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Is there hexavalent chromate in dichromate sealed or chromic acid anodized components?

 

Q. A colleague of mine at a major electronics OEM has indicated that sodium dichromate sealed sulfuric acid anodized aluminum parts can, if not controlled properly, show presence of CrVI in the coating. I'd like to know if anyone else has heard of this happening and if so to what extent?

Apparently this has been a issue plaguing my colleague's chromate immersion process. I am therefore concerned that acid anodized parts that use dichromate sealant may have a similar problem.

Has anyone experienced formation of CrVI within the conversion coating as a result of improperly controlled trivalent anodize/sealant processes? If so:

1)How common is this problem?

2)Can you recommend a lab that can discern CrIII from CrVI within the conversion coating? ....What is the best analytical technique to do this?

3)Is the CrVI found under the anodized coating or sealant and if so how is it best to prep samples for analysis?

4)How much CrVI have you or others found in coatings formed by improperly controlled CrIII processing? ....Are there some literature references on this?

5)What is the process window for critical parameters, e.g., How can one specify correct process constraints to suppliers to prevent this?

Thank you in advance for your consideration.

Mark Newton
- Cupertino, California, USA


 

A. Sodium dichromate is a hexavalent compound, I don't see why you would not have traces of hexavalent chromium in the sealed coating.

tom portrait Tom Pullizzi
Falls Township, Pennsylvania




A. Hi Mark. I have to agree with Tom. Although there are trivalent chrome treatments for certain types of processing (the TCP process for Mil-DTL-5541 conversion coating and proprietary trivalent conversion coating for zinc plated components, for example), the traditional dichromate seal is a hexavalent process (dichromate is Cr2O7-- so the chromium ion is hexavalent).

There may be a proprietary trivalent substitute for it, although I haven't heard of it myself; if you are using a proprietary trivalent substitute though, the vendor may have some data on the issue of the trivalent ions going hexavalent in service, but quantifying the anticipate amount is a tricky issue because there are countless variables. Good luck.

Regards,

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


February 24, 2010

Q. I hope some one can help. Our company has been asked about converting to Tartaric acid/sulphuric anodising from Chromic acid anodising due to our customers desire to comply with the European REACH legislation. As REACH relates to 'substances of concern' being present in the finished article, then is there any Chromate in the surface of a component that has been chromic acid anodised? If not, then surely the REACH legislation should not apply to CAA finishes.
Thanks in advance for any response given.

Mark Lees
company chemist - Isle of Man, British Isles

First of two simultaneous responses -- February 25, 2010

A. Yes, CAA has chromate in the finish, however, for REACH compliance you may be able to claim an "article exemption". Read more about REACH as it applies to metal finishing here: http://www.nmfrc.org/compliance/reach.cfm

Tartaric-sulfuric acid anodizing and boric-sulfuric acid anodizing are both replacement chemistries aimed at eliminating the chrome of chromic anodizing. It may make business sense to switch to one of these processes anyway rather than try to achieve an article exemption under REACH. BSAA is much more of a tried and true process by many shops in the U.S. compared to TSAA.

There is one more thing to consider. A switch to TSAA or BSAA doesn't necessarily ensure REACH compliance as those coatings, last time I checked, still relied on a dilute chromate seal.

Jon Barrows, MSF, EHSSC
Springfield, Missouri



Second of two simultaneous responses -- February 26, 2010

A. Mark,

I am assuming that someone has asked if any of your parts contain Substances of Very High Concern (SVHCs)at concentrations greater than 0.1% in accordance with Article 33 of the regulations.

The current SVHCs listed that contain hexavalent chromium are the following:

Sodium dichromate (both anhydrous and dihydrate)
Lead sulphochromate yellow (C.I. pigment yellow 34)
Lead chromate molybdate sulphate red (C.I. pigment red 104)
Lead chromate

Chromic acid anodising utilises chromium (VI) oxide and is not currently listed as a SVHC and so does not need reporting. Even if some of the chromium (VI) oxide was left on the surface, it still doesn't come under the reporting requirements of REACH.

I will say that I expect chromium (VI) oxide to be included in one of the next two or three SVHC lists (currently being published annually), so it will only be a short stay of execution before it does come under Article 33 reporting requirements.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom
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