Hard chrome plating process using trivalent chromium

Abstract

The present invention relates to a trivalent chromium-based electrolysis bath, a process using such a bath, and a substrate coated with a chromium deposit obtained by such a process. According to the invention, the electrolysis bath comprises 1 to 2.5 mol / L of trivalent chromium, 2 to 12 mol / L of a first inorganic complexing agent in the form of chloride, fluoride, and mixtures thereof, and 1.5 to 7.5 mol / L of a second inorganic complexing agent different from the first complexing agent and selected from chloride, fluoride, sulfate, sulfite, thiosulfate, and mixtures thereof. The bath further comprises 0.001 to 0.100 mol / L of an ionic metallic additive selected from the cationic forms of iron, manganese, aluminum, and mixtures thereof. The electrolysis bath has a pH between 1.5 and 4.0 and is essentially free of organic compounds.

Description

Title of the invention: Hard chrome plating process using trivalent chromium technical field

[0001] The present invention relates to an electrolysis bath based on trivalent chromium and to a method for depositing metallic chromium on a substrate using the aforementioned electrolysis bath. State of the art

[0002] The hard chrome plating process is used in many sectors for its anti-wear or anti-corrosion properties on iron-based substrates. This process generally uses hexavalent chromium, but this is now threatened due to the ban on its use by the European REACH regulation. Hexavalent chromium oxides are known carcinogens and are harmful to the environment.

[0003] As an alternative to the use of hexavalent chromium, it is well known to use processes based on trivalent chromium.

[0004] Known trivalent chromium processes involve electrolytes with much lower chromium concentrations than hexavalent chromium processes due to the limited solubility of trivalent chromium. Furthermore, the reduction of trivalent chromium is very difficult in aqueous media. Trivalent chromium processes therefore require the addition of complexing agents to bind Cr(III) and enable its reduction, ultimately resulting in a metallic chromium deposit. Very often, these complexing agents are organic compounds that are simultaneously reduced with trivalent chromium, causing their incorporation into the deposit and increasing its carbon content. This carbon is either trapped as an insertion within the chromium structure or in the form of defined compounds such as chromium carbide.

[0005] In general, the incorporation of carbon into coatings is detrimental to their properties. Carbon content is often found to be around 15 at.%. Thus, an excessive carbon content can lead to problems with coating fragility and adhesion. By comparison, hexavalent chromium-based coatings do not contain carbon.

[0006] Another difference between the two types of coatings obtained is their oxygen concentration. Coatings based on hexavalent chromium contain very low levels of oxygen, whereas this element can be present at levels of up to 30 at.% in coatings obtained from trivalent chromium-based electrolytes. An excessively high oxygen concentration, synonymous with a high concentration of precipitated trivalent chromium, in the form of hydroxides, will result in the appearance of cracks in the deposit during its aging and dehydration.

[0007] Therefore, generally speaking, high carbon and oxygen contents in the deposit are detrimental to its properties. Indeed, it tends to crack and flake over time.

[0008] In view of the disadvantages resulting, in particular, from known electrolysis baths, it is therefore desirable to improve the process of depositing metallic chromium comprising constituents that do not become trapped in the deposit and do not alter (or only slightly alter) the properties of the latter. Object of the invention

[0009] An object of the present invention is to provide an electrolysis bath and a process allowing a deposit containing no (or fewer) of the impurities mentioned above. General description of the invention

[0010] In order to solve the problem mentioned above, the present invention proposes, in a first aspect, a trivalent chromium-based electrolysis bath for chrome plating, in particular for chrome plating steel parts, comprising:

[0011] of 1 to 2.5 mol / L, preferably of 1.1 to 2.3 mol / L, more preferably of 1.5 to 2.2 mol / L, of trivalent chromium,

[0012] of 2 to 12 mol / L, preferably 2.5 to 10, more preferably 3.1 to 6.3 mol / L, of a first inorganic complexing agent (Complexant 1), the first inorganic complexing agent being selected from chloride, fluoride and mixtures thereof,

[0013] of 1.5 to 7.5 mol / L, preferably 2.5 to 6.5 mol / L, more preferably 3.0 to 6.0 mol / L, of a second inorganic complexing agent (Complexant 2), the second inorganic complexing agent being different from the first inorganic complexing agent and being selected from chloride, fluoride, sulfate, sulfite, thiosulfate and mixtures thereof,

[0014] from 0.001 to 0.100 mol / L, preferably from 0.003 to 0.050 mol / L, more preferably from 0.005 to 0.030 mol / L, of an ionic metallic additive selected from the cationic forms of iron, manganese, aluminum and mixtures thereof, preferably from the cationic forms of iron, manganese and mixtures thereof,

[0015] wherein the electrolysis bath has a pH between 1.5 and 4.0, preferably the pH is between 2.0 and 3.75, even more preferably the pH is between 2.5 and 3.5 and

[0016] the electrolysis bath is essentially free of organic compounds.

