Black zinc plating turning green
A discussion started in 2004 but continuing through 2018(2004)
Q. We are manufacturing 7 GA CRS material, then having them plated Black Zinc. The black turns green in a relatively short time. Is the environment or plating process at fault?Christopher Rouse
sheetmetal forming - Anaheim, California, USA
A. Hi, Christopher. Black chromates are often 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^zinc-iron plating because the presence of the iron allows it to be chromated to a stable black color.
Ted Mooney, P.E.
Pine Beach, New Jersey
P.S.: I meant zinc-iron but didn't recognize my slip until Mr. Nurie's reply below.
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.
- New Delhi, India
With deep regret we
sadly advise that
Asif passed away
on Jan 24, 2016
A. 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.
Anoplex - Dallas, Texas USA
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 sealant coating to minimise this greenish appearance.
plating process supplier
A. 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.
chemical process supplier
Q. 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 preventative oil. Is it because reaction of Rust preventive oil with passivation? Any reaction of sun light (UV rays) on black passivation?Nagendra JK
Plating shop - Bangalore, Karnataka, India
Black chromate turns khaki green (military green) after two weeksOctober 16, 2008
Q. I am doing hexavalent black passivation on zinc cobalt plating on mild steel press component. The problem starts here: the black colour changes to greenish shade (military green) after fifteen days. Can you help me?Avinash Vidhate
Plating shop owner - Nasik, Maharshtra, INDIA
October 17, 2008
A. Hi, Avinash. The black chromate that you are using is apparently based on silver, and these are light sensitive. In my limited experience, the best resolution is to switch from zinc cobalt to zinc iron plating because there are chromates that can take advantage of its iron content to build a stable black color without silver.
Ted Mooney, P.E.
Pine Beach, New Jersey
October 18, 2008
We are doing hexavalent black passivation on Zinc Cobalt Plating for automobile spare parts and exporting it Europe countries. Our black passivated jobs are changing in colour from black to military green. Customer is rejecting the parts, and we are not finding out the exact reason for it. The discolouration occurs within one month to three months period.
Our bath condition is as follows.
Zinc 42 gm/lit
Chloride 165 gm/lit
Boric 37 gm/lit
Cobalt 1.2 gm/lit
We work within pH 4.8 -5.2
can you please guide us?
employee - Nasik, Maharshtra, INDIA
December 4, 2008
A. Dear Avinash,
Your question was repeated. Please read the answer provided above. That is all you need to resolve your problem.
Bangalore, Karnataka, India
Want our cadmium plated yellow chromated machine screws black chromatedApril 30, 2018
Q. I currently have 100 ea #10-24, 1/2" long screws (MS16997-44) that are cad plated to spec with a yellow chromate finish. I need them to be finished in a black chromate. I failed at using a hobby plating chromate. I have conceded and will now be going to a plating shop for help, but am looking at feasibility and an explanation to what might have happened right now.
Question 1: Can the black chromate be applied on top of the yellow chromate (this is what I tried?) It looks like the hobby plating system is designed to be used on zinc or cad substitute (not real cad).
Question 2: Can the yellow chromate be removed /replaced with the black chromate without harming the cadmium plating (not asking for DIY.)
I tried to use the hobby plating black chromate according to directions and it was not a uniform black coating. Eventually the parts went from black to grey and I assume the cadmium and chromate dissolved and I was left with a bare steel parts.
Buyer - Selmer, Tennessee USA
^- Privately contact this inquirer -^
May 2, 2018
A. Hi Larry,
I have used a 10% caustic solution to remove the yellow chromate from cad in the past when I needed the parts dyed green, blue or red, so I'd guess that this approach should work for black dyeing. The one thing you can't do is try to apply the dye over the already chromated part, it just doesn't work.
Aerospace - Yeovil, Somerset, UK
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