US20210355593A1 - Composition for chromium plating a substrate and chromium plating process using such a composition - Google Patents

Composition for chromium plating a substrate and chromium plating process using such a composition Download PDF

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US20210355593A1
US20210355593A1 US17/284,346 US201917284346A US2021355593A1 US 20210355593 A1 US20210355593 A1 US 20210355593A1 US 201917284346 A US201917284346 A US 201917284346A US 2021355593 A1 US2021355593 A1 US 2021355593A1
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Prior art keywords
substrate
mol
chromium
salt
composition
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Inventor
Pierre Bares
Céline Gazeau
Benoit Fori
Cédric Stephan
Patrick Benaben
Clémence MERDRIGNAC
Olivier Brucelle
Claude Rossignol
Rosyne Boyer
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Ratier Figeac SAS
Liebherr Aerospace Toulouse SAS
Mecaprotec Industries SA
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Ratier Figeac SAS
Liebherr Aerospace Toulouse SAS
Mecaprotec Industries SA
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Assigned to MECAPROTEC INDUSTRIES, LIEBHERR-AEROSPACE TOULOUSE SAS, RATIER-FIGEAC reassignment MECAPROTEC INDUSTRIES ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BENABEN, PATRICK, BOYER, Rosyne, MERDRIGNAC, Clémence, BRUCELLE, Olivier, ROSSIGNOL, CLAUDE, BARES, PIERRE, FORI, BENOIT, GAZEAU, Céline, STEPHAN, Cédric
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    • 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 lies in the field of the electrochemical deposition of chromium on the surface of a substrate, with a view to improving the resistance to wear thereof, such a deposition normally being referred to 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 a method for the electrolytic deposition of a chromium coating on the surface of a substrate using such a composition.
  • the invention also relates to a substrate covered on the surface with a chromium coating obtained by such a method.
  • chromium plating The electrolytic deposition of a surface chromium coating on a substrate, also referred to as chromium plating, is used in many fields, in particular in the aeronautical, automobile, mechanical, etc. fields, for improving the resistance to wear of the parts (hard chromium plating is then spoken of), or for producing decorative deposits (decorative chromium plating is then spoken of), the thickness of the surface coating varying from a few tenths of microns for decorative chromium plating to a few hundreds of microns for hard chromium plating.
  • Hard chromium plating with which the present invention is particularly concerned, makes it possible in particular to reduce wear on the parts during their relative movements as well as the coefficient of friction thereof.
  • chromium trioxide CrO 3
  • Hard chromium is obtained by electrolytic reduction of the hexavalent chromium into metallic chromium on the surface of the part to be treated.
  • substances based on hexavalent chromium are toxic for living organisms, for the past several years it has been sought to limit or even completely eliminate use thereof.
  • thermal projection of the HVOF type High Velocity Oxygen Fuel—projection by supersonic flame
  • HVOF type High Velocity Oxygen Fuel—projection by supersonic flame
  • This technology furthermore proves to have high cost: the application thereof would be two to ten times more expensive than hard chromium plating with hexavalent chromium.
  • Electrolytic or chemical deposition methods making it possible in particular to solve the difficulties introduced by complex geometric surfaces, have furthermore been proposed by the prior art. These methods generally use aqueous solutions of nickel or cobalt salts to make metallic depositions, with or without inclusions of particles such as hard, self-lubricating, etc. particles, on the surface of the part to be treated.
  • cobalt salts just like the use of nickel salts, is undesirable, because of a risk of toxicity for living organisms.
  • the current density range in which the chromium plating method can function has in particular great importance, in particular when the parts to be treated have a complex shape. Indeed, during an electrolytic deposition, it is observed that, for a particular current density imposed in the electrolytic bath, the actual current density on the surface of the part being treated may vary locally to a very great extent. In order to guarantee a homogeneous deposition of chromium on the whole of a part, it is therefore important to have available a method that can function with a current density range as wide as possible, this proving all the more important that the shape of the part is more complex.
  • the present invention aims to remedy the drawbacks of the methods proposed by the prior art for hard chromium plating without hexavalent chromium, in particular the drawbacks disclosed above, by proposing a method, and a composition for implementation thereof, that make it possible, without using a substance that is toxic for living organisms, and in particular hexavalent chromium, to form on a substrate a chromium coating of good quality, having in particular resistance to wear that is at least equivalent to that obtained by the hard chromium plating methods of the prior art using hexavalent chromium, this coating being formed on the entire surface thereof, and this whatever the shape of the substrate, including for substrates with a complex shape.
