JP7740879B2 - Electrolytic silver plating bath and electrolytic silver plating method using the same - Google Patents

Description

The present invention relates to an electrolytic silver plating bath and an electrolytic silver plating method using the same.

Silver plating has long been used for decorative purposes. Until now, silver plating was deposited using an alkaline electrolytic silver plating bath containing cyanide.

However, cyanide is highly toxic and requires wastewater treatment, so so-called cyanide-free electrolytic silver plating baths that do not use cyanide have been developed (Patent Documents 1 and 2).

Cyanide-free electrolytic silver plating baths generally contain additives such as brighteners, but if an insoluble anode is used for continuous plating in the same plating bath, the additives such as brighteners are rapidly consumed, and the problem is that bright silver plating quickly becomes unobtainable.

When using an insoluble anode, a method of preventing oxidation of the additive using a diaphragm or the like is usually taken (Patent Document 3).

However, a diaphragm surrounding the anode is not suitable for plating using high current densities, such as sparger plating and hoop plating, or when the plating solution passes through an insoluble anode or when high-speed anode shear liquid flow is required.

Japanese Patent Application Laid-Open No. 2000-192279 Japanese Patent Application Laid-Open No. 2002-121693 JP 2009-149965 A

Therefore, the objective of the present invention is to provide an electrolytic silver plating bath that suppresses the consumption of additives such as brighteners and prevents deterioration in the performance of the plating solution even when plating is performed continuously using the same plating bath.

As a result of extensive research into resolving the above-mentioned issues, the inventors discovered that the above issues could be resolved by combining an electrolytic silver plating bath containing additives such as brighteners with a specific sacrificial reducing agent, leading to the completion of the present invention.

1. An electrolytic silver plating bath containing silver ions, a complexing agent, a conductive salt, a brightener, and a sacrificial reducing agent,
As a sacrificial reducing agent,
[In the formula, _X1 represents hydrogen or an alkali metal.]
and general formula (II)
[In the formula, R ₁ , R ₂ , and R ₃ each independently represent hydrogen (excluding cases where R ₁ , R ₂ , and R ₃ are simultaneously hydrogen), a hydroxyl group, a phenyl group, a C ₁ to C ₆ alkyl group, an alkenyl group, or an alkynyl group, and the alkyl group, alkenyl group, or alkynyl group may have a hydroxyl group or a carboxyl group, and R ₁ and R ₂ may be linked to form a cyclic alkyl or aromatic ring.]
The electrolytic silver plating bath is characterized by containing one or more nitrogen-containing compounds represented by the following formula:

The present invention also provides an electrolytic silver plating method, characterized by immersing an object to be plated in the above-described silver plating bath to perform electrolytic plating.

The electrolytic silver plating bath of the present invention contains a specific sacrificial reducing agent that is preferentially oxidized relative to the oxidizing species generated from the anode, thereby suppressing the consumption of brighteners and preventing deterioration in the performance of the plating solution even during continuous plating.

FIG. 2 is a diagram showing the appearance of the working product and the comparative product in Example 1 after a Hull Cell test using an electrolytic silver plating bath. FIG. 2 is a diagram showing the state of the electrolytic silver plating bath before and after temperature rise in a Hull Cell test of a comparative product (fructose) in Example 1.

The electrolytic silver plating bath of the present invention (hereinafter referred to as the "plating bath of the present invention") is an electrolytic silver plating bath containing silver ions, a complexing agent, a conductive salt, a brightener, and a sacrificial reducing agent,
The sacrificial reducing agent contains one or more nitrogen-containing compounds represented by the following general formula (I) and general formula (II).

The silver ions used in the plating bath of the present invention are not particularly limited, as long as they are generated from a silver ion source such as silver nitrate, silver sulfate, silver acetate, silver lactate, silver citrate, silver oxide, silver phosphate, silver alkanesulfonate, or silver halide. These silver ion sources may be used alone or in combination. The silver ion content in the plating bath of the present invention is not particularly limited, but is, for example, 1 g/L to 80 g/L, preferably 20 g/L to 60 g/L.

