Cadmium Plate Turns Black

We are currently cadmium-plating fasteners in a cyanide bath and are finding that the finish turns black after four to six days. Parts come out of the process more or less bright white. After the cadmium plate we rinse and apply a chromate, rinse and dry. One fastener is a little yellowish after a couple of weeks, but the others that are black were plated only four days ago. Can you make any suggestions as to how to solve this problem?

Q: We are currently cadmium-plating fasteners in a cyanide bath and are finding that the finish turns black after four to six days. Parts come out of the process more or less bright white. After the cadmium plate we rinse and apply a chromate, rinse and dry.

We have been doing this for more than 20 years and sometimes the plated parts have turned yellowish to brown, but never black.

I’ve enclosed some pictures so you can see the magnitude of the black color, together with a freshly plated part.

You can see that one fastener is a little yellowish after a couple of weeks, but the others that are black were plated only four days ago. Can you make any suggestions as to how to solve this problem? M.M.

A: First off, thanks for the photos included with your e-mail. A picture is always worth a thousand words.

Discoloration of a cyanide cadmium plate can be due to a number of causes, but all of them have to do with impurities in one or more of your process tanks. A dark deposit may be due to lead being present in the plating bath. Typically, the lead will cause a problem when the concentration exceeds 0.03 oz/gal. Excess copper in the cadmium bath can do the same thing. Nickel may also be a culprit, although excess nickel usually gives a yellow color. Organic contamination is also a possibility.

To solve this problem you have to go back to basics. Are all of your process tanks within the limits specified for the cadmium plating process? Assuming your cleaning process steps are okay, you must look at the plating bath, bright dip and chromate conversion chemistry. Do you know the concentrations of “tramp” metals in the plating bath? What about your brighteners in the bath? What is the condition of your bright dip? Has it been replaced recently? What is the condition of your chromate conversion process tank? Have you recently changed from a hexavalent to trivalent chromium process? I’m willing to bet that if you get all your process tanks within specifications that this problem will disappear.