How to Clean and Preserve Copper Plating

Question:

We have a little problem with copper plating on torlon plastic parts. The plating seems to be working fine. The problem arises in storage when after time we begin to see small spots or discoloration on the plating. Our customer will reject the parts for having any type of imperfection. We are currently cleaning the parts with a tarn-x solution, rinsing with white vinegar and then a final rinse with DI water. After cleaning we are packaging the parts in individual plastic bags that we purge with nitrogen and heat seal. We then place the parts in a closed container (not necessary air tight) with desiccant bags. Even after all of this, we still find the spots or discoloration. We are not sure if our cleaning procedure is wrong or the storage process. Would you have any ideas or suggestions to help us out? D.W.

Answer:

From what you describe, there does not appear to be a problem with your cleaning and plating process. You will find the copper will tarnish quite easily with very slight amounts of moisture and/or oxygen. This can be on the level of simply an adsorbed water later at the surface. I would suggest two possible routes to try.

The first would be an extension of your cleaning and plating process where you would add a final rinse in a copper anti-tarnish solution. This process tank would contain either one or a mixture of tolytriazole and/or benzotriazole. Both of these are very effective copper inhibitors at extremely low levels. They adsorb to the surface only a few molecules thick and help minimize the tarnish issue you are experiencing. I would discuss this with your current chemical supplier to see if they have anything in their product line for this. The other avenue to explore, either separate or in parallel with the triazole suggestion would be to investigate vapor phase corrosion inhibitors (also known as volatile phase corrosion inhibitors, both with the acronym VCI). They come in a variety of forms where the VCI is applied to a paper or possibly a sponge. It is then used as part of the packaging process to eliminate rust or tarnish formation. I would start with a simple Internet search for vapor phase corrosion inhibitors. I don’t want to endorse any single business, but will let you know I found the top suppliers on that first page of ten hits.