As technology advances and the understanding of corrosion protection evolves, standards for metal surface preparation have been updated or replaced over the years. An earlier standard for “passivation treatments for corrosion resistant steel” was QQ-P-35. First issued by the U.S. Department of Defense in July 1963, it was aimed at standardizing component passivation procedures among existing providers. This standard was applied primarily for parts used in aerospace.

Originally broken up into eight processes, QQ-P-35 only featured passivation techniques containing nitric acid (HNO₃). Types I, III, IV and V were eventually withdrawn and the remaining four had different combinations of process time, temperature, and acid concentration. Type II, unlike other methods, incorporated sodium dichromate (Na₂Cr₂O₇•2H₂O), which is difficult to handle and a health hazard. Under QQ-P-35, choosing the method of passivation was governed by the type of stainless steel being treated. Using the wrong parameters for a particular corrosion resistant steel could damage it.

The Transition from QQ-P-35

QQ-P-35 served as a standard for 35 years. On September 11, 1998 it was withdrawn by Cancellation Notice 3 by the Department of Defense’s Aerospace Materials Division. This was due to the National Technology Transfer and Advancement Act of 1995 (NTTAA). NTTAA was envisioned to help speed up the development of new products and processes by making federal laboratories available for use by the private sector in a collaborative manner. As part of the act, governmental agencies were to use industry standards wherever possible.

Additionally, federal standards were to also become industry standards where applicable. The Cancellation Notice specified that it would be replaced by ASTM A967 and AMS-QQ-P-35. The former was an already existing standard published by the American Society for Testing and Materials (now as ASTM International) while the second was simply a republishing of the standard under the auspices of the Society of Automotive Engineers (SAE), which later became SAE International.

In 2002, seven years after AMS-QQ-P-35 was published, it was cancelled and replaced by AMS 2700 to address issues that had ironically made it unsuitable for use as a government standard. Revision B in 2004 removed the remaining obstacles to its use as a governmental and private industry standard.

While 25 years have passed since QQ-P-35 was cancelled, some legacy documents continue to surface which specify that standard. Currently, ASTM A967 Standard Specifications for Chemical Passivation Treatments for Stainless Steel Parts and AMS 2700 Passivation of Corrosion Resistant Steels are the replacement standards for passivation when QQ-P-35 is requested.

Passivation Protocols: QQ-P-35 vs. ASTM A967 and AMS 2700

One of the ways that both ASTM A967 and AMS 2700 differ from QQ-P-35 is the addition of citric acid (C₆H₈O₇) as a passivation chemical. It is given equal recognition along the existing practice of using nitric acid. Not only is citric acid safer to use than nitric acid, but Astro Pak has found that it provides superior results when properly used.  Both documents also provide in-depth information on selecting the correct passivation method based on the alloy being treated. The graphic below, developed from ASTM A967 Table X2.1 and AMS2700 Table 4, shows suggested passivation treatments for general groups of stainless steel alloys.

While the current standards have similarities, they are not identical. For example, ASTM A967 contains more options for passivation verification tests while AMS2700 provides more guidance on sampling large batches of components for testing and inspection after passivation.