  • the invention also aims for this composition to remain effective over time.
  • An additional objective of the invention is that the cost of this composition and of the implementation of this method should be as low as possible, such implementation moreover being easy.
  • an aqueous liquid composition for the electrolytic deposition of a chromium coating on the surface of a substrate is proposed according to the present invention, this electrolytic deposition also being designated, in the present description, by the term “chromium plating”.
  • the aqueous composition according to the invention contains:
  • the pH thereof is between 0 and 1.
  • the aqueous composition according to the invention is furthermore preferably essentially devoid of hexavalent chromium. What is meant by this is that the composition does not contain hexavalent chromium, or only in the trace state.
  • trivalent chromium means chromium in the +3 oxidation state
  • hexavalent chromium means chromium in the +6 oxidation state
  • the concentration of glycine in the composition may 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 method, makes it possible to obtain, on the surface of the substrate being treated, a high-quality chromium coating, and this over the whole of this surface, including when the substrate has a complex shape.
  • this aqueous composition allows effective operation of the chromium plating method that implements it over a very wide current density range, with an amplitude as high as 72 A/dm 2 , and which may even be greater than 84 A/dm 2 .
  • the chromium plating method using the composition according to the invention can thus function, in order to form a high-quality chromium coating on the surface of the substrate being treated, in a current density range of 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 .
  • these properties are advantageously uniform over the entire surface of the substrate, even when it has a complex shape.
  • the service life 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 equivalent aqueous compositions, i.e. containing the same components in the same concentrations, but the pH of which is greater than 1 and/or the glycine concentration of which 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 therein the equivalent of 22 Ah/L, and even more than 50 Ah/L for certain embodiments, without significant loss of quality of the deposition produced, this without having added in the composition more of one or several of the constituents thereof.
  • Such an advantageous result makes it possible to predict, for industrial implementation, wherein readjustments of the concentrations of the constituents of the aqueous composition used will be made regularly, a service life of the composition that is considerably higher and particularly important for the field of hard chromium plating.
  • the pH of the aqueous composition according to the invention can be adjusted to a value of between 0 and 1 according to any method that is conventional per se for a person skilled in the art, in particular by the addition of 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 a suitable quantity for conferring a pH of between 0 and 1 on said aqueous composition.
  • one or more acids for example hydrochloric acid
  • 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 the constituents thereof in water, and has a low-cost price.
  • aqueous composition according to the invention can furthermore meet one or more of the characteristics described below, implemented individually or in each of the technically operative combinations thereof.
  • the preferential concentration values indicated below are all associated with better still performance of the chromium plating method using the composition according to the invention, in particular in terms of current density range wherein this method functions effectively.
  • the tight concentration ranges indicated below are thus associated with current density ranges wherein the chromium plating method is effective, which are wider.
  • the glycine concentration in the composition is for example between 0.67 and 0.83 mol/l. Optimally, the glycine concentration in the aqueous composition according to the invention may be approximately equal to 0.75 mol/l.
  • trivalent chromium salt means a single trivalent chromium salt or a mixture of various 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 counterion that is conventional per se for chromium plating treatments, or any mixture of such counterions.
  • the trivalent chromium salt may in particular be selected from the group consisting of chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, which may be organic or inorganic, sulfonates, which may be organic or inorganic, thiocyanates, or any one of the mixtures thereof.
  • At least one, preferably several, and preferentially all, of the trivalent chromium salt, the alkali metal salt, the aluminium salt and, where applicable, the ammonium salt is/are selected from chlorides, iodides, fluorides, carboxylates, carbonates, nitrates, nitrites, phosphates, phosphites, acetates, bromides, sulphates, sulphites, sulfamates, which may be organic or inorganic, sulfonates, which may be organic or inorganic, thiocyanates, or any one of the mixtures thereof.
  • Each of the salts forming part of the aqueous composition according to the invention may include a single counterion, or a mixture of a plurality of counterions. Preferentially, each salt is formed with a single counterion.
