Stainless Steel Passivation
Stainless Steel Passivation for Metal Parts
Stainless steel is corrosion-resistant by nature, but it is not entirely impervious to rusting. Abrasion, high heat, exposure to certain chemicals, and processes such as welding and forming can all inhibit its natural protective properties, making the surface more prone to corrosion.
The passivation process is used to improve the corrosion resistance of stainless steel by chemically treating the metal surface. Stainless steel passivation involves using different chemicals to remove exogenous “free iron” and iron compounds from the stainless-steel material’s surface. Once the treated surface is exposed to the air, it develops a thin oxide layer that covers and protects the underlying layers from corrosion.
Stainless steel passivation protects stainless steel and is used to repair and restore the material surface as needed. The passivation process is compatible with grade 303, 304, and 316 alloys and is typically performed to ASTM A9367 and similar industry specifications for chemical passivation.
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. After removal of the surface iron, the remaining elements react to 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.
Passivation of 304 Stainless Steel and 316 Stainless Steel Enhances Corrosion Protection
Passivation of 304 stainless steel is common because this alloy grade does not have the same level of pitting corrosion resistance as 316 stainless steel. Using the passivation process for 316 stainless steel, however, is still recommended to further protect the natural oxide layer from damage caused by manufacturing processes and exposure to seawater or other highly corrosive environments.
Benefits of Passivating Stainless Steel
Stainless steel passivation extends enhances corrosion resistance to protect components from high temperatures and highly corrosive environments. The passivation process provides several benefits, including:
- 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 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 can 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
Arrow Cryogenics can certify to ASTM A967 and numerous other industry standard specifications for the chemical passivation of stainless steel. 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 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 generate the oxide layer simultaneously.
Passivating stainless steel with citric acid is an 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 required specifications and unique needs of your project.
NADCAP Certified for Stainless Steel Passivation Processes
Arrow Cryogenics is Nadcap certified for chemical processing and complies with the AC7004 Aerospace Quality System for the Aircraft, Space and Defense Industry. As part of Nadcap accreditation, our processes were audited based on stringent industry guidelines and are regularly reviewed to maintain compliance.
Stainless Steel Passivation Specifications and Testing
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, including the passivation of 304 stainless steel and other alloys. Our SS passivation certifications include, but are not limited to, the following:
- 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 A967: ASTM standard specification for chemical passivation treatments for stainless steel parts including immersion treatments using nitric acid and citric acid, and electrochemical treatments. Typical tests for ASTM A967 include salt spray testing, copper sulfate testing, high humidity testing, and water immersion testing.
- 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-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
Contact Us for Complete Stainless Steel Passivation Services
Arrow Cryogenics has mastered the art of turning precision components into perfectly finished, top quality parts. Request a quote on stainless steel passivation services for your parts and components, or contact Arrow Cryogenics for more information.