[0017] Since the second inorganic complexing agent is different from the first inorganic complexing agent, this means that if the first inorganic complexing agent is chloride, the second inorganic complexing agent is chosen from fluoride, sulfate, sulfite, thiosulfate, and mixtures thereof; if the first inorganic complexing agent is fluoride, the second inorganic complexing agent is chosen from chloride, sulfate, sulfite, thiosulfate, and mixtures thereof; and if the first inorganic complexing agent is a mixture of chloride and fluoride, the second inorganic complexing agent is chosen from sulfate, sulfite, thiosulfate, and mixtures thereof.

[0018] The first inorganic complexing agent, the second inorganic complexing agent, the ionic metal additive and trivalent chromium can be added to the electrolysis bath in any suitable form allowing the release into solution of the desired chemical species.

[0019] An electrolysis bath based on trivalent chromium according to the present invention therefore comprises 4.0 to 14 mol / L, preferably 5.0 to 12 mol / L, more preferably 6.5 to 11.8 mol / L or 9.0 to 11.5 mol / L of complexing agents. Although having the same function, the complexing agents may be of different chemical natures; therefore, reference is made to a first complexing agent and a second complexing agent in this text. The terms "first" and "second" do not imply the relative importance of one complexing agent over the other and are mentioned solely to distinguish the different complexing agents.

[0020] In a second aspect, the invention proposes a method for depositing metallic chromium on a substrate using said trivalent chromium-based electrolysis bath.

[0021] In a third aspect, the invention proposes a substrate coated with a metallic chromium deposit obtained by said process.

[0022] When using a chromium-based electrolysis bath to chrome plate (i.e. deposit a layer of metallic chromium on) a metal part, for example but not limited to, a steel part, an electric current runs through the part to be chromed placed in the electrolysis bath (or electrolyte) and reduces the Cr3+ cations present near the surface of the part.

[0023] Since the electrolysis bath is an acidic aqueous solution, part of the electric current reduces the H+ protons present in the bath, resulting in a local decrease in pH and an increase in the concentration of hydroxides (OH). These hydroxides are necessary to destabilize the complexes formed by trivalent chromium and its complexing agents, thus enabling the reduction and deposition of trivalent chromium. However, an excessive concentration of hydroxides leads to the precipitation of trivalent chromium in the form of chromium hydroxides, thus weakening the deposits obtained.

[0024] One of the merits of the invention is that it has identified that the addition of an ionic metal additive, in the prescribed concentrations, promotes the reduction of trivalent chromium compared to the reduction of H+ protons. Thus, compared to a conventional electrolysis bath not containing an ionic metal additive, and for a given current density, the present electrolysis bath reduces more trivalent chromium and fewer H+ protons.

[0025] In other words, the reduction yield of trivalent chromium, for a given electric current, is significantly increased. Indeed, the inventors discovered that while in a conventional bath only 10 to 15% of the electric current flowing through the electrolysis bath allows for the reduction of trivalent chromium, in a bath according to the present invention 25 to 30% of the current is used for chromium reduction.

[0026] An electrolysis bath according to the invention therefore allows, compared to a conventional trivalent chromium-based electrolysis bath, operation with lower current densities while maintaining a constant deposition rate, resulting in energy and cost savings. Alternatively, if conventional current densities are applied to the electrolysis bath according to the invention, the chromium deposition rate is higher than for a conventional bath, thus reducing the time required to chrome-plate a part and increasing the number of parts that can be chrome-plated in a given time.

[0027] The improved chromium reduction yield is also advantageously achieved due to the high chromium concentration in the present electrolysis bath, which increases the probability of electrons encountering trivalent chromium at the electrode compared to the probability of electrons encountering H+ protons. Compared to an electrolysis bath containing less trivalent chromium, a bath according to the invention allows for lower current densities while maintaining the same deposition rate.

[0028] In addition, the high concentration of trivalent chromium in the present electrolysis bath allows it to be recharged less often, i.e. to be used for a longer time between two additions of trivalent chromium, which facilitates its handling / use on an industrial scale for chrome plating large parts and / or for chrome plating many parts in succession.

[0029] Another advantage of the present electrolysis bath is that the ionic metallic additive, in particular iron or aluminum, but not limited to it, can precipitate in the electrolysis bath in the form of hydroxide, for example, but not limited to it, in the form of iron or aluminum hydroxide. This reduces the number of hydroxides present in solution near trivalent chromium and thus the formation of chromium hydroxides. The quality of the deposit is improved.

