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ASTM 967E Passivation alternative to chromate?

 

March 16, 2009

We are an OEM for the defense industry and are passivating Stainless Steel parts for our product to the customer. I see that ASTM A967 [link is to spec at TechStreet] E specifies sodium dichromate for the post-cleaning treatment and the Nitric I, Nitric V, Citric IV, and Citric V passivation baths. It worries me as it is a chromic acid salt (and hexavalent chromium?) and the customer's materials list prohibits chromic acid yet we are to meet the applicable mil specs. Is there an alternative to sodium dichromate, or is there a chromic-acid-free and hexavalent chromium free codified process approved by mil spec or federal standards?

I understand from another letter where sodium dichromate goes from hexavalent chromium to trivalent chromium in the passivation bath and/or post-cleaning rinse?

Whit Roberts
engineer/product designer - Westminster, Maryland

First of two simultaneous responses -- March 18, 2009

Whit

If you are referencing Section 10 of ASTM A 967, it is the customer that specifies a post-cleaning treatment (of sodium dichromate).

Also refer to Section 4.2 Ordering Information.

Willie Alexander
- Colorado Springs, Colorado


Second of two simultaneous responses -- March 19, 2009

Whit,

It depends on what your customer states: is it no hexavalent chrome to be present in the final product, or is it to be no hexavalent chrome full stop?

If it is the earlier, then you are OK, assuming you rinse properly there will be no hexavalent chrome in your product.

If it is the latter, then you could use a straight nitric or citric passivate. Again, it comes down to what your customer allows.

Brian Terry
Aerospace - Yeovil, Somerset, UK


March 25, 2009

Thank you for the helpful responses. The customer is dictating no hexavalent chromium in the final product. Regarding the particular process, the customer is pushing that to us and allowing us to decide.

Whit Roberts
- Westminster, Maryland, USA


April 15, 2009

Nitric I includes the dichromate in the bath. You can see from the nitric treatment vs grade chart that this is needed for nitric passivation on the ferritic grades.

No dichromate is ever necessary when passivating with citric acid. Citric passivation completely eliminates hazardous chemistry.

The previous responses are correct, though. All post treatments are listed as "when specified by the purchaser", and with stainless passivation, the dichromate functions as an oxidizer and does not leave hex chrome on the surface of the part. (As opposed to zinc chromate conversion passivation, where it does. You have probably seen the recent industry turnover from hex chrome to trivalent chrome for that process due to OSHA and RoHS.)

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois



July 14, 2011

Thanks for your responses. Before I left that project, we chose the Citric Acid option.

Whit Roberts
- Washington, DC



June 25, 2012

Q. If a customer specifies Passivation in accordance with ASTM A967 does that mean that the Post-Cleaning Treatment specified in section 10 must be performed or does it need to be specified by the customer on the drawing or purchase order?

Ron Rink
Quality management - Souderton, Pennsylvania, USA

June 28, 2012

A. Ron

I recommend you get a copy of the specification if you do not already have one. You will find quite a few "When specified...", "When required...", "Unless specified...".

10.2 begins as "When specified..."

Willie Alexander
- Colorado Springs, Colorado

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