Yellow dichromate vs. Yellow chromate?

Letter 6049

Yellow dichromate vs. Yellow chromate?

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Hello, my name is Mark Thomas. I am an engineer for an OEM that uses a wide variety of plated products. Recently, a question has camp as to the difference between yellow dichromate and yellow/gold chromate. I have seen references to both. What is the difference between the two processes(e.g. salt-spray resistance when paired with zinc electroplate, appearance, application, bath chemistry, etc.).

In addition, is one process used in certain applications while thither is not? Specifically, this question is in reference to zinc electroplated fasteners and similar hardware, however any general information would be appreciated.

Finally, I would like to know what is the best way to note what won't on a part print. Currently, we use a note that says something like,"Zinc and yellow dichromate per ASTM B-633 [link is to spec at TechStreet], Type II, Fe/Zn 12."What is the best way to relay what I want to the plater? Any information would be greatly appreciated. Thanks.

Mark Thomas
- Oshkosh, WI, USA

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Hi Mark, I'm an engineer for a Tier I supplier. The two terms are the same thing. Yellow chrome is yellow chrome. Just ask for "zinc yellow" plating from your plater, give them copies of the correct specifications, and make sure the plating you will get will meet the spec.

By the way, if your application is for model-year 2003 or later,then you can't use yellow chrome. All zinc and zinc-alloy platings used currently also use some form of chrome on top of the plating.This chrome is of the hexavalent variety, and is on your company's restricted substances list. (Go check!) You can specify a TRIVALENT chrome system with a topcoat/sealer which will meet the new material regulations.

We have found that even though specialized groups within the OEMs are aware and dealing with the hex chrome situation, many of the engineers directly working on projects have not yet been informed of these new requirements. So, you are not alone, and you should spread the word to your co-workers. If you would like any further information, I would be happy to help you.

Tim Neveau
- Auburn Hills, MI

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Thanks Tim; I appreciate your response. However, I am still little bit confused. If yellow dichromate and yellow chromate are the same thing, then why is the distinction "yellow dichromate" made as opposed to say "clear chromate." Why not simply refer to all as chromate. Please feel free to go into detail. It is important that Understand what exactly the process is. Thanks again.

Mark Thomas
- Oshkosh, WI USA

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There are different types of hexavalent chromates. Dichromate and chromate refer to the same hexavalent chrome conversion coating.Yellow and gold are about the same too.

However, there are a variety of chromates out there. There are yellow, bronze, clear, olive drab, and other colors out there. Yellow is stronger than clear. Bronze is stronger than yellow. Sometimes the only difference in these chromates is the thickness. Sometimes the chromate bath composition is different, but right now all currently specified chromates are based on hex chrome.

I just saw ASTM B633 this morning while discussing plating specs with a co-worker. Yes, "Zinc and yellow dichromate per ASTM B633,Type II, Fe/Zn 12" is a good way to specify plating on a part print.Type II is for colored chromates like zinc yellow. If you were specifying zinc with clear chromate, you would have to specify Type I.

I suggest that you ask your plater and/or the plater's chemical vendor about trivalent chrome passivation. I also suggest you ask someone in your corporate materials group about the European Union restrictions on hexavalent chrome. This topic is a big issue in fastener groups too, and you might also be able to get internal information there too.

I have had meetings with GM, Ford, and Daimler Chrysler, and all three are working on phasing out the types of chromates you are asking about! You maybe have one more year in which you can continue to specify this type of plating.

Tim Neveau
- Auburn Hills, MI

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I am the Quality Manager at a spring manufacturer. We have a few customers who want their product plated with yellow dichromate or clear Zinc plating. I am looking for information concerning the plating process so that I have a better understanding when discussing issues with our vendors. Any information will be greatly appreciated. Bill Nelson

William L. Nelson
- Adel, Iowa, USA

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In brief, Mr. Nelson, things are zinc plated because it is an inexpensive, corrosion resistant, reasonably attractive finish which offers cathodic protection to steel (it dissolves preferentially to steel, thus protecting the steel, even if scratched).

However, zinc itself corrodes fairly quickly and the corrosion products are unsightly, sticky, white salts. Chromate conversion coatings on the zinc plating are very thin protective coatings that keep the zinc shiny and corrosion free for a time.

Traditionally, clear chromates have usually had a mix of hex and trivalent chromium while yellow chromates have been hexavalent based. The yellow chromates offered substantially better corrosion resistance.

There has been worldwide pressure to eliminate the use of hexavalent chromates for a while because of their toxicity and, in some cases, carcinogeneity. Recently, the European Union issued a directive that autos can have no (or essentially no) hexavalent chromium in them. This greatly intensified the pressure, and most people are trying to eliminate hexavalent chromates from their products; alternatives have recently been developed and improved, but are not as reliable or inexpensive as hexavalent chromates yet.

As for "chromate" vs. "dichromate", this is a semantics/language issue. Although to a chemistry professor chromate is CrO4 and dichromate is Cr2O7, to a plater either there is no difference or they may think of chromates as clear and dichromates as yellow, but it has little basis in fact. Further, as Tim says, all such chromate conversion coatings will soon use trivalent chromium rather than hexavalent anyway and the term trivalent chromate is an oxymoron anyway (CrO4, i.e. 'chromate' can't be trivalent).

I hope this answered your question.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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If anyone is looking for the relevant law on the (zinc) hexavalent chromate issue take at look at the European Union ELV (End of Life Vehicle) Directive 2000/53/EC Article 4 Section 2. (a) Prevention and Annex II Exemptions.

The Directive is available online: europa.eu.int

Sean
Engineering Manager
Ramsey, NJ

Sean McDermott
- Ramsey, NJ

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Tim, I am also struggling with the Hex Chromate issue. Are all Chromate finish (blue, black, clear etc..) contain Hex Chromium? My understanding was, only Yellow contains the Hex Chromium. Also, my understanding is with the trivalent Chromium, you don't need a clear coating. It provides the same 96 hrs of salt spray protection. Is this correct? Your clarification will be greatly appreciated.

Rita Mohant
- Stoughton, MA

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I regards to the Trivalent coating....is this RoHS compliant?

ref: http://www.finishing.com/60/49.shtml

Gene Crevier
- Grand Rapids, MI

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This letter goes back pretty far in time and there have been huge investments and consequent improvements in trivalent chromating in the last few years. So, as an update --

Trivalent chromates have existed for decades but were rarely used because their corrosion resistance was so inferior to hexavalent chromates. But with RoHS regulations and other pressures to remove hexavalent chromium from products, new trivalent chromate processes have been introduced that can match the performance of hexavalent chromate, while conforming to RoHS requirements. All of these are highly proprietary; some involve a post treatment after the chromate whereas some involve a thick film of chromate. None are as easy as hexavalent chromating, nor as cheap, and they are usually not as reliable -- but elimination of hexavalent chromate is necessary and it can be done.

Ted Mooney
finishing.com
Brick, New Jersey

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