[0030] The ionic metal additive can be deposited, as an alloying element, in the metallic chromium deposit on the part to be chromed. Although inevitable, the formation of an alloy is undesirable, and it is therefore necessary to limit the amount of ionic metal additive in the electrolysis bath. The inventors have observed that concentrations of ionic metal additive in the electrolysis bath between 0.001 and 0.100 mol / L advantageously achieve the desired effects regarding the reduction of chromium versus the reduction of H+ protons, while limiting the deposition of this / these additive(s) in the chromium deposit. In particular, the content of ionic metal additive in the chromium deposit is less than 5 wt.S, preferably less than 4 wt.S.

[0031] Another merit of the invention is that it has identified that the present electrolysis bath is more efficient the more essentially it is free of organic compounds. For the purposes of the present invention, the bath is essentially free of organic compounds if it contains less than 500 ppm of total organic carbon (or TOC), preferably less than 250 ppm of total organic carbon, and particularly preferably less than 100 ppm of total organic carbon. According to the present invention, no organic compounds are added to the electrolysis bath during its preparation. However, it is not excluded that the source compounds of chromium, complexing agent, and / or ionic metal additive may contain organic impurities, such that the total organic carbon content in the electrolysis bath is not zero.

[0032] Indeed, the invention is based on the identification of inorganic complexing agents that allow for electrolysis bath conditions which substantially reduce the presence of inclusions / impurities in the deposits. In fact, the inventors have surprisingly discovered that the use of a certain combination of two selected inorganic complexing agents, including at least one chloride or one fluoride, makes it possible to eliminate the need for organic compounds. The present bath is particularly effective when using a fluoride as the first complexing agent and / or a sulfate, a chloride, or mixtures thereof as the second complexing agent. The resulting deposit has very good properties in terms of hardness and adhesion and is free of cracks. Furthermore, the resulting deposit has very few inclusions / impurities, notably a very low oxygen and carbon content.

[0033] Thus, according to certain embodiments, the first complexing agent can be a fluoride. According to these embodiments, the fluoride is advantageously present in the electrolysis bath at a concentration between 2.0 and 7.0 mol / L, preferably between 2.5 and 6.3 mol / L, more preferably between 3.1 and 6.0 mol / L.

[0034] According to the same or other embodiments, the second complexing agent is a mixture of chloride and sulfate, which are advantageously present at a concentration (total, representing the sum of the chloride concentration and the sulfate concentration) of between 2.0 and 7.5 mol / L, preferably between 2.5 and 6.5 mol / L.

[0035] Furthermore, since the present bath is essentially free of organic compounds, the inclusion of organic compounds, and therefore carbon, in the chromium deposit can advantageously be limited, and the carbon content in the deposit can be less than 0.3% by weight. This does not alter either the strength or the adhesion of the deposit. Indeed, the inventors have found that a carbon content of less than 0.3% by weight, 0.2% by weight, or even 0.1% by weight, in the chromium deposit makes it possible to obtain hardnesses between 600 and 1100 Hv for parts as they exit the electrolysis bath, and which are not modified, in particular do not increase, during heating of the chromed part. In other words, the present invention advantageously allows the formation of chromium deposits whose hardness does not increase after the application of a heat treatment.

[0036] Advantageously, such a low carbon content allows the obtaining of thermally stable chromium deposits, i.e. for which no significant increase in hardness (on the order of 20% maximum, or even on the order of 10% maximum) or morphology is observed after heat treatment (at a temperature between 150°C and 600°C for a duration between 1h and 5h), unlike what is obtained for chromium deposits obtained from an electrolysis bath containing more carbon, in particular organic complexants.

[0037] Finally, according to the invention, it is essential that the electrolysis bath have a pH between 1.5 and 4.0 in order to stabilize the bath constituents. For example, the precipitation of chromium in the form of hydroxides during the deposition phase due to the excessively high pH of the solutions (greater than 5) is considered to be partly responsible for the high oxygen content in the deposits resulting from known processes.

[0038] For the purposes of the invention, the pH of the electrolysis bath, which must be between 1.5 and 4.0, is the average pH of the electrolysis bath. However, it is not excluded from the present invention that the electrolysis bath may exhibit local pH inhomogeneities; for example, the pH may increase locally near the electrodes and have local values ​​greater than 4.0, for example, in the range of 4.5, 5.0, 6.0, or even 6.5.

[0039] According to the invention, the ionic metallic additive is selected from the cationic forms of iron, preferably Fe2+ and Fe3+, manganese, preferably Mn4+ and Mn2+, aluminum, preferably Al3+, and mixtures thereof. Since the metallic additive is in cationic form, it may, if necessary, precipitate with hydroxides at the cathode.

[0040] Advantageously, the ionic metal additive is added to the bath in the form of an iron salt, in particular in the form of ferric sulfate (Fe2(SO4)3) and / or ferrous sulfate (FeSO4) and / or ferric chloride (FeCl3) and / or ferrous chloride (FeCl2), a manganese salt, in particular in the form of manganese sulfate (MnSO4, Mn(HSO4)2, Mn(SO4)2 and / or Mn(HSO4)4) and / or manganese chloride (MnCl4 and / or MnCl2), a fluoroaluminate, in particular sodium hexafluroaluminate (Na3 Al1F6), potassium hexafluroaluminate (K3Al1F6) and / or ammonium hexafluroroaluminate ((NH4)3Al1F6), or a mixture thereof.

