WHITE TRIVALENT CHROMIUM PLATING BATH AND WHITE TRIVALENT CHROMIUM PLATING METHOD FOR OBJECT TO BE PLATED USING SAME

Abstract

 A white trivalent chromium plating bath including a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, a pH buffering agent, and one type or two or more types of sulfur-containing organic compounds represented by the following general formula (I) (in which R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or -(CH₂)_n-COOH, n represents an integer of 1 to 4, and X and Y each independently represent CH₂, C=O, or C=S), and a white trivalent chromium plating method using the white trivalent chromium plating bath enable a high plating deposition rate in the trivalent chromium plating bath, and provide an appearance suitable for decorative applications.

Description

Technical Field

[0001] The present invention relates to a white trivalent chromium plating bath and a white trivalent chromium plating method for an object to be plated using the same.

Background Art

[0002] Chromium plating is used as a coating film for decoration because it has a silver-white appearance. Hexavalent chromium has been used for the chromium plating, but in recent years, since this hexavalent chromium affects the environment, its use has been restricted, and there has been a shift to a technique using trivalent chromium.

[0003] Many techniques using trivalent chromium in this manner have been reported. For example, a chromium electroplating solution containing a water-soluble trivalent chromium salt, a complexing agent for trivalent chromium ions such as malic acid, a pH buffering compound, a sulfur-containing organic compound such as thiourea, and a water-soluble compound such as saccharin and having a pH of 2.8 to 4.2 is known (PTL 1).

[0004] However, this trivalent chromium plating bath is not practical because the plating deposition rate is low.

Citation List

Patent Literature

[0005] 

PTL 1: JP5696134B

PTL 2: WO2019/117178

Summary of Invention

Technical Problem

[0006] The present applicant has developed a practical trivalent chromium plating bath in which the plating deposition rate is high (PTL 2: a trivalent chromium plating bath including a trivalent chromium compound, a complexing agent, potassium sulfate and ammonium sulfate as conductive salts, a pH buffering agent, and a sulfur-containing organic compound, in which a carboxylic acid having two or more hydroxy groups and two or more carboxy groups or a salt thereof is used as the complexing agent, and saccharin or a salt thereof and a sulfur-containing organic compound having an allyl group are used in combination as the sulfur-containing organic compounds), and an object of the present invention is to develop a plating technique, in which the composition is simpler than the trivalent chromium plating bath, and with which an appearance also suitable for decorative applications is obtained.

Solution to Problem

[0007] The present inventors conducted intensive studies to achieve the above-mentioned object, and as a result, they found that the above-mentioned object can be achieved by incorporating a sulfur-containing organic compound having a specific structure in a white trivalent chromium plating bath, and thus completed the invention.

[0008] That is, the invention is directed to a white trivalent chromium plating bath including a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, a pH buffering agent, and one type or two or more types of sulfur-containing organic compounds represented by the following general formula (I):



(in which R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or - (CH₂)_n-COOH, n represents an integer of 1 to 4, and X and Y each independently represent CH₂, C=O, or C=S).

[0009] In addition, the invention is directed to a white trivalent chromium plating method for an object to be plated, the method including electroplating the object to be plated in the white trivalent chromium plating bath.

[0010] Further, the invention is directed to a method for improving a deposition rate of white trivalent chromium plating, the method including incorporating a sulfur-containing organic compound represented by the general formula (I) in a white trivalent chromium plating bath containing a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, and a pH buffering agent.

Advantageous Effects of Invention

[0011] Use of the white trivalent chromium plating bath of the invention enables white trivalent chromium plating with a color tone and a deposition rate equivalent to those of hexavalent chromium plating.

Description of Embodiments

[0012] The white trivalent chromium plating bath of the invention (hereinafter referred to as "the plating bath of the invention") contains a trivalent chromium compound, a complexing agent, a sulfate as a conductive salt, a pH buffering agent, and a sulfur-containing organic compound.

