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Silver Plating and Anti-Tarnish Solutions for Jewelry

소개:

This article explains how to prevent silver and silver-plated items from tarnishing, a common issue where surfaces darken due to reactions with sulfur or light. It details various anti-tarnish treatments, including inorganic methods like chromate or tin electrolysis, organic compound soaks, and surfactant applications. The text also covers what electroless silver plating is, exploring both displacement plating for substrates like copper circuits and reduction plating using agents like formaldehyde or hydrazine. Finally, it outlines why and how to analyze key components in cyanide silver plating solutions, such as silver ions and free cyanide.

Section I Silver Tarnish Prevention Treatment

Due to the high reactivity of the Ag plating surface, the silver-plated surface is prone to discoloration when in contact with corrosive media (such as H₂S, SO₂). Light also imparts excess energy to the plating surface, promoting the ionization of Ag and the reaction between Ag and the corrosive media.

Table 2-28 shows the effects of light wavelength and exposure time on the discoloration of Ag. Table 2-29 shows the effects of exposure time on color and chemical composition.

Table 2-28 Effect of Light Wavelength and Exposure Time on the Color Change of Ag

Ray	Exposure time/h
Ray	6	12	18	24	48
2527Å	No change in color	Yellow spots	황갈색	Brownish-black	브라운
3650Å	No change in color	No change in color	No change in color	노란색	-
햇빛	No change in color	No change in color	No change in color	Yellow spots	-

Table 2-29 Effect of Exposure Time on the Color and Chemical Composition of Silver-Plated Surface

2537Å Exposure color of light/h	6	12	18	24	48
색상	실버	노란색	황갈색	Brownish black	블랙
Main chemical composition	Ag（metal）	Ag₂O+AgO	Ag₂O+AgO	AgO+Ag （Superfine）	Ag（Superfine）

From these results, it can be seen that the discoloration products are silver sulfides, oxides, chlorides, or silver particles. The color of the Ag plating after discoloration varies depending on the chemical composition. This type of color change not only affects the decorative appearance of the silver plating, but also increases the electrical resistance of the Ag plating, which is detrimental to the electrical properties of the silver plating.

From these discoloration mechanisms of the Ag plating layer, it can be seen that it is only necessary to avoid contact between the plating surface and oxygen, light, and corrosive media to prevent discoloration of the silver plating layer. At the same time, forming a dense substance (film layer) on the surface that can absorb ultraviolet rays should have an inhibitory effect on the discoloration of the plating layer.

Among the traditional methods for preventing discoloration of Ag plating layers are inorganic compound methods, organic compound methods, surfactant methods, and combined use of these methods.

1. Inorganic Compound Treatment Method

To compare the effects of different anti-tarnish treatments, the test pieces were plated with 2~3μm Ag and then soaked in a concentration of 0.2% ammonium polysulfide solution [(NH4)2SX] for a certain period. Visual inspection was used for evaluation. When visual inspection was impossible, a digital gloss meter was used to measure the specular reflectance, and the difference △ before and after soaking in the ammonium polysulfide solution was calculated. At the same time, for indoor exposure tests, a gloss meter was also used for measurement.

As an anti-tarnish treatment for Ag, metal films or metal oxide films with the same tone as Ag, such as In, Zn, Cd, Cr, Pd, Rh, Sn, Be, Al, Th and Zr, were tested. Among them, the electrolytic treatment of chromate exhibited the most outstanding anti-tarnish function and was widely applied. Secondly, Rh and Be are also among the choices.

(1) Precious Metal Treatment

Electroplating of Rh and Pd alloys was conducted under optimal conditions, followed by their tone changes and discoloration resistance experiments. The results are shown in Table 2-30. The specular reflectance of Rh is 70%, while the specular reflectance of the alloy (Pd80:Ni20) is about 57%. Although the plating thicknesses differ, there may be some correlation. The plating thickness of Rh is 0.1μm, and the plating thickness of Pd-Ni is 0.3μm. When the plating thickness of other metals on Ag increases, the silver color of the underlying silver layer disappears. However, by visually comparing tone, brightness, and relative values of specular reflectance, it was found that when the specular reflectance is 80%, the difference from the silver color becomes obvious, and 70% is its limit. Below this, the silver color disappears and changes to a different metal color. Therefore, from the tone perspective, a Rh plating thickness below 0.1μm is more suitable. Still, its discoloration resistance is insufficient, while the Pd-Ni alloy plating layer is thicker and has good discoloration resistance. Using precious metals means increased cost, but they offer good drug and wear resistance. The better the discoloration resistance, the more the original silver color disappears. Hence, enhancing discoloration resistance without changing the tone and brightness of the silver plating is very difficult.

(2) Treatment of Tin and Tin Alloys

Alloys of Cu and Sn are called mirror alloys. When the Sn content is above 60%, their color is silver-white, and the specular reflectance is also high, with good corrosion resistance. Therefore, they have been studied since ancient times and used as mirror surfaces in reflecting telescopes. When used as silver anti-tarnish coatings, they perform quite well against tarnishing for 5~6h. However, the specular reflectance drops to about 65%, clearly losing the silver color. The usability of mirror alloys depends on the degree of tint. Although cheaper than precious metals, when the film thickness increases to several thousand angstroms, a potential difference arises between the precious metal Ag and the Cu-Sn alloy, which may cause corrosion resistance issues.

Besides Sn alloy plating, Sn metal or Sn compounds precipitated by electrolytic or soaking treatments of 50~100Å can also have tarnish resistance and improved weather resistance. As a tarnish treatment, it can maintain the stability of the contact resistance of contacts for a long time.

The experimental results are shown in Table 2-30. Compared with alkaline and acidic solutions, parts treated with acidic solutions have a misty white precipitate and cannot be used for decorative purposes. In contrast, parts treated with alkaline solutions and electrolytic treatments show no problems in the short term.

Table 2-30 Inorganic Compounds Resistance to Discoloration on Silver-Plated Surfaces

일련 번호	Solution composition	Processing conditions	Condition to the occurrence of apparent color	참고
1	PNP (Nissin Kasei, Japan) Pd-Ni alloy plating	DK lA/dm² 실내 온도 1min	Above 6h	Loss of silver color, poor CN^- resistance
2	Pewter(Daiya Shokai) Cu-Sn alloy plating	3V 55℃ 40s	Above 6h	Loss of silver color
3	Na₂Cu(CN)₃ 24g/L Na₂SnO₃ 90g/L Free NaCN 16g/L NaOH	D_k 2A/dm² 60℃ 30s	Above 6h	Loss of silver color, better than silver color of 1 and 2.
4	Na₂SnO₃ 90g/L CH₃COONa 0g/L,5g/L,10g/L	D_K 0.5 암페어/디엠비² 실내 온도 15〜90s	10 〜20s	Adding CH₃COONa can stabilize the solution, and the longer the electrolysis time, the more stable it is, but it will lose its silver color.
5	SnCl₂ 20g/L 0. 025NHCl 50mL/L	실내 온도 1min	10분
6	SnCl₂ 20g/L 0.025NHCl 50mL/L	D_K 0. 1 〜 2A/dm² Room temperature, 10s	10분	흰색
6	SnCl₂ 20g/L 0.025NHCl 50mL/L	Room temperature ,1min, 10min		Slightly yellowed
7	Tin fluoroborate 3%	D_K 0. 1 ~ 2 A/dm² Room temperature , 10s	-	흰색
7	Tin fluoroborate 3%	Room temperature ,1min, 10min	10분	Slightly yellow
8	Stannous sulfate 20g/L Iodic acid 30mL/L	D_K 0. 1 〜 2 A/dm² Room temperature ，10s	-	흰색
9	Stannous fluoroborate 20mL/L Fluoric acid 30mL/L Fluoroboric acid 20g/L	Room temperature ，1min, 10min D_K 0. 1 〜2 A/dm² Room temperature ，10s	10분 -	Slightly yellowish 흰색
9		Room temperature, 1min, 10min	10분	Slightly yellow
10	BeSO₄ , 4H₂O 2g/L pH = 5. 7〜5. 8	D_K 0. 007 A/dm² Room temperature 3〜10min	Above 1h
11	BeSO₄ , 4H₂O 2g/L pH = 5. 8	1 〜2. 4V 20s~4min	20min
12	EverShine S(Tamura Chemical）	6.5V 실내 온도 90s	15min
13	K₂CrO₄ 15g/L NaOH 30g/L	2〜6V Room temperature 15〜120s	10 〜30s	The longer the electrolysis, the better
14	K₂CrO₄ 30g/L NaOH 40g/L	D_K 4A/dm² Room temperature 30〜40s		White fogging No silver color
15	K₂CrO₄ 30g/L NaOH 40g/L Heating 20h			White fogging No silver color
16	Al₂(SO₄)₃ 4g/L Ammonium oxalate 2g/L pH = 5. 8	D_K 0.06A/dm² Room temperature 30s〜1min	20 〜30s	Same time as without treatment
17	Sodium carbonate
18	Na₂SnO₃ 8. 5g/L CH₃COONa 5g/L (No. 4)	D_K 0. 5A/dm² 15 〜30s 실내 온도	Above 6h
18	K₂CrO₄ 15g/L KOH 30g/L (No. 13)	2〜3V 30s 실내 온도	Above 6h
19	K₂CrO₄ 15g/L KOH 30g/L K₂CO₃ 50g/L	6V 15 〜60s	10 〜30min

