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Zinc Plating with Blue Chromate is turning yellow over time
Current question and answers:October 24, 2021 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Zinc barrel plating with Cr3 Blue passivation on mild steel pressed components having complex shape is changing to dark pink /yellow shades on LCD area after 3 months in 300 pcs out of the total assembled 8000 components in one particular lot supplied.
The components are assembled with copper terminals. Conducted XRF spectrum and found 0.2 micron copper on the pink shaded area. In the same spectrum, iron shows 35% .This could be due to contamination of bath? or improper passivation? Due to Galvanic Corrosion? First time occurs in 8 years. Conducted 15 various trials in passivation, changing different concentration and time. Awaiting SEM analysis report! Can anyone help in fix the exact root cause please. Thanks.Ganesan Ponnusamy
- Maraimalai Nagar, India
November 1, 2021
A. Hello there,
We need more information about your zinc tank. Is it alkaline non-cyanide, or is it Acid zinc ? I assume is acid zinc since you have co-deposited impurities in the LCD area. Because of the geometry of your part plus being a barrel plating, your LCD area have very low current density, if its lower than 0,3 A/dm2 you have co-deposition of Iron. (Imagine doing dummy plating but with your parts instead of panels).
If you want correct passivate film on your zinc parts, the thickness should be minimum 5 µm. Also keep your Iron level low in both the tank and the passivation tank.
To check your bath for Iron, you simply can add a drop of H2O2 in a jar with sample from your bath: if it becomes cloudy brown, then your iron is high and you should treat it with H2O2. For barrel plating you can treat directly in the tank but don't add more than 0,1 ml/L at once. The iron will precipitate (considering your pH is higher than ~5.3-5.5) and will drop on the bottom of the tank. Repeat these steps until there is no precipitation occurring.
Chemical Engineer - Bucharest, Romania
Closely related historical postings, oldest first:December 13, 2008
Q. Dear sir,
We are facing a problem in Zinc plating Blue passivation, After 10 to 12 Weeks, passivation is converting to yellow passivation; we are not able to find out the problem.
plating division - Bangalore, Karnataka, India
December 18, 2008
Could you please provide us with some more information? For example is this a hex or tri clear? What types of parts are these? What conditions are the parts subjected to during these 10-12 weeks? Is the corrosion performance still up to par after the yellowing? What are the chemical properties of the bath? Etc.
The more information that you can give us the better.
David J. Wolf
Medina, Ohio ^
December 26, 2008
Q. Dear Sir,
We are fasteners platers from chennai, India and we are facing the problem of discoloration of trivalent blue passivation on storage. The silver-tone color changes to pinkish yellow on storage of 3-5 weeks. The Salt spray like is unaffected. The discoloration takes place even in new passivation bath which means zinc and iron contamination cannot be the reasons. Hexavalent passivation from the same plating baths are good and meets the salt spray expectations.
Can any one help. Thanks in advance.
process incharge - Chennai, Tamilnadu, India
June 26, 2012
I am seeing a yellowish tinge in blue passivated parts, mainly on the edges. How could that happen -- is it because of a cyanide bath? Normally RoHS compliant passivations use non cyanide bath ... is it that reason?
- Kochi, Kerala, India
A. Dear Arun,
When there are yellowish tinges on the edges, mainly is a iron content problem of the zinc bath, or the blue chromate concentration is too high.
We see that problem in barrel pieces and solve it with diluted (5%) hydrogen peroxide. Do you test the zinc bath with Hull cell? If you do, you could test it in Hull cell (2 A, 5 minutes, with agitation) and chromate the plates with and without peroxide added. Then you could make a conclusion!
Hope you could solve your issue. Best regards,
Self-Drilling Fasteners Manufacturer - Cańuelas, Buenos Aires, Argentina
Two acid zinc tanks give perfect blue but two give yellowish tintFebruary 8, 2014
Q. We have 4 Rack acid zinc plating tanks. Material from two tanks gives perfect blue tint after blue passivation. While the material from other two tanks have yellowish tint after blue passivation.
Can you suggest a remedy?
- Lahore, Punjab, Pakistan
A. Hi Shazer. You are fortunate to have a rather consistent issue rather than an off and on problem. I think Step 1 is to do Hull Cell testing of the four baths. Because of the consistent nature of the problem this testing ought to quickly reveal whether the problem is in the chemistry of the bath vs. elsewhere. Good luck and please get back to us.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 22, 2014
A. When I used to have this problem it was mostly due to iron contamination of the bath. But maybe there could be another reasons for that problem.Daniel Hernandez Guerra
- Bucaramanga Santander Colombia
February 24, 2014
A. The yellow chromate of a blue chromate formation coming out of a chloride solution is most likely high iron contamination.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
March 10, 2018
Q. I have a problem, after blue zinc and pack with plastic bag the next day it become yellowishJames kt
electroplating Malaysia - johor Malaysia
A. Hi James. I think the first step is to leave a part unwrapped, wrap a part in paper, & wrap a part in different plastic and see whether the problem is related to the wrapping material/method or whether the yellowing always appears irrespective of the wrapping. Good luck.
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Blue passivation turns into silvery colour after a weekSeptember 4, 2018
Q. We are doing plating of MS component (OD & ID (CNC ground surface) in Acid zinc plating and plating with blue trivalent passivation without top coat). With top coat also, we have tried. Topcoat itself is changing colour to silvery yellow. After a week, 10 ~ 20% of the parts are changed into yellow in colour and rest of the parts are blue in colour. We have checked the iron content in the bath and found to be 0.05 ppm only. The pH of the bath is 5.1 and passivation solution is 2.3 only. Shall we take this partial colour change is the inherent characteristics of blue passivation? Could this affect the performance of the product? What is the reason for this partial change except iron content?SWAMY RAM
- chennai, TAMIL NADU, INDIA
September 10, 2018
A. Hello Swamy,
Most likely iron content in your blue trivalent PASSIVATION tank, not in your plating bath. If it were in your tank, you would see black staining in high current zones, but yellow staining may be because of high iron content in your chromate :)
Best of luck!
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina
June 19, 2019
A. Good day.
We had that kind of problem a few years ago.
The main reason is the dissolved iron in passivate, activation or sealer.
Please try to analyze iron by sulfosalisilic acid (Fe+3) or phenantroline + sodium disulfite (Fe+2).
Once a week take out the passivation to a relay tank and clean the walls and remove the pieces from the bottom.
In my experience the concentration over 150 ppm of iron (Fe+3) generates the problem of yellow pieces
- Qro, Qro, Mexico
March 1, 2021
Q. Hi sir ,
I had new complaints from customer that parts are in Zn trivalent blue passivization turns into the yellowish on center of the part; its CRCA.
Is this issue from packing material lifetime, or finishing the parts?
Please help me.
- bangalore, karnataka, India
August 12, 2021
A. This defect is related to copper contamination in your plating bath. Use titanium anode hooks instead of copper anode hooks.Alireza Barmeh
- Isfahan, Iran