Research on Decorative Silver Plating Process

After the silver, the historic precious metal an application, content in the crust is small, accounting for only 0.07ppm, sterling silver is a beautiful silvery-white metal, jewelry and utensils that have good reflectivity, polished It can achieve high brightness and is widely used in jewelry and home decoration. The plating of some silver on some metal surfaces can also achieve the decorative effect of pure silver. The appearance is bright and detailed, and it is wear-resistant, corrosion-resistant, and has strong discoloration resistance, so it has a wide range of applications. The previously used non-bright silver plating process has poor appearance, low anti-corrosion force, and particularly poor ability to resist sulfur and discoloration. In order to emit light, chemical polishing or copper brushing and reimmersion silver are usually used, which wastes electricity and time. And pollute the environment. In order to solve the above problems, two methods are commonly used in the country. One is to use a bismuth potassium tartrate with organic additives (mostly sulphur compounds) to obtain a bright silver plating layer. This method makes the coating easily discolored and brittle due to the effects of antimony and sulfur. Large, solderability is not ideal. The other is the use of cyanide bright silver plating, this method uses a non-sulfur organic brightener and the appropriate amount of potassium antimony tartrate in combination to obtain a bright silver layer, to solve the coating is easy to change color, brittle, can be welded The problem of unsatisfactory sexuality, while reducing the consumption of raw materials . However, due to the large number of organic complexes required in this method, the number of factors influencing electroplating increases, and the number of processes increases. In order to find a way to save time and labor and materials, a reform was made on the basis of the original process, and an impact silver plating process was recommended. That is, at a higher current density, silver plating and silver layers were completed in a few seconds. Thin and uniform light, the composition of the electroplating solution is simple, no other organic ingredients are needed, the process is completed once, the time is short, and the production efficiency is high.

Save raw materials (the amount of silver required is only about 4% of the original cyanide bright silver plating).

1 Experimental methods and results discussion

1.1 Electrodes and Experimental Conditions

Anode: stainless steel plate 10 × 2cm2;

Cathode: copper piece (a number) 10 × 2cm2;

Temperature: No stirring at room temperature.

1.2 Process flow

Electroplating degreasing→hot water washing→cold water washing→chemical derusting→water washing→cyanide top copper plating→water washing→acid bright copper plating→washing→bright nickel plating→washing→impact silver plating→deionized water (or distilled water)→ Passivation → Immersion 400 Organic Film → Drying → Dipping Light Protective Film → Drying → Inspection.

1.3 Electrolyte

The silver plating electrolyte includes AgCN and KCN electrolytes. Therefore, the content of AgCN and KCN has a great influence on the impact plating method. The amount of fixed AgCN used in the experiment was 1.5 g/L. The content of KCN in the electrolyte was changed to determine the proper ratio of AgCN and KCN. It can be seen from Table 1 that the content of KCN in the electrolyte is 80 g/L, that is, the ratio with AgCN is 160:3. At this time, the silver plating is bright and detailed, the appearance is ideal white, and the content of KCN is low. At this content, the coating layer is formed into a mist so that the silver plating layer is blurred, the gloss is insufficient, and the crystallization of the plating layer is not fine. If it is higher than this content, the plating layer becomes yellow.

This ratio relationship between AgCN and KCN is maintained because the main components of the impact silver plating electrolyte are complex silver salts and free potassium cyanide. The formation reaction of the complex silver salt is as follows: AgCN+KCN=K[Ag(CN)2]; disassociation reaction of the complex silver salt: K[Ag(CN)2]=K++[Ag(CN)2]-

Since the unstable constant of [Ag(CN)2]- is very small (K instability = 8 x 10-22), the concentration of Ag in the electrolyte is very low, so the deposition of the silver layer on the workpiece is mainly [Ag(CN)] 2] - The direct reduction.

Potassium cyanide acts as a complexing agent in the electrolyte and complexes with silver to form [Ag(CN)2]-. Due to its strong complexing ability, the stability of the cyanamide ion is good. Keeping a certain amount of potassium cyanide free in the electrolyte can ensure the stability of the [Ag(CN)2]-complex ion, increase the role of cathode polarization, and make the coating uniform and detailed. If the content of AgCN in the electrolyte is too high, the yellowing of the coating will be caused. If the content is too low, the complexation of silver ions with potassium cyanide will be unstable, the cathode polarization will be small, and the coating will not be detailed. On the basis of a large amount of experiments, this method can determine the requirement of impact plating when the content of AgCN is 1.5 g/L, which saves the amount of silver and meets the requirements of decorative silver plating.

