As stainless steel is machined, formed, or fabricated, free iron can be introduced, leaving the surface vulnerable to corrosion. Gleco offers ASTM A967 and AMS 2700 citric passivation to remove this iron so companies can achieve longer-lasting corrosion resistance in their parts and products.
Passivation is a widely used metal finishing process in which the chemical treatment of the stainless steel creates a protective oxide layer (known as a passivation film). This oxide layer prevents chemical reactions, so the passivated stainless steel can better resist rust.
Passivation is sometimes referred to as “controlled corrosion” because the acid in the bath actually corrodes the free iron from the surface. This normally occurs in a uniform, controlled manner. A flash attack happens when the chemical corrosion is not properly controlled, and the metal develops a dark, heavily etched surface.
The Passivation Process
The passivation process only involves three steps:
- The stainless steel surface is cleaned to remove surface contaminants such as oil and grease.
- The stainless steel surface is chemically treated by being dipped into an acid bath. The immersion time is typically about 20-30 minutes, and the acid bath is typically heated to between 120-150 degrees Fahrenheit.
- The passivated stainless steel is tested to verify passivation success.
Common tests for passivation include (from fastest to slowest) copper sulfate, salt spray, water immersion, and high humidity. The copper sulfate test involves placing the passivated piece into a copper sulfate path for a minimum of 6 minutes. The salt-spray test involves placing the passivated piece into a salt-spray testing chamber for a minimum of 2 hours. The water immersion test involves placing the passivated piece into a tank of distilled water for a minimum of 24 hours. The high humidity test involves placing the passivated steel into a humidity chamber for a minimum of 24 hours.
The Benefits of Citric Passivation
Passivation is an ideal post-fabrication process for stainless steel parts and products because it offers a number of unique benefits:
- Chemical barrier against rust
- Surface contaminant removal
- Extended product life
- Reduced maintenance costs