By definition, stainless steel is not supposed to rust. However, when it is machined, formed, or fabricated, free iron can adhere to its surface. This makes a corrosion-resistant metal vulnerable to the very deterioration it was designed to avoid. Fortunately, a chemical process can be used to reverse it: passivation.
Gleco offers ASTM A967 and AMS 2700 passivation to companies seeking better rust prevention on their machined stainless steel parts and products. Known as “controlled corrosion,” passivation is a widely-used metal finishing process that uses an acid chemical treatment to create a protective oxide layer or passivation film. This layer or film helps to prevent chemical reactions on the surface of the stainless steel, making it rust-resistant.
Two Types of Passivation
Nitric acid is the original and most common chemical used for passivating. Unfortunately, it poses significant environmental and safety hazards. That’s why citric acid passivation was developed. Adopted in the 1990s, citric acid does not produce noxious nitrogen oxide vapors and doesn’t require special handling or regulatory oversight because it’s not hazardous.
While the military originally used nitric acid passivation, citric acid passivation was originally adopted by the aerospace industry under the QQ-P-35 standard. In the 2000s, this standard was replaced by AMS 2700 and implemented by the medical device industry.
Today both nitric acid and citric acid are widely used, industry-accepted methods of passivation of stainless steel.
The Passivation Process
Passivation is a relatively simple process that involves three steps:
- Cleaning: First, the stainless steel is cleaned to remove surface contaminants.
- Passivating: Next, it is immersed into a 120-150 degrees Fahrenheit acid bath for about 20-30 minutes to be chemically treated.
- Testing: Finally, the passivated stainless steel is tested.
Following are the most common testing methods for passivation:
- Copper sulfate testing: Placing the passivated piece into a copper sulfate path for a minimum of 6 minutes.
- Salt spray testing: Placing the passivated piece into a salt-spray testing chamber for a minimum of 2 hours.
- Water immersion testing: Placing the passivated piece into a tank of distilled water for a minimum of 24 hours.
- High-humidity tested: Placing the passivated steel into a humidity chamber for a minimum of 24 hours.
The Benefits of Passivation
Today both nitric acid and citric acid are widely used by a number of industries due to their ability to:
- Provide a chemical barrier to prevent rust
- Remove contaminants from the surface of the stainless steel
- Reduce ongoing maintenance costs
- Extends the overall product life
In addition, citric acid has the added benefit of being environmentally safe and free from regulatory compliance issues.