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topic 11950

Trivalent Chromate Coatings


(2001)

How many different colored trivalent coatings are commercially available (not including organic overlays)? What are the typical neutral salt spray test (ASTM ASTM B117 [link by ed. to spec at TechStreet] ) duration requirements for each color trivalent chromate? Are US electroplaters stopping the application of hexavalent chromate on all parts they produce or only those for the automotive industry? Are companies willing to pay extra for trivalent chromate or is the push to use trivalent chromate in lieu of hexavalent chromate based purely on environmental factors?

Graham Dodd
Melbourne, Australia


 

Yes, the US automotive industry is in the process of elimination of hexavalent chrome. This complies with the European Parliament's ELV Directive restricting hexavalent chrome to 2 grams per vehicle. (PS - The 2 gram limit may be voted down or eliminated before the   deadline)

Most trivalent systems are clear. This will appear silver since the passivate goes on top of silver zinc or zinc-alloy. There are trivalent yellows available, but most of these are achieved using dyes, not a true yellow. Many people are working on a black trivalent, but I have not seen one ready for production use.

Salt spray performance varies greatly. There are many systems out there incorporating different topcoats and sealers in addition to the passivation. Each supplier can give you a ballpark figure for their systems. You can get resistance equivalent to hexavalent chrome.

No, automotive companies are NOT willing to pay for these systems. Everyone has to change anyway even though it costs more. Yes, this creates a big political problem. Everyone on the chain from OEM down will need to play "hot potato" with the added cost.

tim neveau
Tim Neveau
Rochester Hills, Michigan


 

To answer your questions, maybe not in order I will relate a little information I have gathered as a supplier. Several suppliers are marketing trivalent chromate processes. The ones that re blue bright have low (17 hr) to moderate (75 hour) salt spray resistance. The yellow chromate replacements are available in all chromate formulations from a couple of suppliers, these will exceed 120 hours to white salt. Other suppliers are marketing a combination of polymers and trivalent chrome compounds which are mixed into the same tank for a one step process. Still other suppliers are using a trivalent chromate and a post dip. As usual the auto industry is looking for cost reduction while instituting standards for a more expensive process. This is the reason why the vast majority of companies plating for the auto industry are waiting for the trivalent finishes to be required! The environmental aspects are minimal in waste treatment for many plants but could make the difference in the classification of sludge and the resultant cost of disposal. The European auto industry wants to limit hex chromium in cars from a recycling standpoint to keep the hex chrome out of the recycling chain. This may not be based upon good science (my opinion) since the real danger from chromium comes from aerosol mists ( a recent study of chromium in drinking water in California bears this out).

I hope I have been helpful, not too boring or confusing.

Gene Packman
process supplier - Great Neck, New York


 

If there are trivalent chromates capable of achieving salt spray performance comparable to hexavalent chromates without the addition of top coats is the torque tension relationship affected when trivalent chromates are applied to fasteners?

Do the dyes, additives and top coats used currently and under investigation affect the torque tension relationship to any degree?

Are the Japanese OEM's and their US transplants using trivalent coatings? If not are they reducing the number of zinc plated parts with hexavalent chromate replacing them with organic or similar coatings?

Graham Dodd
- Melbourne, Australia


 

Do not assume that a trivalent passivate can get equivalent corrosion performance to a hexavalent passivate without additional topcoats or additives. I said earlier that you can get equivalent performance, but that usually requires extra steps.

Friction of the surface is greatly affected by the finish. One good thing is that the majority of topcoats and sealers can be adjusted for friction requirements.

I have not yet seen any desire or requirements from Japanese OEMs or their transplants to eliminate hexavalent chrome. I have gotten the feeling that they really like the plating systems they use now because of cost, performance, and color and would probably resist changing away from these.

tim neveau
Tim Neveau
Rochester Hills, Michigan

 

BUT...BUT...BUT doesn't Toyota, Datsun, & etc. have to meet the European directive if they want to sell cars to the EU?

pooky
tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania

(2001)  Yes exactly, Tom. It will be interesting to see if there are any last minute shifts in OEM requirements (all OEMs not just Japanese) around the 1st of July.

tim neveau
Tim Neveau
Rochester Hills, Michigan


(2001) 

As I work for one of those transplants I hope the change away from hexavalent if implemented is given plenty of notice.

Graham Dodd
- Melbourne, Australia


March 19, 2010

Chromium as Chromate or dichromate is considered hexavalent or in the oxidation state of VI. When one changes this to Chromium (III) ... in chemistry, we don't call it chromate anymore. So ... calling trivalent chromium chromate is an abuse of the word chromate and should be considered bad practice by manufacturers. I have a PhD in chemistry and find the improper use offensive.

Joe Ryan
- Sonora, California


March 19, 2010

You are absolutely correct, of course, Dr. Joe. But for each PhD or ScD, this industry has a thousand plating line operators, many without high school chemistry or much command of English, who don't have enough energy after a hard day's work to learn enough about chromium chemistry to adopt a new name to replace the oxymoron "Trivalent Chromate" :-)

It is a disinformation phrase but I don't think you and I can fix it.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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