The soaking method utilizes the strong reducibility and strong adsorption of Sn ions. Except for the low concentration of chloride ions, all factors cause Ag to turn yellow, and it only has a certain degree of discoloration resistance, so it has not been used. Compared with soaking treatment, electrolytic treatment is more advantageous. Their common point is the low hardness of metallic Sn. The hardness of the electroplated thick film is Hv10~20. Although it has good resistance to sulfur and chemicals, its wear resistance is insufficient and easily scratched.

(3) Beryllium Treatment

Attaching a beryllium oxide film to Au, Cu and brass as a color-changing treatment can achieve good effect. The principle utilized by this method is: the isoelectric point of beryllium hydroxide is pH=5.8, and the following electrolytic reaction can occur:

When the film thickens through this treatment, interference colors (neon colors) can be obtained, and it is more likely to appear in areas of high current density. Therefore, this treatment usually results in a patterned interference film. The hardness of this film is comparable to that of Sn; it is relatively soft but has better color fastness. The drawback of this method is the poor stability of the solution, and the high toxicity of beryllium makes it unusable.

(4) Aluminum Treatment

There are reports of oxide film formation similar to Be; additional experiments showed no difference between treated and untreated samples, and oxide film could not be formed.

(5) Chromate Treatment

Silver plating is the most common method for the anti-tarnish treatment of brass plating. There are two chromate treatment methods: cathodic electrolysis and soaking, but the cathodic electrolysis method is much superior.

Additives used in chromate electrolytes include: K₂CO₃, Na₂CO₃, KCN, NaS₂O₃, KCNS, NaCl, KI, EDTA, etc. Among them, the K₂CO₃ experimental results found that it can also act as a conductive salt, thereby shortening the electrolysis processing time compared to when it is not added. The effects of the other additives showed no significant differences.

The disadvantage of chromate electrolysis treatment is that the film has poor UV resistance and will turn brown after being left for a long time.

2. Treatment with Organic Compounds and Surfactants

Organic compounds were systematically classified, and their anti-discoloration effects were studied. At the same time, surfactants sold on the market were also tested. The experimental method used various experimental solutions, soaking under optimal conditions, washing with water, then soaking with untreated silver-plated samples in an 0.2% ammonium polysulfide solution for 3 minutes, and comparing and evaluating the degree of discoloration of the two types of samples. The results are shown in Tables 2-31 and 2-32.

