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topic 55350

Blue or yellow passivation better after Zinc plating?

A discussion started in 2010 but continuing through 2019

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

aff. link
Zinc Plating
by Herb Geduld
from Abe Books
info on Amazon

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, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

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
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 - Pine Beach, New Jersey
Striving to live Aloha

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 contact this inquirer -^

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 - Pine Beach, New Jersey
Striving to live Aloha

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. I think the corrosion resistance of today's trivalent chromium conversion coating is just as good as hexavalent chromium.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

September 4, 2019

thumbs up sign Ok Thanks Ted

Yasir Zoha [returning]
Alsons Group - Karachi, Pakistan is possible thanks to our supporting advertisers, including this shop:none


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