Black Spots on Acid Chloride Plated Fasteners

Q. We have an auto chloride zinc plant for fasteners where we do trivalent clear and iridescent passivation. Sometimes we get black dots on fasteners after a few days of storing. This is commonly observed during rainy and winter seasons where humidity in our area will be around 95 percent. Is this problem related to drying or storage?  —V.O.

A. It could be that your rinsing is not adequate and small amounts of material on the surface turn dark in a short period of time. Since this problem seems to surface primarily in the humid and wet winter seasons, the most likely cause is improper storage of the parts. If they are being stored in a warehouse that does not have climate control, the excess humidity will speed the formation of black spots. How can you solve this problem? The parts should be stored in a climate-controlled environment. If they are stored individually in plastic bags, a desiccant should be added to each bag to control the humidity. If the parts are stored in corrugated boxes, the sulfur compounds that are commonly found in corrugated and paper materials can also hasten the appearance of these black spots. Try storing your parts in plastic containers with a desiccant in each container.