Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/12—Electroplating: Baths therefor from solutions of nickel or cobalt

Definitions

• 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".
• 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.
• chromium plating 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.
• chromium trioxide (Cr03) 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.
• substances based on hexavalent chromium are harmful to living organisms, it has been sought for several years to limit, or even completely eliminate, their use.
• Several technologies have been proposed by the prior art to replace the current hard chromium plating processes based on hexavalent chromium.
• 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.
• cobalt salts like that of nickel salts, is undesirable due to the risk of toxicity to living organisms.
• 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.
• the invention also aims to ensure that this composition remains effective over time.
• 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.
• 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:
• 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.
• the glycine concentration in the composition can in particular be between 0.63 and 0.9 mol / l, in particular between 0.63 and 0.83 mol / l.
• the glycine concentration in the composition is between 0.6 and 2.8 mol / l, in particular between 0.63 and 2.8 mol / l, and for example between 0.67 and 2.8 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.
• 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 density range of current up to more than 100 A / dm 2 , in particular as wide as going from 13 A / dm 2 to more than 100 A / dm 2 .
• these properties are advantageously uniform over the entire surface of the substrate, even when it has a complex shape.
• the lifetime of the aqueous composition according to The invention having a pH of between 0 and 1, and a glycine concentration of between 0.6 and 0.9 mol / l, is significantly greater than for the equivalent aqueous compositions, that is to say containing the same components in the same concentrations, but whose pH is greater than 1 and / or the glycine concentration is greater than 0.9 mol / l.
• the aqueous composition according to the invention can thus advantageously be used, for the electrolytic deposition of chromium on the surface of a substrate, after having imposed the equivalent of 22 Ah / L, and even more than 50 Ah / L for certain embodiments, 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.
• the aqueous composition according to the invention may in particular contain one or more acids, for example hydrochloric acid, in an amount sufficient to confer on said aqueous composition a pH of between 0 and 1.
• 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.
• aqueous composition according to the invention can also respond to one or more of the characteristics described below, implemented in isolation or in each of their technically operative combinations.
• the preferred concentration values indicated below are all associated with even better performance of the chromium plating process implementing 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 concentration of glycine in the composition is 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.
• 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 can be organic or inorganic, sulfonates, which can be organic or inorganics, thiocyanates, or any of their mixtures.
• 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.
• each salt is formed with a single counterion.
• 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.
• 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, xH20, (CH3C02) 2Cr, xH20, (CH3C02) 7Cr3 (0H) 2, xH20, CrF3, xH20, etc.
• the trivalent chromium salt is a chromium chloride, for example CrCl3,6H20.
• the concentration of trivalent chromium salt in the composition is for example between 0.41 and 0.86 mol / l.
• the concentration of trivalent chromium salt in the aqueous composition according to the invention can be approximately equal to 0.79 mol / l.
• 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 a or more of the counterions listed above.
• the aluminum salt is an AICI3 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.
• the concentration of aluminum salt in the aqueous composition according to the invention can be approximately equal to 0.26 mol / l.
• 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.
• 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.
• ammonium salt means a single ammonium salt or a mixture of different ammonium salts.
• ammonium salt contained in the aqueous composition according to the invention may contain, in addition to the ammonium ion, any conventional counter-ion in itself for chromium-plating treatments, or any mixture of such counter-ions, in particular a or more of the counterions listed above.
• 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.
• the aqueous composition contains:
• 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.
• 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.
• 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, even for compositions devoid of ammonium salt.
• a particular aqueous composition according to the invention contains:
• trivalent chromium for example trivalent chromium chloride
• alkali metal salt for example sodium chloride
• ammonium salt for example of ammonium chloride
• Another particular aqueous composition according to the invention contains:
• trivalent chromium salt for example trivalent chromium chloride
• alkali metal salt for example sodium chloride
• aluminum salt for example aluminum chloride; its pH being between 0 and 1;
• composition being devoid of ammonium salt.
• 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.
• the aqueous composition according to the invention may for example contain one or more surfactants.
• the aqueous composition according to the invention is substantially free from one or more, preferably all, of the following substances: boric acid / borate, or other compound based on boron, quaternary ammonium, oxalate, vanadium, manganese, iron , cobalt, molybdenum, nickel, tungsten, indium.
• 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.
• 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:
• 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,
• the electrolytic reduction of trivalent chromium into hard metallic chromium occurs on the surface of the part to be treated.
• 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.
• 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.
• 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. It is for example around 45 ° C or 50 ° C.
• 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 ⁇ m. 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.
• 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.
