Stainless steel is naturally resistant to corrosion, but manufacturing processes can introduce free iron and other contaminants onto the surface. Passivation is a chemical treatment used to remove these contaminants and promote the corrosion-resistant passive surface of stainless steel.
Passivation is commonly used for stainless steel components manufactured through CNC machining, forging, casting, stamping, and fabrication, particularly when corrosion resistance and surface cleanliness are important.
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Stainless steel contains chromium, which allows a thin passive oxide film to form naturally when a clean surface is exposed to oxygen. This passive film provides much of stainless steel’s corrosion resistance.
Manufacturing operations can introduce free iron and other contaminants onto the surface. Sources may include tooling, fixtures, grinding equipment, handling, and other manufacturing operations.
Passivation chemically removes free iron and other surface contaminants without significantly affecting the underlying stainless steel. Once the surface is properly cleaned, the passive oxide film forms naturally in an oxygen-containing environment.
Passivation is not a coating. Unlike plating or painting, it does not add a separate layer of material to the part.
Passivation is used when surface contamination could reduce the expected corrosion resistance or cleanliness of a stainless steel component.
Common reasons for passivation include:
Passivation can be particularly useful after manufacturing operations such as CNC machining, grinding, forming, and fabrication, where surface contamination can occur.
The exact process depends on the stainless steel grade, part requirements, and applicable specification, but passivation generally involves several steps.
The parts are cleaned to remove oil, grease, machining fluids, dirt, and other surface residues.
The cleaned stainless steel is treated with a controlled chemical solution. ASTM A967/A967M covers several accepted methods, including:
The appropriate treatment depends on the material and specified requirements.
The parts are thoroughly rinsed after treatment to remove residual chemicals and stop the chemical reaction.
Depending on the specification, passivated parts may be tested to verify the effectiveness of the treatment. ASTM A967/A967M includes several test methods for evaluating surface contamination and passivation effectiveness.
Nitric acid has historically been one of the most common passivation treatments. It is effective at removing free iron and other contaminants but requires appropriate chemical handling and environmental controls.
Citric acid is another established passivation method and is specifically covered by ASTM A967/A967M.
Neither chemistry is universally appropriate for every application. The choice should be based on the stainless steel grade, component requirements, customer specification, and finishing supplier’s process.
Passivation is used with many stainless steel grades, including:
The appropriate passivation treatment can vary by alloy, so the material grade should be identified when specifying the process.
Passivation can be incorporated into the manufacturing process for stainless steel components produced through a variety of methods. The need for passivation depends on the material, manufacturing process, surface condition, and application requirements.
CNC machining is a common manufacturing process for stainless steel components that require passivation. Cutting tools, fixtures, machine equipment, grinding, and handling can introduce trace amounts of iron or other contaminants onto the surface. Passivation can be performed after machining to provide a clean, corrosion-resistant surface.
TFG manufactures stainless steel components using CNC machining, including multi-axis milling and turning.
Stainless steel forgings may undergo machining, grinding, heat treatment, and other secondary operations before passivation. Passivation can be specified after these processes when surface cleanliness and corrosion resistance are important.
Stainless steel forging can produce strong, durable components for demanding industrial applications, with secondary operations available based on part requirements.
Stainless steel castings can require several secondary processes, including machining, grinding, heat treatment, and cleaning. Passivation can be used after these operations when required by the component specification.
TFG’s stainless steel casting capabilities include secondary operations such as machining, heat treatment, grinding, cleaning, and passivation.
Passivation and pickling are related but different processes.
Pickling is generally more aggressive and is used to remove scale, oxides, heat tint, and other surface conditions that can result from welding or heat treatment.
Passivation primarily addresses free iron and other surface contamination and promotes a clean passive stainless steel surface.
Pickling or descaling may be necessary before passivation when significant scale or heat tint is present.
Electropolishing is another stainless steel finishing process, but it serves a different purpose.
Passivation | Electropolishing | |
How it works | Chemically removes free iron and other surface contaminants | Electrochemically removes a small amount of material from the surface |
Primary purpose | Clean and passivate the stainless steel surface | Smooth and improve the surface finish |
Changes surface finish? | Generally minimal | Yes |
Removes material? | Very little | Yes |
Improves corrosion resistance? | Yes | Yes |
If a component requires a specific surface roughness or highly polished finish, electropolishing may be more appropriate than passivation alone.
Two ASTM standards are commonly referenced when specifying stainless steel passivation.
ASTM A967/A967M, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, covers chemical passivation treatments and testing.
It covers nitric acid, citric acid, and electrochemical treatments, along with several methods for evaluating passivation effectiveness.
ASTM A380/A380M, Standard Practice for Cleaning, Descaling, Pickling, and Passivation of Stainless Steel Parts, Equipment, and Systems, provides broader guidance for stainless steel surface preparation and treatment.
A drawing or purchase order may specify:
PASSIVATE PER ASTM A967/A967M
Additional requirements may identify the treatment method, testing requirements, acceptance criteria, or documentation.
Passivation is commonly used for stainless steel components in applications where corrosion resistance and surface cleanliness are important, including:
The appropriate stainless steel grade and finishing process should be selected based on the component’s operating environment and performance requirements.
Passivation helps maintain the corrosion resistance of stainless steel by removing surface contamination and promoting a clean passive surface. It does not make stainless steel completely immune to corrosion.
No. Passivation is a chemical surface treatment, not a conventional coating such as plating, paint, or powder coating.
Yes. 304 stainless steel is commonly passivated when required by the application or specification.
Yes. 316 and 316L stainless steel can be passivated and are commonly used where corrosion resistance is important.
Yes. 17-4 PH stainless steel components can be passivated after machining and other manufacturing operations when required.
ASTM A967/A967M is a primary specification for chemical passivation treatments of stainless steel parts. ASTM A380/A380M provides broader guidance for cleaning, descaling, pickling, and passivation.
Passivation is an important secondary finishing process for many stainless steel components. By removing free iron and other surface contaminants, it helps stainless steel maintain the corrosion resistance associated with its alloy composition.
The process can be applied to stainless steel parts manufactured through CNC machining, forging, casting, stamping, and fabrication. The appropriate treatment depends on the stainless steel grade, surface condition, application, and applicable specification.
When required, specifying a recognized standard such as ASTM A967/A967M provides a clear starting point for defining the passivation process and testing requirements.
