Method for hard chrome plating using trivalent chromium

Abstract

The present invention relates to a trivalent chromium-based electrolysis bath, a method using same and a substrate coated with a chromium deposit obtained by means of such a method. 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 chosen from chloride, fluoride, sulphate, sulphite, thiosulphate and mixtures thereof. The bath further comprises 0.001 to 0.100 mol / L of an ionic metal additive chosen from the cationic forms of iron, manganese, aluminium and mixtures thereof. The electrolysis bath has a pH of 1.5 to 4.0 and is essentially free of organic compounds.

Description

Hard chrome plating process from trivalent chromium Technical field

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

[0002] The hard chrome plating process is used in many industries for its anti-wear and anti-corrosion properties on iron-based substrates. This process generally uses hexavalent chromium, but this is now under threat due to the ban on the use of hexavalent chromium by the European REACH regulation. Hexavalent chromium oxides are proven 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-based processes involve electrolytes with a much lower chromium concentration than hexavalent chromium-based processes due to the limited solubility of trivalent chromium. In addition, the reduction of trivalent chromium is very difficult in an aqueous medium. Trivalent chromium-based processes then require the addition of complexing agents to complex the Cr(III) and allow its reduction and the final production of a metallic chromium deposit. Very often, these complexing agents are organic compounds that are simultaneously reduced with the trivalent chromium causing their incorporation into the deposit and the increase in the carbon content therein. This carbon is either trapped by insertion into the chromium structure or in the form of defined compounds of the chromium carbide type.

[0005] Generally speaking, the incorporation of carbon into deposits is detrimental to their properties. Carbon contents are often found to be around 15%. a t. Thus, too high a carbon content can lead to problems of fragility and adhesion of the deposit. By comparison, coatings based on hexavalent chromium do not contain carbon.

[0006] Another difference between the two types of coatings obtained is their oxygen concentration. In fact, coatings obtained from hexavalent chromium contain very low oxygen contents, while this element can be present at contents of up to 30%. at in coatings obtained from trivalent chromium-based electrolytes. Too high an 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 resulting drawbacks, in particular, of known electrolysis baths, it is therefore desirable to improve the process for deposition of metallic chromium comprising constituents which do not become trapped in the deposit and do not alter (or only slightly) its properties. Subject of the invention

[0009] An object of the present invention is to provide an electrolysis bath as well as a method allowing a deposit not comprising (or less than) the impurities mentioned above. General description of the invention

[0010] In order to solve the above-mentioned problem, the present invention provides, in a first aspect, a trivalent chromium-based electrolysis bath for chromium plating, in particular for chromium plating of steel parts, comprising: - from 1 to 2.5 mol / L, preferably from 1.1 to 2.3 mol / L, more preferably from 1.5 to 2.2 mol / L, of trivalent chromium, - from 2 to 12 mol / L, preferably from 2.5 to 10, more preferably from 3.1 to 6.3 mol / L, of a first inorganic complexing agent (Complexant 1), the first inorganic complexing agent being chosen from chloride, fluoride and their mixtures, - 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, of a second inorganic complexing agent (Complexant 2), the second inorganic complexing agent being different from the first inorganic complexing agent and being chosen 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, more preferably from 0.005 to 0.030 mol / L, of an ionic metal additive chosen from the cationic forms of iron, manganese, aluminum and their mixtures, preferably from the cationic forms of iron, manganese and their mixtures, - wherein the electrolysis bath has a pH of 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 being essentially free of organic compounds, i.e. it comprises less than 500 ppm of total organic carbon.

[0011] The second inorganic complexing agent being different from the first inorganic complexing agent, this means that if the first inorganic complexing agent is chloride, the second inorganic complexing agent is selected from fluoride, sulfate, sulfite, thiosulfate, and mixtures thereof; if the first inorganic complexing agent is fluoride, the second inorganic complexing agent is selected 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 selected from sulfate, sulfite, thiosulfate, and mixtures thereof.

[0012] The first inorganic complexing agent, the second inorganic complexing agent, the ionic metal additive and the trivalent chromium may be added to the electrolysis bath in any suitable form allowing the release in solution of the desired chemical species.

