Summary for astm a967 passivation alternative Discussion
ASTM A967E passivation alternative to chromate? The thread explains that sodium dichromate in ASTM A967 is not always mandatory; Willie Alexander notes that Section 10 and Section 4.2 make the post-cleaning treatment customer-specified. Brian Terry says the key question is whether the customer forbids hexavalent chromium in the final product or forbids it entirely during processing. If only the final product must be free of hex chrome, a properly rinsed dichromate process may still be acceptable; if hex chrome is prohibited outright, a straight nitric or citric passivation should be used. Whit Roberts later confirms the customer is dictating no hexavalent chromium in the final product and that the process choice is being left to the supplier. He also notes that nitric I uses dichromate for some ferritic grades, while citric passivation needs no dichromate and avoids hazardous chemistry.
We are an OEM for the defense industry and are passivating Stainless Steel parts for our product to the customer. I see that ASTM A967E 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
March 16, 2009
First of two simultaneous responses — March 18, 2009
Whit
If you are referencing Section 10 of ASTM A967, 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
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
March 25, 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
April 15, 2009
Thanks for your responses. Before I left that project, we chose the citric acid option.
Whit Roberts
– Washington, DC
July 14, 2011
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 25, 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
June 28, 2012
this text gets replaced with bannerText