[0041] Thus, the ionic metal additive is advantageously added to the electrolysis bath in the form of a salt that releases sulfates or fluorides into the electrolysis bath, which are inorganic complexing agents according to the present invention. In other words, the counterion of the salt of the ionic metal additive can advantageously be used as a complexing agent for trivalent chromium, limiting the number of species present in solution and the potential for undesirable side reactions.

[0042] For the same reasons, trivalent chromium is advantageously added to the electrolysis bath in the form of chromium(III) sulfate, chromium(III) chloride, chromium(III) fluoride or a mixture of these.

[0043] According to certain embodiments, the source compound of the ionic metal additive and / or the source compound of trivalent chromium may also be a source compound of an inorganic complexing agent, in particular a source of fluoride, chloride, or sulfate. Those skilled in the art know how to consider the different possible sources for the same chemical species and adjust the quantity (of the first and / or second) of inorganic complexing agent to be added to the electrolysis bath to obtain the desired concentration, depending on the respective quantity of these agents released into the electrolysis bath as a counter-ion of the ionic metal additive and / or trivalent chromium.

[0044] According to the same or other embodiments, the present electrolysis bath further comprises 0.05 to 0.20 mol / L, preferably 0.08 to 0.15 mol / L, more preferably about 0.10 mol / L, of an antioxidant. Advantageously, the antioxidant limits or even prevents the oxidation of trivalent chromium, since this results in the formation of hexavalent chromium, which is toxic.

[0045] Thus, the present trivalent chromium-based electrolysis bath can be essentially free of hexavalent chromium. For the purposes of the invention, the term "essentially free of hexavalent chromium" means that no hexavalent chromium is added to the electrolysis bath. However, it is not excluded that the bath may contain a negligible amount of hexavalent chromium, typically less than 10 mg / L, because trivalent chromium can be oxidized to hexavalent chromium at the anode during the use of the present electrolysis bath.

[0046] The inventors have observed that an antioxidant chosen from among the compounds having a lower redox potential than the redox potential of the Cr67Cr3+ couple, such as, for example but not limited to, bromide or iodide, added to the bath in the form of a salt of bromide or iodide, was particularly effective in preventing the oxidation of trivalent chromium and the formation of hexavalent chromium.

[0047] Thus, according to certain embodiments, the antioxidant is added to the electrolysis bath in the form of bromide and / or iodide. It is possible that the bromide and / or iodide could also complex the trivalent chromium present in the electrolysis bath. However, the bromide and / or iodide are present in small quantities in the electrolysis bath compared to the complexing agents, and the amount of bromide is not sufficient for it to fulfill the role of the first or second complexing agent. Moreover, unlike chloride and fluoride, bromide and / or iodide have a suitable redox potential to prevent the oxidation of trivalent chromium and are mostly consumed at the anode.

[0048] According to some embodiments, the molar ratio between the first complexing agent and trivalent chromium ([Complexant 1] / [Cr3+]) is from about 0.8 to about 6.0, preferably between 1.0 and 5.0, more preferably between 1.5 and 4.0, more preferably between 2.0 and 3.0. Alternatively or additionally, the molar ratio between the second complexing agent and trivalent chromium ([Complexant 2] / [Cr3+]) is from about 0.5 to about 4.0, preferably between 1.0 and 3.5, more preferably between 1.2 and 3.2 or between 1.2 and 2.7 or between 1.2 and 2.8 or between 1.5 and 3.0.Such molar ratios between trivalent chromium and its complexing agents advantageously allow the formation of chromium complexes that are both sufficiently stable to solubilize chromium and limit its oxidation at the anode during the use of this electrolysis bath, and sufficiently fragile to be destabilized by hydroxides near the cathode, thus allowing the reduction and deposition of chromium at the cathode during the use of this bath. In this text, the molar ratio between two compounds is understood to be the ratio between the respective molar concentrations of each of the two compounds.

[0049] According to some embodiments, the first inorganic complexing agent may be a mixture of chlorides and fluorides. The concentration of the first inorganic complexing agent [Complexing Agent 1] taken into account in the molar ratio between the first complexing agent and trivalent chromium corresponds to the sum of the concentrations of the chlorides and fluorides.

[0050] Similarly, according to some embodiments, the second inorganic complexing agent can be a mixture of several compounds and the concentration of the second inorganic complexing agent [Complexing 2] taken into account in the molar ratio between the second complexing agent and trivalent chromium corresponds to the sum of the concentrations of the compounds in the mixture.

