Electroplating bath for depositing chromium or chromium alloy and method for depositing chromium or chromium alloy

Abstract

The invention relates to an electroplating bath for depositing chromium or chromium alloys and to a method for depositing chromium on a substrate.

Description

[0001] This invention relates to an electroplating bath for depositing chromium or chromium alloys and a method for depositing chromium on a substrate.

[0002] In the field of chromium plating, trivalent chromium baths have gained significant use due to their lower toxicity. However, despite their advantages, the use of trivalent chromium baths is not without its drawbacks.

[0003] One of these drawbacks is that the bath requires close monitoring of its efficiency. Trivalent chromium baths are not always stable and require regular replacement.

[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 for obtaining trivalent chromium electroplating electrolytes containing boric acid.

[0005] Zeng, Zhixiang, Liang, Aiming, and Zhang, Junyan. (2009). A Review of Recent Patents on Trivalent Chromium Plating. Recent Patents on Materials Science. 2.50-57.10.2174 / 1874465610902010050. This paper provides a review of patents in the field of trivalent chromium plating. A section specifically discusses the reduction of hexavalent chromium and its elimination via electrolytic purification.

[0006] US 4,167,460 A discloses an aqueous acidic trivalent chromium electroplating solution and a method for depositing chromium plating in a bath containing trivalent chromium, a complexing agent, a reducing agent, and a controlled effective amount of anionic or nonionic surfactant selected from organic monoesters, diesters, or triesters of phosphates. The anionic or nonionic surfactant helps to improve the operating characteristics and effectiveness of the electroplating bath and enhances the uniformity of the chromium deposit.

[0007] US 4,184,929 A discloses an aqueous acidic trivalent chromium electroplating solution and a method for forming a chromium plating, the method employing a bath containing trivalent chromium, formate ions as a complexing agent, and a bath of a soluble reducing agent selected from the group consisting of: formaldehyde, glyoxal, formaldehyde bisulfite, glyoxal dibisulfite, sodium formaldehyde bisulfite, and mixtures thereof.

[0008] None of these existing technical documents focus on the effect of specific agents as stabilizers on improving the lifespan of electroplating baths.

[0009] Therefore, the object of the present invention is to provide an electroplating bath for depositing chromium or chromium alloys, wherein the trivalent chromium bath has improved stability.

[0010] This problem is solved by an electroplating bath having the features described in claim 1 and having the features described in claim 1.

[0011] The method for depositing chromium or chromium alloys on a substrate, as described in claim 13, is solved. Further dependent claims refer to preferred embodiments.

[0012] According to the present invention, an electroplating bath for depositing chromium or chromium alloys from a trivalent chromium bath, the electroplating bath comprising:

[0013] a) At least one trivalent chromium ion at a concentration of 0.2 mol / L to 1 mol / L,

[0014] b) At least one complexing agent with a concentration of 2 mol / L to 10 mol / L

[0015] c) At least one halide salt with a concentration of 0.01 mol / L to 0.5 mol / L.

[0016] d) At least one stabilizer, different from the complexing agent, at a concentration of 0.01 mol / L to 1 mol / L.

[0017] The electroplating bath has a pH of 4 to 7 and is substantially free of divalent sulfur compounds and boric acid, its salts and / or derivatives, and the molar ratio of the complexing agent to the trivalent chromium ions is 8:1 to 15:1.

[0018] The at least one stabilizer is selected from the group consisting of: oxalic acid, glycolic acid, tartaric acid, ascorbic acid, pyruvic acid, glyoxylic acid, mercaptoacetic acid, and mixtures thereof.

[0019] 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.

[0020] Surprisingly, it was also found that the stabilizer not only improved the bath's lifespan, but also significantly reduced the amount of hexavalent chromium, which is usually present as an impurity in trivalent chromium baths.

[0021] Metallic impurities, especially hexavalent chromium, in trivalent chromium baths need to be closely controlled. Furthermore, hexavalent chromium salts are listed under SVHC (Substances of Very High Consent) in the European REACH (Regulation, Evaluation, Authorization and Restriction of Chemicals) regulations and should be banned from use starting in 2024. Hexavalent chromium is typically produced at the anode from trivalent chromium ions in undesirable oxidation reactions.

[0022] The formation of hexavalent chromium must be reduced or even suppressed because its presence has a negative impact on the coating. For example, in a trivalent chromium plating bath, an amount of hexavalent chromium such as 5 mg / L is sufficient to make the deposition completely unstable and is therefore unacceptable.

[0023] In a preferred embodiment, the stabilizer is present in an amount of 0.01 mol / L to 0.5 mol / L, preferably in an amount of 0.02 mol / L to 0.3 mol / L.

