Clear Zinc Chromate coating is discoloring

First a disclaimer, Stanley: these are not 'expert opinions' -- that's a legal term which requires diligence going far beyond what is practical in a public forum like this.

A. Zinc chromate is normally not considered a decorative finish with need for high aesthetics, although sometimes it is. But my guess is the parts are being stacked for drying or packaging in such a way that the outside dries fairly well while moisture accumulates inside. Good luck.

A. What Ted suggests is a likely cause.

I have also seen this problem on enclosures, on the inside, resulting from inadequate plating thickness. Sometimes, if you look carefully, there's little or no plating there.

If this is the problem, have your shop give the parts more time in the plating tank, or use a little internal anode to get more thickness on the inside.

A. On second thought, I think David has offered a better guess than mine. It is common to plate chassis like this with auxiliary anodes either during the main processing or as a separate first step before re-racking. But it is costly, so your supplier will not do it if they don't have to.

A. Both answers from Ted and Dave are distinct possibilities. One way for a plater to cost effectively plate enclosures/chassis in zinc is to use an alkaline bath. These baths have much better coverage in the low current areas (recesses, inside areas, holes, etc) than the acid type baths and consequently there is not as much need for auxilliary anodes to get thickness on the interior of the enclosure. Further, the steps between plating and chromating can have an effect on the clarity of the conversion coating. You might want to discuss the manner in which the parts are processed with your vendor as well as try other vendors to see if you can get a superior finish.

A. Hi Stephen. I know of a case where bicycle wheels were bright acid zinc plated, clear chromated, and then lacquered as a cheaper alternative to nickel-chrome plating. This process may or may not still be done, I don't know. But the point is, there are occasional cases where zinc plating must meet stringent aesthetic requirements. Usually, however, zinc plating is considered a functional finish rather than a decorative one.

Unfortunately, the issue of what aesthetic requirements are implied by a contract when it doesn't address aesthetic requirements is a thorny one. Was the plating supposed to be done in accord with any particular spec?

Regards,