[0051] In another aspect, the invention proposes a method for depositing metallic chromium on a substrate using the trivalent chromium-based electrolysis bath as described above.

[0052] The method according to the invention comprises the following steps:

[0053] (i) provide a trivalent chromium-based electrolysis bath as described above,

[0054] (ii) immerse a substrate in the electrolysis bath and place it at the cathode, and

[0055] (iii) apply an electric current density to the substrate in order to deposit the chromium metallic on this one.

[0056] The electrolysis bath, during deposition, is preferably at a temperature between 25 and 65°C, more preferably between 40 and 60°C.

[0057] The process can be implemented by applying an appropriate current density. In particular, it will be carried out by applying a current density between 5 A / dm² and 70 A / dm², preferably between 10 and 50 A / dm², and more preferably between 15 and 35 A / dm². Conventionally, current density is defined as the electric current flowing through the substrate (the substrate being located at the cathode). One of the advantages of the invention is therefore, as mentioned above, the possibility of working with reduced current densities without inducing a slowdown in the deposition rate.

[0058] Initial tests carried out according to the process of the invention enabled the formation of essentially impurity-free / inclusion-free metallic chromium deposits at a rate of between 20 and 50 pm / h (increasing in deposit thickness per hour). In other words, the deposition rate is preferably in the range of 20 to 50 pm / h. The inventors thus observed that, despite the purity of the deposited chromium, it is possible to achieve very interesting deposition rates, similar to known processes using organic complexing agents, the deposits of which contain, respectively, carbon contents of at least 0.5 wt% and oxygen contents of at least 1 wt%.

[0059] In another aspect, the invention provides a substrate coated with a metallic chromium deposit obtained by said metallic chromium deposition process. The inventors have found that with the described process, it is possible to obtain chromium deposits with very few impurities and a hardness between 600 and 1100 Hv, preferably between 800 and 900 Hv. Due to the use of the electrolysis bath according to the invention, the deposit on the substrate has an oxygen content of less than 1% by weight and / or a carbon content of less than 0.3% by weight, preferably less than 0.2% by weight, more preferably less than 0.1% by weight. According to certain embodiments, the carbon content in the deposit on the substrate can be less than 2% by weight.

[0060] According to some embodiments, a relative difference between the hardness of the deposit after heat treatment at a temperature between 150°C and 600°C for a duration between 1h and 5h and the hardness of the deposit before heat treatment (i.e. at the exit of the electrolysis bath) does not exceed 20%, preferably does not exceed 10% or even does not exceed 5%, in particular there is no significant increase in the hardness of the deposit during the heat treatment.

[0061] For the purposes of the present invention, the first inorganic complexing agent, the second inorganic complexing agent, the ionic metallic additive and the antioxidant agent respectively refer to a species (or mixture of species) free in solution, and available to perform the desired function.

[0062] The present invention is generally applicable for producing chromium coatings to impart anti-wear and / or anti-corrosion properties. It finds application in numerous sectors, particularly in the automotive and aerospace industries. The parts that can be treated (the substrates) are typically iron-based (various steels and alloys). The process finds particular application in parts made of steels such as 15CDV6, and especially in parts such as those subject to rotation and / or friction. Brief description of the drawings

[0063] Other features and characteristics of the invention will become apparent from the detailed description of some advantageous embodiments presented below by way of illustration, with reference to the accompanying drawings. These show:

[0064] [Fig.1] Comparison of fractographies (scanning electron microscope images) of the deposits obtained according to the present invention (c), with a reference electrolysis bath based on hexavalent chromium (a) and with a commercial conventional electrolysis bath comprising trivalent chromium and organic complexing agents (b);

[0065] [Fig.2] SEM images allowing comparison of the morphologies of the deposits obtained according to the present invention (c), with a reference electrolysis bath based on hexavalent chromium (a), with a commercial conventional electrolysis bath comprising trivalent chromium and organic complexing agents (b), and with an electrolysis bath comprising less chromium than in the present invention (d);

[0066] [Fig.3] Views of the chromium deposit obtained with a bath according to the invention at different pH levels and by applying different current densities;

[0067] [Fig.4] A graph representing the hardness of the deposits obtained according to the present invention (c), with a reference electrolysis bath based on hexavalent chromium (a), with a commercial conventional electrolysis bath comprising trivalent chromium and organic complexants (b) before heat treatment, and after different heat treatments;

[0068] [Fig. 5] Comparison of fractographs (scanning electron microscope images) of the deposits obtained according to the present invention (a) before heat treatment, according to the invention after heat treatment (b), with a commercial conventional electrolysis bath comprising trivalent chromium and organic complexing agents before heat treatment (c) and after heat treatment (d); and

[0069] [Fig.6] Comparison of fractographies (scanning electron microscope images) of deposits obtained with a comparative bath comprising inorganic ligands and carbon (a) before heat treatment and after heat treatment (b), and with a bath according to the invention before heat treatment (c) and after heat treatment (d). Description of preferred executions

[0070] I) Materials and methods

[0071] Chrome plating tests were carried out with a bath according to the invention and comparative baths not conforming to the present invention. For each of the tests, the substrates were rectangular test specimens made of 15CDV6 steel with dimensions of 120mm x 80mm x 6mm.