[0013] The sulfur-containing organic compound is represented by the following general formula (I):



(in which R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or-(CH₂)_n-COOH, n represents an integer of 1 to 4, and X and Y each independently represent CH₂, C=O, or C=S).

[0014] Preferred examples of the sulfur-containing organic compound represented by the general formula (I) include a compound represented by the following general formula (II):

(in which R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or-(CH₂)_n-COOH, more preferably -H or -NH₂, n represents an integer of 1 to 4, more preferably an integer of 1 or 2, and X₁ and Y₁ each independently represent C=O or C=S).

[0015] More preferred examples of the sulfur-containing organic compound represented by the general formula (I) include thiazolidine, rhodanine, thiazolidin-2-one, 3-methylthiazolidine-2-thione, 4-thioxo-1,3-thiazolidin-2-one, 2-mercaptothiazoline (also known as 2-thiazoline-2-thiol), 2,4-thiazolidinedione, 3-methyl-1,3-thiazolane-2,4-dione, 3-aminorhodanine, 3-methylrhodanine, 3-ethylrhodanine, 3-allylrhodanine, rhodanine-3-acetic acid, and rhodanine-3-propionic acid, and among these, rhodanine, 2,4-thiazolidinedione, 3-aminorhodanine, and 3-ethylrhodanine are preferred. Among these sulfur-containing organic compounds, one type can be used, or two or more types can be used in combination.

[0016] The content of the sulfur-containing organic compound represented by the general formula (I) or the like in the plating bath of the invention is not particularly limited, but is, for example, 1 to 500 mg/L, and preferably 5 to 100 mg/L.

[0017]  The trivalent chromium compound used in the plating bath of the invention is not particularly limited, but is, for example, basic chromium sulfate, chromium sulfate, chromium chloride, chromium sulfamate, or chromium acetate, and is preferably basic chromium sulfate or chromium sulfate. Among these trivalent chromium compounds, one type can be used, or two or more types can be used in combination. The content of the trivalent chromium compound in the plating bath of the invention is not particularly limited, but is, for example, 1 to 25 g/L, and preferably 1 to 15 g/L in terms of metallic chromium.

[0018] The complexing agent used in the plating bath of the invention is not particularly limited, and examples thereof include dicarboxylic acids such as malic acid and tartaric acid, and salts of dicarboxylic acids such as sodium malate, sodium tartrate, and diammonium tartrate, and malic acid or diammonium tartrate is preferred. Among these complexing agents, one type can be used, or two or more types can be used in combination. The content of the complexing agent in the plating bath of the invention is not particularly limited, but is, for example, 1 to 90 g/L, and preferably 2 to 50 g/L.

[0019] The conductive salt used in the plating bath of the invention is a sulfate such as potassium sulfate, ammonium sulfate, or sodium sulfate. Among these sulfates, potassium sulfate is preferred. Among these sulfates, one type can be used, or two or more types can be used in combination. The content of the sulfate in the plating bath of the invention is not particularly limited, but is, for example, 100 to 300 g/L, and preferably 120 to 240 g/L.

[0020] The pH buffering agent used in the plating bath of the invention is not particularly limited, and examples thereof include boric acid, sodium borate, potassium borate, phosphoric acid, and dipotassium hydrogen phosphate, and among these, boric acid and sodium borate are preferred. Among these pH buffering agents, one type can be used, or two or more types can be used in combination. The content of the pH buffering agent in the plating bath of the invention is not particularly limited, but is, for example, 30 to 150 g/L, and preferably 50 to 110 g/L.

[0021] It is preferred to further incorporate saccharin or a salt thereof in the plating bath of the invention in order to stabilize the deposition of chromium. Examples of the saccharin or a salt thereof include saccharin and sodium saccharinate. As such saccharin or a salt thereof, one type can be used, or two or more types can be used in combination. The content of saccharin or a salt thereof in the plating bath of the invention is not particularly limited, but is, for example, 0.5 to 10 g/L, and preferably 2 to 8 g/L. Naturally, the content of saccharin in the plating bath of the invention is not included in the content of the sulfur-containing organic compound represented by the general formula (I) or the like.

