AMS 2700, “Passivation of Corrosion Resistant Steels,” is a widely used specification for small stainless steel components fabricated for the aerospace industry. AMS stands for Aerospace Material Specification. ASTM A380 and A967 focus on small components and large systems of stainless steel, while AMS 2700 is primarily for processing large batches of small components through chemical bath immersion. AMS 2700 is currently in revision F and is published by the Society of Automotive Engineers (SAE) International. The specification supersedes the now-obsolete AMS-QQ-P-35.
This article aims to provide an overview of the standard, its key features, and how it differs from other standards, such as ASTM A380 and A967. Additionally, we will dive into the specific procedures and requirements outlined in AMS 2700, such as the nitric acid citric acid passivation methods and the verification tests. We will also discuss the advantages and benefits of using AMS 2700 for passivating corrosion-resistant steels, especially for the aerospace industry. If you are looking to passivate small components, AMS 2700 provides a comprehensive guide to ensure that your corrosion-resistant steel parts remain in top condition.
Differences Between AMS 2700 and Other Standards
Passivation, as defined in Section 1.1 of this standard, is “…a process to assure removal of free iron or other less noble contaminants from the surfaces of corrosion-resistant steel parts.” AMS 2700 has no guidelines for descaling or chemical cleaning processes detailed in ASTM A380 and ASTM A967; AMS 2700 is strictly a specification on passivation processes. ASTM A967 describes a passivation procedure must follow cleaning and / or descaling. Any contamination not removed prior to passivation reduces a part’s corrosion resistance to below the level its alloy was designed for. AMS 2700 primarily focuses on processing batches of small parts in a chemical bath, while ASTM A967 gives more consideration to passivating already fabricated stainless steel systems.
Passivation Requirements in AMS 2700
All procedural information in AMS 2700 is in Section 3, “Requirements.” AMS 2700 begins Section 3 with eight parameter sets of varying concentrations, temperatures, and time for nitric acid passivation. The list also includes an electrochemical passivation procedure in which a DC current is briefly applied to the stainless steel workpiece. AMS 2700 does not allow for a user-defined set of passivation parameters (A967 Nitric Methods 4 & 5, Citric Methods 4 & 5) even if they are proven to pass a verification test. This is AMS 2700’s greatest procedural difference from ASTM A967. With AMS 2700’s larger variety of qualified parameters, one of the sets must be followed.
AMS 2700 only provides one method of citric acid passivation in Section 3. However, this method allows for the inclusion of “…additional wetting agents and inhibitors as applicable,” allowing for some process customization like ASTM A967 Citric Methods 4 & 5. AMS 2700 recommends processing citric acid passivation baths between 70 and 160 degrees Fahrenheit. Process time varies between a minimum of 4 minutes through 30 minutes, depending on the processing temperature.
Verification Test in AMS 2700
AMS 2700 contains four verification tests to confirm a well-passivated surface. The four tests are the high humidity test, water immersion test, copper sulfate test, and salt spray test. Three tests from ASTM A967, the boiling water, damp cloth and ferroxyl tests, are not included in AMS 2700. As mentioned in our post on ASTM A967, these tests are qualitative. They demonstrate that the passive layer exists in a clean and continuous state. They do not demonstrate how resistant the passive layer is. Also included in this part of the standard is a visual appearance check. Visual inspection shall have no evidence of “…etching, pitting, smutting, frosting, dimensional changes, or other chemical attack on the parts…” According to this paragraph, the only acceptable visual change because of processing is the loss of temper color on the metal surface. Since it is more focused on batch processing small parts than ASTM A967, AMS 2700 provides more information on the verification test frequency of part lots. Two subgroups of processed parts must be pulled, one group is to be visually inspected while the other subgroup is tested with one of the four verification tests.
Additional Information in Section 8 of AMS 2700
Section 8 is the final yet one of the longest sections of AMS 2700. It is titled “Notes,” providing supplemental information on best practices for passivation processing. In Section 8 are recommendations for rinse water quality, post rinse treatments, passivation of nickel-based alloys, and criteria for re-passivation. An important note in this section is the type of nitric acid used. Both AMS 2700 and ASTM A967 base their nitric acid passivation methods on using 42° Baume nitric acid. The Baume scale is a measurement of specific gravity. Positive degrees of Baume indicate chemicals or solutions denser than water. 42° Baume nitric acid corresponds to a 1.4 specific gravity solution of nitric acid that is roughly 67% by weight. A passivation process calling for 20% by volume nitric acid already accounts for the fact that the nitric acid is not 100% concentrated. No extra consideration on the weight percent is required and acid should be diluted to the specified volume percentage.
Astro Pak’s Capabilities in Passivation
No matter what standard or specification, Astro Pak has the capability and know-how to passivate whatever stainless steel components and systems you have. Our Ultra Pass® chemistry is verified by multiple acceptance tests mentioned in AMS 2700 and ASTM A967 and provides further corrosion resistance than nitric acid passivation. Reach out today to begin the process toward a more resistant, more reliable, stainless steel system.