[0072] Generally, the electrolytic deposition process involves preparing an electrolysis bath, immersing the substrate to be coated, and applying an electric current to the substrate placed at the cathode. The invention uses standard electrolysis equipment, which will therefore not be described here.

[0073] The compositions and general characteristics of the different electrolysis baths are summarized in Table 1 below. [Tables 1] Electrolysis bath Hexavalent Chromium (Reference) Chromium derivative and organic complexant (Commercial product) Chromium derivative and inorganic complexant (Invention - example no. 1) Chromium derivative and inorganic complexant (counter-example no. 1) Chromium derivative and inorganic complexant (counter-example no. 2) Chromium derivative and inorganic complexant (counter-example no. 3) Chromium derivative and organic complexant + metallic additive [Cr3+] -120 g / L 20 g / L 90-110 g / L 40-52 g / L 20 g / L 110 g / L 94 g / L Molar ratio [Complexan 11 / Cr] / 12 2-2.5 1.5-2.5 / / / PH <0.5 5.2-5.3 2.3 - 2.7 2.1-2.7 / / / Conductivity >300 mS / cm2 180 mS / cm2 -200 mS / cm2 -200 mS / cm2 / / /

[0074] The electrolysis baths in the first two columns of Table 1 are conventional and will not be described in detail. The first column concerns a reference bath with hexavalent chromium. The second column concerns a conventional bath of trivalent chromium with organic complexing agents (commercial product).

[0075] The third column corresponds to a trivalent chromium electrolysis bath according to the invention. The fourth column corresponds to a trivalent chromium electrolysis bath comprising less chromium than according to the present invention and constitutes a counterexample. The fifth and sixth columns correspond to trivalent chromium-based electrolysis baths comprising less complexing agent than according to the present invention and constitute counterexamples. The seventh column corresponds to the bath of the first column (commercial product) to which iron (ionic metallic additive) is added at a concentration of 0.18 mmol / L.

[0076] The compositions of the baths during their assembly, i.e. their preparation, of the third to sixth columns are described in detail in Table 2.

[0077] For the example according to the invention (Invention - example no. 1), the pH of the electrolysis bath shown in Table 2 is maintained at a value between 2.3 and 2.7 and the bath temperature is maintained between 40 and 60°C. For counterexamples no. 1 to no. 3, the pH of the electrolysis bath shown in Table 2 is maintained at a value between 2.1 and 2.7 and the bath temperature is maintained between 30 and 50°C. [Tables 2] Function of the compound Chemical nature Concentration Invention - example no. 1 Concentration Counter-example no. 1 Concentration Counter-example no. 2 Concentration Counter-example no. 3 Chromium salt Cr2(SO4)3 Cr3+: 2.1 mol Cr3+: 1 mol / Cr3+: 2.1 mol Cr3+: 1.8 mol e 1 / L 1 / L 1 / L First complexing agent Free fluoride, introduced by the complexing agent NH4F 5.3 mol / L 2.5 mol / L 5.9 mol / L 1.8 mol / L Second complexing agent Sulfate SO42 and Chlorides Cl- 4.30 mol / L Sulfates SO42 1.5M / / Antioxidant Potassium bromide, KB r 0.1 mol / L 0.1 mol / L 0.1 mol / L 0.1 mol / L Ionic metal additive Ferric sulfate Fe2(SO4)3e if applicable Potassium fluoroalum inate K3A1F6 0.01 mol / L iron 0.01 mol / L iron 0.01 mol / L aluminum / / TOC (total organic carbon) < 200 ppm 500 ppm 500 ppm 500 ppm II) Characterization Methods

[0078]

[0079] The metallic chromium deposits obtained with the different baths (reference bath based on hexavalent chromium, commercial product with or without metallic additive, comparative baths based on inorganic complexing agents, and invention bath) were characterized to determine the carbon and oxygen content, hardness, and the possible presence of cracks. The different characterizations were carried out as follows:

[0080] The carbon and oxygen content of the deposit, in particular the % by weight, is measured using a carbon and oxygen elemental analyzer, by infrared analysis of the gases produced by reactions / combustions of the sample in a specific reactor. Additionally or alternatively, the % at can be determined by X-ray photoelectron spectrometry (XPS).

[0081] The hardness of the deposit is measured using a TABER abrasimeter according to ASTM D 4060-19.

[0082] The deposits are observed by optical microscopy and scanning electron microscopy.

