Stainless Steel Passivation
What is Stainless Steel Passivation?
Stainless steel passivation is a process that gives stainless steel even greater resistance to corrosive environmental factors such as air and water. While it is corrosion-resistant by nature, stainless steel is not entirely impervious to rusting. Iron embedded in the surface during part forming and machining can corrode and create rust spots, called “rouging”. Passivating stainless steel removes exogenous “free iron” and/or iron compounds from the material’s surface, and helps create a new corrosion resistant oxide layer over the affected areas. It also reinforces the stainless steel’s naturally occurring oxide layer.
Stainless steel’s protective oxide layer can be damaged by abrasion, high heat, or exposure to certain chemicals. However, through the passivation process, it can be repaired and restored as needed
How Does the Passivation of Stainless Steel Work
Passivating stainless steel requires a two-step process. First, the stainless steel is submerged in a special chemical bath to clean its surface. The material is then submerged in a second bath that uses nitric acid, citric acid, or similar chemicals to fully dissolve any embedded iron contaminants without significantly affecting the surface of the stainless steel.
The stainless steel is then removed from the bath and allowed to react with oxygen, which forms a new, protective layer of chromium oxide across the treated surface.
Benefits of Passivating Stainless Steel
- Improved corrosion resistance without plating
- Superior surface cleanliness
- Eliminates iron contamination reactions with other materials
- No discoloration from rust
- No dimensional change
- Repairs damage to the naturally-occurring chromium oxide layer
Passivation & Cleaning Specifications
Arrow Cryogenics certifies to many passivation and cleaning specifications, including, but not limited to:
- AMS 2700
- ASTM B912
- ASTM B849
- ASTM F86
- ASTM A380
- CGA G-4.1
- ASTM B117
Please contact Arrow Cryogenics to inquire about other specifications. We can meet the requirements of almost any customer or industry-standard specification for the passivation of stainless steel.
Testing for Free Iron
After parts have been passivated, Arrow Cryogenics tests the surface for free iron. We use a variety of tests to detect free iron, including high humidity testing, salt spray testing, and copper sulfate testing.
When requested, Arrow Cryogenics will provide certification of stainless steel passivation and testing.
Stainless Steel Passivation Chemicals
To treat your stainless steel parts, we will use either a citric acid or a nitric acid passivating chemistry. Nitric acid passivation is the more common process, as this strong mineral acid quickly dissolves all iron compounds and other trace minerals from the stainless steel’s surface, and serves as a potent oxidizer, which helps to simultaneously generate the oxide layer.
Passivating stainless steel with citric acid is alternate option. Citric acid is not as corrosive as nitric acid, though still powerful enough to remove free iron, etc.
Our expert team can help you determine whether citric acid or nitric acid passivating is better for your specific application, based on the specification that needs to be met and the unique needs of your project.