The complexing agent used in the plating bath of the present invention is not particularly limited, and examples include imide compounds represented by the following general formula (III):

[In the formula, B represents nitrogen or carbon, and R ₄ and R ₅ each independently represent hydrogen or a methyl group.] Among these complexing agents, succinimide, hydantoin, and hydantoin derivatives such as 5,5-dimethylhydantoin are preferred, with 5,5-dimethylhydantoin being more preferred. These complexing agents may be used alone or in combination of two or more. The content of the complexing agent in the plating bath of the present invention is not particularly limited, but is, for example, 10 g/L to 400 g/L, preferably 100 g/L to 360 g/L, and particularly preferably 120 g/L to 320 g/L.

The conductive salts used in the plating bath of the present invention are metal salts of inorganic and organic acids, and are not particularly limited. Examples include metal salts of sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, sulfamic acid, carbonic acid, boric acid, acetic acid, lactic acid, methanesulfonic acid, and citric acid. Among these conductive salts, sodium and potassium salts of carbonate, nitric acid, and methanesulfonic acid are preferred, with potassium carbonate being more preferred. These conductive salts may be used alone or in combination of two or more. The content of the conductive salt in the plating bath of the present invention is not particularly limited, but is, for example, 1 g/L to 80 g/L, and preferably 10 g/L to 60 g/L.

The brightener used in the plating bath of the present invention is not particularly limited, and examples include thiosulfuric acid, 2-mercaptoethanesulfonic acid, 3-mercaptopropanesulfonic acid, thiodiglycolic acid, dithiodiglycolic acid, thiourea, or metal salts thereof. Among these brighteners, thiosulfuric acid or metal salts of thiosulfuric acid, or thiodiglycolic acid or metal salts thereof are preferred, with thiosulfuric acid or metal salts of thiosulfuric acid being particularly preferred. These brighteners may be used alone or in combination of two or more. The content of the brightener in the plating bath of the present invention is not particularly limited, and is, for example, 1 mg/L to 1000 mg/L, preferably 10 mg/L to 600 mg/L.

The sacrificial reducing agent used in the plating bath of the present invention is a nitrogen-containing compound represented by the following general formulas (I) and (II).

In the formula, _X1 represents hydrogen or an alkali metal. Among alkali metals, lithium, sodium, or potassium is preferred, and sodium or potassium is more preferred. The compound represented by general formula (I) may be in the form of an acid or a metal salt. Among the compounds represented by this formula, compounds containing trivalent nitrogen that are oxidized to pentavalent nitrogen during plating are preferred. Examples of such compounds include nitrous acid, lithium nitrite, sodium nitrite, and potassium nitrite.

In the formula, _R1 , _R2 , and _R3 each independently represent hydrogen (excluding cases where _R1 , _R2 , and _R3 are simultaneously hydrogen (ammonia)), hydroxyl, phenyl, _C1 - _C6 alkyl, alkenyl, or alkynyl groups, and the alkyl, alkenyl, and alkynyl groups may contain hydroxyl or carboxyl groups. _R1 and _R2 may be linked to form a cyclic alkyl or aromatic ring. The number of carbon atoms in the alkyl group or the like shown here is preferably _C1 - _C3 , and more preferably _C2 . Among the compounds represented by this formula, compounds containing trivalent nitrogen and that are oxidized to pentavalent nitrogen during plating are preferred. Examples of such compounds include glycine, iminodiacetic acid, nitrilotriacetic acid, ethanolamine, 2,2'-iminodiethanol, triethanolamine, and aniline.

Of the nitrogen-containing compounds listed above, sodium nitrite, glycine, iminodiacetic acid, nitrilotriacetic acid, ethanolamine, 2,2'-iminodiethanol, and triethanolamine are preferred, and from the standpoint of plating brightness and stability, sodium nitrite, glycine, iminodiacetic acid, and nitrilotriacetic acid are more preferred, with sodium nitrite being particularly preferred.

