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"Blue or yellow passivation better after Zinc plating?"

(to provide context, hopefully helping readers more quickly understand the Q&A's)

Zinc plating virtually always needs & receives a subsequent "chromate conversion coating" (sometimes called 'passivation') to deter white rust. In the 'old days', pre-1990s, there were two types:
1. a very robust hexavalent chromate which was usually yellow/golden. Hexavalent chromate is inherently yellow, so the yellow color had a cause and a meaning.
2. a much less corrosion resistant trivalent chromate which was usually clear or blue.

As many of us now know and accept, hexavalent chromates are toxic and carcinogenic, so there has been worldwide pressure to reduce or eliminate their usage, and many rules have been established (RoHS, REACH, and automotive standards for example) which restrict or forbid hexavalent chromates.

Fortunately, new developments in the late 1990's and beyond have now made proprietary trivalent chromates (usually with an additional 'sealant' step) the equal of hexavalent chromates, at least in salt-spray test corrosion resistance. Many of these proprietary trivalents are inherently clear or almost clear such that a yellow color usually only indicates the presence of dye, not additional corrosion resistance.

Current question:

September 2, 2021

Q. Dear Sir,

We are doing low cyanide Zinc plating on steel parts of our that are exported to Europe. Due to REACH law we cannot do hexavalent Chromium Passivation. We have therefore used a special (branded) passivation with trivalent chromate + Silica nano particles. Before 96 hrs NSST (ISO-9227 ) there is white rust and black spots on the parts (see attached picture). What is the reason for these black spots and white rust? Some of the other parts we do zinc plating and hexavalent passivation do not show any black spots in SST. Does Silica nano particles cause this black spots ? Please provide your opinion and feedback regarding this. What can we do to avoid this problem? Our target is no black spots / white rust for 96 hrs in SST.


Best Regards

Yasir Khan [returning]
Manager Electroplating - Karachi, Pakistan

September 2021

A. Hi Yasir. I don't have any insight into the electroplating process supply situation in Pakistan, but it does seem like the trivalent chromate available to you is simply not up to the job.

sidebar I know it can be hard for people from countries where names are not in english to always use the same anglicized version of their name, but it is helpful to know when people have long-running problems, so please try. For example, we see that you are the same person as 'Yasir Zoha', and have been having this problem for two years now.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

"REACH: Proposition 65 on steroids". Diagnosis: steroid psychosis :-)

Closely related Q&A's, oldest first:

July 13, 2010

Q. Dear Sir,

I want to know the comparison between blue and yellow passivation in Zn plating and which is the best one to follow. Explain me with some justification. Do the needful in this regard.


Hari Haran
QA/QC Engineer - Riyadh. Kingdom of Saudi Arabia

July 13, 2010

affil. link
Zinc Plating
by Herb Geduld
from Abe Books

A. Hi, Hari. A few years ago the answer would have been that yellow chromate is heavier and contains more hexavalent chromium, and offers about 2-1/2x the corrosion resistance.

But today, what with RoHS standards, most "chromates" are trivalent and basically colorless, and most of the color you see is just dye. Further, the chromates are virtually all proprietary and based on several different technologies: thin film vs. thick film, sealer vs. no sealer, etc. One of the minor pieces of fallout from this generally beneficial change is that the color gives you no indication at all of the corrosion resistance.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

September 19, 2010

A. Welcome all

You can use yellow but if green available you can use it also.


Yasser Alshafee, Senior Chemist
steel factory - Cairo, Egypt

Zinc Plating: clear, blue, yellow, green passivation differences (trivalent or hexavalent)

September 9, 2018

Q. I want to know about the processing of zinc plating, different types of passivations (clear, blue, yellow, green), (trivalent or hexavalent), how this is being processed also their advantages and disadvantages.

Himanshu Vashishtha
- Gurgoan, Haryana & India

September 2018

Digital version

(No longer published, but a copy is on Academia.edu)
Download it before it disappears.

A. Hi Himanshu. These days you usually must use trivalent chromates because hexavalent chromates are not RoHS compatible. There is usually very little difference in the chromates except for their color, as it is usually just dye, although there are exceptions like proprietary thick-film trivalent chromates.

If you tell us who you are, what you do, why you want to know, and what kind of parts you have in mind we can probably help you arrive at some decisions. But without that it's hard to get started ... books about zinc plating are hundreds of pages long and we have a hard time knowing whether to start with a discussion about rack vs. barrel plating, cyanide vs. acid vs. alkaline zinc plating, pretreatment for plating, chromates & sealers, or what. Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

What treatment should we give to pregalvanized EMT conduit?

January 20, 2019

Q. The same question for our EMT Conduit Tubes made out of pregalvanized steel sheets. Which passivation should I use to give it anticorrosive properties as well as a clear surface keeping its shine?

Amed Hassan
El Delta El Masreya - Cairo Egypt

  ^- Privately respond to this RFQ -^

January 2019

A. Hi Amed. The appropriate chromate quench or clear chromate conversion treatment will have already been applied by the galvanizer. It may be difficult or impractical to apply a chromate a substantial time later because the conduit is no longer clean and active -- and to the best of my knowledge it is not done.

I think you dare not apply an organic clearcoat because you will be defeating the expectation that galvanized conduit is conductive.

If I am misunderstanding, and you are fabricating EMT conduit in house from pre-galvanized material, there will be no zinc left at the weld line, and the whole surface is likely to be dirty and inactive after fabrication; so I think you need to clean, activate, zinc electroplate, and chromate post-treat it after fabrication. As for which chromate, you probably should use a RoHS-compatible proprietary clear chromate. There may be ASTM, ISO, UL and similar standards for EMT tubing that dictate what must be done. Good luck.

Decades ago I worked for a company which manufactured conduit plating machines and I saw them in the field, but I was never personally involved in designing them and I don't know exactly what steps they executed, how thick the plating was, or whether they started with pre-galvanized material. That 'they exist' is about the limit of my knowledge on that subject :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

September 3, 2019

Q. Trivalent chromium Passivation was invented and used for more then 10 years now but still its corrosion resistance is very weak as compared to that of Hexavalent Chromium? Is this true? And if yes why is this so ?

Yasir Zoha
Alsons Group - Karachi, Pakistan

September 2019

A. Hi Yasir. No, I don't think that's true. Although trivalent chromium conversion coating is not as simple, reliable, and overall robust as hexavalent chromium, I think the corrosion resistance of today's proprietary trivalent chromium conversion coating processes is just as good as hexavalent chromium.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

September 4, 2019

thumbs up sign Ok Thanks Ted

Yasir Zoha [returning]
Alsons Group - Karachi, Pakistan

May 31, 2020

A. High cobalt base trivalent passivation gives good corrosion protection.

Devikant Choudhary
- Pune, Maharashtr, India

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