[0022] The plating bath of the invention in a preferred aspect may be, for example, a plating bath containing the above-mentioned components, but it goes without saying that it may be a plating bath containing only the above-mentioned components (a plating bath composed of the above-mentioned components).

[0023] The plating bath of the invention may further contain a polymer compound such as polyethylene glycol, ascorbic acid, sodium ascorbate, hydrogen peroxide, a surfactant, chloral hydrate, iminodiacetic acid, or the like as long as the effects of the invention are not impaired.

[0024] The pH of the plating bath of the invention is not particularly limited as long as it is acidic, and is preferably, for example, 2 to 4.5, and more preferably 3 to 4.5.

[0025] The method for preparing the plating bath of the invention is not particularly limited, and the plating bath can be prepared, for example, by adding and mixing a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, a pH buffering agent, and a sulfur-containing organic compound (saccharin or a salt thereof if necessary) in water at 50 to 60°C, and finally adjusting the pH with sulfuric acid, aqueous ammonia, or the like if necessary.

[0026] The plating bath of the invention enables white trivalent chromium plating on an object to be plated by electroplating the object to be plated in the plating bath of the invention in the same manner as in a conventional chromium plating bath.

[0027] Electroplating conditions are not particularly limited, but for example, electroplating may be performed at a bath temperature of 30 to 60°C using carbon or iridium oxide as an anode at a cathode current density of 2 to 20 A/dm² for 1 to 15 minutes.

[0028] Examples of the object to be plated that can be electroplated include metals such as iron, stainless steel, and brass, and resins such as ABS and PC/ABS. The object to be plated may be subjected to a treatment such as copper plating or nickel plating in advance before being plated in the plating bath of the invention.

[0029] When white trivalent chromium plating is performed using the plating bath of the invention in this manner, the deposition rate is 0.03 to 0.17 um/min, preferably 0.04 to 0.13 um/min, which is equal to or higher than the deposition rate when conventional hexavalent chromium plating is performed.

[0030] Therefore, the deposition rate of white trivalent chromium plating can be improved by incorporating the sulfur-containing organic compound represented by the general formula (I) or the like in a white trivalent chromium plating bath containing a trivalent chromium compound, a complexing agent, a sulfate as a conductive salt, and a pH buffering agent.

[0031] A chromium-plated product thus obtained is white, and has an L* value of 80 or more, preferably 82 or more, more preferably 83 or more, and particularly preferably 83 to 84, as measured using a color difference meter, and the color tone is equivalent to that of hexavalent chromium plating.

[0032] The white chromium-plated product can be used for the same applications as conventional hexavalent chromium-plated products, but is particularly suitable for products for decorative applications such as cars, motorcycles, and metal fittings.

Examples

[0033] Hereinafter, the invention will be described in detail with reference to Examples, but the invention is by no means limited to these Examples.

Example 1

White Chromium Plating:

[0034] A trivalent chromium plating bath was prepared by dissolving the components shown in Table 1 in water and adjusting the pH with aqueous ammonia. A Hull cell test was performed on the trivalent chromium plating bath using a nickel-plated brass plate. The Hull cell test conditions are as follows: current: 5 A; and plating time: 3 minutes. After plating, the film thickness of the brass plate was measured with X-ray fluorescence at a point where the current density is 10 A/dm², and the deposition rate was calculated. Further, the appearance after plating was evaluated based on an L* value, an a* value, and a b* value obtained using a color difference meter (manufactured by Konica Minolta, Inc.). These results are also shown in Table 1.

