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How to fix lack of uniformity in yellow chromating


I have problems in obtaining uniformity of color in my yellow chromate over alkaline zinc plate. My customer needs a total finishing yellow. How do I do it.

Alejandro Tovar
plating shop - Tijuana, B.C., Mexico


For consistent color you need consistent application. This means a proper zinc plate to start with, free of organic film, efficient rinsing, top quality chromating chemicals, air agitation for proper mixing and replenishment, and (sometimes) a heater to maintain a slightly warm temperature (maybe 80 °F, depending on supplier recommendations).

It's hard to tell from this distance in which of those areas your shop might be deficient, but if you can provide more info maybe we can provide more help.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


We have many of our parts yellow chromate plated. What we are finding is the plating is not consistent. At times a blueish color comes out and the yellow is not as bright as we have seen on parts coming in from overseas. We are desperately trying to compete with the China material and their plating seems to be quite consistent. I have a meeting scheduled to discuss our concerns with our plater and would like any information anyone may have. Thanks

Tom Bradley
Starter and Alternator manufacturer - West Chicago, Illinois, USA


You can't compete with China on cost, so your metal finishing must be better than theirs, not just as good as theirs.

First you need to decide what your spec is. Are your starters and alternators expected to be free of hexavalent chromium per RoHS / European Parliament EOL requirements? Are your competitors' parts expected to be? If they are going on new autos, or out of the country, they'd better be. If they are parts for the replacement market, maybe you have breathing room on that issue.

Generally, zinc plated from an acid zinc plating process is brighter than zinc plated from other processes. Is your plater aware of your desire for a bright finish?

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Ted listed most all of the causes of color variations. Here is another one.

If the zinc plating is from an acid electrolite, then there will be iron co-depositing with the zinc, more iron in the low current density areas, and less in the high current densities, with a color variation following the function of current density distribution.

And as Ted said with a different emphasis, the tenacious wetting agent from the acid zinc formulations will not rinse off, you must pre-dip ahead of the chromate in either Nitric (about 1/4 %/vol) or Hydrochloric Acid (about 1/2%/vol), then since these dips are very dilute, you must replace them frequently.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

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