[0024] In a preferred embodiment, 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) glycolate, 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 basic forms, preferably selected from the group consisting of: chromium(III) sulfate, chromium(III) acetate, chromium(III) glycolate, chromium(III) formate, chromium(III) formate-chromium(III) sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof in acidic or basic forms.

[0025] In a more preferred embodiment, at least one trivalent chromium salt source is selected from the group consisting of: chromium(III) sulfate in acidic or basic forms, chromium(III) formate sulfate, and mixtures thereof.

[0026] In a preferred embodiment, at least one trivalent chromium ion is present in an amount of 0.25 mol / L to 0.8 mol / L.

[0027] In a preferred embodiment, at least one stabilizer 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.

[0028] In a preferred embodiment, the electroplating bath is substantially free of chloride ions and / or substantially free of aluminum ions.

[0029] In a preferred embodiment, the amount of at least one complexing agent is from 2 mol / L to 10 mol / L, preferably from 3 mol / L to 7 mol / L and / or the molar ratio of the complexing agent to trivalent chromium ions is from 9:1 to 14:1, preferably from 10:1 to 13:1.

[0030] In a preferred embodiment, at least one halide salt is selected from the group consisting of bromide, chloride, iodide, and fluoride salts, preferably bromide, iodide, and fluoride salts, and more preferably potassium bromide, sodium bromide, ammonium bromide, and mixtures thereof.

[0031] In a preferred embodiment, the presence of at least the halide salt is from 0.01 mol / L to 0.5 mol / L.

[0032] In a preferred embodiment, the bath contains at least one additive at a concentration of 1 mg / L to 10 g / L, preferably 5 mg / L to 500 mg / L, wherein the additive is preferably selected from the group consisting of:

[0033] ● Whitening agents, such as polyamines or mixtures of polyamines including quaternary ammonium compounds,

[0034] ● Wetting agents, such as electrically neutral surfactants, cationic surfactants, and amphoteric surfactants, and

[0035] ● Their combination.

[0036] In a preferred embodiment, the anion of the trivalent chromium ion is the anion of a volatile or electrochemically consumable acid.

[0037] 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.

[0038] In a preferred embodiment, the anion of trivalent chromium ions is sulfate anion.

[0039] According to the present invention, a method for depositing chromium or chromium alloy from a trivalent chromium bath onto a substrate is also provided, the method comprising the following steps:

[0040] ● Provides an electroplating bath as described in any of the preceding claims,

[0041] ●Immerse the substrate in the electroplating bath, and

[0042] ● Apply an electric current to deposit it onto the substrate.

[0043] In a preferred embodiment, the electroplating bath is separated from the anode by a membrane, which is preferably an anion or cation exchange membrane or a porous membrane, more preferably a cation exchange membrane, defining the anolyte and the catholyte.

[0044] In a preferred embodiment, electroplating is performed using pulsed current.

[0045] In a preferred embodiment, the anolyte comprises chromium(III) sulfate.

[0046] In a preferred embodiment, the temperature of the bath is 20°C to 70°C, preferably 30°C to 65°C, and more preferably 40°C to 60°C.

[0047] Referring to the following figures and embodiments, the subject matter of the invention is intended to be explained in more detail, but is not intended to limit the subject matter to the specific embodiments shown herein.

[0048] Figure 1 The Hull bath plating, used as a control, is shown, achieved with a bath free of organic acids.

[0049] Figure 2 The Hull bath plating achieved using a bath containing 5 g / L ascorbic acid is shown.

[0050] Figure 3 The Hull bath plating achieved using a bath containing 5 g / L glycolic acid is shown.

[0051] Figure 4 The Hull bath plating achieved using a bath containing 10 g / L glycolic acid is shown.

[0052] Figure 5 The Hull bath plating achieved using a bath containing 5 g / L pyruvate is shown.

[0053] Figure 6 The Hull cell plating achieved using a bath containing 5 g / L tartaric acid is shown.

[0054] Figure 7 The Hull bath plating achieved using a bath containing 5 g / L mercaptoacetic acid is shown.

[0055] Figure 8 A graph showing the plating thickness depends on the current density used is shown. Each line represents a different stabilizer. Example

[0056] High-performance liquid chromatography (HPLC) was achieved on a Shimadzu Nexera XR with an Allsep Anion 7μm 4.6x250mm Alltech 3091917.1 column.

[0057] XRF spectrometer from FISCHER X-RAY The coating thickness was measured by X-ray fluorescence using an internal calibration method on 222.

[0058] Evaluation of the role of glycolic acid as a stabilizer :

[0059] Prepare a bath using the following components:

[0060] -0.38M trivalent chromium ions

[0061] -5.43M formic acid

[0062] -5.3M NH3

[0063] -0.085M KBr

[0064] -1g / l quaternary ammonium compound

[0065] Complexation was performed at 60°C for 2 hours.