[0013] Advantageously, the concentrations mentioned above for the different components of the bath, in particular trivalent chromium, the first inorganic complexing agent, the second inorganic complexing agent, and the additive ionic metal, correspond to the concentrations of the bath (at least) during its assembly (i.e. its preparation) i.e. before any use of it.

[0014] A trivalent chromium-based electrolysis bath according to the present invention therefore comprises from 4.0 to 14 mol / L, preferably from 5.0 to 12 mol / L, more preferably from 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 nature, reference is made to a first complexing agent and a second complexing agent in the present text. The terms "first" and "second" do not take precedence over the relative importance of one complexing agent over the other and are only mentioned to distinguish the different complexing agents.

[0015] In a second aspect, the invention provides a method for depositing metallic chromium on a substrate using said trivalent chromium-based electrolysis bath, in particular a method for depositing a layer of metallic chromium having a hardness of between 600 and 1100 HV, preferably between 800 and 900 HV, for an applied load of 50 g or 100 g.

[0016] In a third aspect, the invention provides a substrate coated with a metallic chromium deposit obtained by said method.

[0017] 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 flows through the part to be chrome plated placed in the electrolysis bath (or electrolyte) and reduces the Cr cations 3+present near the surface of the part. Advantageously, according to the present invention, all the Cr cations 3+ are reduced to metallic chromium Cr° by the effect of the electric current, i.e. there is no reduction by chemical means. Thus, according to certain embodiments, the electrolysis bath is free (i.e. comprises less than 500 ppm, preferably less than 250 ppm, more preferably less than 100 ppm) of a trivalent chromium reducing compound.

[0018] Since the electrolysis bath is an acidic aqueous solution, part of the electric current reduces the H protons + present in the bath, which leads to a local lowering of the pH and an increase in the concentration of OH- hydroxides. These hydroxides are necessary to destabilize the complexes formed by trivalent chromium and its complexing agents, and thus allow the reduction and deposition of trivalent chromium. However, too high a concentration of hydroxides leads to the precipitation of trivalent chromium in the form of chromium hydroxides, thus weakening the deposits obtained.

[0019] One of the merits of the invention is to have identified that the addition of an ionic metallic additive, in the prescribed concentrations, favors the reduction of trivalent chromium in the face of the reduction of H protons. + . Thus, compared to a conventional electrolysis bath not including an ionic metal additive, and for a given current density, the present electrolysis bath makes it possible to reduce more trivalent chromium and fewer H protons. + .

[0020] In other words, the reduction efficiency of trivalent chromium, for a given electric current, is significantly increased. Indeed, the inventors discovered that while for a conventional bath, only 10 to 15% of the electric current flowing through the electrolysis bath allowed the reduction of trivalent chromium, in a bath according to the present invention 25 to 30% of the current is used for the reduction of chromium.

[0021] An electrolysis bath according to the invention therefore makes it possible, compared to a conventional trivalent chromium-based electrolysis bath, to work with lower current densities while maintaining a constant deposition rate, which allows energy and financial savings. Alternatively, if usual current densities are applied to the electrolysis bath according to the invention, the chromium deposition rate is higher than for a conventional bath, which makes it possible to reduce the time required to chrome-plated a part and to increase the number of parts that can be chrome-plated in a given time.

[0022] The better chromium reduction efficiency is also advantageously the result of the high chromium concentration of the present electrolysis bath, which makes it possible to increase at the electrode the probability of encounter between electrons and trivalent chromium compared to the probability of encounter between electrons and H protons. + Compared to an electrolysis bath comprising less trivalent chromium, it is possible with a bath according to the invention to work with lower current densities while having the same deposition speed.

[0023] In addition, the high concentration of trivalent chromium in this electrolysis bath allows it to be recharged less often, i.e. to be used for longer 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 numerous parts in a row.

[0024] Another advantage of the present electrolysis bath is that the ionic metal additive, in particular iron or aluminum, but not limited to, can precipitate in the electrolysis bath in the form of hydroxide, for example but not limited to, in the form of iron or aluminum hydroxide. This reduces the number of hydroxides present in solution near the trivalent chromium and thus the formation of chromium hydroxides. The quality of the deposit is improved.

