JPH0361393A - Silver plating method - Google Patents

Abstract

PURPOSE:To form a lustrous silver plating film by adding at least an N-contg. carboxylic acid deriv., aldehyde and a compd. contg. C=S as additives to a silver plating bath contg. an acidic silver complex salt as an essential component. CONSTITUTION:Silver nitrate is dissolved in water and the resulting soln. is mixed with an aq. soln. of sodium bisulfite and further mixed with an aq. soln. of sodium thiosulfate to prepare a silver plating bath contg. silver thiosulfate complex anions. Sodium sulfate as an electrically conductive agent, boric acid as a stabilizer and sodium nicotinate as a buffer are added to the silver plating bath. When the silver plating bath is used, a lustrous silver plating film is obtd. without using cyanide.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a silver plating method using an acidic silver complex salt as a main component.

(Prior Art) Conventionally, a cyanide bath has often been used when performing silver plating. In recent years, regulations regarding cyanide wastewater have become stricter, and acid silver plating is also being used to deal with this.

(Problems to be Solved by the Invention) However, acidic silver plating has various disadvantages when industrialized compared to cyanide baths. The biggest problem with this is the lack of gloss. In particular, mechanical polishing after silver plating is not only uneconomical to scrape off a thick silver film, but also requires considerable skill because the silver film is soft.

The brighteners used in cyanide baths often contain sulfides, such as carbon disulfide. Added to this are funnel oil and formalin. However, it has not yet achieved a definitive specular luster.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a silver plating method using an acidic silver complex salt as a main component, which contains at least a nitrogen-containing carboxylic acid derivative and an aldehyde C=S-containing compound as an additive. It is a method of using things.

The structure of the bath of the present invention will be explained. The silver salt used in the method of the present invention may be silver nitrate, silver chloride, sulfate, etc., but it is advantageous to use silver with a high content of gold dust, and it is preferable to use one that is soluble in an organic or inorganic aqueous solution. It needs to be easy to do.

In addition to the above-mentioned nitrogen-containing carboxylic acid derivatives and aldehyde C=S-containing compounds, additives include complexing agents, conductive salts, stabilizers, buffers, and the like.

Complexing agents include sodium thiosulfate, sodium pyrosulfite,
Sodium hydrogen sulfite is used and reacts with silver salts to form complex salts. This is possible for nitrate roots, chloride roots, sulfate roots, etc.

Sodium sulfate, ammonium sulfate, soda carbonate, etc. are used as the conductive agent, boric acid is used as the stabilizer, and sodium nicotinate, malic acid, tartaric acid, Rossel's salt, etc. are used as the buffering agent. In the present invention, a bath is constituted by each of the above chemicals.

(Function) The bath preparation used in the plating method of the present invention and the interaction effect will be explained. In the method of the present invention, acidic complex ions are used, unlike conventional cyanides. Sodium thiosulfate or potassium must be used to create this complex ion. It has long been believed that potash is better for silver plating than soda, but the present invention is not inspired by this.

Sodium thiosulfate (NaaSzOa) decomposes in an acidic aqueous solution as sulfur is precipitated as shown in the following formula.

S20□'-+H4HSO3-+S...(1
) To suppress this decomposition as much as possible, use sodium pyrosulfite (N
aaSzOa) or sodium bisulfite (Na
It is desirable to add H3Os). These salts react in the following manner in an aqueous solution.

520s"-+HpO2H8Oa--(2) N a H
S Ox Na4+HS Ox--(3)(2),
By the reaction of formula (3), the decomposition of S2's2- as shown in formula (+) can be suppressed by the common ion of 2H8O3. Therefore, in order to prepare a bath, the following dissolution step is required to avoid this decomposition, and the general dissolution method does not result in bath preparation due to precipitation and decomposition.

First, silver nitrate is dissolved in water alone. Next, a single aqueous solution of sodium bisulfite is mixed, and then a single aqueous solution of sodium thiosulfate is mixed. It is necessary to go through this mixing order. If a single aqueous solution of sodium thiosulfate is mixed first, a black precipitate of A g 2 S will be formed as shown in the following formula.

2 A g N O * + N a 2 S 20 x
=AgzS20t+NaN0z −・
=(4n (whiteness R) ↓ AgzS20t+HtO→A g 2 S + H=S
Oa (black precipitate) (5) Precipitation of sulfide m (Ag=S) as shown in equation (5) means that silver ions are reduced in the atmosphere.

