Electroplating bath for depositing chromium or chromium alloys and process for depositing chromium or chromium alloys - Patents.com

Abstract

The present invention relates to electroplating baths for depositing chromium or chromium alloys and to processes for depositing chromium on substrates.

Description

[Technical field]

[0001] The present invention relates to electroplating baths for depositing chromium or chromium alloys and to processes for depositing chromium on substrates.

[0002] In the field of chromium plating, trivalent chromium baths play an important role in the chromium plating industry due to their lower toxicity. However, despite their advantages, the use of trivalent chromium baths is not without drawbacks.

[0003] One of these disadvantages is the need to closely monitor the baths for their efficiency: trivalent chromium baths are not always stable and must be replaced periodically.

[0004] Benaben, P..(2011).An overview of hard chromium plating using trivalent chromium solutions.National Association for Surface Finishing Annual Conference and Trade Show 2010,SUR / FIN 2010.2 discloses different baths used to obtain trivalent chromium plating electrolytes containing boric acid.

[0005] Zeng, Zhixiang & Liang, Aiming & Zhang, Junyan. (2009). A Review of Recent Patents on Trivalent Chromium Plating. Recent Patents on Materials Science. 2.50-57.10.2174 / 1874465610902010050 provides an overview of patents in the field of trivalent chromium plating. Some are identified with respect to the reduction of hexavalent chromium and its removal by electrolytic refining.

[0006] U.S. Pat. No. 4,167,460(A) discloses an aqueous acidic trivalent chromium electroplating solution and process for depositing chromium plating using a bath containing trivalent chromium, a complexing agent, a reducing agent, and a controlled effective amount of an anionic or nonionic surfactant selected from the class of organic mono- or di- or triester phosphates that contributes to improving the working characteristics and efficiency of the electroplating bath and enhances the homogeneity of the chromium deposit.

[0007] U.S. Pat. No. 4,184,929(A) discloses an aqueous acidic trivalent chromium electroplating solution and a process for forming a chromium plate using a bath containing trivalent chromium, formate ions as a complexing agent, and a bath-soluble reducing agent selected from the group consisting of formaldehyde, glyoxal, formaldehyde bisulfite, glyoxal disulfite, sodium formaldehyde sulfoxylate, and mixtures thereof.

[0008] None of these prior art documents focus on the effect of specific agents as stabilizers to improve the life of electroplating baths.

[0009] It was therefore an object of the present invention to provide an electroplating bath for depositing chromium or chromium alloys using a trivalent chromium bath with improved stability.

[0010] This problem is solved by an electroplating bath having the features of claim 1 and by a process for depositing chromium or a chromium alloy on a substrate having the features of claim 13. The further dependent claims refer to preferred embodiments.

[0011] According to the invention, there is provided an electroplating bath for depositing chromium or a chromium alloy from a trivalent chromium bath, the trivalent chromium bath comprising: a) 0.2 to 1 mol / L of at least one kind of trivalent chromium ion; b) 2 to 10 mol / L of at least one complexing agent; c) 0.01 to 0.5 mol / L of at least one halogen salt; d) 0.01 to 1 mol / L of at least one stabilizer different from the complexing agent; the electroplating bath has a pH of 4 to 7, is substantially free of divalent sulfur compounds and boric acid, its salts and / or derivatives, and has a molar ratio of complexing agent to trivalent chromium ions of 8:1 to 15:1; the at least one stabilizing agent is selected from the group consisting of oxalic acid, glycolic acid, tartaric acid, ascorbic acid, pyruvic acid, glyoxylic acid, thioglycolic acid, and mixtures thereof; The electroplating bath, wherein the at least one complexing agent is selected from the group consisting of formic acid, acetic acid, propionic acid, lactic acid, malic acid, citric acid, succinic acid, gluconic acid, glycine, aspartic acid, glutamic acid, and mixtures thereof.

[0012] Surprisingly, it has been found that the stabilizer not only improves bath life, but also can significantly reduce the amount of hexavalent chromium that is typically present as an impurity in trivalent chromium baths.

[0013] Trivalent chromium baths must be strictly controlled for metallic impurities, especially hexavalent chromium, whose salts are furthermore listed as Substances of Very High Concern (SVHC) whose use should be banned from 2024 onwards in the European REACH (Regulation, Evaluation, Authorisation and Restriction of Chemicals) regulatory project. Hexavalent chromium is typically produced at the anode from trivalent chromium ions in an undesirable oxidation reaction.