Table 2-31 Anti-discoloration Experiment of Organic Compounds

Categorization	일련 번호	Organic compounds	물리적 속성	Solubility	Processing conditions	모양	Hydrophobicity	Anti-discoloration effect
Aliphatic amine	1	Ethylamine (70% aqueous solution) CH₃CH₂NH₂	M_w 45. 09 d¹⁵ 0. 6892 Melting point 83.3℃ Boiling point 16℃	(Aqueous) alcohols Ether	0. 1mol/L (0. 65mL/L) 0. 01mol/L (6. 5mL/L) Room temperature，20min,60min	pH 10〜11 Colorless Transparent O	X	X
	2	Dodecylamine CH₂(CH₂)₁₁NH₂	M_w 18. 5 Melting point 25℃ Boiling point 247℃	Acetone (Ethanol)	0. 1mol/L (18. 5g/L) Room temperature, 20min,60min	Colorless Transparent Water stain	X	X
	3	Tributylamine (CH₃CH₂CH₂CH₂)₃N	M_w 185 d 0. 7782 Boiling point 216.5℃	(Ethanol) Ether	0. 1mol/L (24mL/L) Room temperature, 20min, 60min	Colorless Transparent O	X	X
Aromatic amines	4	O-Toluidine	M_w 107. 16 d²⁰ 0. 9989 Boiling point 199. 7℃	Water 1. 5（25）（Ethanol） Ether	0. 1mol/L (24mL/L) Room temperature, 20min, 60min	Reddish brown transparent O	X	X
Aromatic amines	5	Diphenylamine	M_w 169.23 d 1.159 Boiling point 302℃	（Ethanol） 56 Ether	0. 1mol/L (17g/L) Room temperature, 20min, 60min	Colorless transparent Water stain	X	X
Diamines, Polyamines	6	H₂N(CH₂)₂NH₂	M_w 45.09 d³⁵ 0.892 Melting point 8.5℃ Boiling point 117℃	（water） 에탄올	0.1mol/L (6. 7mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 10〜11 O	X	X
Diamine, polyamine	7	Diethylenetriamine H₂ N(CH₂ )₂ NH(CH₂ )₂ NH₂	M_w 103 Boiling point 208℃	(water) 에탄올	0. 1mol/L (10mL/L) Room temperature, 20min,60min	Colorless Transparent pH 10〜11 O	X	X
Diamine, polyamine	8	Triethylenetetramine H₂ N(CH₂ )₂ NH(CH₂ )₂ NH₂ (CH₂ )₂ NH₂	M_w 146 Melting point 208℃ Boiling point 174℃	(water)	0.1mol/L (15mL/L) Room temperature, 20min,60min	Colorless Transparent pH 10〜11 O	X	X
Amino alcohols	9	Triethylenetetramine H₂N(CH₂)₂NH(CH₂)₂ NH(CH₂)₂NH₂	M_w 189	(water)	0. 1mol/L (19mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 10〜11 O	X	X
	10	Ethanolamine HO(CH₂)₂NH₂	M_w 61.09 d²⁷ 1.0111 Boiling point 171℃	(water) 에탄올	0. 01mol/L (0.61mL/L) 0.1mol/L (6.1mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 9〜10 O	X	X
	11	Diethanolamine (HOCH₂CH₂)₂NH	M_w 105.14 d²⁰ 1.0916 Melting point 28℃ Boiling point 268℃	(water) 에탄올	0. 01mol/L (0.96mL/L) 0.1mol/L (9.6mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 9〜10 O	O	X
	12	Triethanolamine (HOCH₂CH₂)₃NH	M_w 149.19 d²⁰ 1.124 Melting point 21.2℃ Boiling point 227℃	(water) 에탄올	0. 01mol/L (1.3mL/L) 0.1mol/L (13mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 9〜10 O	O	X
	13	2N-Diethylamine (CH₂)₂NCH₂CH₂OH	M_w 93.1 Boiling point 39.5℃	(water)	0. 1mol/L (0.89mL/L) 0. 01mol/L (8.9mL/L) Room temperature, 20min, 60min	Colorless Transparent pH 9〜10 O	O	X
	14	2-Amino-2-ethyl-1,3-propanediol	M_w 105	(water)	0.01mol/L (1.1g/L) 0. 1mol/L (11g/L) Room temperature, 20min, 60min	Colorless Transparent pH 9〜10 O	O	X
Amide	15	Acetamide	M_w 59.07 d 1.159 Melting point 52.62℃ Boiling point 221℃	(water) 에탄올	0. 1mol/L(5.1mL/L) Room temperature, 20min, 60min	Colorless Transparent O	△	X
	16	Acrylamide	M_w 71.1 d 1.122 Melting point 85℃	(water) 에탄올 Ether Trichloromethane	0. 1mol/L(7.1mL/L) Room temperature, 20min, 60min	Colorless Transparent O	△	X
	17	Benzylamine	M_w 121.14 d 1.341 Melting point 128℃ Boiling point 290℃	(water) 에탄올	0. 1mol/L(12g/L) 30도 Room temperature, 20min, 60min	Colorless Transparent O	O	X
Oxime	18	Butanedione oxime	M_w116.12 Melting point 240℃	(Ethanol) Ether (Water)	0. 01mol/L (0.12g/L) 0. 001mol/L (1.2gmL/L) Room temperature, 20min, 60min	Colorless Transparent O	X	X
Oxime	19	Benzohydrin oxime	M_w227 Melting point 154℃	(Ethanol) (Ammonia)	0.01mol/L (0.23g/L) 0. 001mol/L (2.3g/L) Room temperature, 20min, 60min	Colorless Transparent △	X	X
Pyridine	20	Pyridine	M_w 79.10 d²⁰ 0.977 Boiling point 115.5℃	(water) 에탄올	0. 001mol/L (0.1mL/L) 0.1mol/L (8mL/L) Room temperature, 20min, 60min	Colorless Transparent O	X	X
Quinoline	21	Quinoline	M_w 127.16 d²⁰ 0.938 Melting point -15℃ Boiling point 238℃	Hot water, dilute acid, ethanol, ether, carbon disulfide (6mol/L HCl) 2~20mol/L	0.1% (1g/L) 1% (10g/L) Room temperature, 20min, 60min	Colorless Transparent pH 2~4 O	O	X
Quinoline	22	Carboxyquinoline	M_w 145 Melting point 75~76℃	Ethanol, acetone, trichloromethane, benzene (6mol/L HCl) 2~20mol/L	0.1% (1g/L) 1% (10g/L) Room temperature, 20min, 60min	Yellow Transparent pH 2~4 O	X	X
Quinoline	23	Butanedione oxime	M_w 256	Inorganic acid (6mol/L HCl) 40mol/L 에탄올	0.1% (1g/L) 1% (10g/L) Room temperature, 20min, 60min	Yellow Transparent pH 10~11 O	X	X
Diazo compounds	24	p-Ethoxy-2,4-diaminoazobenzene	M_w 256	Inorganic acid (6mol/L HCl) 40mol/L 에탄올	0.1% (1g/L) 1% (10g/L) Room temperature, 20min, 60min	Red Transparent O	O	X
Hydroxycarboxylic acid	25	Tannic acid	Yellowish white, light color, powder	(water) 에탄올 Acetone	0.1% 0.1% 5%	Colorless Transparent pH 6 Yellow transparent pH 4 Yellowish brown transparent pH 2	X	X
Thiourea	26	1-Acetyl-2-thiourea	M_w 118.16 Melting point 165℃	(water) Alcohol	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 7 O	O	X
Thiourea	27	Aminothioureas	M_w 95.6 Melting point 81~183℃	(6mol/L HCl) 50mL/L	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 1 O	X	X
Thiourea	28	Dithizone	M_w 256	(Trichloromethane) 사염화탄소	10g/L Room temperature, 20min, 60min	Blue water stain Turns yellow	O	X
Monosaccharide	29	Ascorbic acid (vitamin C)	M_w 176 Melting point 190~192℃	(water)	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 1 O	X	X
Imidazoles	30	1-Acetyl-2-thiourea	M_w 155 Decomposition 287~288℃	Methanol Water 45mL	1g/L, 10g/L Room temperature, 20min, 60min	Colorless Transparent pH 1 O	X	X
Imidazoles	31	2-Heptadecylimidazole		Methanol 55mL Water 45mL Dissolution Alcohol, acid	1g/L, 10g/L Room temperature, 20min, 60min	White suspension 불균일 White adherence	O	O
Benzimidazoles	32	2-mercaptopyridine	M_w 150 Melting point 301~302℃	Methanol 15mL Water 45mL 온수 에탄올 NaOH	1g/L, 10g/L Room temperature, 60℃ 20min, 60min	Colorless transparent O	O	X
Triazoles	33	3-Amino-1,2,3-triazole	M_w 159 Melting point 159℃	(water) 에탄올 Trichloromethane	Room temperature, 60℃ 1min, 10min, 60min	O	-	X
Benzotriazole	34	Benzotriazole	M_w 119.13 Melting point 99℃	온수 Alcohol	12g/L, pH 6 60℃ 1min, 3min, 10min	Colorless Transparent O	-	X
Triazine	35	Triethylenediamine	M_w 140 Sublimation above 230°C	(water) Hydrate (CN₂)₂N Trichloromethane Alcohol	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 8 O	X	X
Oxazole	36	2-Oxobenzazole	M_w 1151 Melting point 143.2℃	암모니아 6mol/L ammonia 200mL/L Acetic acid Ether	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 10 O	O	X
(Oxo)zines	37	Morpholine	M_w 87.12 d¹³ 1.0007 Boiling point 128℃	(water) Alcohol Ether	10g/L Room temperature, 20min, 60min	Colorless Transparent pH 9~10 O	X	X
Thiazole	38	Ortanin	M_w 264 Melting point 200~246℃ (Decomposition) Red needle-like crystals	Strong acids Dense Trichloromethane Ether Benzene (acetone)	1g/L Room temperature, 20min, 60min	Red transparent (Red precipitate) Brown water stain	O	X
Benzothiazole	39	Benzothiazole	M_w 135.39 d¹² 1.2349 Boiling point 231℃	(Ethanol) Ethyl ether	1g/L Room temperature, 3min, 60min	Reddish brown 투명 O	△	X
Benzothiazole	40	2-Hydrophobic benzothiazole	M_w 167 Melting point 177℃	에탄올 Ethyl ether Benzene Acetic acid Potassium carbonate Potassium hydroxide Sodium hydroxide 2g/L	1g/L Room temperature, 3min, 60min	White turbid pH 10 O	X	O
Naphthalenes	41	Naphthalene sparing agent	M_w 217 Melting point 109.5℃	Water (20℃) 0.01g/L	0.2g/L (Precipitation) Room temperature, 2min, 10min, 30min	O	-	X
				Isoacetone	10g/L Room temperature, 10min, 60min	Yellow transparent Water stain	O	O
				Isoacetone 50% Water 50%	5g/L Room temperature, 10min, 60min	Yellow transparent Water stain	O	O