Table 4 Effect of Other Conditions on Plating Appearance

Plating phenomenon

cause

Solution

Yellow

Electroplating solution contains organic impurities and sulfur; AgCN content is too low; KCN content is too high

Low KCN content; high current density

Treated with activated carbon; supplemented with AgCN; adjusted KCN content

Supplement KCN; appropriately reduce the current density

White fog, hair paste, lack of gloss

Low KCN content; organic magazine accumulation

Add KCN; activated carbon filter

From the skin, coating scorch, hair flower

Poor cleaning before plating; too high current density; incomplete degreasing; organic impurities before plating; unactivated before plating

Improve pretreatment; Reduce current density; Strengthen degreasing before plating; Treat with activated carbon;

1.4 Effect of Current Density on the Appearance of Plating

The experiments were carried out with impact plating at different current densities (see Table 2). Experiments have shown that the current density is too large to cause the coating to peel and scorch, the coating is dark, and the crystal of the coating is thick; the current density is too small to reach the effect of impact plating, and the required time is long enough to have an effect; reasonable current density is 5.0 ~6.0A/dm2; The experiment also shows that with the plating solution in the AgCN and KCN content of different current density should be appropriate changes, the general KCN content increases, the current density is also appropriate to increase.

Table 1 Effect of KCN content on the appearance of silver plated layer

KCN content (g/L)

Plating appearance phenomenon

25

Coating is dull and dull

55

Plating is lack of haze gloss

80

Plating white and fine

135

Most of the white light coating is locally yellowish

175

Plating yellow rough matt

1.5 Effect of plating time on the appearance of plating Compared with the plating effect at different times, the best time for striking silver plating is sought. Impact plating time is not easy to be too long, 7 ~ 9 seconds is appropriate, and because of the different size of plating parts, the impact plating time should be slightly different, plating parts can be more red plating 2 ~ 3 seconds, plating can be small Less flush plating for 2 to 3 seconds. If the impact time is too long, it will cause the coating to be hazy. Although white but not bright, the time is too short, the coating has no obvious phenomenon, and the appearance is dark, which is mainly due to the non-uniform plating, which does not completely cover the nickel underlayer.

1.6 The effect of temperature on the appearance of the coating

The impact plating experiments at different temperatures are used: if the temperature of the electrolyte is properly increased, the cathode current density can be correspondingly increased and the deposition rate can be increased. However, the temperature is too high, it will speed up the evaporation of the electrolyte, emit toxic gases, so that the electrolyte instability, generally can be operated at room temperature, without heating nor without stirring (see Table 3).

Table 2 Effect of current density on the apparent appearance of silver plated layer

DX (A/dm2)

Plating appearance phenomenon

2.0

No phenomenon

4.0

Small bubbles, uneven

5.0

Uniform bubble, smoother speed, bright

6.0

Uniform bubble, smooth speed, bright

9.0

Bubbles are too fast, matt

10.0

Dramatic air bubble, scorched

1.7 Discussion of Other Influence Conditions (See Table 4)

Table 3 Effect of Temperature on Plating Appearance

T(°C)

Plating appearance phenomenon

25

Bai Liang

35

Bai Liang

40

Bai Liang

55

White uneven brightness

65

Some yellow and matt

2 process recipe

Based on the above experiments, the process recipe for impact silver plating was obtained. Decorative silver plating formula is as follows: AgCN (g / L) 1.5-3; KCN (g / L) 80-160; T (°C room temperature; T (s) 5-11; DK (A / dm2) 5.0 ~ 6.0; Voltage (V) 3-4 Through experiments, the impact of silver plating process has the following advantages: short cycle, production efficiency is greatly improved, easy and easy to operate plating; reduce operating procedures, save raw materials, electroplating electrolyte only two kinds of AgCN and KCN The finished products are bright, wear-resistant, corrosion-resistant and anti-tarnish.The coated products meet the decorative requirements and can be applied to large-scale production at the same time.This method is applied to the electroplating of decorations and jewelry, and has strong practicality.

references

[1] Zhang Dachang Plating and Protection [M]

[2] Wu Shuangcheng Plating and Environmental Protection [M]

[3] Shen Shunbao Surface Technology [M]

About the author: Ma Shihua (1970-), female, Fushun, Liaoning, Lecturer, Fushun Technician College