We plate aluminum (6061) parts with cyanide copper followed with an alkaline tin plate. We are using an acid etch to prepare the parts (10 min at 160F) prior to plating. We have a problem with blisters that appear a few days after the plating process is complete. What is causing this? W.G.
Let’s start at the beginning. The aluminum has to be prepared by soak cleaning followed by an acid dip. Nitric acid is usually used for the acid dip. The acid usually will contain a small amount of fluoride either from hydrofluoric acid or a fluoride salt. Immersion of your parts for 10 min is a rather long time. A more typical time is 1–2 min. Excess etching can damage your parts and cause pitting.
After the acid dip, the parts must be zincated. I recommend the commercially available zincate baths that are available from a number of different vendors. A list of vendors can be found in the Products Finishing Directory and Technology Guide under Plating Processes for Aluminum. Most plating shops will use a double zincate process, which is basically zincating the aluminum parts, stripping the zincate layer using 50% nitric acid and then zincating again.
The parts are then plated in a copper strike bath followed by the tin plate. Please keep in mind that thorough rinsing is required between each of the steps mentioned above.
As for the most likely causes of the latent blisters, consider reducing the time that parts are in the acid etch bath. Also, review your rinsing procedures and zincating procedures.