[0025] The ionic metal additive can be deposited, as an alloying element, in the metallic chromium deposit on the part to be chrome-plated. Although inevitable, the formation of an alloy is not desired, and it is therefore necessary to limit the amount of ionic metal additive in the electrolysis bath. The inventors have noted that concentrations of ionic metal additive in the electrolysis bath between 0.001 and 0.100 mol / L advantageously make it possible to have the desired effects concerning 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% en weight, preferably less than 4% en weight.

[0026] Another of the merits of the invention is to have identified that the present electrolysis bath is all the more efficient as it is essentially free of organic compounds. For the purposes of the present invention, the bath is essentially free of organic compounds in that it comprises less than 500 ppm of total organic carbon (or TOC). According to certain embodiments, the bath comprises less than 250 ppm of total organic carbon, preferably less than 100 ppm of total organic carbon. According to the present invention, no organic compound is 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 comprise organic impurities, so that the total organic carbon content in the electrolysis bath is not zero. The measurement of total organic carbon (TOC) can be carried out in a conventional manner, which consists of first acidifying the solution to remove the inorganic carbon, then oxidizing the acidified solution (by heating). The organic carbon reacts with oxygen to form CO2 which is measured, e.g. via an infrared detector.

[0027] Indeed, the invention is based on the identification of inorganic complexing agents allowing electrolysis bath conditions that substantially reduce the presence of inclusions / impurities in the deposits. Indeed, the inventors surprisingly discovered that the use of a certain combination of two inorganic complexing agents from among those selected, comprising at least one chloride or one fluoride, made it possible to dispense with the use of organic compounds. The present bath is particularly efficient using a fluoride as the first complexing agent and / or a sulfate, a chloride or their mixtures as the second complexing agent. The deposit obtained has very good properties in terms of hardness, adhesion and no longer exhibits cracks. In addition, the deposit obtained has very few inclusions / impurities, in particular a very low oxygen and carbon content.

[0028] Thus, according to certain embodiments, the first complexing agent may be a fluoride. According to these embodiments, the fluoride is advantageously present in the electrolysis bath at a concentration of 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.

[0029] 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.

[0030] Furthermore, since the present bath is essentially free of organic compounds, the inclusion of organic compounds and therefore of carbon in the chromium deposit can advantageously be limited and the carbon content in the deposit be less than 0.3%. enweight. This does not affect the strength or adhesion of the deposit. In fact, the inventors realized that a carbon content of less than 0.3% en weight, 0.2% en weight or even 0.1% en weight, in the chrome deposit made it possible to obtain hardnesses of between 600 and 1100 Hv for parts leaving the electrolysis bath, and which are not modified, in particular which do not increase, during heating of the chrome-plated part. In other words, the present invention advantageously allows the formation of chrome deposits whose hardness does not increase after application of a heat treatment (for example treatment at a temperature between 150°C and 600°C for a duration between 1 h and 5 h).

[0031] Advantageously, such a low carbon content makes it possible to obtain thermally stable chromium deposits, i.e. for which no significant increase in hardness is observed (of the order of 20% maximum, or even of the order of 10% maximum) or in morphology after heat treatment (at a temperature between 150°C and 600°C for a duration between 1 h and 5 h), unlike what is obtained for chromium deposits obtained from an electrolysis bath comprising more carbon, in particular organic complexing agents.

[0032] Finally, according to the invention, it is essential that the electrolysis bath has a pH between 1.5 and 4.0 in order to stabilize the bath constituents. For example, it is considered that the precipitation of chromium in the form of hydroxides during the deposition phase due to the pH of the solutions being too high (greater than 5) is partly responsible for the high oxygen contents in the deposits resulting from the known processes.

[0033] 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 has local pH inhomogeneities, for example the pH may increase locally near the electrodes, and locally have values ​​greater than 4.0, for example of the order of 4.5; 5.0; 6.0 or even 6.5.

[0034] According to the invention, the ionic metal additive is chosen from the cationic forms of iron, preferably Fe 2+ and Fe 3+ , manganese, preferably Mn 4+ and Mn 2+ , aluminum, preferably Al 3+ , and their mixtures. The metallic additive being in cationic form, it can, if necessary, precipitate with the hydroxides at the cathode.