This is industrially uneconomical. Therefore, in order to suppress black precipitate in the prepared bath, the various components of the prepared bath described above are properly dissolved. Thereby, the silver as a bath forms a silver thiosulfate complex anion. From this, CA g (S 20 *
) )- and (Ag (S20s))"- are estimated to coexist in the bath. The chemical reaction is expressed by the following equation, (B)
, (7) and (8). Or, similarly for sodium pyrosulfite, as shown in the following formula, 2 A g N O3+
N at S 20 s= A g 2 S 20
s + N a N Os + A g 2 S 2
0 s (white) +2NatSzO* “”’ (G) A g
2820S+N aas20嘗2Na (Ag (S20a)) 10Nats*Os -・・・-+(7)
A g 2 S 20 s + 4 N a a S
2032Na (Ag (32gs)2)”+Naa
S*Os''...(8) The instability constant of the produced silver thiosulfate complex anion is shown below.

CAg (320m) -K K = 1. 5X
10-”(Ag (S20s) 2) ”-K i
=3.5X 10-14 Therefore (Ag (-3ag
s)2] ”− coexists in large numbers, CAg (S203))
- is rare. This is considered to be due to the instability constant mentioned above.

The above is about the case where silver nitrate is used. The same applies to silver chloride (AgC1) and silver nitrate (AgN Oa ). However, the difference is that silver chloride has a high solubility in sodium thiosulfate. The formation of silver thiosulfate complex ions during the reaction process is the same as with silver nitrate, and the reaction formula is as follows.

AgCl+2Naas20a Nag (Ag (S2sh) 2) +NaC1...
(9) (Example) Tables 1 and 2 below show examples of bath compositions used in the silver plating method of the present invention.

This compositional formulation is capable of depositing silver from acidic to neutral conditions, producing a film equivalent to silver plating from a cyanide bath.

Table 1. Silver plating bath composition (silver nitrate plating bath) (Note 1) The additive is a 50% epoxy polymerized water-soluble product of sodium nicotinate and/or potassium nicotinate.

Table 2. Silver plating bath composition (silver chloride plating bath) (Note 1) The additive is a 50% polymerized water-soluble epoxy of sodium nicotinate and/or potassium nicotinate.

In order to make silver shiny, mirror luster can only be obtained by depositing smooth metal polycrystals in which fine crystal grains are preferentially oriented in all plating films.

In addition to gloss, the following conditions are also taken into consideration.

That is, smoothing, crystal refinement, gloss, improvement of uniform electrodeposition, reduction of residual stress, prevention of bits, prevention of anode passivation, etc. are taken into consideration.

The additives used in the method of the present invention include all surfactants, brighteners, and leveling agents, including nitrogen-containing carboxylic acid derivatives, aldehydes, C=S-containing compounds, or tautomers thereof. This will help you understand the interaction of the bath preparation formulations and the roles played by additives as described above.

Plating conditions are important and have a large impact on productivity and bath management. The requirements for the bath are that it be stable during operation and have a long life. It changes rapidly and if it decomposes, a uniform product cannot be obtained.

The conditions for the plating method of the present invention include: ■ion concentration, ■current density, ■pH, ■temperature, ■stirring, ■time, and ■additives (
(including brighteners).

■Ion concentration: The silver ion concentration required in combination with each formulation must be appropriate. There should be no precipitation or release due to excessive addition.

An example of this is the silver complex Nag (Ag (Sl!O))
) is easily dissolved in water, but N a (A g (
S2(h)) is difficult to dissolve in water (precipitates). However, it dissolves well in excess of sodium thiosulfate (NaaS20i).

2 moles of thiosulfate are required for 1 mole of silver in the silver salt. This is a factor that influences the stability of ion concentration.

■Current density: It is desirable to plate with a high current density. However, since silver is a highly conductive metal, there is a decrease in current density, current efficiency, and polarization effect due to hydrogen generation.

(2) pH: It is related to the precipitation rate, and when it is high, it becomes difficult to dissolve the electrode plate. When it is low, a lot of hydrogen is generated and the precipitation rate is low.

■Temperature: It is preferable to keep the temperature as low as possible.

Difficult to manage at high temperatures. In this study, 2
The condition is 0 to 35°C or less.

■ Stirring: Stirring speeds up ion diffusion and increases the polarization effect, thereby improving coverage.

■Time: Since time has a close relationship with plating thickness, the required thickness can be obtained by calculation.

■Additives (including brighteners): Additives play different roles in making up the entire bath.

In particular, in the present invention, we experimented with a number of commercially available brighteners to obtain a smooth specular gloss, but we found that 50% water-soluble epoxy resins of sodium nicotinate and/or potassium nicotinate were more effective. The amount added is shown in Table 1.

[Effects of the Invention] According to the silver plating method of the present invention, a shiny silver plating film can be obtained without using a cyanide.

Claims (1)

[Claims] (1) A silver plating method using an acidic silver complex salt as a main component, characterized in that the additive contains at least a nitrogen-containing carboxylic acid derivative and an aldehyde C═S-containing compound. (2) The silver plating method according to claim 1, wherein the additive includes at least sodium nicotinate and/or a 50% epoxy polymerized water-soluble product of potassium nicotinate.