• the chroming process according to the invention proves to be particularly advantageous for use on steel substrates.
• the present description includes, in the term steel, steel alloys, in particular stainless steels.
• the chromium plating process according to the invention can also, for example, be implemented on substrates made of nickel-based superalloy, of cobalt-based superalloy, of bronze, of aluminum alloy, of magnesium alloy, of titanium alloy, etc.
• 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.
• 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.
• 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 density of current 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:
• the chrome-plating process according to the invention may also comprise 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 of between 20 and 200 minutes .
• 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 ⁇ m, in particular between 15 and 450 ⁇ m.
• 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 3 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.
• FIGS. 1 to 4 in which:
• FIG. 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;
• FIG. 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;
• FIG. 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 values of current density 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 ;
• FIG. 4 shows photographs of steel substrates treated by a chromium plating process, respectively using an aqueous composition containing a glycine concentration of 0.75 mol / l (a /), an aqueous composition comprising a glycine concentration of 1 mol / l (b /) and of an aqueous composition comprising a glycine concentration of 1.25 mol / l (c /), each aqueous composition used having different stages of aging (expressed in Ah / L), and the lower part of the treated substrates having been rubbed with abrasive paper.
• Example 1 shows photographs of steel substrates treated by a chromium plating process, respectively using an aqueous composition containing a glycine concentration of 0.75 mol / l (a /), an aqueous composition comprising a glycine concentration of 1 mol / l (b /) and of an aqueous composition comprising a glycine concentration of
• 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:
• 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.
• 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:
• 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.
• 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 ⁇ m 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.
• each substrate is subjected to a degassing step for 3 h at 190 ° C.
• 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.
• 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.
• the thickness of the coating measured by electron microscopy, is between 5 and 500 ⁇ m.
• 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.
• the coating is not peeled off after a grinding thickness greater than 100 ⁇ m.
• the resistance to wear of a coating is equivalent to that obtained for the coatings obtained by the processes using hexavalent chromium of the prior art.
• Example 2 XC38 steel substrates identical to those described in Example 1 are treated, in accordance with the present invention, as indicated in Example 1, with the difference that the pH of the aqueous composition used has been adjusted to a value of 1, by adding hydrochloric acid to the composition.
• 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.
• 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.
• 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.
• the properties of the coatings formed on the substrates are similar to those described above for the coatings of Example 1.
• 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.
• a chromium coating with a thickness of 50 ⁇ m is obtained on the surface of the substrate.
• This coating has:
• Example 2 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.
• C1 to C13 or not in accordance with the invention (called C14 to C20) are tested to assess the current density range in which a chromium plating process using them can operate.
• 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.
• brass plates are used at the cathode and an iridized titanium grid is used at the anode.
• the temperature of the composition is 45 ° C., and a current intensity of 8 A is applied for 1 min 30 s
• I represents the current intensity at A crossing 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
• aqueous composition in accordance with the invention described in Example 1, here called C1 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.
• compositions in accordance with the present invention are all associated with very wide current density ranges, the most effective composition being composition C1.
• the results obtained 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 l water:
• 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.
• composition B1 a concentration equal to 0.75 ml / l (composition B1), a concentration equal to 1 mol / l (composition B2) and a concentration equal to 1.25 mol / l (composition B3).
• a method is used for substrates as described in Example 1 according to the conditions described in Example 1, except for the pH value which is equal to 1 and the glycine concentration which is equal to 0.75 mol / l for the composition B1, to 1.00 mol / l for the composition B2 or at 1.25 mol / l for composition B3.
• composition B1, B2 and B3 For each composition B1, B2 and B3, the following experiment is carried out.
• 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 ⁇ m on the surface of the substrate, which for each coating corresponds to an amount of electrical charge imposed per volume of aqueous composition of between 2.2 and 2.3 Ah / L.
• each substrate is subjected to a degassing step for 3 h at 190 ° C.
• FIG. 4 shows photographs of the substrates thus obtained, respectively in a / for composition B1, in b / for composition B2 and in c / for composition B3.
• composition B1 according to the invention, a loss of adhesion of the metallic coating (indicated by a white arrow in the figure) is observed in the area rubbed with abrasive paper for the coatings.
• metals which have been formed in the electrolytic baths in which the equivalent of 24.6 Ah / L or more has been imposed.
• the metal coating remains adherent.
• composition B2 the loss of adhesion of the coating is observed at much shorter aging times, immediately after
• the electrolytic baths based on the compositions containing 1 mol / l of glycine and more have a greatly reduced lifetime, compared to the baths formed on the basis of the compositions according to the invention containing at most 0.9 mol / l of glycine.
• the latter advantageously have a long service life, during which they make it possible to form on the surface of the substrate a metallic coating of chromium having good adhesion.

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• 2018
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