[0035] Advantageously, the ionic metal additive is added to the bath in the form of iron salt, in particular in the form of ferric sulfate (Fe2(SO4)s) and / or ferrous sulfate (FeSCU) and / or ferric chloride (FeCh) and / or chloride ferrous (FeCl2), manganese salt, in particular in the form of manganese sulfate (MnSCU, Mn(HSO4)2, Mn(SÛ4)2 and / or Mn(HSO4)4) and / or manganese chloride (MnCk and / or MnCl2), fluoroaluminate, in particular sodium hexafluoroaluminate (NasAIFe), potassium hexafluoroaluminate (KsAIFe) and / or ammonium hexafluoroaluminate ((NF jsAIFe), or a mixture thereof. The addition of the ionic metal additive to the electrolysis bath in the form of a salt advantageously ensures the presence of this additive in the bath, at a known and controlled concentration to obtain the desired effects. Although low, the concentration of ionic metal additive does not result from the presence of impurities (uncontrolled / controllable) but is deliberately added to the electrolysis bath to promote the reduction of trivalent chromium in the face of the reduction of H protons +, thus increasing the reduction efficiency of trivalent chromium for a given electric current.

[0036] In addition, the ionic metal additive is advantageously added to the electrolysis bath in the form of a salt which 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 possible undesirable side reactions.

[0037] 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.

[0038] According to certain embodiments, the compound that is a source of ionic metal additive and / or the compound that is a source of trivalent chromium may also be a compound that is a source of inorganic complexing agent, in particular a source of fluoride, chloride or sulfate. A person skilled in the art knows how to consider the different possible sources for the same chemical species and adapt the quantity (of first and / or second) of inorganic complexing agent to be added to the electrolysis bath to obtain the desired concentration as a function of the respective quantity of these agents released into the electrolysis bath as a counter-ion of the ionic metal additive and / or trivalent chromium.

[0039] According to the same or other embodiments, the present electrolysis bath further comprises from 0.05 to 0.20 mol / L, preferably from 0.08 to 0.15 mol / L, more preferably about 0.10 mol / L of an antioxidant agent. Advantageously, the antioxidant agent makes it possible to limit or even avoid the oxidation of trivalent chromium, since this results in the formation of hexavalent chromium, which is toxic.

[0040] Thus, the present electrolysis bath based on trivalent chromium may 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 comprises a negligible amount of hexavalent chromium, typically less than 10 mg / L, since trivalent chromium can oxidize to hexavalent chromium at the anode when using the present electrolysis bath.

[0041] The inventors noticed that an antioxidant agent chosen from compounds having a lower oxidation-reduction potential than the oxidation-reduction potential of the Cr couple 6+ / Cr 3+ , such as but not limited to bromide or iodide, added to the bath in the form of bromide or iodide salt, was particularly effective in preventing the oxidation of trivalent chromium and the formation of hexavalent chromium.

[0042] Thus, according to certain embodiments, the antioxidant agent is added to the electrolysis bath in the form of bromide and / or iodide. It is not excluded that the bromide and / or iodide may also complex the trivalent chromium present in the electrolysis bath. However, the bromide and / or iodide is present in reduced quantities in the electrolysis bath compared to the complexing agents, and the quantity of bromide is not suitable for it to fulfill the role of the first or second complexing agent. In addition, unlike chloride and fluoride, the bromide and / or iodide has a suitable redox potential to prevent the oxidation of trivalent chromium and is mainly consumed at the anode.

[0043] According to certain embodiments, the molar ratio between the first complexing agent and the trivalent chromium ([Complexing agent 1] / [Cr 3+]) 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 ([Complexing agent 2] / [Cr 3+]) 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 make it possible to obtain chromium complexes that are both sufficiently stable to solubilize the chromium and limit its oxidation at the anode when using the present electrolysis bath, and sufficiently fragile to be destabilized by the hydroxides near the cathode and allow the reduction and deposition of the chromium at the cathode, when using the present bath. In the present text, the term "molar ratio" between two compounds is understood to mean the ratio between the respective molar concentrations of each of the two compounds.

[0044] According to certain embodiments, the first inorganic complexing agent may be a mixture of chlorides and fluorides. The concentration of first inorganic complexing agent [Complexing Agent 1] taken into account in the molar ratio between the first complexing agent and the trivalent chromium corresponds to the sum of the concentrations of the chlorides and the fluorides.

