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Silver plating of conductors and parts. HAM Tips

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Radio amateur V. Prokopenko (Petushki, Vladimir region) uses the method described below to apply a thin layer of silver on printed circuit board conductors, on wire for winding high-frequency coils and hinged connections in equipment, and other copper parts. The method is based on the recovery of metal from a salt solution.

Consider the case of silvering a copper wire. For work, you will need three faience or glass vessels with a volume of 0,5 liters and running water. Concentrated sulfuric acid with a density of 1,84 g/cm3 is poured into the first vessel to decapitate the wire surface. The second vessel, into which water is constantly poured, is needed to flush the wire before silvering and then after it.

The third vessel is filled with a solution for silvering. It consists of silver nitrate - no more than 10 g, medical glucose - 5 g and aqueous ammonia 25% - 20 ml. Silver nitrate is dissolved in 250 ml of distilled water and then aqueous ammonia is poured in. After the brown precipitate of silver oxide formed at the first moment is completely dissolved, glucose dissolved in a separate bowl in 200 ml of water is added to the vessel, stirring the solution.

It must be remembered that the preparation of the solution on tap water is unacceptable. The water temperature for solutions is 20 °C, at this temperature the silvering process is carried out.

The content of ammonia greatly affects the quality of the coating, so the minimum amount is given in the recipe, and the solution must be corrected with samples before starting work. To do this, small pieces of decapitated wire are immersed in a silver bath for 5–10 s, increasing the amount of ammonia in the bath by 1–2 ml after each test until a dense, shiny, mechanically resistant white coating with a slightly golden tint is achieved. Solution adjustment can be simplified with a universal pH indicator; this indicator should be equal to 8...9. Instead of ammonia, a 10% solution of sodium hydroxide or caustic potassium can be used for adjustment.

The solution is not critical to the silver content, therefore, with a small amount of work, the amount of starting substances can be proportionally reduced with the same volume of water to dissolve them.

For silvering, the wire is twisted on a cylindrical mandrel into a large-turn spiral and immersed in a vessel with a reagent, holding it by the bent end of the wire.

Before silvering, the product must be mechanically cleaned of dirt and oxide, degreased in detergent and decapitated. The success of the case largely depends on the preparation of the surface for coating. This issue is well covered in the literature and is not presented here.

In conclusion, we note the most characteristic deviations from the normal process. If the coating is a black washable coating, then this means that either the wire has not been pickled, or there is little ammonia in the solution, or the product has not been washed after pickling. When the coating has a cold bluish tint and in some places the silver layer is removed by friction in the form of flakes, there is little ammonia in the solution.

The snow-white, matte color of the coating, the formation of cracks in the place of a steep inflection indicates that there is a lot of ammonia in the solution and it must be neutralized by introducing a few drops of strong nitric acid into the solution. The same happens when the product is overexposed in the bath, the resulting thick layer of silver is fragile. If the product is poorly cleaned or after decapitation it stays in water or in air for a long time, an oxide film will form on the surface again, which will lead to gray spots on the coating and its local absence.

The disadvantages of the coating include the difference in the elasticity of the layer and the base, which can be eliminated only by special heat treatment, which is impossible in amateur conditions. It must be remembered that only a thin layer coating is the most resistant to deformation.

Before the final fifteen-minute rinsing with running water, it is desirable to passivate the product for 20 minutes in a 1% potassium bichromate solution at room temperature.

Silver solution can be stored for no more than a week. Long-term storage of the solution is dangerous due to the possible formation of a precipitate of silver fulminate compounds.

Author: V. Prokopenko

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