[Table 1]

	Comparative Product 1	Example Product 1	Example Product 2	Example Product 3	Comparative Product 2	Example Product 4	Example Product 5	Example Product 6	Comparative Product 3	Example Product 7	Comparative Product 4	Example Product 8
Basic chromium sulfate (g/L)	19.5	19.5	19.5	19.5	19.5	19.5	19.5	19.5	64	64	64	64
Chromium concentration (g/L)	3	3	3	3	3	3	3	3	10	10	10	10
Malic acid (g/L)					3	3	3	3
Diammonium tartrate (g/L)	4	4	4	4					30	30	30	30
Potassium sulfate (g/L)	150	150	150	150	150	150	150	150	150	150	150	150
Boric acid (g/L)	70	70	70	70	70	70	70	70	70	70	70	70
Saccharin (g/L)	3	3	3	3	3	3	3	3	3	3
Thiazolidine (mg/L)						10
Rhodanine (mg/L)		20										30
2,4-Thiazolidinedione (mg/L)			20
3-Aminorhodanine (mg/L)				10			20			20
Thiazolidin-2-one (mg/L)								10
pH	3.7	3.7	3.7	3.7	3.7	3.7	3.7	3.7	3.7	3.7	3.7	3.7
Bath temperature (°C)	55	55	55	55	55	55	55	55	55	55	55	55
Color tone L*	83.45	83.45	83.24	83.01	82.9	80.24	83.17	82.14	82.97	83.47	80.57	83.62
Color tone a*	-0.7	-0.66	-0.68	-0.81	-0.81	-0.65	-0.61	-0.66	-0.82	-0.58	-0.52	-0.5
Color tone b*	-0.13	-0.74	-0.55	-0.68	0.1	0.53	-0.58	-0.32	0.05	-0.68	0.8	-1.12
Film thickness (µm)	0.061	0.131	0.15	0.27	0.093	0.158	0.332	0.169	0.104	0.234	0.04	0.178
Deposition rate (µm/min)	0.020	0.044	0.050	0.090	0.031	0.053	0.110	0.056	0.035	0.078	0.013	0.059

[0035] From these results, it was found that use of the compound represented by the general formula (I) as the sulfur-containing organic compound in the plating bath of the invention enables a high plating deposition rate, a white appearance, and an L* value of 80 or more. In particular, it was found that use of the compound represented by the general formula (II) as the sulfur-containing organic compound enables a high plating deposition rate, a white appearance, and an L* value of 83 or more. It was also found that the deposition of chromium is stabilized by further incorporating saccharin or a salt thereof in the plating bath of the invention.

Industrial Applicability

[0036] The plating bath of the invention can be used for various applications in the same manner as white plating using hexavalent chromium.

Claims

1. A white trivalent chromium plating bath comprising a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, a pH buffering agent, and one type or two or more types of sulfur-containing organic compounds represented by the following general formula (I) :

(wherein R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or-(CH₂)_n-COOH, n represents an integer of 1 to 4, and X and Y each independently represent CH₂, C=O, or C=S).

2. A white trivalent chromium plating bath comprising a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, a pH buffering agent, and one type or two or more types of sulfur-containing organic compounds represented by the following general formula (II) :

(wherein R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or-(CH₂)_n-COOH, n represents an integer of 1 to 4, and X₁ and Y₁ each independently represent C=O or C=S).

3. The white trivalent chromium plating bath according to claim 1 or 2, wherein the complexing agent is a dicarboxylic acid or a salt thereof.

4. The white trivalent chromium plating bath according to claim 3, wherein the dicarboxylic acid is malic acid and/or tartaric acid.

5. The white trivalent chromium plating bath according to any one of claims 1 to 4, further comprising saccharin or a salt thereof.

6. A white trivalent chromium plating method for an object to be plated, the method comprising electroplating the object to be plated in the white trivalent chromium plating bath according to any one of claims 1 to 5.

7. A method for improving a deposition rate of white trivalent chromium plating, the method comprising incorporating a sulfur-containing organic compound represented by the following general formula (I):

(wherein R represents -H, -NH₂, -OH, -CH₃, -(CH₂)_n-CH₃, or-(CH₂)_n-COOH, n represents an integer of 1 to 4, and X and Y each independently represent CH₂, C=O, or C=S) in a white trivalent chromium plating bath containing a trivalent chromium compound, a complexing agent, a sulfate serving as a conductive salt, and a pH buffering agent.