Understanding Passivation for Aircraft Stainless Steel Parts
What Is Passivation of Stainless Steel?
Passivation of stainless steel is a process intended to make the metal surface less reactive to its environment and more resistant to corrosion, rust, and other irritants. The process creates a passive oxide layer on the metal surface, enhancing its corrosion resistance.
Stainless steel parts can pick up very small contaminant fragments of metal oxides, fabrication debris, tamp iron, and inclusions, all of which can make the metal surface more vulnerable to corrosion. Stainless steel passivation serves as a corrosion prevention measure by removing surface contamination. This is critical in the aerospace industry, where components are regularly exposed to harsh service environments that can introduce countless corrosive elements into aerospace systems.
Passivation can also extend the service life of stainless steel aerospace parts. Corrosion prevention makes stainless steel parts less vulnerable to issues that can lead to failure. The result of passivation is more reliable parts with longer maintenance intervals. Stainless steel passivation also enhances the appearance of stainless steel aerospace parts, which is valuable in some applications.
Passivation is compatible with many metals, including stainless steel, and is one of several methods for aerospace coatings that can be NADCAP accredited.
Understanding the Passivation Process
The passivation process has three primary stages: cleaning, an acid bath, and testing. Each stage plays a critical role in ensuring effective corrosion prevention.
Cleaning
Stainless steel parts destined for passivation are cleaned thoroughly to remove any and all contaminants that may have been picked up during machination. This may include oils, greases, and fabrication debris, among others. Depending on the severity and variety of contamination, the cleaning step of the stainless steel passivation process may also include grinding or pickling, which work well for removing thermal oxides and other tough debris.
Acid Bath
Once the cleaning stage is complete, stainless steel parts are coated in acid, usually via submersion in a tank bath. The acid reacts with the metal surface to enhance the protective oxide layer.
A tank bath is not the only available method for the acid bath stage of stainless steel passivation. Spray, gel, and circulation methods may also be considered. However, submersion in a tank bath allows for maximum penetration of the part.
In the context of aerospace parts, acid baths are usually performed with nitrous acid. Citric acid may be used occasionally but is more commonly deployed in the medical device industry.
Testing
The final stage of the passivation process is to test the passivated component and ensure that the process was successful. There are several industry standards in circulation designed to assess the quality of passivated parts:
- ASTM A967-01 Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. ASTM A967 includes recommendations for the passivation of stainless steel parts and details a number of tests designed to assess the efficacy of the process, including water immersion, humidity, and salt spray tests.
- ASTM A380/A380M-17 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems. ASTM A380 covers a wide range of stainless steel components, from individual parts to equipment and full systems.
- AMS2700 Passivation of Corrosion Resistant Steels. AMS2700 covers passivation solution types and passivation verification classes for more specific assessments.
- AMS-STD-753B Corrosion-Resistant Steel Parts: Sampling, Inspection and Testing for Surface Passivation. AMS-STD-753B is modeled after a U.S. military standard and is considered a stabilized specification, since it covers mature technology.
Benefits of Passivation
Passivation offers several key benefits, including corrosion prevention and resistance, reduced contamination risk, extended component lifespan, and improved performance.
Enhanced Corrosion Resistance
Passivation improves the corrosion prevention and resistance of stainless steel aerospace components, so they can withstand harsh environments and function as intended even in extreme conditions.
Reduced Contamination Risk
The cleaning stage of the passivation process removes all manner of unwanted debris from the surface of stainless steel components. This ensures a clean surface and keeps unwanted and potentially harmful contaminants out of sensitive systems and assemblies.
Extended Component Lifespan
Passivation extends the lifespan of aerospace components by protecting stainless steel parts from corrosion. Extending the longevity of these critical components can reduce downtime and cut costs associated with maintenance, repair, and replacement.
Choose NAP for Reliable Passivation Services
National Aerospace Processing (NAP) is a long-time provider of NADCAP passivation services for aerospace components. Our facility is equipped with a state-of-the-art, ten-tank system to ensure thorough cleaning and meticulous surface preparation before submersion in the passivation tank bath, and every stage is closely monitored by our experienced team to ensure a strong passive layer. Additionally, our quality control system is compliant with AMS 2700 and ASTM A967, so you can trust that your passivated parts meet the highest standards in the industry.
Aerospace OEMs, tier 1 suppliers, and defense contractors trust NAP for NADCAP passivation and other aerospace coatings. To get started on your own passivation, contact us >