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Non Hexavalent Chromium Conversion (Clear)

July 9, 2009

We have a current requirement that our supplier provide Non Hexavalent Chromium Conversion coating with a gold tint per ASTM B921-02. They have notified us that due to environmental concerns that they will no longer use the gold tint.
My question is, would they be eliminating the gold tint strictly due to environmental concerns or as an easy differentiation between finishes and will the finish still be in compliance with ASTM B921 -02? Also, is there a way to determine if the finish truly exists.
Thanks
Jim

Jim Callen
OEM - Duryea, Pennsylvania, USA

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July 9, 2009

Hi, Jim. The only hexavalent-free conversion coating for aluminum which meets the MIL spec is "TCP". This may be what your vendors have switched to. It is clear, very thin, and cannot be dyed.

Regards,

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

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Thanks for the response. Can you define "TCP".
By MIL spec do you mean the ASTM?
If so then am I to assume that if we receive material that IS clear non-hexavalent conversion that it CANNOT be in accordance with ASTM B921 -02?

Thanks
Jim

Jim Callen
- Duryea, Pennsylvania, USA
July 14, 2009

Hi. NAVAIR developed a non hexavalent chromate conversion coating process for aluminum that they call TCP, got it approved per the military spec MIL-DTL-5541 [on DLA], and licensed it to four chemistry suppliers. This process is almost colorless and cannot be dyed. This is the only non-hex process that meets the MIL spec, and so I suspect that your vendor switched to this, and I have no reason to question your vendor's statement.

Sorry, I don't have a copy of ASTM B921 -02 handy so I cannot interpret what it says, and what finishes might comply with it, and whether they can be dyed yellow.

But the fact is that the traditional yellow color was never a dye; it was the inherent color of the hexavalent chromium toxin. Many plating shops want to switch to TCP, so I think it will be ever harder for you to continue to make your parts look like they are carcinogenic when they are not :-)

I don't think you need to give up on ASTM compliance, but should consider getting away from the gold colored dye.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July , 2009

I have just checked today with several chem. companies. None add yellow to their product. We have complaints from both the DoD and the FAA that their inspectors cannot tell if the Non-hex chromate clear conversion coating is on the metal or not. Maybe somebody can add a UV trace to the coating similar to parylene conformal coating--there is a UV trace added.

Fred Barber
- Las Vegas, Nevada
September 16, 2010

Metalast International (Minden, NV), the first licensee of the TCP process has a product called TCP-HF. It is a trivalent chromium conversion process. Additionally, we have two blue dyes available the Metalast CC-600 and the Metalast CC-600P (Powdered.) Both impart a clearly visible blue color to the trivalent chromate conversion coating and do not reduce salt spray testing hours. The color produced is not homogenous as you would expect from a dye, but is useful for an inspection aid. The Metalast CC-600 and 600P are only sold in combination with the Metalast TCP-HF as compatibility with other chromate conversion coatings cannot be assured.

Kevin Baranowski
- Chicago, Illinois, USA
January 9, 2012