Table 2-32 Surfactants’ Anti-Discoloration Experiment

일련 번호	Name (company name)	상태	집중	Processing conditions	모양	Hydrophobicity	Inhibition effect	비고
1	Rust Preventive Agent MC- 501 (Japan Lion Grease)	Light red transparent， d 1.06 pH 8. 0〜8. 5	2%,10%	Room temperature 20min, 60min	O	O	Ag：x
1	Rust Preventive Agent MC- 501 (Japan Lion Grease)	Light red transparent， d 1.06 pH 8. 0〜8. 5	2%	Room temperature 15s, 10min	O	O	Cu：Tap water immersion X, exposure test △	15s better
2	Enajiko-ru CNS (Japan Lion Grease)	Amphoteric pH 8〜9	1%	Room temperature 20min, 60 min (Ag)15s, 10min(Cu)	O	O	Ag：x Cu：Tap water immersion X, exposure test：	1% 15s good
			15% Sodium benzene sulfonate 15% (Neutral)	Room temperature 20min,60min (Ag)15s, 10min(Cu)	Brown, white precipitate adhesion	O	Ag：x Cu：Tap water immersion X, exposure test：	1% 15s good
			10% Potassium pyrophosphate 5% (Alkaline)	Room temperature 20min,60min (Ag) 15s, 10min(Cu)	White turbidity O	O
3	Ripa-ru OH- 104P (Japan Lion grease)		0.1%	Room temperature 20min,60min	Colorless and transparent O Yellowish-white turbid O	O △	Ag：x
			1%
			10%
4	Dyuomin CD (Japan Lion Grease)		0.1%	Room temperature 20min,60min	White cloudy O Yellow muddy streaks O	O	Ag：x
			1%
			10%
5	Dyuomin CDA-50 (Lion Grease, Japan)		0.1%	Room temperature 20min,60min	Colorless and transparent O	△ O	Ag：x	Slightly better at 60min
			1%		Colorless and transparent O	△ O		Slightly better at 60min
			10%		Turbid yellow O
6	FC-98 (Kanto Chemical, Japan)	Anionic white powder containing fluorine	0.1% 1%	Room temperature 20min,60min	Colorless and transparent pH 5~7 Mottled marks	O	×(Ag)
7	FC-134 (Kanto Chemical, Japan)	Fluoride-containing oxygen ion tea-brown powder	0.1% 1% (Ethanol)	Room temperature 20min, 60min 5min, 30s	Yellow transparent, Transparent yellowish-brown pH 6 Mottled marks	O	○(Ag)	Marks disappear when temperature is lowered
8	Soft-data-jento W (Japan Lion grease)	Cationic White powder	1%	Room temperature 20min	White turbid pH 7 O	X	X	OH RCH₂CH(CH₂)_nSO₃Na (MIX) α-Allylsulfonate
9	Ripomin COH (Japan Lion Grease)	Amphoteric liquid	1%	Room temperature 20min	White turbid pH 8 O	O	X	Imidazolyl
10	Perettex TR (Kao, Japan)	Cationic Colorless transparent	1%	Room temperature 20min	White turbid pH 10 O	O	X	RODC—CH—SO₃N₄R—OOC—CH₃ Sodium dipropylsulfosuccinate
11	Ema-ru 20C (Kao, Japan)	Cationic Light yellow liquid	1%	Room temperature 20min	Colorless turbidity pH 7 O	X	X	R—O(CH₂ OH)_nSO₃Na Polyoxyethylene alkyl acids
12	Koutamin 24P (Kao, Japan)	Anionic White solid	1%		Colorless and transparent pH 7 O	O	X	Trimethylamine
13	Perettex # 1222 (Kao, Japan)	Nonionic	1%	Room temperature 20min	White cloudy O	O	X
14	Perettex # 1265 (Kao, Japan)	Nonionic	1%	Room temperature 20min	Colorless and transparent O	O	X
15	Soft-kurin QA-1 (Miyoshi, Japan)	Amphoteric White Solid	1%	Room temperature 20min	Colorless turbidity pH 7 O	O	X	β-alanine type
16	Soft-kurin MA-3-70 (Miyoshi, Japan)	Amphoteric	1%	Room temperature 20min	Colorless and transparent pH 8 O	O	X
17	Marusenokku AgT (Japan Ma-ruseru)	Silver anti-tarnish agent	3mL/L 30mL/L	Room temperature 30s, 60min	White cloudy pH 8 Adhesion	O	△	Refer to the instruction manual
18	Dainshiruba-SS (Yamato Kasei, Japan)	Silver anti-tarnish agent	10% Isopropyl Ketone 50% Water 10%	25~30℃ 4min, 20min, 60min	Colorless and transparent pH 8 O	O	X	Refer to the instruction manual
19	T611 (Uemura Kogyo, Japan)	Silver anti-tarnish agent	Liquid 10% 90%	Room temperature 30s, 2min, 10min, 60min	Colorless transparent spots	O	O	Refer to the instruction manual
20	Roukorinsu- eido (Aikorouko, Japan)	Silver anti-tarnish agent	1%	Room temperature 31min, 5min, 30min	White cloudy pH 7 O	O	X	For use in electronic factories

Among organic compounds, those with an anti-Ag discoloration effect when soaked in 0.2% ammonium polysulfide solution are 2-17 alkanimidazole, 2-mercaptobenzimidazole, and complexing agents.

Organic compounds with these anti-discoloration effects share the following characteristics:

① Treatment is carried out in an alkaline region near pH 7.

② Contains hydrophobic groups (long-chain alkyl, phenyl).

③ Contains dimethylamine groups or —SH groups or contains double bonds.

④ Insoluble in water.

⑤ Colorless, close to solid.

However, some cases meet the above conditions but do not have anti-discoloration effects, such as 2-mercapto benzimidazole and 2-thiazolyl benzimidazole.
The mutual relationship between them is still not very clear.

The effects of FC-134 and T611 have been confirmed regarding the anti-discoloration effect of surfactants. At the same time, experiments were conducted on the combined effect of organic compounds and surfactants (FC-134), and no significant effect was found (Table 2-33). The discoloration resistance of 0.2% polysulfide ammonium solution lasts only about 10 minutes at most. Meanwhile, the solvent (diluent) resistance is also relatively weak. For discoloration resistance of decorative items due to the presence of stains, considering these issues comprehensively, the only possible usage is 2-mercapto benzimidazole with FC-134 (see Table 2-34).

Table 2-33 Combined Effects of Organic Compounds and Surfactants on the Anti-Discoloration of Silver-Plated Layers

일련 번호	구성	Processing conditions	모양	Hydrophobicity	Anti-color change property	비고
1	2-Heptadecamidazolelg/L FC-134 1g/L EtOH : H₂O = 1 ：1	실내 온도 1min, 60min	White precipitate pH7 Adhesion	O	O Less effective than when used alone
2	2-Mercaptobenzimidazole 1g/L FC-134 1g/L NaOH 2g/L	실내 온도 1min, 60min	O	X	O Better than when used alone	Reduced foaming power, the longer the better
3	Mercaptone 5g/L FC-134 0. 5g/L Isoacetone：H₂O = 1 ：1	실내 온도 1min, 60min		O	O Better than when used alone	Reduced foaming power, the longer the better

Table 2-34 Effect of Solvents on Anti-Discoloration Film

Organic compounds	Processing conditions	Solvent	모양	Anti-discoloration effect
2-Heptadecamidazole	1g/L (C₂H₅OH : HO = 1 ： 1) 20min		Stains	O
		희석제 (5min)	Stain disappears O	X
		Alcohol (5 min)	Stain disappears O	X
		Coarse spray	Scratch	O Only the scratched part is discolored
2-Mercaptobenzimidazole	10g/L NaOH 2g/L Room temperature 20min		O	O
		Thinner (5min)	O	X
		Alcohol (5min)	O	△
		Coarse spray	Scratch	O Only the scratched part is discolored

The above experimental results are summarized as follows.

① It is difficult to ensure that Ag’s unique tone and luster are not damaged when applying other precious metals on Ag for anti-tarnish treatment.

② Using Sn or Sn alloy methods, when the thickness of Sn is 50~10Å, the anti-tarnish effect can be achieved, but the effect of this method alone is limited.

③ Treatments with Be and Al have little effect.

④ Classical treatments like chromate treatments have relatively good effects. However, their biggest drawback is that the treated film has weak UV resistance and may turn brown over long-term protection.

⑤ Among organic compounds, those with anti-discoloration properties include 2-heptadecylimidazole, 2-alkyl benzimidazole, and mercapto naphthalene agents. However, other organic compounds with the same structure do not necessarily have the same anti-discoloration ability.