[0066] First, the stabilizing effect of different concentrations of glycolic acid on the bath was observed. Baths with different glycolic acid concentrations (0, 0.039M, 0.11M, 0.39M, 0.78M) were prepared. The baths were left at ambient temperature for one month, and their appearance was observed. Only the bath without glycolic acid showed precipitation of trivalent chromium.

[0067] Next, the effect of glycolic acid on the concentration of hexavalent chromium was investigated. For this purpose, a bath with a glycolic acid concentration of 0.11 M was prepared as described above.

[0068] A predetermined concentration of hexavalent chromium in the form of sodium dichromate (Na2Cr2O7·2H2O) was added, and the concentration of hexavalent chromium was monitored by HPLC (using diphenylcarbazide for ultraviolet analysis at 540 nm).

[0069] For all embodiments, a concentration of hexavalent chromium was selected that was higher than the concentration typically found in operating trivalent chromium plating baths (in the range of approximately mg / L). The reduction of hexavalent chromium was measured appropriately with a stabilizer.

[0070] When the value approaches zero, make further additions to check until it is no longer possible to decrease it.

[0071] Compare the reactions at room temperature and at the bath working temperature (55°C - maintained only during the workday).

[0072] Table 1: Cr in the plating bath Ⅵ Concentration monitoring

[0073]

[0074]

[0075] At 55°C, the concentration of hexavalent chromium was observed to decrease with increasing reduction rate. All components of the bath were analyzed, and a decrease in glycolic acid concentration was observed after the introduction of hexavalent chromium into the bath. This decrease in glycolic acid was monitored.

[0076] Table 2: Monitoring of glycolic acid concentration in the plating bath

[0077]

[0078] In the absence of hexavalent chromium in the bath, the concentration of glycolic acid remains unchanged. At higher temperatures, the decrease in glycolic acid concentration becomes more significant.

[0079] During the reduction of hexavalent chromium, glycolic acid is certainly oxidized in glyoxylic acid, but since glycolic acid is unstable at that pH, it is definitely degraded into oxalic acid.

[0080] As expected, oxalic acid was not found in the new bath, but it was 0.65 g / L in a bath with hexavalent chromium added and maintained at 55°C.

[0081] To confirm, a new bath free of glycolic acid was prepared and approximately 4.5 g / L of hexavalent chromium was introduced.

[0082] Chromium content was analyzed after 15 days at 55°C (the temperature maintained during working hours).

[0083] Table 3: Changes in hexavalent chromium concentration in a bath containing no glycolic acid

[0084]

[0085] A decrease in hexavalent chromium concentration was observed, but this was significantly different from previous measurements.

[0086] The bath was then divided in two. One half remained as is, and the other half had an "additional concentration" of glycolic acid added.

[0087] The concentration of hexavalent chromium was measured after leaving the container at room temperature for 21 days.

[0088] Table 4: Comparison of hexavalent chromium concentrations in baths containing and without glycolic acid

[0089] glycolic acid Glycolic acid-free J+21 0.45g / L 2.1g / L

[0090] The role of glycolic acid in the reduction of hexavalent chromium has been clearly demonstrated; even if glycolic acid alone does not have an effect, it certainly acts as a catalyst for the reduction reaction.

[0091] Therefore, if formed, hexavalent chromium is reduced with glycolic acid in a trivalent chromium bath and has higher efficiency at the working bath temperature.

[0092] Testing new molecules as stabilizers:

[0093] To complete the previous experiments, the ability of several molecules with structures similar to glycolic acid to reduce hexavalent chromium in a trivalent chromium bath was evaluated. The selected molecules were acetic acid, propionic acid, malic acid, tartaric acid, lactic acid, glyoxylic acid, pyruvic acid, succinic acid, thioglycolic acid, benzoic acid, and ascorbic acid.

[0094] Similar tests were performed using those stabilizers, and the concentration was measured 3 weeks after the addition of hexavalent chromium to obtain an initial concentration of 4 g / L.

[0095] Table 5: Cr depending on stabilizer VI final concentration

[0096] Molecules being tested <![CDATA[Adding Cr VI Concentrate 3 weeks later]]> glycolic acid 3.5g / L ascorbic acid 0.02g / L Pyruvic acid 3.2g / L glyoxylic acid 1g / L tartaric acid 3.4g / L Thioglycolic acid 3.2g / L

[0097] Interestingly, for the same amount of hexavalent chromium, not all compounds are consumed in the same proportion.

[0098] The effect of this compound on coating was evaluated using a Hull cell (10A-10min), and it can be used to evaluate the effect of the compound on coating. Figures 1 to 7 As observed, no adverse effects on the plating were observed except for thioglycolic acid.