[0045] Similarly, according to certain embodiments, the second inorganic complexing agent may be a mixture of several compounds and the concentration of second inorganic complexing agent [Complexant 2] taken into account in the molar ratio between the second complexing agent and the trivalent chromium corresponds to the sum of the concentrations of the compounds of the mixture.

[0046] In another aspect, the invention provides a method for depositing metallic chromium on a substrate using the trivalent chromium-based electrolysis bath as described above. Advantageously, the invention provides a method for depositing a layer of metallic chromium having a hardness of between 600 and 1100 HV, preferably between 800 and 900 HV, on a substrate using the trivalent chromium-based electrolysis bath as described above.

[0047] The method according to the invention comprises the following steps: (i) provide a trivalent chromium electrolysis bath as described above, (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 the metallic chromium thereon.

[0048] The electrolysis bath, during deposition, is preferably at a temperature between 25 and 65°C, more preferably between 40 and 60°C.

[0049] The method can be implemented by applying an appropriate current density. In particular, we will work by applying a current density between 5 A / dm 2 and 70 A / dm 2 , preferably between 10 and 50 A / dm 2 , more preferably between 15 and 35 A / dm 2 . Conventionally, current density is defined as the electric current flowing through the substrate (the substrate being at the cathode). One of the merits of the invention is therefore, as mentioned above, the possibility of working with reduced current densities without inducing a slowdown in the deposition rate.

[0050] Initial tests carried out in accordance with the method according to the invention have enabled the formation of metallic chromium deposits essentially free of impurities / inclusions at a rate of between 20 and 50 pm / h (increase in the thickness of the deposits per hour). In other words, the deposition rate is preferably of the order of 20 to 50 pm / h. The inventors have thus found that despite the purity of the deposited chromium, it is possible to achieve very interesting deposition rates similar to known methods using organic complexing agents whose deposits contain carbon contents of at least 0.5% respectively. en weight and oxygen contents of at least 1% en weight.

[0051] In another aspect, the invention provides a substrate coated with a metallic chromium deposit obtained by said metallic chromium deposition method. The inventors have found that with the described method, it is possible to obtain chromium deposits with very few impurities, and having a hardness of between 600 and 1100 Hv, preferably between 800 and 900 Hv. Due to the use of the electrolysis bath according to the invention, it results that the deposit on the substrate has an oxygen content of less than 1%. en weight and / or a carbon content of less than 0.3% en weight preferably less than 0.2% en weight, more preferably less than 0.1% en weight. According to some embodiments, the carbon content in the deposit on the substrate may be less than 2% a t.

[0052] According to certain embodiments, a relative difference between the hardness of the deposit after a heat treatment at a temperature between 150°C and 600°C for a duration of between 1 h and 5 h and the hardness of the deposit before heat treatment (i.e. at the outlet 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.

[0053] For the purposes of the present invention, the first inorganic complexing agent, the second inorganic complexing agent, the ionic metal additive and the antioxidant agent respectively refer to a species (or a mixture of species) free in solution, and available to perform the desired function.

[0054] The present invention is generally applicable for the production of chromium layers to provide anti-wear and / or anti-corrosion properties. It finds application in many sectors of activity, particularly in the automotive and aeronautics industries. The parts that can be treated (the substrates) are typically iron-based (steels and various alloys). The method finds particular application to parts made of steels such as 15CDV6, and in particular for parts such as rotating and / or friction parts. Brief description of the drawings

[0055] Other features and characteristics of the invention will emerge from the detailed description of some advantageous embodiments presented below, by way of illustration, with reference to the appended drawings. These show: [Fig. 1] Comparison of the 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 conventional commercial electrolysis bath comprising trivalent chromium and organic complexing agents (b); [Fig. 2] SEM images allowing the 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 conventional commercial electrolysis bath comprising trivalent chromium and organic complexing agents (b), and with an electrolysis bath comprising less chromium than in the present invention (d); [Fig. 3] Views of the chromium deposit obtained with a bath according to the invention at different pH and by applying different current densities; [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 conventional commercial electrolysis bath comprising trivalent chromium and organic complexing agents (b) before heat treatment, and after different heat treatments; [Fig. 5] Comparison of fractographies (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 conventional commercial electrolysis bath comprising trivalent chromium and organic complexing agents before heat treatment (c) and after heat treatment (d); and [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 favorite executions

[0056] I) Materials and methods

[0057] Chrome plating tests were carried out with a bath according to the invention and comparative baths not in accordance with 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.