[0014] Its presence has a detrimental effect on the plating layer, making it essential to reduce or even inhibit its formation: for example, an amount of hexavalent chromium of 5 mg / L in a trivalent chromium plating bath is enough to completely destabilize the deposit and is therefore unacceptable.

[0015] In a preferred embodiment, the stabilizer is present in an amount of 0.01 to 0.5 mol / L, preferably in an amount of 0.02 to 0.3 mol / L.

[0016] In a preferred embodiment, the at least one trivalent chromium salt source is selected from the group consisting of chromium(III) sulfate, chromium(III) chloride, chromium(III) acetate, chromium(III) hydroxyacetate, chromium(III) formate, chromium(III) formate sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof, preferably from the group consisting of chromium(III) sulfate, chromium(III) acetate, chromium(III) hydroxyacetate, chromium(III) formate, chromium(III) formate sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof, in acidic or alkaline form.

[0017] In a more preferred embodiment, the at least one trivalent chromium salt source is selected from the group consisting of acid or alkaline forms of chromium(III) sulfate, chromium(III) formate sulfate, and mixtures thereof.

[0018] In a preferred embodiment, the at least one trivalent chromium ion is present in an amount of 0.25 to 0.8 mol / L.

[0019] In a preferred embodiment, the at least one stabilizing agent is selected from the group consisting of glycolic acid, tartaric acid, ascorbic acid, pyruvic acid, oxalic acid, and mixtures thereof, or salts thereof and mixtures thereof.

[0020] In a preferred embodiment, the electroplating bath is substantially free of chloride ions and / or substantially free of aluminum ions.

[0021] In a preferred embodiment, the at least one complexing agent is present in an amount of 2-10 mol / L, preferably 3-7 mol / L, and / or the molar ratio of complexing agent to trivalent chromium ions is 9:1-14:1, preferably 10:1-13:1.

[0022] In a preferred embodiment, the at least one halogen salt is selected from the group consisting of bromide salts, chloride salts, iodide salts, fluoride salts, preferably bromide salts, iodide salts, fluoride salts, more preferably potassium bromide, sodium bromide, ammonium bromide, and mixtures thereof.

[0023] In a preferred embodiment, at least the halogen salt is present in an amount of 0.01 to 0.5 mol / L.

[0024] In a preferred embodiment, the bath contains from 1 mg / L to 10 g / L, preferably from 5 to 500 mg / L, of at least one additive, the additive being Brighteners, for example polyamines or mixtures of polyamines containing quaternary ammonium compounds, Wetting agents such as neutral, cationic and amphoteric surfactants, and and combinations thereof.

[0025] In a preferred embodiment, the anion of the trivalent chromium ion is the anion of a volatile or electrochemically consumable acid.

[0026] In a more preferred embodiment, the anion of the volatile or electrochemically consumable acid is selected from the group consisting of formate, acetate, propionate, glycolate, oxalate, carbonate, citrate, or mixtures thereof.

[0027] In a preferred embodiment, the anion of the trivalent chromium ion is a sulfate anion.

[0028] According to the present invention, there is provided a process for depositing chromium or a chromium alloy on a substrate from a trivalent chromium bath, comprising the steps of: - providing an electroplating bath according to any one of the preceding claims, - immersing the substrate in an electroplating bath; and applying an electric current to deposit on the substrate.

[0029] In a preferred embodiment, the electroplating bath is separated from the anode by a membrane, preferably an anion or cation exchange membrane or a porous membrane, more preferably a cation exchange membrane, that defines the anolyte and catholyte.

[0030] In a preferred embodiment, the electroplating is performed using a pulsed current.

[0031] In a preferred embodiment, the anolyte comprises chromium(III) sulfate.

[0032] In a preferred embodiment, the bath temperature is 20 to 70°C, preferably 30 to 65°C, and more preferably 40 to 60°C. [Brief description of the drawings]

[0033] With reference to the following figures and examples, the subject matter according to the invention is intended to be explained in more detail, without wishing to limit said subject matter to the specific embodiments shown therein. [Figure 1] 4 shows Hull Cell plating achieved in a bath containing no organic acid to serve as a reference. [Diagram 2] 4 shows Hull Cell plating achieved in a bath containing 5 g / L ascorbic acid. [Diagram 3] 4 shows Hull Cell plating achieved in a bath containing 5 g / L glycolic acid. [Figure 4] 4 shows Hull Cell plating achieved in a bath containing 10 g / L glycolic acid. [Diagram 5] 4 shows Hull Cell plating achieved in a bath containing 5 g / L pyruvic acid. [Figure 6] 4 shows Hull Cell plating achieved in a bath containing 5 g / L tartaric acid. [Figure 7] Shown is Hull Cell plating achieved in a bath containing 5 g / L thioglycolic acid. [Figure 8] 1 shows a graph depicting plating thickness depending on the current density used, with each line representing a different stabilizer. EXAMPLES

[0034] High performance liquid chromatography (HPLC) was performed on a Shimadzu Nexera XR equipped with a column Allsep Anion 7 μm 4.6×250 mm Alltech 3091917.1.

