One of our plating vendors sent us a batch of zinc plated parts that looked "good" at the time of receipt and were passed by our quality inspectors. However, when the parts were taken from storage a few weeks later, they contained fingerprints all over the surface. What's going on? C.P.
You have a classic case of improper handling of the parts during the production and/or inspection process. The parts were handled with bare hands, and small amounts of body oils and salts were left on the plated surfaces. Depending on how the parts were stored, the oils and salts picked up moisture from the environment, and through ever-present phenomenon of corrosion, the parts were stained and damaged.
There are a couple of things that can be done so this doesn't happen in the future. One is to require that all handling of the parts be done wearing cotton gloves. The second is to store the parts if they are not going to be used immediately in closed containers. The containers should include a drying agent to further control moisture.
One last comment: If the fingerprints are of good quality, you might try matching them with those of your quality inspectors and/or the employees of your plating vendor. J.K.!