[0058] Generally speaking, the electrolytic deposition process involves the preparation of an electrolysis bath, the immersion of the substrate to be coated and the application of an electric current to the substrate placed at the cathode. The invention uses the usual electrolysis equipment, which will therefore not be described here.

[0059] The compositions and general characteristics of the different electrolysis baths are summarized in Table 1 below. [Table 1] 0060] 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).

[0061] 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 metal additive) is added at a concentration of 0.18 mmol / L.

[0062] 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.

[0063] For the example in accordance with the invention (Invention - example no. 1), the pH of the electrolysis bath presented in Table 2 is maintained at a value between 2.3 and 2.7 and the temperature of the bath is maintained between 40 and 60°C. For counter-examples no. 1 to no. 3, the pH of the electrolysis bath presented in Table 2 is maintained at a value between 2.1 and 2.7 and the temperature of the bath is maintained between 30 and 50°C.

[0064] It should be noted that the measurement of total organic carbon (or TOC) is well known in analytical chemistry and can be done according to the classical analytical method consisting of first determining the total inorganic carbon content (TIC) by acidification (e.g. by measuring the amount of CO2 released) and then determining the total organic carbon content (TOC) by oxidation (e.g. by oxidizing the acidified solution by heating and again measuring the amount of CO2 released). The CO2 determination can be done via an infrared detector. The sum TIC + TOC represents the total carbon content (TC). The analysis can be done for example according to the ASTM D7573-18AE1 standard (January 2019). Table 2] 0065] II) Characterization methods

[0066] The metallic chromium deposits obtained with the different baths (reference based on hexavalent chromium, commercial product with or without metallic additive, comparative based on inorganic complexing agents and invention) were characterized in order to determine the carbon and oxygen content, the hardness and the possible presence of cracks. The different characterizations are carried out as follows:

[0067] The carbon and oxygen levels of the deposit, in particular the % en weight, are measured using an elemental carbon, oxygen analyzer, by infrared analysis of gases resulting from reactions / combustions of the sample in a specific reactor. Additionally or alternatively, the % a t can be determined by X-ray photoelectron spectrometry (XPS).

[0068] The abrasion resistance of the deposit is measured using a TABER abraser according to ASTM D 4060-19

[0069] The hardness of the deposit is assessed using a micro-durometer on the surface of the deposit or on a metallographic section (through the thickness of the deposit). For surface measurements, the applied load is 50 g or 100 g. For sectional measurements, the applied load is 50 g so that the trace left by the indenter represents a maximum of one third of the thickness of the deposit. Hardness measurements comply with standard NF EN ISO 4516:2002-10.

[0070] The deposits are observed by optical microscopy and scanning electron microscopy.

[0071] III) Results

[0072] Table 3 below shows the properties of the deposits obtained for certain baths. [Table 3] na: not rated

[0073] 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 macroscopic cracking. The appearance of the deposits is matt and a homogeneous and non-cracked structure is observed, and this for different pH values ​​of the electrolysis bath between 2.5 and 3.5, and different current densities applied to the electrolysis bath, between 26 and 52 A / dm 2

[0074] Concerning the properties of the deposits obtained (Table 3) as well as the fractographies of these deposits (Fig. 1, a) - c)), we observe a greater resemblance of the deposits obtained with the electrolysis bath according to the present invention with those obtained with a bath based on hexavalent chromium than those obtained with an electrolysis bath based on trivalent chromium comprising organic complexing agents (commercial product). We can note the absence of through cracks (Fig. 1 and Fig. 2), contrary to what we can observe for the deposits obtained using the bath with organic complexing agents, for which through cracks are clearly visible Fig. 2, b).

[0075] Concerning baths based on trivalent chromium and inorganic complexing agents, in the absence of ionic metal additive (counter-examples no. 2 and no. 3), no chromium deposit is obtained.

