Zinc Plating: clear, yellow, green passivation differences

A. Hi Abhishek. The pattern is distinct enough and regular enough that it's certainly not random. It's probably echoing the barrel holes in some way. Perhaps the zinc plating is different at the holes. Excessive iron manifests itself this way, although in my limited experience it turns black rather than white. Or possibly it's related to the area of the parts against the hole getting more exposure to chromating solution.

I guess I'd suggest plating a Hull Cell panel with your acid zinc and then dipping it in the green chromate to see if the defect is related to something that happens at high current density -- perhaps you'll get a white stripe or a white end on your panel. But if you can't do that for some reason, I'd suggest maybe a peroxide treatment to reduce the iron would help.

Luck & Regards,

A. This defect is related to high voltage.

Thanks, Alireza, you're probably right. It's probably 'burning' at the high current area at the barrel holes. A Hull Cell panel may demonstrate it.

Luck & Regards,

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A. Hi Muthusamy. In general, the darker the passivation, the greater the corrosion resistance because the heavier the coating. Typically 12, 96, 150 salt spray hours respectively for clear, yellow, olive drab. Many plating shops would not charge more for yellow than clear, despite the somewhat greater chemical usage. Many shops don't have olive drab, so the ones that do probably do charge a bit extra as a "special". These chromates are proprietary chemicals, so you can get pricing from your plating chemical supplier.

But Jim is certainly right that answering all your questions would take hundreds of pages, and that if you don't have a zinc plating book you probably need to start with one. Good luck!

A. Hi Suresh. So that the earlier discussion doesn't mislead you: back in 2003 and earlier, chromate conversion coatings were most commonly based on hexavalent chromium compounds, and the corrosion resistance and the depth of color were proportional to the thickness of the chromate film, so olive drab was thicker and more corrosion than yellow, which was thicker and more corrosion resistant than clear/blue. With the need for RoHS compliance came newer technology trivalent chromating, but a muddled proprietary situation of thin film vs. thick film, topcoated vs. non-topcoated, inherent color vs. dyed colors vs. post-dyed colors. Yes, olive drab COLOR RoHS-compatible trivalent chromating is available, but textbook references to its greater thickness and corrosion resistance are not necessarily true anymore. Other shades of green are available as well.

Nylon is not as resistant as many other plastics to the acids and alkalies used in electroplating, but to my knowledge green passivated nyloc nuts should be available. Green passivation is not as common as yellow and clear though.

Regards,