To the readers: My February 2004 column contained a question and answer regarding passivating to a standard QQ-P-35 (www.pfonline.com/articles/clinics/0204cl_clean2.html). The information below was in response to that column and provides additional background and insight. I appreciate you reading and taking the time to respond. Although the standard does not allow for citric acid passivation, I thought it would be worth it to find out if the previous writer could talk to their customer regarding their passivation needs. For instance, if they are supplying a defense/government contract, the citric acid does not appear to be an option. However, if the customer was simply putting it on the document as “boilerplate” and just wanted the part passivated, the citric acid could provide additional options to all parties.
Regarding the Question on Passivation to QQ-P-35:
Most military standards that require passivation still specify QQ-P-35, even though the military “cancelled” the standard and turned it over to SAE International to maintain as AMS-QQ-P-35. The two standards, except for some format changes so it looks like other SAE standards, are identical. Even if citric acid is superior to nitric acid, you can’t legally certify to a standard if you don’t fulfill the standard’s requirements. Citric acid is not an allowed substitution when QQ-P-35 is called for.
B) Type II passivation is designed for use with 400 series stainless, screws machining grades (303, 303se, 347se. et. al.) and precipitation hardenable steels (paragraph 184.108.40.206 of AMS-QQ-P-35 and QQ-P-35c-1988).
C) For cold heading and hot heading austenitic 200 and 300 series grades like 304, 305 or 384 with 17% or more chromium, types VI and VII are recommended (paragraph 220.127.116.11 and 18.104.22.168).
Table II at the back of the standard lists a wide range of stainless steel grades and the recommended passivation type for each. The new vendor may have read the spec more carefully than the old and realized type II is not recommended for the grades this fasteners manufacturer supplies.