[0083] III) Results

[0084] Table 3 below shows the properties of the deposits obtained for certain baths. [Tables 3] Electrolysis bath Hexavalent Chromium (Reference) Trivalent Chromium and organic complexing agent (Commercial product) Trivalent Chromium and inorganic complexing agent (Invention - example no. 1) Trivalent Chromium and inorganic complexing agent (Counter-example no. 1) Cracking Microcracking Through cracking Non-through cracking Through cracking Chromium content (max) 99% by weight 95.4% by weight 95% by weight 95% by weight Carbon content (max) <0.1% by weight 1.8% by weight <0.1% by weight >0.2% by weight Oxygen content (max) <1% by weight 2.3% by weight <1.5% by weight <l,5%en poids taux d’hydrogène (te neur max) 0,12% en 0,4% <0,25 %en poi ds n.a. dureté 750 - 1200 hv 800 1000 600-1100 800-1100 vitesse de dépôt 15 20 pm h 2-50 50 40 adhérence 1kg n / a: not evaluated

[0085] As can be seen in [Fig. 3], a metallic chromium deposit obtained using an electrolysis bath according to the invention (example no. 1) does not exhibit cracking. macroscopic. The appearance of the deposits is matte and a homogeneous and uncracked structure is observed, and this for different pH of the electrolysis bath between 2.5 and 3.5, and different current densities applied to the electrolysis bath, between 26 and 52 A / dm2.

[0086] Regarding the properties of the deposits obtained (Table 3) and the fractographs of these deposits ([Fig. 1], a) - c)), a greater resemblance is observed between the deposits obtained with the electrolysis bath according to the present invention and those obtained with a hexavalent chromium-based bath than with those obtained with a trivalent chromium-based electrolysis bath containing organic complexing agents (commercial product). The absence of through-fractures ([Fig. 1] and [Fig. 2]) is noteworthy, unlike what can be observed for the deposits obtained using the organic complexing agent bath, for which through-fractures are clearly visible [Fig. 2], b).

[0087] Regarding baths based on trivalent chromium and inorganic complexants, in the absence of ionic metallic additive (counterexamples no. 2 and no. 3), no chromium deposit is obtained.

[0088] Furthermore, the carbon and oxygen contents of metallic chromium deposits obtained using a trivalent chromium-based electrolysis bath according to the present invention are very close to the carbon and oxygen contents of deposits obtained using a hexavalent chromium-based bath. Indeed, in a deposit obtained according to the invention, the carbon content is <0.1 wt% (as for a deposit obtained using the reference hexavalent chromium-based bath) and the oxygen content is <l,5%poids. de plus des mesures teneurs en hydrogène pour le dépôt obtenu à partir de l’exemple n°l selon l’invention indiquent <0,25%enpoids(contre 0,12%en poidspour un l'aide du bain référence base chrome hexavalent). ces valeurs sont bien inférieures celles obtenues réalisé utilisant d’électrolyse trivalent employant complexants organiques.

[0089] Adding an ionic metallic additive to a trivalent chromium-based electrolysis bath using organic complexing agents alters the macroscopic appearance of the chromium deposit, which takes on a black appearance, similar to a burnt deposit. Such a deposit exhibits through-cracks.

[0090] The deposit obtained using an electrolysis bath according to the present invention also exhibits a hardness - at the exit of the bath - close to that obtained for deposits made using an electrolysis bath based on hexavalent chromium or using an electrolysis bath based on trivalent chromium and organic ligands (Table 3 and [Fig.4]).

[0091] After heat treatment at 500°C for 2 hours or at 200°C for 2 hours, no significant variation in the hardness of a deposit made using an electrolysis bath according to the present invention is observed, contrary to what is observed for the deposit obtained using the electrolysis bath based on trivalent chromium and organic ligands, for which the hardness more than doubles ([Fig.4]).

[0092] As can be seen in [Fig.5], a chromium deposit obtained using an electrolysis bath according to the present invention does not crack when subjected to a heat treatment of 2h at 500°C ([Fig.5], b) after heat treatment compared with [Fig.5], a) - before heat treatment), unlike a deposit obtained using an electrolysis bath based on trivalent chromium and organic ligands after a heat treatment of 4h at 200°C ([Fig.5], d) after heat treatment compared with [Fig.5], c) - before heat treatment).

[0093] The presence of total organic carbon in a non-negligible quantity in the electrolysis bath based on trivalent chromium and inorganic ligands (counter-example no. 1) alters the morphology of the chromium deposit, which exhibits micro-cracks before heat treatment and through-cracks after heat treatment at 500°C for 2h ([Fig.6], a) - b)), unlike the deposit obtained using a bath according to the invention (example no. 1 - [Fig.6], c) - d)) which does not exhibit cracks, even after heat treatment of 2h at 500°C.

