Stainless Steel Passivation
Stainless steel equipment that has been in operation can lose the passive layer that protects the steel and develop free iron on the surface. The regeneration of this passive protective layer is achieved by cleaning any organics on the metal and applying citric acid or nitric acid to the surface. This surface oxidizer forms an oxide layer and the thickness will depend on the method chosen. The Chromium oxide layer on a stainless-steel surface treated with Nitric acid can be up to 25% thicker than Citric acid applications.
Chemical passivation using Nitric Acid also requires high temperatures to ensure a complete and effective reaction. Due to the aggressive environment that is created, it may be necessary to replace gaskets within the system.
On-site Passivation Service
The passivation process is ideal for a variety of industries including aerospace & defense, medical, food & beverage, and oil & gas. We provide on-site cleaning and passivation of stainless steel equipment. The following is an example of how we process stainless steel aboveground storage tanks.
- Prepare a site-specific health and safety plan
- Set up cleaning in place system (CIP)
- Cycle cleaning/degreasing fluid through tanks, piping and temporary circulation loops
- Passivate tanks with a citric acid solution (create chromium oxide passive layer)
- Circulate acid solution through tank interiors via a 360° rotating ball
- Rinse tank and leave clean
Stainless Passivation Specifications:
- ASTM A 967, Chemical Passivation Treatments for Stainless Steel Parts
- ASTM A 380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless-Steel Parts, Equipment, and Systems
- Federal Specification QQ-P-35C, Passivation Treatments for Corrosion Resistant Steels
- ASTM B 912, Passivation of Stainless Steels Using Electropolishing
Why is Passivation needed?
Even the best grades of stainless steel with the best passivation treatments can develop rust while in service if they are in highly corrosive or high chloride environments. Swimming pools, marine areas, or regular cleaning with bleach can cause stainless steel to rust. Stainless steel subjected to these conditions should be monitored and passivated at the first sign of corrosion. Regular wash downs with fresh water can also help slow the pace of environmental chloride attacks. It’s also common for stainless equipment to collect iron contamination on the surface during fabrication or while they’re waiting to be shipped to the end-user. If this equipment isn’t passivated, you will see rust developing quickly after installation. Welds that haven’t been passivated tend to rust quickly as well.
How do I check my passivation?
We check if surface iron was removed by using a copper sulfate test. By placing a drop of copper sulfate on the treated surface, we can tell within 6 minutes if iron remains. If the drop changes to a copper color, it indicates that the surface still has iron and will rust. No color change to the drop within 6 minutes indicates the passivation process is complete and the equipment is ready for service. A passivation meter is another method we use to test the surface of stainless steel. Passivation meters detect free iron on surfaces using a galvanic process that is measured in voltage. This device allows for the testing of larger surface areas.