substrate obtained by a method according to any one of claims 10 to 13, covered on the surface with a chrome coating of thickness between 5 and 500 μιτι, having a Vickers hardness greater than 800 Hv.