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topic 54441

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


A discussion started in 2002 & continuing through 2017

(2002)

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


(2002)

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 pullizzi portrait
Tom Pullizzi
Falls Township, Pennsylvania


(2002)

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,

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



Is there a Trivalent Chromic Anodize?

March 19, 2015

Q. Has anyone out there ever heard of a trivalent chromic anodize? Are any research facilities even looking into this? I know there is MIL-A-8625, TYPE IIB as a non-chromic alternative but I am hoping someone is developing a trivalent that meets the requirements of Type I or AMS 2470.

Tim Hamlett
Tim Hamlett, CEF
    aerospace metals distributor
Tamarac, Florida, USA




April 2, 2015

A. Hi Tim,

As far as I am aware research has really only gone in three directions, all are modified sulfuric acid anodizing solutions.

There is the boric-sulfuric system, which is a Boeing process (problem with introducing this solution into the EU as Boric Acid is proposed for REACH Annex XIV Authorization and may well be heavily regulated)
There is the tartaric-sulfuric system, preferred by Airbus and finally there is Thin Film Sulfuric Acid Anodizing (TFSAA).
One problem with all of these solutions is that the resultant coatings need to be sealed in a dichromate solution, so negating the benefits of moving away from the Cr(VI) anodizing solution!

You could look at rather more exotic coatings such as Tagnite, Tiodize and Keronite, these all work on a very different system of anodizing, but worth a look.

Brian Terry
Aerospace - Yeovil, Somerset, UK



Chromic Acid Anodising REACH push back

February 23, 2017

Q. Does anyone have any information on the proposed push back for CAA in the UK?

We are currently running some TSA trials but almost everyone I talk to in the Industry is waiting for the push back rather than looking for an alternative.

We are awaiting results from running TSA using Surtech, a trivalent chrome based additive, for sealing. Has anyone used this or any other product?

Eagerly awaiting your responses.....Julian

Julian Rees
Treatments shop manager - Redruth, Cornwall UK


February 27, 2017

A. Hi Julian,

I think it depends which business you are talking about. Aerospace has pushed back heavily because of the cycle time of a product, often well beyond 50 years. It's not that we don't want to change, but the change is going to mean some major commitment to re-qualify products and get customer approval for the changes. 7 years is probably not going to be enough time for us to complete this task. My company has over 75,000 separate part numbers that are chromic acid anodised. The solution is not a drop-in replacement, so we will have to look at what level of re-qualification is necessary to change.

That is not to say we are not looking at replacements. My company started looking at thin film sulphuric acid anodising a while back, alongside TSA, both with a self-formulated low temperature trivalent chrome seal (can't give you any information on that as it is IP protected at the moment). The problem, as you are well aware, is the cost of the research and implementation of changes.

I'm sure that your customers will be very interested in the work you are doing and what your results are. I think your customers are struggling with finding drop-in replacements as much as you are suffering from push back on chromic acid anodising replacement technologies.

Brian Terry
Aerospace - Yeovil, Somerset, UK



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