We plate different steel components with black zinc. We've been having difficulty with the finished product turning green after exposure to any type of light. We’ve spent time and money on various UV blockers and other products that are supposed to prevent this. Unfortunately though, we’ve been unable to find a black chromate without silver in it. Is there such a product? C.M.
Anybody who has done black chromate onzinc can attest to the problem you are experiencing. The black color is due to the size of the silver particles in the chromate coating.The silver has to be evenly dispersed andthe particles must be very small in size(0.5 microns or less). If the silver particles are larger than 1.0 micron in size or arenot evenly distributed, you will get the green color that you observe. It has been suggested that the cause of this phenomenon is due to process control parameters. Low pH during the drying cycle coupled with a higher-than-normal concentration of hexavalent chromium in thechromate coating layer causes some of the finesilver particles to dissolve and then reprecipitate. In this process, larger silver particlesare formed and once this happens, the green color will dominate. In a few words, the color change is due to a change in the physical state of the silver. I suspect that a similar typeof mechanism can occur in the presence of light.
I do not have any “silver bullet” solutions to your problem except to suggest that you carefully review all of your operating parameters and be certain that you are following your chemical vendors' recommendations. More than likely, your vendor has dealt with this issue at other companies and should be able to help. If your vendor is unable to help, you might want to talk with a new vendor. Look for other vendors at PF Online's suppliers database.
Regarding non-silver containing black chromates, a number of patents describe suchmaterials but I am not aware of any commercially-available processes. Perhaps one ofour readers has such information and can share it with us.