[0035] The plating thickness was measured by X-ray fluorescence on a Fischer XRF spectrometer FISCHERSCOPE® X-RAY XAN® 222 using an internal calibration method.

[0036] Evaluation of the effect of glycolic acid as a stabilizing agent: The bath was made using the following ingredients: -0.38M trivalent chromium ions -5.43M formic acid -5.3M NH3 -0.085M KBr -1g / L of quaternary ammonium compounds

[0037] The complexation was carried out at 60° C. for 2 hours.

[0038] First, different concentrations of glycolic acid were used to check the stabilizing effect on the bath. Different baths were prepared with glycolic acid concentrations (0, 0.039M, 0.11M, 0.39M, 0.78M). The baths were left at ambient temperature for one month and the appearance of the baths was observed. Only the bath without glycolic acid shows precipitation of trivalent chromium.

[0039] The effect of glycolic acid on the concentration of hexavalent chromium was then investigated. For this purpose, a bath was run as described above with a glycolic acid concentration of 0.11 M.

[0040] Hexavalent chromium (Na) in the form of sodium dichromate at a given concentration 2 Cr 2 O 7, 2H 2 O) was added and its concentration was monitored by HPLC (UV analysis at 540 nm with diphenylcarbazide).

[0041] For all examples, concentrations of hexavalent chromium (approximately in the mg / L range) were chosen that were higher than those typically found in working trivalent chromium plating baths in order to adequately measure the reduction of hexavalent chromium by the stabilizer.

[0042] If the value was close to 0, further additions were made and checked until no further reduction was possible.

[0043] Reactions at room temperature and at bath working temperature (55° C., maintained only during the working day) were compared.

[0044] [Table 1]

[0045] A high rate of decrease in the concentration of hexavalent chromium was observed at a temperature of 55° C. All components of the bath were analyzed, and a decrease in the concentration of glycolic acid was observed after the hexavalent chromium was introduced into the bath. This decrease in glycolic acid was monitored.

[0046] [Table 2]

[0047] In the absence of hexavalent chromium in the bath, the concentration of glycolic acid does not change. At higher temperatures, the reduction in the concentration of glycolic acid becomes more significant.

[0048] Glycolic acid is certainly oxidized in glyoxylic acid during hexavalent chromium reduction, but since it is unstable at this pH, it is certainly decomposed to oxalic acid.

[0049] As expected, no oxalic acid was found in the fresh bath, but 0.65 g / L was found in the bath spiked with hexavalent chromium and maintained at 55°C.

[0050] To confirm, a new bath was made without glycolic acid and loaded with approximately 4.5 g / L of hexavalent chromium.

[0051] After 15 days at 55°C (temperature was maintained throughout the working time), the chromium content was analyzed.

[0052] [Table 3]

[0053] A decrease in hexavalent chromium concentrations was observed, but far from what had been previously measured.

[0054] The bath was then split in half: one was left as is and the other had a "make-up" concentration of glycolic acid added.

[0055] After an additional 21 days at room temperature, the hexavalent chromium concentrations were measured.

[0056] [Table 4]

[0057] The effect of glycolic acid on the reduction of hexavalent chromium has been clearly demonstrated, even though it has no effect on its own and certainly acts as a catalyst for this reduction reaction.

[0058] Thus, when formed, hexavalent chromium is reduced more efficiently in trivalent chromium baths with glycolic acid at the operating bath temperatures.

[0059] Testing novel molecules as stabilizers To complete the previous tests, several molecules with a similar structure to glycolic acid were evaluated for their ability to reduce hexavalent chromium in a trivalent chromium bath. The molecules selected were acetic acid, propionic acid, malic acid, tartaric acid, lactic acid, glyoxylic acid, pyruvic acid, succinic acid, thioglycolic acid, benzoic acid, and ascorbic acid.

[0060] Similar tests were conducted using these stabilizers, and the concentrations were measured three weeks after the addition of hexavalent chromium, giving a starting concentration of 4 g / L.