⑥ The effectiveness of FC-134 in surfactants sold on the market has been confirmed.

3. Anti-Discoloration Effect of Combined Treatment with Tin Electrolysis and Chromic Acid Electrolysis

From the above results, although various anti-discoloration treatment methods have been studied, no optimal solution has been found. However, tin plating treatment has shown some relative effectiveness. Some explanations on the tin electrolysis treatment and chromic acid electrolysis treatment experiments will be provided next.

(1) Discoloration Promotion Test and its Discoloration Measurement Method

The conditions for the discoloration promotion test are shown in Table 2-35.

Table 2-35 Conditions for Anti-Discoloration Promotion Test

Solution	Ammonium sulfide solution (yellow) (Showa Chemical) （NH₄)₂S_x（2%）
온도	20℃
Soaking time	2h

Methods for measuring discoloration include visual inspection, weighing, optical methods, and contact resistance measurement. However, from a usage perspective, the reflectance (measuring angle 60°)before and after the discoloration test was measured by visual inspection and gloss meter method (same as an inorganic compound treatment method) at a measurement angle, and the change amount △ was calculated using the formula (2-1). The smaller the value △, the more difficult the discoloration, meaning better sulfur resistance.

△(%) = L₁– L₂ (2-1)

공식에서,

L₁— reflectance before the color change test, %;

L₂— reflectance before the color change test, %.

(2) Tin Electrolyte

Through experiments on the effects of treatment conditions of the tin electrolyte, it was determined that the concentration of sodium stannate, electrolysis time, current density, and treatment temperature could be ignored regarding their impact on the discoloration resistance characteristics. At the same time, sodium acetate was added as a stabilizer in the alkaline Sn plating. The composition and optimal conditions for Sn electrolysis are shown in Table 2-36.

Table 2-36 Composition and Optimal Conditions for Sn Electrolysis

Composition of plating solution	NaSnO₃ • 3H₂O 8. 5g/L CH₃COONa ・ 3H₂O 5g/L
치료 조건	Current density 0. 5A/dm² Electrolysis time 15s (10〜30s) Temperature 20℃ (7〜30℃) Anode material Stainless steel plate

(3) Relationship Between Emissivity and Electrolysis Time

The relationship between reflectivity and electrolysis time is shown in Figure 2-8. As the electrolysis time increases, the reflectivity decreases sharply, which detracts from the hue of Ag. An electrolysis time of about 90 seconds will show a tin-white color, and at about 180 seconds of electrolysis, light brown streaks appear on the surface. At the same time, when the electrolysis time exceeds 60 seconds, the electrical properties also deteriorate. The experimental results obtained according to equation (2-1) are shown in Figure 2-9. As the electrolysis time increases, the △ value decreases.

(4) Electrolytic Tin Film Thickness and Electrolysis

Considering the cathode current efficiency according to Faraday’s law, the relationship between electrolysis time and film thickness is shown in Figure 2-10. The tin thickness is about several tens of angstroms.

(5) The Influence of Impurities

The effects of possible impurities (CN, Ag, Cu, Ni, etc.) that may be mixed into the electrolyte in Table 2-36 on the appearance and sulfide resistance of the electrolytic membrane were confirmed through experiments. The results are shown in Table 2-37.

Table 2-37 Effect of Impurities on Sulfur Resistanc

불순물	Concentration/×10^-6	Electrolysis Time /s
불순물	Concentration/×10^-6	30	60	120
없음	0	O	O	△(white)
CN	20 200 2000 20000	O O O △，white	O △，white △，white △，white	△，white △，white △，white △，white
Ag	20 50 100 150 200 400	O O △ X X X	O △，red X, red-black X，red-black X，red-black X, black	△，red △，red-black X, red-black X, red-black X, black X, black
Cu	20 200 300 400 500 600 1000	O O O △，red △，red x，red x，red	O O △，white △，red X，red X，red X，red	△，white △，white △，red △，red X，red X，red X，red
Ni	20 200 1000	O O O	O O O	O O O
Note: 1. Compounds used: CN—NaCN；Ag—KAg(CN)₂；Cu—KCu(CN)₂； Ni—Ni (CN)₂ • 2KCN - 2H₂O. 2. Degree of color change: ○ no appearance change; △ slight color change; × color change.

From the appearance of the membrane after electrolysis, the allowable concentrations of various impurities are as follows: Ag 50X10^-6, Cu 300X10^-6. The presence of CN and Ni is not problematic, but their sulfur resistance will be affected.

(6) Chromate Electrolysis

Besides chromate, inorganic salts in the electrolyte, such as (Na₂CO₂, KCN, KI, Na₂S₂O₃, etc.), are used as additives. These aqueous solutions have long been reported as anti-tarnish agents for silver. The basic composition of the electrolyte is shown in Table 2-38.

Table 2-38 Cr Electrolytic Process Conditions

Electrolyte Composition	K₂ CrO₄ 15g/L NaOH 30g/L
Electrolyte Composition	Used after 30h heating and maturing process
Treatment Conditions	Voltage 6V (2 〜 6V) Electrolysis time 15s (60s) Temperature 20℃ Anode material Stainless steel plate

(7) The Relationship between Reflectivity and Electrolysis Time

The relationship between reflectivity at a voltage of 2~6V and electrolysis time is shown in Figure 2-11. As the voltage increases, the thickness of the Cr film increases while the reflectivity decreases. Unlike the case with Sn electrolyte, under the same voltage conditions, the increase in electrolysis time does not affect reflectivity.
7. The Relationship between Reflectivity and Electrolysis Time
The relationship between reflectivity at a voltage of 2~6V and electrolysis time is shown in Figure 2-11. As the voltage increases, the thickness of the Cr film increases while the reflectivity decreases. Unlike the case with Sn electrolyte, under the same voltage conditions, the increase in electrolysis time does not affect reflectivity.

(8) Sulfide Resistance

The comparison △ results under the condition of soaking in 0.2% (NH₄)2S_x solution for 2 hours are shown in Figure 2-12. The higher the voltage, the better the sulfur resistance, mainly due to the thickening of the chromium oxide film. If the electrolysis time is prolonged without improvement, it is because there are too many pinholes in the electrolytic film, resulting in an increase in chromic acid mixed into the pinholes.

(9) Discoloration Caused by ultraviolet light

The discoloration and sulfur resistance results of Cr electroplated films caused by ultraviolet irradiation are shown in Table 2-39 (Cr electroplating conditions: 6V, 30s). After ultraviolet irradiation, the Cr electroplated film is completely damaged, turning reddish-purple and bluish-purple. At the same time, the sulfur resistance also sharply decreases, dropping to the same level as without anti-discoloration treatment. There is little difference between the anti-discoloration-treated samples and those exposed to sunlight due to the difference in ultraviolet intensity compared to general ultraviolet irradiation experiments.

Table 2-39 Color Change Caused by Ultraviolet Irradiation

일련 번호	자외선		L₁/%	L₂/%	L₃/%	△₁/%	△₂/%	비고
일련 번호	1	-	L₁/%	L₂/%	L₃/%	△₁/%	△₂/%	비고	-	86. 3	-	84. 8	0	1.5	(Turns yellow)
2 3 4	UV^①	0.5 1 3	86. 0 86. 2 86. 5	86. 0 83. 2 58〜65	67 53 —	0 3 21〜28	19 33 —	파란색 - Violet
5 6	햇빛^②	3 6	86.6 86.4	86.6 86.4	85.6 85.5	0 0	0.8 0.9	(Turns yellow) -
① The photoelectric gloss meter uses a hydrogen discharge tube. ② Outdoor exposure during the daytime: L₁ —— Reflectance before irradiation; L₂ —— Reflectance after irradiation; L₃ —— Reflectance after soaking in 0. 2% (NH₄ )₂ S_x for 30 minutes following irradiation. △₁ =L₁ - L₂ △₂ =L₁ - L₃

카피라이팅 @ 소블링.쥬얼리 - Sobling. 맞춤형 주얼리 제조업체, OEM 및 ODM 주얼리 공장

(10) The Impact of Impurities

The test results of impurity effects in the basic composition are shown in Table 2-40.

