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Black zinc plating turning green
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We are manufacturing 7 GA CRS material, then having then plated Black Zinc. The black turns green in a relatively short time. Is the environment or plating process at fault?
Christopher Rousesheetmetal forming - Anaheim, California, USA
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Both and neither, Christopher. Black chromates are silver based and tend toward this problem. Poor plating can accentuate the problem as can excessively bright light. My understanding is that the right answer is to shift the plating to
nickel-iron* because the presence of the iron allows it to be chromated to a stable black color.
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
* P.S. 11/12/04: I meant zinc-iron but didn't recognize the typo until Mr. Nurie's reply below.
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Christopher
Another cost effective route is Zinc Iron Alloy plating. The iron runs best at 0.8 percent in the allot to produce really wonderful jet black long lasting deposits which accept both Silver and Non Silver black Chromates .
Regards
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Asif Nurie Atlanta Global  New Delhi, India |
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I'm a big fan of using sealers for black zinc plating. We started this and have greatly reduced the fading and premature corrosion problems. Also, the pH is critical. I keep it under 2 and check it regularly, with frequent small additions of both components.
Jim Conner- Fort Worth, Texas
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Hi,
There are post dips after black chromate which enhances the jet black colour and also the brightness (chromic base), and if you go for zinc iron with a silver and non silver passivations , Iron ratio to zinc is most important factor to obtain best results - practically if you consider in production level and a sealent coating to minimise this greenish appearrance .
Praveen- Mumbai
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The black chromate on zinc-iron is NOT silver based so it does not tend to get greenish with light. These chromates are even not based on hex-chrome but on tri valent chromium. And yes, for a perfect appearance you need a sealer.
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Some time after 15 - 30 days latter the colour of black passivation on Zinc plating is getting discoloured/changed to greenish yellow. Some greenish salt residue also remains on the surface. Finally the surface is applied by a rust prevetive oil. Is it because reaction of Rust preventive oil with passivation. Any reaction of sun light (UV rays) on black passivation.
Nagendra JKPlating shop - Bangalore, Karnataka, India