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EP-4739822-A1 - METHOD FOR HARD CHROME PLATING USING TRIVALENT CHROMIUM

EP4739822A1EP 4739822 A1EP4739822 A1EP 4739822A1EP-4739822-A1

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.

Inventors

  • ESMILAIRE, Roseline
  • FRAYRET, Jérôme
  • LEGRAND, Benjamin
  • Tardelli, Joffrey
  • HIHN, JEAN-YVES
  • Gigandet, Marie-Pierre
  • MARCELET, Martin

Assignees

  • Institut de Recherche Technologique Matériaux, Métallurgie, Procédés

Dates

Publication Date
20260513
Application Date
20240705

Claims (13)

  1. 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, in which 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. 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. 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. 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. 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 6+ /Cr 3+ couple.
  6. 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 their mixtures.
  7. 7. A trivalent chromium electrolysis bath according to any preceding claim, wherein the bath is essentially free of hexavalent chromium.
  8. 8. A trivalent chromium electrolysis bath according to any preceding claim, wherein the molar ratio of the first complexing agent to 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. 9. A trivalent chromium electrolysis bath according to any preceding claim, wherein the molar ratio of the second complexing agent to 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. 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. 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 2 and 70 A/dm 2 , preferably between 10 and 50 A/dm 2 , more preferably between 15 and 35 A/dm 2 is applied; and/or the deposition rate is of the order of 20 to 50 pm/h.
  12. 12. Substrate coated with a deposit of metallic chromium obtained by the method according to claim 10 or 11.
  13. 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% by weight and/or the carbon content is less than 0.3% by weight, preferably less than 0.2% by weight.

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 deposition of metallic chromium on a substrate using the mentioned electrolysis bath. State of the art [0002] The hard chrome plating process is used in many sectors of activity for its anti-wear or 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] The known trivalent chromium-based processes involve electrolytes that are much less concentrated in chromium than the hexavalent chromium-based processes because of the limited solubility of trivalent chromium. In addition, the reduction of trivalent chromium is very difficult in an aqueous medium. The 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. It is often found that carbon contents are of the order of 15% at 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. Indeed, the coatings obtained based on hexavalent chromium contain very low oxygen contents, whereas this element can be present at contents of up to 30% a t in the coatings obtained from electrolytes based on trivalent chromium. 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. In fact, 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, that is to say 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 fluori