Table 2-40 Effects of Impurities

불순물	Concentration /×10^-6	모양
CN	200 400 600 800 1000	O O O O O
Ag	10 20 30 40 50	O O O X（black） X（black）
Sn	200 400 600 800 1000	O O O O O

The presence of CN and Sn mixed in does not affect the appearance. When the Ag content is 40×10^-6, black spots may appear, but there is no adverse effect on sulfur resistance.

(11) Sn Electrolysis Treatment Plus Cr Sulfur Resistance during Electrolytic Treatment

The methods introduced above each have pros and cons and cannot be considered complete anti-discoloration methods. Therefore, the following approach is considered: immediately performing electrolytic Sn treatment after plating Ag, followed by electrolytic Cr treatment.

Under constant current conditions ( D_K = 0. 5A/dm² ), the electrolysis time of Sn was changed, and during the subsequent Cr electrolysis treatment, its voltage and electrolysis time were varied. The results are shown in Figure 2-13. The reflectivity reduction under each condition is caused by changes in Sn electrolysis time. The electrolysis time must be kept within 30 seconds to maintain the silver tone.

When in the same Sn electrolysis treatment time state, extending the Cr electrolysis time causes the color of the Cr underlying Sn to show through, resulting in a white or gray uneven state. At the same time, when Cr electrolysis is at a high voltage (6V), this phenomenon appears earlier; at 6V, it can be seen in 5 seconds, whereas at 2V, it can last more than 60 seconds. The extension of Sn electrolysis time shows the same tendency. The longer the electrolysis time, the more likely whitening occurs.

This phenomenon appears when the charge passed during the Cr treatment of a certain Sn electroplated layer reaches a certain value. The redox reaction during Cr electroplating plays a certain role on the Sn electroplated layer, causing changes in the crystallization morphology of Sn or causing changes in the contrast between the Sn electroplated layer and the Cr electroplated layer.

Soaking in 0.2% (NH₄)₂S_x solution for 2 hours, the △ value obtained through equation (2-1) is shown in Figure 2-14. It is lower than when Sn electroplating and Cr electroplating are performed separately (close to 0), and it also has good sulfur resistance. After soaking in 0.2% (NH₄)₂S_x solution for 2 hours, no discoloration occurs.

When no color change occurs, it is related to the extension of Sn electrolysis time. The electrolysis time is 15~30s+ electrolysis time over 30 seconds especially has good sulfur resistance, but there will be appearance issues. The combination of Sn electrolysis at 0.5A/dm² for 15~30s, plus Cr electrolysis at 2~3V for 30s was the optimal condition.

The results of the indoor exposure test are shown in Figure 2-15. Yellowing after 1~2d for untreated and 7~10d for Cr electrolytic treatment. In contrast, the parts treated with Sn electrolysis followed by Cr electrolysis only began to partially turn gray after 60~100d, showing good effectiveness. Soaking was conducted using a 0.2% (NH₄)₂S_x solution equivalent to one month of indoor exposure testing. The Sn electrolysis treatment followed by the Cr treatment is abbreviated as SGP (silver guard process).

① The effect of various tin salts on sulfur resistance

The test results of the effects of various tin salt surface activators in the SGP method are shown in Table 2-41 and Figure 2-16.

Table 2-41 Comparison of the Effects of Various Tin Salts

일련 번호	조성 및 공정 조건	Sn electrolysis time /s	Cr Electrolysis time/s	L₁/%	△/%	비고
1	SnCl₂ 4g/L HCl 0.025mol/L （immersion）	13 30 60	15	90. 2 89. 8 89. 2	5. 4 4. 8 5. 8	Yellowing Yellowing Yellowing
2	Na₂SnO₃ -3H₂O 8. 5g/L CH₃COONa·3H₂O 5g/L 3. 5V(0. 1A/dm²)	15 30 60	15	89. 7 87. 3 85. 2	0. 7 0. 1 0	Partial yellowing - Tin color
3	No. 2 plus Peretekkusu^# 1232^① 0. 1g/L 3. 5V (0. 15A/dm²)	15 30 60	15	87. 4 85.0 76. 6	0 0. 1 0	- - Tin color
4	K₂SnO₃-3H₂O 8. 5g/L CH₃COONa·3H₂O 5g/L 3. 5V (0. 1A/dm²)	15 30 60	15	89. 6 88. 9 85. 4	0. 9 0 0	Partial yellowing - Tin color
① Peretekkusu^# 1232 is a product of Miyoshi Oil & Fat in Japan. Note: Cr electrolyte 3 V.

The results of sulfur resistance are shown in Figure 2-17. When the soaking time in the stannous chloride electrolyte (No. 1) is extended, unlike the sodium stannate electrolyte, it does not cause a decrease in reflectivity. The sulfur resistance is also independent of soaking time, maintaining a certain △ value (electrolysis from this electrolyte results in a whitened product, which detracts from the appearance). This is due to the adsorption of Ag-Cl-Sn reaching equilibrium.

Adding a non-ionic surfactant (No. 3) to the sodium stannate electrolyte significantly improves current efficiency, accelerates the electrolysis rate (No. 2), reduces reflectivity, and enhances sulfur resistance.

Although the results of the potassium stannate electrolyte (No. 4) are the same as those of the sodium stannate electrolyte, there are certain differences between the effects of sodium salts and potassium salts in alkaline tin plating solutions.

② The effect of chromium electrolyte on sulfur resistance was mainly studied from aspects such as electrolyte concentration, the effect of surfactants, and the influence of current density on the appearance after treatment.

Table 2-42 Effect of Concentration on Sulfur Resistance

Concentration /(g/L)	Electrolysis time /s			Current value at constant voltage (6V)/A
Concentration /(g/L)	5	15	30	Current value at constant voltage (6V)/A
K₂CrO₄ 15 NaOH 30	X	X	X	4. 0 (40A/dm²)
K₂CrO₄ 7.5 NaOH 15	O	X	X	2.4 (24A/dm²)
K₂CrO₄ 1.5 NaOH 3.0	O	O	X	0.6 (6A/dm²)
Note: O indicates no change in appearance; × indicates a change in appearance (whitening).

Effect of concentration: The standard solution was diluted to 1/2, 1/10, and the electrolysis time at which a tin color appeared during electrolysis at 6 V was investigated. The results are shown in Table 2-42.

Effect of Surfactants: Standard solutions of surfactant (Perettekusu#1265, a product of Miyo-shi Oils and Fats Co., Ltd., Japan) were added with ionic surfactant at concentrations of 1/1 and 1/2, and the electrolysis time was investigated at the time when it began to show a tin color. The results are shown in Table 2-43. Compared to no addition, the effect was better at high current density when surfactants were added. At low current density, there was little difference from the case without addition, but gas hindered the progress of electrolysis. When surfactants were present, their concentration and current density affected the time for whitening to start, increasing management difficulties. At the same time, bubbles were produced due to the large amount of hydrogen generation. It was easier to manage without addition.

