Stainless Steel Passivation
Stainless Steel Passivation for Metal Parts
Stainless steel is corrosion-resistant by nature, but it is not entirely impervious to rusting. Passivation of stainless steel is a process used to improve corrosion resistance by chemically treating a metal surface. The passivation process removes exogenous “free iron” and/or iron compounds from the stainless-steel material’s surface. Once the treated surface is exposed to the air, it develops an oxide layer that covers and protects the underlying layers from corrosion. The passivation process also reinforces the stainless steel’s naturally
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.occurring oxide layer.
The Stainless Steel Passivation Process
The corrosion resistant surface of stainless steel can eventually wear out, exposing the iron within the steel to oxygen, which can lead to corrosion and eventual rusting. Stainless steel passivation provides an extra layer of protection, essentially shielding the inner steel and iron from oxygen.
The passivation process begins by submerging the stainless steel 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. Once the surface iron is removed the leftover elements then react to the oxygen in the air to form a layer of chromium oxide that protects the rest of the steel from corrosion.
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
Testing and Industry Specifications for the Passivation of Stainless Steel
Industry specifications set mutually agreed upon expectations and help industries set best practices for the delivery of quality industrial components, products and services. The specifications are set by professional organizations including SAE International and ASTM International, as well as military and aerospace associations.
Arrow Cryogenics certifies to many passivation and cleaning specifications for metal passivation and cleaning processes. Our stainless steel passivation certifications include, but are not limited to:
- AMS 2700: SAE International certification that defines standards and requirements for removing surface contaminants to reinforce the protective properties of stainless steel and improve corrosion resistance.
- ASTM B912: ASTM International specification for passivation of stainless steel alloys using electropolishing. It places standards on the passivation process and secondary processes to ensure products meet performance requirements and are free of visible defects.
- ASTM B849: ASTM International standard specification for pre-treatment of iron and steel for reducing the risk of hydrogen embrittlement.
- MIL-STD-753B: Military standard for corrosion-resistance steel parts including sampling, inspection and testing for surface passivation.
- MIL-STD-753C: Supersedes MIL-STD-753B military standard for corrosion-resistance steel parts including sampling, inspection and testing for surface passivation.
- ASTM F86: ASTM International standard for the surface preparation and marking of metallic surgical implants manufactured from iron, cobalt, titanium or tantalum-based materials.
- AMS-QQ-P-35: SAE International specification for the passivation of corrosion-resistant stainless steel. This certification was superseded by SAE AMS2700.
- ASTM A380: ASTM International standard for chemical treatments including cleaning, descaling and passivating stainless steel parts.
- CGA G-4.1: Compressed Gas Association standard for cleaning equipment used in the production, storage and distribution of gaseous oxygen.
- MIL-S-13165C: Military specification for shot peening metal parts to improve resistance to fatigue, stress corrosion cracking, and galling. This certification was superseded by AMS-S-13165)
- MIL-STD-808A: Military standard outlining acceptable requirements for finishes, materials and processes for corrosion prevention and control in support equipment.
- ASTM B117: ASTM International standard practice for operating salt spray apparatus to create a controlled corrosive environment for testing.
- MIL-F-14072D: Military specification outlining requirements for finishes that are necessary to protect electronic equipment from deterioration caused by climatic conditions in military applications.
- MIL-STD-171D: Military standards for finishing metal and wood surfaces, including general finish codes and serving as a guide to aid in the selection of suitable materials, procedures and systems for cleaning and finishing.
- WS19168A: Surface specification for metal passivation
- A967: ASTM standard specification for chemical passivation treatments for stainless steel parts including immersion treatments using nitric acid and citric acid, and electrochemical treatments.
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 Metal Passivation
To ensure quality and compliance with industry standards, we can run several tests to ensure the passivation process was successful. After parts have been passivated, Arrow Cryogenics has the capability to test the surface for free iron using high humidity testing, salt spray testing, and copper sulfate testing that will comply the requirements of the specification we are asked to meet operating conditions.
Certification for Parts Treated Using Stainless Steel Passivation
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.