  • the counterion or counterions of the trivalent chromium salt, the counterion or counterions of the alkali metal salt, the counterion or counterions of the aluminium salt, and where applicable the counterion or counterions of the ammonium salt are identical.
  • the aqueous composition according to the invention is then advantageously more stable over time.
  • the trivalent chromium salt may for example be selected from the following trivalent chromium salts: CrCl 3 .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.
  • the trivalent chromium salt is a chromium chloride, for example CrCl 3 .6H 2 O.
  • 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 may be approximately equal to 0.79 mol/l.
  • aluminium salt means a single aluminium salt or a mixture of various aluminium salts.
  • the aluminium salt contained in the aqueous composition according to the invention may contain, in addition to the aluminium ion, any counterion conventional per se for chromium-plating treatments, or any mixture of such counterions, in particular one or more of the counterions listed above.
  • the aluminium salt is an aluminium chloride AlCl 3 .
  • the concentration of aluminium 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 aluminium salt in the aqueous composition according to the invention may be approximately equal to 0.26 mol/l.
  • alkali metal salt means a single alkali metal salt or a mixture of various alkali metal salts.
  • the alkali metal is preferably sodium or potassium, or one of the mixtures 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 counterion conventional per se for chromium-plating treatments, or any mixture of such counterions, in particular one or more of the counterions listed above.
  • the alkali metal salt is a sodium chloride NaCl and/or a potassium chloride KCl.
  • 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 various ammonium salts.
  • ammonium salt contained in the aqueous composition according to the invention may contain, in addition to the ammonium ion, any counterion conventional per se for chromium-plated treatments, or any mixture of such counterions, and in particular one or more of the counterions listed above.
  • ammonium salt is ammonium chloride NH 4 Cl.
  • 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:
  • the aqueous composition is devoid of ammonium salt, and contains a concentration of alkali metal salt 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-plated method, enables this method to function, by forming on the surface of the substrate a high-quality chromium coating, within a very wide current density range, even for compositions devoid of ammonium salt.
  • a particular aqueous composition according to the invention contains:
  • Another particular aqueous composition according to the invention contains:
  • the aqueous composition according to the invention may, optionally, contain substances other than those listed above, to the exclusion of hexavalent chromium, but these substances should however not interfere with the action of the essential constituents of the aqueous composition listed above, for the electrolytic deposition of chromium on the surface of the substrate.
  • the aqueous composition according to the invention may for example contain one or more surfactants.
  • the aqueous composition according to the invention is substantially devoid of one or more, preferably all, of the following substances: boric acid/borate, or another compound based on boron, quaternary ammonium, oxalate, vanadium, manganese, iron, cobalt, molybdenum, nickel, tungsten and indium.
  • Substantially devoid herein means that the aqueous composition does not contain these substances, except in the trace state, i.e., in a non-operative quantity.
  • the present invention relates to a method for the electrolytic deposition of a chromium coating on the surface of a substrate, referred to as a chromium plating method.
  • This method comprises:
  • the electrolytic reduction of the trivalent chromium into hard metallic chromium occurs in the bath, on the surface of the part to be treated.
  • the chromium coating thus formed by the method according to the invention on the surface of the substrate has the entirely advantageous properties disclosed above.
  • the current applied between the substrate and the anode may be of the pulsed type. In preferred embodiments of the invention, a continuous current is applied between the substrate and the anode.
  • Such a feature advantageously makes it possible to be free from the difficulties of use of pulsed currents on an industrial scale, and to simplify the implementation of the chromium plating method according to the invention.
  • the method according to the invention is in particular entirely adapted to industrial use, in a simple way.
  • the continuous 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 preferentially 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 preferentially between 40 and 60° C. It is for example approximately 45° C. or 50° C.
  • the applying of a current between the substrate and the anode is carried out for a duration that is suitable for forming on the surface of the substrate a chromium coating with a thickness of between 5 and 500 ⁇ m. It is within the skills of a person skilled in the art to be able to choose this duration, according in particular to other operating parameters, in particular the temperature of the aqueous composition, the exact composition thereof and the current density applied.
  • a person skilled in the art will for this purpose be able to test a plurality of periods of application of the current, and then measure, for each sample substrate treated, the thickness of the chromium coating formed on the surface thereof, for example by electron microscopy, and to deduce therefrom the appropriate duration of application of current corresponding to these operating parameters and to the thickness of the chromium coating sought.