Table 2-43 Effects of Surfactants

Concentration/(g/L)	Perettekusu^# 1265/(mL/L)	Voltage/V	D_K/(A/dm²	Electrolysis Time/s
Concentration/(g/L)	Perettekusu^# 1265/(mL/L)	Voltage/V	D_K/(A/dm²	5	10	15
K₂CrO₄ 15 NaOH 30	0	6 5 6 3	40 29 19 7	X X X X	X X X X	X X X X
	0.01	6 5 6 3	40 29 19 7	O O O O	X X O O	X X X X
	0.1	6 5 6 3	40 29 19 7	O O O O	O O O X	X X X X
	1	6	40	O	X	X
K₂CrO₄ 7.5 NaOH 15	0	6 5 6 3	24 17 10 3	X X X O	X X X O	X X X X
K₂CrO₄ 7.5 NaOH 15	0.01	6 5 6 3	24 17 10 3	O O O O	O O X X	X X X X

Influence of Current: Density The time when the tin color first appears at various current densities during electrolysis is shown in Figure 2-18. At the same time, the E-I curve is shown in Figure 2-19. The current density is more appropriate when the electrolysis time is 0.5A/dm2 around 30 seconds. Meanwhile, the time without the appearance of tin color is also affected by electrolyte concentration, anode current density, electrode spacing, and other factors. Concentration management is done by calculating the size of the tank, and it is possible to derive the intrinsic power of the tank that does not appear tin-colored at a certain voltage, for ease of management.

The anti-tarnish management of Ag is diverse. Compared with other methods, the SGP method does not damage the color of Ag and has sulfur resistance. The process is shown in Figure 2-20. The sulfur resistance results of this process are shown in Figure 2-21.

At the same time, using detergents (surfactants) containing α-diketone and its salts to wash or soak silver (or silver-plated items) and its alloys (tin alloy, zinc alloy, indium alloy, palladium alloy, etc.) can greatly inhibit surface discoloration or corrosion.

The basic structure of its additive is shown in Figure 2-22.

Where X¹ is an oxygen or nitrogen atom, X² is a carbon or oxygen atom; when X¹ is oxygen, Y¹ does not exist) C¹~C¹² is an alkyl group; Y²~Y¹⁰ is H, OH, or an alkyl group of C₁~C₁₂ (where halogen atoms or OH can also substitute the H in the alkyl group), but when X² it is O, Y⁸ does not exist. Other relevant components can also be added as needed, such as pH buffers, foaming agents, preservatives, etc. The anti-tarnish effect is better when iodide ions are present. Some specific examples are shown in Table 2-44.

Table 2-44 Formulation Examples of Diketo Surfactants for Silver Tarnish Prevention

구성	1번	2번	3번	4번	5번	6번	7번	8번
Chlorovanillic acid	10
Polyoxyethylene alkyl ether	0.5
Sodium dihydrogen phosphate	5
Polyethylene glycol	1
Tretinoin		5
Pyrazole chloride		5
Sodium bicarbonate		5
Protease		1
요오드화칼륨		0.05
Tetrahydroxy-1,4-benzoquinone			0.5
2-Alkyl-N-carboxymethyl-N-hydroxymethylimidazoline			10
Thiourea			0.5
Glycerol			1
Sodium 3-ethyl-2-hydroxy-2-cyclopentane-1-carboxylate				0.05
Potassium alkyl sulfonate				5
Isocyanuron salt				5
N-Acylated amines				1
3-Hydroxy-2-methyl-4-pyrone					20
구연산					5
Sodium benzoate					5
Menthol					0.1
Calcium rosmarinic acid						2
3-Methyl-1,2-Cyclopentanaminium						20
Polyoxyethylene castor oil						5
Steroidal Glycosides						1
3-Hydroxy-1,2-dimethyl-4(1H)-pyridine							6
Mercaptoalanine							2
Glucoamylase							5
Sorbitol							1
2,5-Dihydroxy-1,4-benzoquinone								10
Alkyl benzyl ammonium chloride								30
Potassium sodium tartrate								5
Papain								1

It has been confirmed through experiments that all solutions have anti-tarnish and anti-corrosion effects on silver plating and its alloys.

Section II Electroless Silver Plating

Because silver has low resistivity and is much cheaper than other precious metals, silver plating is widely used in the electronics field.

As a precious metal, silver has a more positive redox potential than other metals, allowing it to be displaced and deposited on metals such as copper, iron, and zinc, forming a silver plating layer on these metals. It will be mentioned later that this type of reaction is used on printed circuit boards to replace the copper with silver so that the copper pattern is covered by a layer of silver, thereby increasing solderability and reducing contact resistance. However, this displacement deposition generally results in a relatively thin plating layer, about 2μm or less. In contrast, the plating layer obtained through a reduction reaction can be somewhat thicker.

In the application of silver plating, the most famous should be the silver mirror reaction. The silver mirror reaction was developed by Drayton in 1830 and was applied to the mirror manufacturing industry. In addition, chemical silver plating (especially reduction silver plating) is widely used in electronics, thereby expanding the industrial applications of silver plating. Chemical silver plating can be applied not only to metals but also to non-metals.

1. Displacement of Silver Plating and its Applications

Displacement silver plating utilizes the potential difference between the base metal and silver.

The conductor patterns on printed circuit boards are usually made of copper, but during storage, before component mounting, an oxide film forms on the copper surface. This oxide film affects the solderability of the circuit board. High-end products or even Ni/Au, Ni/Pd, Ni/Pa/Au plating are generally used to enhance the brazing ability of copper circuits, which naturally increases the cost. However, Sn plating lacks sufficient rust resistance. Silver plating is close in price to Sn and not only prevents rust and is suitable for brazing but can also be used for metal wire connections, meaning silver can be displacement-plated on copper circuits. However, the thickness must be increased to compensate if silver is plated directly on copper due to uneven plating. This makes the plating fail the tape test, with plating peeling off easily, especially after storage. However, this problem can be solved by two-step plating. The method is to first plate a layer of metal, which is more expensive than copper, on the copper, then displacement plate silver. The first metal layer can be Pd or Ag. The thickness is so thin, on the order of nanometers (i.e., several nanometers thick), that the presence of the plating cannot be recognized with the naked eye. The displacement reaction is:

Cu + Ag⁺ → Cu⁺ + Ag

The silver displacement solution must also contain a complexing agent for monovalent copper to stabilize the displaced copper ions. When the first coating is displacement silver, its displacement rate must be much slower than that of the second Ag plating layer, equivalent to less than 15% of its speed. If it is too fast, it is easy to cause a decrease in coating adhesion, resulting in coating peeling during the tape test. The following is an example of silver plating.

When the first plating layer is Pd, the Pd plating solution is

PdSO₄ 50mg/L (accounted as Pd)

H₂SO₄ 5%

Temperature Room temperature

Time About 30s

The composition of silver plating and its process conditions are shown in Table 2-45.

Table 2-45 Composition and Process of Displacement Silver Plating (first plating layer)

구성 및 공정 조건	Formulation and component	구성 및 공정 조건	Formulation and component
Silver methanesulfonate (as Ag)	1g/L	Lurotex A25	20mg/L
NaBr	320g/L	pH	5.5
Diethylenetriamine pentylenephosphonic acid	30mL/L	온도	50℃
Polyethylene glycol 400	80mL/L	시간	5 min
2,2'-Bipyridine	90mg/L

After plating, the plated parts undergo heat treatment at 150℃ for 4 hours, followed by a wettability test, which is qualified.

When the first plating layer is Ag, the plating solution and its process are shown in Table 2-46.

Table 2-46 Composition and Process of Displacement Silver Plating (Second Coating)

구성 및 공정 조건	Formulation and component	구성 및 공정 조건	Formulation and component
Silver methanesulfonate (as Ag)	1g/L	Polyethylene glycol 400	80mL/L
NaBr	320g/L	2,2'-Bipyridine	90mg/L
Diethylenetriamine pentylenephosphonic acid	30mL/L	pH	5.5

The plating solution for the first and second plating layers can have the same composition, but the temperature and immersion time differ. The conditions for the first plating layer are as follows: 25℃, 30s. The conditions for the second plating layer are as follows: 50℃, 6min. The resulting plating layers are uniform, silver bright, and have good brazing weldability.

Hutchinson et al. proposed a plating solution method using alkyl polyethylene glycol ether as a brightener, ethylenediamine, and 1-hydroxyethylene-1,1-diphosphonic acid as copper complexing agents, and the obtained plating layer can meet the soldering conditions of printed circuit boards.

