Silver plating of aluminum and its alloys is much more difficult than normal copper plating and the process is also more complicated. The main reason is that the properties of aluminum and its alloys are not the same as those of other metals. Aluminum is an amphoteric metal and reacts with both acids and bases. A slight mishandling of the pretreatment causes excessive surface corrosion. Furthermore, aluminum and its alloys are prone to oxide films, either in air or in solution. If the oxide film is not removed, it will affect the adhesion of the coating.
The key to the silver plating of aluminum and its alloys is the bonding force between the plating and the base metal. Therefore, a special treatment must be taken to obtain a silver coating with good bonding strength on the aluminum substrate. The method of dipping zinc can better solve this difficulty.
Zinc leaching is based on the principle that aluminum has a relatively negative potential and is easily displaced in the electrolyte solution, displacing a thin layer of zinc. This layer of zinc is sandwiched between the base metal and the silver plating layer, and serves to enhance the bonding strength between the plating layer and the base metal.
In the silver plating process of aluminum and its alloys, the following points must also be noted:
1) Whether it is degreasing or caustic washing, the content of NaOH should not be too high, and the time should not be too long to avoid excessive corrosion of the surface.
2) The process of zinc immersion is the key to satisfactory plating. Zinc leaching is performed twice because the zinc layer is rough after the first zinc immersion. After removing it with 1:1 HN0, a second zinc immersion is performed. After the second zinc immersion, only the uniform, fine zinc layer with good bonding with the substrate can be obtained until the next step is completed.
3) During the zinc immersion process, attention should be paid to swinging to prevent the parts from overlapping each other and causing local zinc-free layers.
4) If it is found that the quality of zinc leaching is not good, use 1:1 HN0, and then re-immersion zinc.
5) After the zinc-impregnated parts are charged into the cyanide copper plating solution, they shall be charged with a human tank, and shall be plated with high current for 2 minutes before returning to normal current. If it is found that the surface of the parts is black and dark during plating, the parts may be taken out and then electroplated.
6) After the aluminum parts are plated with copper, the normal process of silver plating of the copper parts can be performed.
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