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topic 41663

What is Blue Chromate Plating?

A discussion started in 2006 but continuing through 2019


Q. I am a young Mechanical Engineer at a heavy manufacturing plant, I am fairly new to the game and I am very inexperienced in materials finishes. I have a customer spec calling out a blue chromate finish for a part. Can anybody give me a brief description of this and if possible some references where I can goto to learn more. I am planning on hitting up the library to reference the ASM collection, is that a good enough resource or is there a better resource out there?

Kevin Engelstad
- West Fargo, North Dakota, USA


A. "Blue chromate plating" is an example of somewhat misleading lingo or trade vernacular, Kevin, and it can indeed be hard to get your bearings in such cases. As a young engineer involved in surface finishing, please give strong consideration to attending a local AESF meeting ( if possible. You do not need to be a member, you will be welcomed, and you will rub shoulders with "old hands" who will be delighted to answer your every question in whatever detail you need. If you can get to "Sur/Fin" this year, you can attend the industry's largest exhibition and conference.

To answer your question without misleading you can be tricky, but I'll try. Many steel components, including the majority of nuts & bolts, are zinc plated for corrosion resistance and an acceptable appearance. But the zinc in turn will quickly "white rust". To deter this, zinc plated components are almost always chromate conversion coated, and that chromate can be of several colors -- black, iridescent yellow, and blue being examples. The blue is a slightly blue toned bright metallic finish, it is certainly not blue in the sense of blue paint. Traditionally these chromate coatings have involved hexavalent chromium. These days they are usually trivalent to comply with RoHS requirements.

So what does "blue chromate plating" mean? Almost surely it means zinc plating followed by a blue-toned chromate conversion coating, hopefully trivalent chromate. Our "must have" booklist is more targeted towards these processes than simply starting at ASM. Good luck!

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

December 20, 2011

Q. Thanks for your article about the Blue Chromate. I am struggling with a cost associated with blue chromate and I am considering taking the product and converting it to Stainless. My customer is concerned about the conductivity of stainless with respect to that of Blue Chromate.


Ron Thompson
Engineer - Armagh, Pennsylvania, USA

May 2014

A. Hi Ron. Zinc plating with blue chromate is the least expensive electroplating of all, and although stainless steel construction might be better in some ways, it would be extremely rare for solid stainless steel to be less expensive than zinc plated and blue chromated steel.

As for conductivity, there are usually two issues, the bulk conductivity/resistivity and the surface resistance. For example, copper and silver have excellent bulk conductivity, but steel and titanium and stainless steel have poor bulk conductivity. Gold has excellent surface conductivity because it doesn't tarnish, whereas a painted steel item would have extremely poor surface conductivity if any.

Neither zinc plated steel nor stainless steel has really good surface conductivity, so neither is suitable as a surface for electrical contacts, so you are probably thinking of bulk conductivity (if the item must carry a ground current for example). The conductivity of carbon steel is about 10% of the conductivity of silver; the conductivity of stainless steel varies according to type, but it is about 1/4 that of steel. Good luck.


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

January 17, 2019

Q. Hello Gentlemen. I am restoring my 82 year old Ariel Square Four motorcycle and I've been reading your exchanges regarding Blue Chromate Plating. I am fully aware of the zinc plating processes and am trying to buy Blue and Yellow Chromates.
I can find only one supplier, [name deleted by editor]. Is this the only supplier or are there other US suppliers you might recommend?
Thank you

Patrick Volum
- Miami, Florida, USA
  ^- Privately contact this inquirer -^

January 2019

A. Hi Patrick. Sorry, but one thing we can't do here is recommend or dis-recommend specific suppliers (why?); hopefully a supplier will contact you in private. There are numerous competing industrial suppliers, but it is true that most are unwilling to sell to individuals over liability concerns and the lack of economy of scale. Good luck.


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

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