ITO (a transparent indium tin oxide) is widely used in liquid and plasma crystal displays. As the size of liquid crystal displays and plasma displays has increased in recent years, the resistance of ITO itself has become relatively high. To solve this problem, a silver layer with lower conductivity can be added to the ITO. This cannot be implemented by silver electroplating directly because it is difficult to directly deposit catalytically active metals on ITO. Therefore, a layer of tin is first deposited on the ITO, followed by a catalytically active metal deposition, and finally, silver plating. The composition of the plating solution and process conditions for silver plating on ITO are shown in Table 2-47.

Table 2-47 Composition of the Plating Solution and Process Conditions for Silver Plating on ITO

Immersion Tin Liquid	SnCl₂ HCl	70g/L 230mL/L
Activation Fluid	AgNO₃	10g/L
Silver plating solution	AgNO₃ NH₄OH (NH₄)₂SO₄ Co(SO₄)₂ Mg(OH)₂ Ag thickness Specific resistance Specific resistance (250℃, 30min after treatment)	0. 03〜0. 08mol/L 7〜10mol/L 0. 3〜0. 8mol/L 0. 1〜0. 2mol/L 0. 01〜0. 05mol/L 2900〜3200Å 3〜3. 5μΩ• cm 1. 6〜2. 5μΩ• cm

Among them, Co(SO₄)₂ acts as a reducing agent, and its reaction is as follows:

Ag⁺ + Co²⁺ → Ag + Co³⁺

The purpose of adding Mg²⁺ is to prevent Ag agglomeration and precipitation, ensuring Ag deposition’s uniformity.

2. Reduction of Silver Plating and its Applications

The composition of the reduction silver plating solution is similar to that of general chemical plating, with the main components being: Ag salt, complexing agent, reducing agent, pH adjuster, stabilizer, etc. The reducing agents for silver include glucose, saccharin, glyoxal, ascorbic acid, gluconic acid, sorbitol, hydrazine, HCHO, KBH₄, NaBH₄, DMAB and others.

The reactions of some representative silver-reducing agents are as follows.

(1) HCHO (when using ammonia solution as a complexing agent)

2AgNO₃+ 2NH₄OH → Ag₂O + 2NH₄아니요₃ + H₂O

Ag₂O + 4NH₄OH → 2[Ag(NH₃)₂]OH + 3H₂O

2[Ag(NH₃)₂]OH + HCHO → 2Ag + 4NH₃+ HCOOH + H₂O

(2) Tartrate salts

4AgNO₃ + 4NH₃ + C₄H₄O₆NaK + H₂O → 4Ag + 4NH₄아니요₃ + C₃H₂O₆NaK + CO2

(3) Hydrazine

4[Ag(NH₃)₂]NO₃+ N₂H₄ → 4Ag + 4NH₄아니요₃ + 4NH₃ + N₂

(4) Borohydride

N₂H₄BH₃ + 3Ag⁺ + 4OH^– → 3Ag + N₂H₄ + B(OH)₄^–+ 3/2H₂

In addition, there have been attempts to use organic solvents to replace cyanide with non-cyanide silver plating. This plating solution can be used on non-conductive materials that are unstable or hydrophobic in aqueous solutions. The formula is shown in Table 2-48.

Table 2-48 Formulations Using Organic Solvents for Electroless Silver Plating

재료 및 공정 조건	1번	2번
Dimethyl sulfoxide/mL Ethylene glycol/ml Ethanol/mL d-(+)Glucose/mL Triethylamine/mL 온도/°C Time/min Plating materia	300 200 - - 15 70 20 Al₂O₃ powder	200 - 300 5g 2. 5mL/min 60 10 Nylon cloth

Among them, diols and glucose are used as reducing agents. At the same time, since dimethyl sulfoxide has a slight reducing ability, the plating solution does not require aging treatment. The resulting plated parts have silver-like metallic luster.

Organic compounds containing two monothiol groups are used as complexing agents, with the structural formula as follows:

X—R¹ —S—R² —R³ —Y

In the formula, R¹ 그리고 R² are alkylene groups containing 1~5 carbon atoms; R³ are an alkylene group containing 2~8 carbon atoms X and Y can be carboxyl, sulfonic acid, amino, alkyl groups, etc.

The reducing agents include aldehydes, hydrazines, borohydride compounds, ascorbic acid, etc. Table 2-49 shows examples of the use of this series.

Table 2-49 Process Conditions of Silver Reduction Plating Solution Using Two Monothiol Organic Compounds as Complexing Agents

조성 및 공정 조건	1번	2번	3번
질산은/(g/L) 1,2-Bis(2-carboxyethylthio)ethane/(g/L) 1,4-bis(2-carboxyethylthio)butane/(g/L) 1,2,2'-(Ethylthio)diethyl mercaptan/(g/L) Hydrazine hydrate/(g/L) Formalin/(g/L) 차아인산나트륨/(g/L) pH (adjusted with NaOH) 온도/°C	0. 17 10 - - 8 - - 10 35	0. 31 - 15 - - 4 - 11 50	0. 34 - - 20 - - 10 10. 5 50

The above plating solution can be used for silver plating on copper plates, alumina ceramic pieces, and epoxy resin (activation treatment must be performed in advance when plating on ceramics and resin), and a uniform plating layer can be obtained. After plating, if the plating solution is left at room temperature for 4 weeks and then used again, a good plating film can still be obtained, with no pH changes or precipitation observed.

Section III Analysis of Cyanide Silver Plating Solution

1. Analysis of Silver

See Table 2-50.

Table 2-50 Silver Ion Analysis Method for Silver Plating Solution

Operation sequence	Notes and Instructions
(1) Take 5ml of plating solution into 300 beaker, add 20mL of sulfuric acid and 5mL of nitric acid while observing. (2) Heat until white smoke is produced (3) Add 100mL of water and 3mL of 10% ammonium iron sulfate. (4)Titrate with 0.1mol/L potassium thiocyanate. Calculation formula: Ag(g/L) = mL × 2. 158× f AgCN(g/L) = Ag(g/L) ×1. 2402 In the formula, mL——0. 1mol/L titer of potassium thiocyanate; f——0. 1mol/L potassium thiocyanate solution coefficient	Due to the generation of toxic hydrogen cyanide gas, it is necessary to operate in a local ventilation area. Change from brown smoke to white smoke Titration endpoint: light red color 0. 1mol/L KCNS 1mL = 0. 01079g Ag

Operation sequence

Notes and Instructions

(1) Take 5ml of plating solution into 300 beaker, add 20mL of sulfuric acid and 5mL of nitric acid while observing.

(2) Heat until white smoke is produced

(3) Add 100mL of water and 3mL of 10% ammonium iron sulfate.

(4)Titrate with 0.1mol/L potassium thiocyanate.

Calculation formula:

Ag(g/L) = mL × 2. 158× f

AgCN(g/L) = Ag(g/L) ×1. 2402

In the formula, mL——0. 1mol/L titer of potassium thiocyanate;

f——0. 1mol/L potassium thiocyanate solution coefficient

Due to the generation of toxic hydrogen cyanide gas, it is necessary to operate in a local ventilation area.

Change from brown smoke to white smoke

Titration endpoint: light red color

0. 1mol/L KCNS 1mL = 0. 01079g Ag

2. Methods for Analyzing Free Cyanide

See Table 2-51.

Table 2-51 Analytical Methods for Free Cyanide

Operation sequence	Notes and Instructions
(1)Take 5ml of plating solution into 300m beaker, add water 50mL (2)Add 10% potassium iodide solution 1~2mL (3)Titrate with 0.1mol/1 silver nitrate solution Calculation formula. Free KCN (g/L) = mL×2.60×f Free NaCN (g/L) = mL×1.96× f In the formula, f - coefficient of 0.1mol/L silver nitrate solution	Titration endpoint: light red color 0. 1mol/L AgNO₃ 1mL = 0. 0130g KCN = 0. 0098g NaCN

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