The plating bath of the present invention can contain one or more of the above-mentioned nitrogen-containing compounds. The content of the sacrificial reducing agent in the plating bath of the present invention is not particularly limited, but is, for example, 1 g/L to 100 g/L, preferably 5 g/L to 60 g/L.

The plating bath of the present invention may further contain a known leveling agent and/or pH adjuster used in electrolytic silver plating.

Examples of leveling agents include polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and polyethyleneimine. The content of the leveling agent in the plating bath of the present invention is not particularly limited, but is, for example, 0.1 mg/L to 1000 mg/L, preferably 10 mg/L to 600 mg/L.

Examples of pH adjusters include sulfuric acid, nitric acid, methanesulfonic acid, sodium hydroxide, and potassium hydroxide. The amount of pH adjuster contained in the plating bath of the present invention is not particularly limited, but it is typically adjusted to a pH of 6.0 to 12.5, preferably 9.0 to 11.5.

The plating bath of the present invention may further contain known crystal refiners, anti-tarnish agents, and the like used in electrolytic silver plating. Examples of crystal refiners include selenium and antimony compounds such as selenious acid and potassium antimony tartrate, as well as sulfur-containing compounds such as thiourea and potassium thiocyanate. Examples of anti-tarnish agents include nitrogen- or sulfur-containing compounds with heterocyclic rings such as benzotriazole, 2-mercaptobenzothiazole, and 1,2,3-benzothiadiazole.

The plating bath of the present invention described above can be prepared by mixing the components according to conventional methods. Since the plating bath of the present invention can be prepared using only the components listed above, it can be a so-called cyanide-free electrolytic silver plating bath that does not contain cyanide compounds. However, it goes without saying that the same effect can be achieved by adding cyanide compounds that have been used in conventional silver plating baths.

The plating bath of the present invention, like conventional electrolytic silver plating baths, can be used to electrolytically plate an object by immersing the object in the bath and electroplating it.

There are no particular limitations on the objects that can be plated using the plating bath of the present invention, but examples include metals and objects made of copper, nickel, iron, tin, zinc, or alloys of these. There are also no particular limitations on the shape of the objects to be plated.

The conditions for electrolytic silver plating are not particularly limited, but examples include a bath temperature of 20 to 70°C, a current density of 0.5 to 70 A/ ^dm2 , and an electrolysis time of 3 seconds to 1 hour.

Electrolytic silver plating using the plating solution of the present invention can be performed using conventional electrolytic plating equipment, but equipment using an insoluble anode is particularly preferred. Examples of such equipment include hoop plating equipment and sparger plating equipment.

In addition, the electrolytic silver plating bath of the present invention suppresses consumption of brighteners by preferentially oxidizing the specific sacrificial reducing agent contained therein over the oxidizing species generated from the anode, thereby preventing deterioration in the performance of the plating solution even during continuous plating.

The electrolytic silver plating obtained using the plating solution of the present invention as described above is a film with excellent electrical properties, such as low contact resistance, and excellent hardness. Therefore, this electrolytic silver plating is preferable for applications such as connectors.

The present invention will be explained in detail below using examples, but the present invention is not limited to these examples in any way.

Example 1
Electrolytic silver plating:
Electrolytic silver plating baths were prepared by mixing the basic compositions of the electrolytic silver plating baths shown in Table 1 below with one of the sacrificial reducing agents. A Hull Cell test was conducted using these electrolytic silver plating baths. The Hull Cell test was conducted using a copper Hull Cell cathode and an iridium oxide-coated titanium anode, with a current of 2 A, a plating time of 2.5 minutes, air agitation, and a bath temperature of 50°C. This test was repeated up to five times. The silver ion source, which was lost during plating, was replenished after each plating run. The appearance after the Hull Cell test is shown in Figure 1.

*1 40g/L as silver
*2 An appropriate amount of potassium hydroxide was added to dissolve each component and adjust the pH.