[0099] Except for the uncoated mercaptoacetic acid Figure 8 Minor differences in coating thickness are also shown.

[0100] Therefore, all the tested molecules except for mercaptoacetic acid have the function of stabilizing agents.

Claims

1. An electroplating bath for depositing chromium or chromium alloys from a trivalent chromium bath, said electroplating bath comprising: a) At least one trivalent chromium ion at a concentration of 0.2 mol / L to 1 mol / L, (b) At least one complexing agent at a concentration of 2 mol / L to 10 mol / L, 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, or salts thereof and mixtures thereof. c) At least one halide salt with a concentration ranging from 0.01 mol / L to 0.5 mol / L. d) At least one stabilizer at a concentration of 0.01 mol / L to 1 mol / L, wherein the at least one stabilizer is selected from the group consisting of: glycolic acid, tartaric acid, pyruvic acid and mixtures thereof, or salts thereof and mixtures thereof. e) At least one additive at a concentration of 0 to 10 g / L, wherein the at least one additive is selected from the group consisting of: ●Whitening agents, ●Wetting agents, and ●Their combination, The electroplating bath has a pH of 4 to 7 and is free of divalent sulfur compounds and boric acid, its salts and / or derivatives, and the molar ratio of the complexing agent to the trivalent chromium ions is 8:1 to 15:

1.

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

3. The electroplating bath according to claim 1 or 2, wherein the at least one trivalent chromium ion is a trivalent chromium salt selected from the group consisting of: chromium(III) sulfate, chromium(III) chloride, chromium(III) acetate, chromium(III) glycolate, chromium(III) formate, chromium(III) formate-chromium(III) sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof in acidic or basic forms.

4. The electroplating bath according to claim 1 or 2, wherein the presence of the at least one trivalent chromium ion is from 0.25 mol / L to 0.8 mol / L.

5. The electroplating bath according to claim 1 or 2, wherein the electroplating bath is free of chloride ions and / or free of aluminum ions.

6. The electroplating bath according to claim 1 or 2, wherein the at least one complexing agent is present in an amount of 2 mol / L to 10 mol / L, and / or the molar ratio of the complexing agent to the trivalent chromium ions is 9:1 to 14:

1.

7. The electroplating bath according to claim 1 or 2, wherein the at least one halide salt is selected from the group consisting of bromides, chlorides, iodides, fluorides, and mixtures thereof.

8. The electroplating bath according to claim 1 or 2, wherein: ●The whitening agent is selected from polyamines or mixtures of polyamines including quaternary ammonium compounds, and / or ●The wetting agent is selected from neutral surfactants, cationic surfactants and amphoteric surfactants.

9. The electroplating bath according to claim 1 or 2, wherein the anion of the trivalent chromium ion is an anion of a volatile or electrochemically consuming acid, wherein the volatile or electrochemically consuming acid anion is selected from the group consisting of formate, acetate, propionate, glycolate, oxalate, citrate, or mixtures thereof.

10. The electroplating bath according to claim 1 or 2, wherein the anion of the trivalent chromium ion is a sulfate anion.

11. The electroplating bath according to claim 2, wherein the stabilizer is present in an amount of 0.02 mol / L to 0.3 mol / L.

12. The electroplating bath according to claim 6, wherein the at least one complexing agent is present in an amount of 3 mol / L to 7 mol / L, and / or the molar ratio of the complexing agent to the trivalent chromium ions is 10:1 to 13:

1.

13. The electroplating bath according to claim 7, wherein the at least one halide salt is selected from the group consisting of potassium bromide, sodium bromide, ammonium bromide, and mixtures thereof.

14. The electroplating bath according to claim 3, wherein the at least one trivalent chromium ion is a trivalent chromium salt selected from the group consisting of: chromium(III) sulfate in acidic or basic form, chromium(III) acetate, chromium(III) glycolate, chromium(III) formate, chromium(III) formate-chromium(III) sulfate, chromium(III) hydroxyformate, chromium(III) carbonate, chromium(III) methanesulfonate, potassium chromium(III) sulfate, and mixtures thereof.

15. A method for depositing chromium or a chromium alloy on a substrate from a trivalent chromium bath, the method comprising the steps of: ● Provide an electroplating bath according to any one of the preceding claims, ●Immerse the substrate in the electroplating bath, and ● Apply an electric current to deposit the chromium onto the substrate.

16. The method of claim 15, wherein the electroplating bath is separated from the anode by a membrane, defining the anolyte and the catholyte.

17. The method according to claim 16, wherein the membrane is an anion or cation exchange membrane or a porous membrane.

18. The method of claim 17, wherein the membrane is a cation exchange membrane.

Images