  • the substrate to which the chromium plating method according to the invention is applied is formed from metal material or any other material having an electrically conductive surface.
  • the chromium plating method according to the invention proves to be particularly advantageous for implementation on steel substrates.
  • steel includes steel alloys, in particular stainless-steel alloys.
  • the chromium plating method according to the invention may also for example be implemented on substrates made from nickel-based superalloy, cobalt-based superalloy, bronze, aluminium alloy, magnesium alloy, titanium alloy, etc.
  • the substrate may have been covered with one or more undercoats, for example an undercoat of nickel, by any method conventional per se.
  • the anode used in the chromium plating method according to the invention can be formed from any material conventional per se for the electrolytic deposition of metal, in particular of chromium, on a substrate. It may for example be formed from an inert conductive material such as graphite, iridium-titanium, platinised titanium or titanium covered with a metal oxide mixture (MMO) or any other conductive material covered with one of these materials.
  • an inert conductive material such as graphite, iridium-titanium, platinised titanium or titanium covered with a metal oxide mixture (MMO) or any other conductive material covered with one of these materials.
  • the method according to the invention may comprise prior steps of degreasing, in particular alkaline degreasing, and/or pickling, of the substrate.
  • degreasing and pickling steps may be implemented in accordance with any method conventional per se for a person skilled in the art.
  • the method according to the invention comprises a step of alkaline degreasing of the substrate, by putting the substrate in contact, in particular by immersion, in an alkaline composition, such as the composition sold under the name Presol 7045 by the company Coventya.
  • This contacting is for example implemented for a period of 20 minutes, the composition being at a temperature of approximately 60° C.
  • This prior degreasing step may be followed by a pickling step.
  • a step of pickling 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 pickling step may be implemented in a conventional manner, for example be implemented at ambient temperature, i.e., 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 cathodic phase, a current density of 30 A/dm 2 for four minutes.
  • the method according to the invention may furthermore comprise final steps of:
  • the chromium plating method according to the invention may also comprise a step of heat treatment of the substrate covered with a chromium coating obtained, for example at a temperature of 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 method 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 the chromium coatings formed by the chromium plating methods 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 heat treatment at 300° C. for 120 minutes.
  • This substrate may be any mechanical part, including a part with a complex shape.
  • FIG. 1 shows a photograph of steel substrates treated by a chromium plating method according to the invention, the aqueous composition used having a pH of 0.5 and being at various ageing stages (expressed in Ah/L), the lower part of the substrates treated having been rubbed with abrasive paper;
  • FIG. 2 shows a photograph of steel substrates treated by a chromium plated method according to the invention, the aqueous composition used having a pH of 1 and being at various stages of ageing (expressed in Ah/L), the lower part of the treated substrates having been rubbed with abrasive paper;
  • FIG. 3 shows a photograph of a substrate partially covered with a chromium coating at the end of a Hull cell test using an aqueous composition according to the invention, the associated current density values 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 method, by means respectively of an aqueous composition comprising a concentration of glycine of 0.75 mol/l (a/), an aqueous composition comprising a glycine concentration of 1 mol/l (b/) and an aqueous composition comprising a glycine concentration of 1.25 mol/l (c/), each aqueous composition used being at various ageing stages (expressed in Ah/L), and the lower part of the substrates treated having been rubbed with abrasive paper.
  • Cylindrical substrates made from XC38 steel, of 20 mm in diameter and 200 mm long, are subjected to the following steps of a chromium plating method according to the invention:
  • Electrolytic pickling in a sulfuric medium by immersing the substrate in a composition of sulfuric acid and ethylene glycol at ambient 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 iridium-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 been previously adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.
  • the temperature of the aqueous composition is 45° C.
  • a current density of 40 A/dm 2 is imposed between the substrate and the anode for a suitable duration for forming on the surface of the substrate a chromium coating with a thickness of 50 nm, which for each coating corresponds in this precise case to a quantity of electrical load 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.
  • a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being of metal grey colour, homogeneous, devoid of any black marks that would testify to the presence of chromium oxides instead of metallic chromium.
  • the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.
  • FIG. 1 shows a photograph of the substrates thus obtained.