[0076] Furthermore, the carbon and oxygen contents of the 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% by weight (as for a deposit obtained using the reference hexavalent chromium-based bath) and the oxygen content is <1.5%. pO ids- In addition, measurements of the hydrogen contents for the deposit obtained from example no. 1 according to the invention indicate contents <0.25% by weight (compared to 0.12% en weight for a deposit obtained using the reference bath based on hexavalent chromium). These values ​​are much lower than those obtained for a deposit made using an electrolysis bath based on trivalent chromium using organic complexing agents.

[0077] Adding an ionic metal additive to a trivalent chromium electrolysis bath using organic complexing agents changes the macroscopic appearance of the chromium deposit, which has a black appearance, similar to a burnt deposit. Such a deposit has through cracks.

[0078] The deposit obtained using an electrolysis bath according to the present invention also has a hardness - at the bath outlet - 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).

[0079] 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, unlike 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).

[0080] 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 2 h at 500°C (Fig. 5, b) after heat treatment in comparison with Fig. 5, a) - before heat treatment), unlike a deposit obtained using an electrolysis bath based on trivalent chromium and organic ligands after heat treatment of 4 h at 200°C (Fig. 5, d) after heat treatment in comparison with Fig. 5, c) - before heat treatment).

[0081] The presence of total organic carbon in a significant 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 2 hours (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 for 2 hours at 500°C.

[0082] The trivalent chromium-based electrolysis bath and the hard chromium plating process according to the invention therefore make it possible to obtain metallic chromium deposits having fewer impurities (carbon and oxygen) than the deposits obtained using a trivalent chromium-based electrolysis bath employing organic complexing agents, thus 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 cracks is observed.

Claims

Claims 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, of trivalent chromium, from 2.0 to 12.0 mol / L, preferably from 2.5 to 10.0, more preferably from 3.1 to 6.3 mol / L, of 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, of 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 metal additive chosen from the cationic forms of iron, manganese, aluminum and mixtures thereof,wherein the electrolysis bath has a pH of 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 comprising 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 iron salt, in particular in the form of ferric sulfate and / or ferrous sulfate and / or ferric chloride and / or ferrous chloride, manganese salt, in particular in the form of manganese sulfate and / or manganese chloride, fluoroaluminate, in particular sodium hexafluoroaluminate, potassium hexafluoroaluminate and / or ammonium hexafluoroaluminate, or a mixture thereof.

3. A trivalent chromium-based electrolysis bath according to any preceding claim, the bath comprising less than 250 ppm of total organic carbon, preferably less than 100 ppm total organic carbon.

4. A trivalent chromium-based electrolysis bath according to any preceding claim, wherein the bath further comprises from 0.05 to 0.20 mol / L of an antioxidant agent.

5. Electrolysis bath based on trivalent chromium according to the preceding claim, in which the antioxidant agent is chosen from compounds having a lower oxidation-reduction potential than the oxidation-reduction potential of the Cr couple 6+ / Cr 3+ .

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

7. A trivalent chromium electrolysis bath according to any preceding claim, wherein the bath is essentially free of hexavalent chromium.

8. Trivalent chromium-based electrolysis bath according to any one of the preceding claims, in which the molar ratio between the first complexing agent and the trivalent chromium ([Complexing agent 1] / [Cr 3+ ]) 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.

9. A trivalent chromium-based electrolysis bath according to any one of the preceding claims, wherein the molar ratio between the second complexing agent and the trivalent chromium ([Complexing agent 2] / [Cr 3+ ]) 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 1.5 and 3.

0.

10. A method of depositing metallic chromium on a substrate comprising the following steps: (i) providing a trivalent chromium 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 thereon.

11. Method according to the preceding claim, in which the electrolysis bath is at a temperature between 25 and 65°C, preferably between 40 and 60°C; and / or a current density between 5 A / dm is applied 2 and 70 A / dm 2 , preferably between 10 and 50 A / dm 2 , more preferably between 15 and 35 A / dm 2; and / or the deposition rate is of the order of 20 to 50 pm / h.

12. Substrate coated with a deposit of metallic chromium obtained by the method according to claim 10 or 11.

13. Substrate coated with a metallic chromium deposit according to the preceding claim, in which the deposit has a hardness of 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% en weight and / or carbon content is less than 0.3% en weight preferably less than 0.2% en weight.

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