As shown in the Hull Cell test in Figure 1, when no sacrificial reducing agent is included, a clear semi-bright white appearance is obtained for the first two consecutive plating passes, but by the third consecutive pass, a cloudiness characteristic of low current density begins to appear, and by the fifth pass, a slightly yellowish silver plating is obtained, confirming a decrease in the brightener components. When testing is performed with a plating solution containing the sacrificial reducing agent shown in Table 1, the same appearance as the first plating pass is obtained even after five consecutive plating passes, demonstrating that the brightener components are not impaired.

It was also found that the appearance of sodium hypophosphite and formaldehyde changed with repeated Hull Cell tests, and the desired effect was not achieved. On the other hand, while fructose showed a certain degree of effectiveness in the Hull Cell test, as shown in Figure 2, the plating solution decomposed during the temperature rise stage before the Hull Cell test, causing the decomposition to float in the bath, making it cloudy, and to precipitate in the plating tank after plating. Using such an inappropriate substance with strong reducing power as a sacrificial reducing agent would be unsuitable for industrial use.

In addition to the sacrificial reducing agents mentioned above, other reducing agents that have shown decomposition include hydroquinone, L(+)-sodium ascorbate, benzaldehyde, and sugars such as D(+)-glucose and maltose. Sacrificial reducing agents that have not shown the desired effect include phosphorous acid, sodium formate, and methanol.

These results demonstrate that the use of a sacrificial reducing agent in an electrolytic silver plating bath containing silver ions, complexing agents, conductive salts, and brighteners can suppress the decomposition of brightener components. Furthermore, in terms of plating brightness, accumulation due to replenishment, and stability, it was found that sodium nitrite, iminodiacetic acid, and nitrilotriacetic acid are superior sacrificial reducing agents, with sodium nitrite being particularly superior.

Example 2
Sparger electrolytic silver plating:
Sparger electrolytic silver plating was performed using a known sparger plating apparatus and the electrolytic silver plating bath of Example 1, which used sodium nitrite as a sacrificial reducing agent. Sparger plating was performed using a copper plate as a test piece, with the plating solution being sprayed from a nozzle opening of φ4 mm at a flow rate of 4 L/min against a mask opening of φ5.5 mm, under conditions of 10 ASD to 30 ASD and 27 seconds to 9 seconds. The obtained silver plating film had an appearance equivalent to that of a high current density area in a Hull cell test, without oxidation of the brightener components.

This invention can be used for silver plating.

Claims (7)

1. An electrolytic silver plating bath containing silver ions, a complexing agent, a conductive salt, a brightener, and a sacrificial reducing agent,
The sacrificial reducing agent is selected from sodium nitrite, iminodiacetic acid, and nitrilotriacetic acid.
and one or more nitrogen- containing compounds ,
The brightener is thiosulfuric acid or a metal salt of thiosulfuric acid, or thiodiglycolic acid or
is a metal salt,
The complexing agent is represented by the following general formula (III):
[wherein B is nitrogen or carbon, and R ₄ and R ₅ each independently represent hydrogen or a methyl group]
represents.]
is an imide compound represented by the formula:
Electrolytic silver plating bath characterized by: The electrolytic silver plating bath according to claim 1, wherein the conductive salt is a metal salt of an inorganic acid or an organic acid. The imide compound represented by the general formula (III) is a hydantoin and a 5,5-dimethyl hydantoin.
1 or 2 of the hydantoin derivatives selected from the group consisting of hydantoin and hydantoin.
3. The electrolytic silver plating bath according to claim 2 . The electrolytic silver plating bath according to any one of claims 1 to 3, further comprising a leveling agent and/or a pH adjuster. 5. The electrolytic silver plating bath according to claim 1, which is used for silver plating using an insoluble anode. An electrolytic silver plating method comprising immersing an object to be plated in the silver plating bath according to any one of claims 1 to 5 to perform electrolytic plating. 7. The method for electrolytic silver plating according to claim 6 , wherein an insoluble anode is used as the anode for electrolytic plating.