  • the values expressed in Ah/L associated with each substrate correspond to the stages of ageing of the aqueous composition at the end of the treatment of this substrate, the various substrates having been successively treated in the composition.
  • the electrolytic bath according to the invention has a long service life, and that the chromium plating method makes it possible to form on the surface of the substrate a metallic chromium coating having good adhesion.
  • the thickness of the coating measured by electron microscopy, is between 5 and 500 ⁇ m.
  • the Vickers hardness, measured for an applied load of 100 g, is greater than 800 Hv.
  • substrates made from XC38 steel 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 was adjusted to a value of 1, by adding hydrochloric acid in the composition.
  • a chromium coating of uniform thickness is obtained on the surface of the substrate, this coating being metal grey in colour, homogeneous, devoid of any black marks.
  • the adhesion of the coating is evaluated by rubbing the lower part of the substrate with abrasive paper.
  • FIG. 2 shows a photograph of the substrates thus obtained.
  • the values expressed in Ah/L associated with each substrate correspond to the stages of ageing 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 wherein the equivalent of 22.4 Ah/L has been imposed, and this without having added in the bath additional quantities of one or more of its constituents.
  • a loss of adhesion is found afterwards, as shown by the white arrows, which designate the regions 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 in accordance with 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 is obtained with a thickness of 50 ⁇ m on the surface of the substrate.
  • This coating has:
  • aqueous compositions according to the invention (named C1 to C13) or not in accordance with the invention (named C14 to C20) are tested in order to evaluate the current density range in which a chromium plating method using them can function.
  • 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 way that is not parallel to each other.
  • the other two walls are parallel and insulating.
  • brass plates are used at the cathode and an iridium-titanium grid is used at the anode.
  • the temperature of the composition is 45° C., and a current density of 8 A is applied for 1 min 30 s.
  • the current density applied at a precise point of the cathode can be determined by means of the following formula:
  • extreme current density values are thus for example determined, for the operating current density range 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 according to the present invention are all associated with very wide current density ranges, the composition with the highest performance being composition C1.
  • the results obtained are significantly inferior 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 according to the invention containing, in solution in water:
  • This solution is devoid of ammonium salt.
  • the pH thereof has previously been adjusted to a value of 0.5 by adding a suitable quantity of hydrochloric acid in the composition.
  • composition B1 a concentration equal to 0.75 ml/l
  • composition B2 a concentration equal to 1 mol/l
  • composition B3 a concentration equal to 1.25 mol/l
  • Example 1 For substrates as described in Example 1, a method according to the conditions described in Example 1 is implemented, with the exception of the value of the pH, which is equal to 1, and the glycine concentration, which is equal to 0.75 mol/l for composition B1, to 1.00 mol/l for composition B2 or to 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 suitable period for forming on the surface of the substrate a chromium coating 50 ⁇ m thick, which, for each coating, corresponds to a quantity of electrical load imposed per volume of aqueous composition of between 2.2 and 2.3 Ah/L.
  • a plurality of substrates are thus treated successively in the same bath at various stages of ageing, until an ageing of the bath of 33.6 Ah/L is reached.
  • each substrate is subjected to a degassing step for 3 h at 190° C.
  • FIG. 4 shows photographs of the substrates thus obtained, respectively at a/ for composition B1, at b/ for composition B2 and at c/ for composition B3.
  • composition B1 according to the invention, a loss of adhesion of the metallic coating (indicated by a white arrow on the figure) is observed at the region rubbed with abrasive paper for the metallic coatings that were formed in the electrolytic baths in which the equivalent of 24.6 Ah/L and more has been imposed. For the baths in which the equivalent of 22.4 Ah/L or less has been imposed, the metallic coating remains adherent.
  • composition B2 the loss of adhesion of the coating is observed at much shorter ageing times, as soon as after 15.6 Ah/L (loss of adhesion indicated by a black arrow).
  • composition B3 the loss of adhesion occurs after having imposed in the bath an even lower electrical load, equivalent to 6.8 Ah/L.
  • the electrolytic baths based on compositions containing 1 mol/l of glycine and more have a greatly reduced service life compared with baths formed from compositions according to the invention containing no more than 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 chromium coating having good adhesion.

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US17/284,346 2018-10-12 2019-10-11 Composition for chromium plating a substrate and chromium plating process using such a composition Abandoned US20210355593A1 (en)

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