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Brownish Cadmium on Fasteners

 

Gentlemen,

We are Cadmium plating fasteners in a Cyanide Bath and regularly we get complaints that the parts are turning dark brown (they start as a light yellow and get darker) over time. After Cad plate parts are soaked in Nitric dip and then chromate.

We have parts in our shop for months and are bright and white. We can¥t give a chromate that has a final yellowish appearance, our clients want white cadmium.

Also, it happens more on the smaller fasteners and in the bigger ones seldom happens. Thickness is above 6 um.

The small fasteners are 1/4" by 2-1/2", and the brown color is always on the threads, not the head. If they are larger it doesn't appear on the body of the fastener, only on the threaded parts. We think the problem might start with only a few pieces and over time the rest in the same container get "contaminated" in some way.

Can this be due to the rust in the Cadmium? Thickness on the treads? Why it happens more on the small fasteners? Could it be bad rinses?

Thank you in advance for your help

Enrique Segovia
- Monterrey, Mexico

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If I had to bet on this I would put money on the rinses, try some washed well in a basket with a hose. If they are fine you have the answer the rinse are dirty

Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England