[0061] [Table 5]

[0062] Interestingly, it can be seen that for the same amount of hexavalent chromium, not all compounds are consumed at the same rate.

[0063] Their effects on plating were evaluated using a Hull Cell (10 A-10 min) and can be seen in Figures 1 to 7. With the exception of thioglycolic acid, no adverse effects on plating are observed.

[0064] FIG. 8 also shows small differences in plating thickness, except for thioglycolic acid, which was not plated.

[0065] Thus, all molecules tested except thioglycolic acid have an effect as a stabilizer.

Claims

1. 1. An electroplating bath for depositing chromium or a chromium alloy from a trivalent chromium bath, the trivalent chromium bath comprising: a) 0.2 to 1 mol / L of at least one trivalent chromium ion; b) 2 to 10 mol / L of at least one complexing agent; c) 0.01 to 0.5 mol / L of at least one halogen salt; d) 0.01 to 1 mol / L of at least one stabilizing agent different from the complexing agent; the electroplating bath has a pH of 4 to 7, is free of divalent sulfur compounds and boric acid, its salts or derivatives, and has a molar ratio of the complexing agent to the trivalent chromium ions of 8:1 to 15:1; the at least one stabilizing agent is selected from the group consisting of oxalic acid, glycolic acid, tartaric acid, ascorbic acid, pyruvic acid, glyoxylic acid, and mixtures thereof; 13. The electroplating bath of claim 12, wherein the at least one complexing agent is selected from the group consisting of formic acid, acetic acid, propionic acid, lactic acid, malic acid, citric acid, succinic acid, gluconic acid, glycine, aspartic acid, glutamic acid, and mixtures thereof.

2. 2. The electroplating bath of claim 1, wherein the stabilizer is present in an amount of 0.01 to 0.5 mol / L.

3. The electroplating bath of claim 1, wherein the stabilizer is present in an amount of 0.02 to 0.3 mol / L.

4. The electroplating bath of claim 1, wherein the at least one trivalent chromium salt source is selected from the group consisting of acidic or alkaline forms of chromium(III) sulfate, chromium(III) chloride, chromium(III) acetate, chromium(III) hydroxyacetate, chromium(III) formate, chromium(III) formate sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof.

5. The electroplating bath of claim 1, wherein the at least one type of trivalent chromium ion is present in an amount of 0.25 to 0.8 mol / L.

6. The electroplating bath of claim 1, wherein the at least one stabilizer is selected from the group consisting of glycolic acid, tartaric acid, ascorbic acid, pyruvic acid, oxalic acid, salts thereof, and mixtures thereof.

7. The electroplating bath according to claim 1, wherein the electroplating bath does not contain chloride ions and does not contain aluminum ions.

8. The electroplating bath according to claim 1, wherein the at least one complexing agent is present in an amount of 3 to 7 mol / L and the molar ratio of the complexing agent to the trivalent chromium ions is from 9:1 to 14:

1.

9. The electroplating bath of claim 1, wherein the at least one halogen salt is selected from the group consisting of bromide salts, chloride salts, iodide salts, fluoride salts, and mixtures thereof, and the halogen salt is present in an amount of 0.01 to 0.5 mol / L.

10. The bath comprising 1 mg / L to 10 g / L of at least one additive, the additive comprising: a brightener selected from the group consisting of polyamines or mixtures of polyamines containing quaternary ammonium compounds; Wetting agents, and Combinations of these The electroplating bath according to any one of claims 1 to 9, wherein the electroplating bath is selected from the group consisting of:

11. The electroplating bath of claim 10, wherein the bath comprises 5 to 500 mg / L of the at least one additive.

12. The electroplating bath according to claim 1, wherein the anion of the trivalent chromium ion is the anion of a volatile or electrochemically consumable acid.

13. The electroplating bath of claim 12, wherein the anion of the volatile or electrochemically consumable acid is selected from the group consisting of formate, acetate, propionate, glycolate, oxalate, citrate or mixtures thereof.

14. The electroplating bath according to claim 1, wherein the anion of the trivalent chromium ion is a sulfate anion.

15. A process for depositing chromium or a chromium alloy on a substrate from a trivalent chromium bath, comprising the steps of: - providing an electroplating bath according to any one of claims 1 to 14; - immersing a substrate in said electroplating bath; applying an electric current to deposit said chromium on said substrate.

16. The process of claim 15, wherein the electroplating bath is separated from the anode by a membrane that defines an anolyte and a catholyte.

17. The process of claim 16, wherein the membrane is an anion or cation exchange membrane, or a porous membrane.

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