[0094] The trivalent chromium-based electrolysis bath and the hard chrome plating process according to the invention thus make it possible to obtain metallic chromium deposits with fewer impurities (carbon and oxygen) than deposits obtained using a trivalent chromium-based electrolysis bath employing organic complexing agents, thereby improving the mechanical properties of the deposits obtained. They also make it possible to obtain more stable chromium deposits when subjected to heat treatment, i.e., for which neither a significant variation in hardness nor the appearance of through-cracking is observed.

Claims

Demands

1. An electrolysis bath based on trivalent chromium comprising, from 1.0 to 2.5 mol / L, preferably from 1.1 to 2.3 mol / L, more preferably from 1.5 to 2.2 mol / L, trivalent chromium, from 2.0 to 12.0 mol / L, preferably from 2.5 to 10.0 mol / L, more preferably from 3.1 to 6.3 mol / L, a first inorganic complexing agent, the first inorganic complexing agent being selected from chloride, fluoride and mixtures thereof, from 1.5 to 7.5 mol / L, preferably from 2.5 to 6.5 mol / L, more preferably from 3.0 to 6.0 mol / L, a second inorganic complexing agent, the second inorganic complexing agent being different from the first inorganic complexing agent and being selected from chloride, fluoride, sulfate, sulfite, thiosulfate and mixtures thereof, from 0.001 to 0.100 mol / L, preferably from 0.003 to 0.050 mol / L, of an ionic metallic additive selected from the cationic forms of iron, manganese, aluminium and mixtures thereof,wherein the electrolysis bath has a pH between 1.5 and 4.0, preferably the pH is between 2.0 and 3.75, even more preferably the pH is between 2.5 and 3.5, and the electrolysis bath comprises less than 500 ppm of total organic carbon.

2. Trivalent chromium electrolysis bath according to claim 1, wherein the ionic metal additive is added to the bath in the form of an iron salt, in particular in the form of ferric sulfate and / or ferrous sulfate and / or ferric chloride and / or ferrous chloride, a manganese salt, in particular in the form of manganese sulfate and / or manganese chloride, a fluoroaluminate, in particular sodium hexafluoroaluminate, potassium hexafluoroaluminate and / or ammonium hexafluoroaluminate, or a mixture thereof.

3. Trivalent chromium-based electrolysis bath according to any one of the preceding claims, the bath comprising less than 250 ppm of total organic carbon, particularly preferably less than 100 ppm of total organic carbon.

4. Trivalent chromium electrolysis bath according to any one of the preceding claims, wherein the bath further comprises 0.05 to 0.20 mol / L of an antioxidant agent.

5. Electrolysis bath based on trivalent chromium according to the preceding claim, wherein the antioxidant agent is selected from compounds having a lower redox potential than the redox potential of the Cr67Cr3+ couple.

6. Trivalent chromium electrolysis bath according to any one of the preceding claims, wherein the first inorganic complexing agent is fluoride and / or the second inorganic complexing agent is selected from chloride, sulfate and mixtures thereof.

7. Electrolysis bath based on trivalent chromium according to any one of the preceding claims, wherein the bath is essentially free of hexavalent chromium.

8. Trivalent chromium electrolysis bath according to any one of the preceding claims, wherein the molar ratio between the first complexing agent and trivalent chromium ([Complexing agent 1] / [Cr3+]) is about 0.8 to about 6.0, preferably between 1.0 and 5.0, more preferably between 1.5 and 4.0, more preferably between 2.0 and 3.

0.

9. Trivalent chromium electrolysis bath according to any one of the preceding claims, wherein the molar ratio between the second complexing agent and trivalent chromium ([Complexing 2] / [Cr3+]) is about 0.5 to about 4.0, preferably between 1.0 and 3.5, more preferably between 1.2 and 3.2 or between 1.5 and 3.

0.

10. A method for depositing metallic chromium onto a substrate comprising the following steps: (i) providing a trivalent chromium-based electrolysis bath according to any one of the preceding claims; (ii) immersing a substrate in the electrolysis bath and placing it at the cathode; and (iii) applying an electric current density to the substrate in order to deposit a layer of metallic chromium on it.

11. A method according to the preceding claim, wherein the electrolysis bath is at a temperature between 25 and 65°C, preferably between 40 and 60°C; and / or

12.

13. a current density of between 5 A / dm² and 70 A / dm² is applied, preferably between 10 and 50 A / dm², more preferably between 15 and 35 A / dm²; and / or The deposition rate is in the order of 20 to 50 pm / h. Substrate coated with a metallic chromium deposit obtained by the process according to claim 10 or 11. Substrate coated with a metallic chromium deposit according to the preceding claim, wherein the deposit has a hardness between 600 and 1100 HV, preferably between 800 and 900 HV, without modification after heat treatment and / or the oxygen content is less than 1% by weight and / or the carbon content is less than 0.3% by weight, preferably less than 0.2% by weight.

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