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Oiled silver plated parts turning green

1999

We silver plate some copper electrical connectors. After the plating they are tumbled in a dilute oil mixture (either Castrol SafT film (30% oil/70% water) or a customer specified proprietary oil (0.7%)). The oil solution is allowed to drain from the tumbler and then heat is applied.

We have had problems off and on in the summer with the parts turning green several days or more after plating. The green may be spots or may be an overall tint on the part. Scrapings of the green spots seemed to indicate minimal copper. Our boiler company (who also treats cooling towers) indicated that the green was not biological. Even with this indication we use several bacteriocides/fungicides in the oil. After use for a while the oil turns greenish also. Recently we have had trouble with the SafT film coating in the part turning brown. We've begun changing the oil solutions much more often, but I am not confident this will be the solution, since the problem is seasonal.

I believe the silver plating is typically 0.00005 to 0.00010 inches (hope I have the decimal in the right place). Altho the parts are copper, we used to copper strike the parts. The customer requested we stop that several years ago to cut costs. As an experiment we have very recently resumed copper striking the parts but I do not know that this has helped. The parts are silver struck in a 0.5 troy oz/gal bath, and then plated in one of two silver baths (about 5 troy oz/gal; one with no brightener one with a small amt. of brightener). Tanks are very high in carbonates but otherwise are in good balance chemical wise.

Non-oiled parts from the silver bath apparently do not have this problem. The problem seems minimal in the winter. These facts plus the indication that copper wasn't present, seems to suggest that the problem is not "spotting out" of the copper through the silver.

Conrad Bowers
industrial plating - Lafayette, Indiana