Passivated Stainless Steel: All You Need to Know

Stainless steel is valued for its durability and ability to resist rust, but it can still gain from additional protection. Passivation, a treatment involving chemicals, improves its ability to withstand corrosion, increasing its longevity in tough conditions. In marine, industrial, or medical settings, passivated stainless steel outperforms untreated versions.

In this article, we’ll cover the types of stainless steel that need passivation, the benefits it provides, the process, how to test it, and the grades best suited for this treatment.

What is Passivation of Stainless Steel?

Stainless steel passivation is a chemical treatment that improves resistance to corrosion by eliminating surface iron, in accordance with ASTM A967 and AMS 2700 guidelines. Unlike the natural oxide layer, passivation quickly forms a protective chromium oxide film. It’s recommended in environments with harsh chemicals or humidity, and is widely used in medical, food processing, and aerospace applications.

History of the Passivation Process

Passivation began in the 19th century when it was found that metals like iron and stainless steel naturally form protective oxide layers. In the 20th century, controlled chemical passivation methods were developed, driven by the growing use of stainless steel in industrial and military applications. Standards like ASTM A967 and AMS 2700 later ensured consistent and effective results across industries.

Why Passivate Stainless Steel?

Iron Content:
Stainless steel contains 60-70% iron, depending on the grade. While chromium naturally forms a protective oxide layer, the high iron content limits corrosion resistance.

Contamination During Manufacturing:
During manufacturing, extra iron contaminants often come from contact with carbon steel tools or airborne iron dust, further reducing the material’s ability to resist corrosion.

Damage to Oxide Layer:
The natural oxide layer can be damaged by welding or scratches, making the steel more prone to corrosion.

Passivation Process:
Passivation removes surface iron and contaminants, allowing the formation of a more stable chromium oxide film.

Improved Protection:
The enhanced chromium oxide layer provides better protection against corrosive environments, extending the durability of stainless steel.

Types of Stainless Steel Passivation

Stainless steel passivation methods vary by application. Key chemical methods include nitric, citric, phosphoric, and chromic acid passivation. Electrochemical and mechanical methods are used in specific cases. The following are listed by invention date.

Mechanical Passivation

Definition:
Mechanical passivation involves processes like polishing or sandblasting to remove surface contaminants and promote natural oxide layer formation.

Advantages:

• No hazardous chemicals are involved.
• Can be combined with other surface treatments for enhanced results.

Disadvantages:

• Does not chemically enhance the protective layer.
• Less effective for certain corrosion-resistant applications.

Nitric Acid Passivation

Definition:
Nitric acid passivation utilizes a mixture of nitric acid to eliminate free iron from the stainless steel exterior.

Advantages:

• Proven and widely used method.
• Effective in enhancing corrosion resistance.
• Complies with industry standards like ASTM A967.

Disadvantages:

• Involves hazardous chemicals.
• Requires proper disposal and safety precautions.

Chromic Acid Passivation

Definition:
Chromic acid passivation uses chromic acid to form a protective oxide layer.

Advantages:

• Offers enhanced protection in specific high-demand applications.
• Creates a durable oxide layer.

Disadvantages:

• Environmental concerns due to toxic waste.
• Decreasing use due to strict regulations.

Phosphoric Acid Passivation

Definition:
Phosphoric acid passivation not only enhances corrosion resistance but also improves surface adhesion for coatings.

Advantages:

• Improves corrosion resistance and coating adhesion.
• Suitable for pre-coating preparation.

Disadvantages:

• Less common than other methods.
• May require additional safety measures during handling.

Electrochemical Passivation

Definition:
Electrochemical passivation applies electrical current to speed up the creation of a protective oxide coating on stainless steel.

Advantages:

• Provides precise control over the passivation process.
• Suitable for high-quality surface finishes.

Disadvantages:

• Requires specialized equipment.
• More expensive than chemical methods.

Citric Acid Passivation

Definition:
Citric acid passivation is a greener method that utilizes citric acid to accomplish the same effect as nitric acid.

Advantages:

• Safer and less toxic compared to nitric acid.
• Eco-friendly with fewer disposal concerns.
• Provides effective corrosion resistance.
• It’s simpler to manage, making it one of the most widely used passivation techniques.

Disadvantages:

• May require more stringent process control for consistent results.
• Slightly less aggressive in removing iron contaminants than nitric acid.

What Does Passivation Do to Stainless Steel

Passivation offers several benefits for stainless steel. Passivated stainless steel has increased corrosion resistance, improved electrochemical stability, and cleaner surfaces compared to unpassivated stainless steel. These advantages make passivated stainless steel more durable and better suited for challenging environments, where unpassivated stainless steel would be more prone to corrosion and surface contamination.

Corrosion Resistance:

• Passivated stainless steel has 5 to 20 times better corrosion resistance.
• The passivation film prevents localized corrosion, like pitting, which extends the material’s lifespan in challenging environments.

Electrode Potential Change:

• After passivation, stainless steel’s electrode potential moves in a positive direction.
• This reduces its reactivity and slows metal dissolution, increasing corrosion resistance.

Surface Cleanliness:

• Passivation removes impurities, including free iron, leading to a much cleaner surface.
• A cleaner surface is less prone to corrosion, improving the material’s chemical stability.

Oxide Layer Thickness:

• The process thickens the chromium oxide layer, enhancing its protective abilities.
• A thicker layer offers better protection against chemicals and oxidation.

Magnetism:

• Passivated stainless steel, particularly austenitic grades like 304, is typically non-magnetic or has low magnetism.
• After cold work, some magnetic properties may appear, but this is also true for non-passivated stainless steel after cold work.

Surface Smoothness:

• Passivation often improves surface smoothness, as the process removes contaminants and free iron. A smoother surface can reduce friction in some applications and improve aesthetic qualities.

Hardness:

• Passivated stainless steel generally has slightly higher hardness than non-passivated steel.
• The passivation process can enhance surface hardness, reducing the risk of wear and scratches. However, the exact improvement depends on the material composition, such as chromium and nickel content, and the processing method, like cold working or heat treatment.

Fatigue Strength:

• Passivation can positively impact fatigue strength by improving the surface quality and eliminating contaminants.
• A cleaner surface with fewer defects helps increase the material’s fatigue life. The level of improvement relies on the steel’s original condition and the surrounding environment in which it operates.

Surface Durability:

• Passivated stainless steel tends to have better surface durability, as the passivation layer offers extra protection against physical wear and abrasion.
• This durability is especially useful in applications involving repeated contact or movement, like rotating machinery parts.

Impact Resistance:

• Passivated stainless steel may show slightly improved resistance to minor impacts, as the passivation layer can protect the surface from damage.
• However, this effect is minimal, and the overall impact resistance remains largely dependent on the alloy type and heat treatment.

Electrical Conductivity:

• Passivated stainless steel may have slightly lower electrical conductivity. The passivation layer raises surface resistance, though the impact is typically minor and doesn’t greatly affect overall conductivity.

Oxidation Resistance:

• Passivation enhances oxidation resistance by forming a stable oxide layer. This layer protects the stainless steel from reacting with oxygen, reducing the risk of surface oxidation.

Reduced Maintenance Needs:

• Passivation creates a cleaner, more stable surface that is less prone to corrosion. This means less frequent maintenance is required, reducing downtime in critical applications.

Long-term Economic Benefits:

• Passivated stainless steel lasts longer in corrosive environments, which helps lower replacement and repair costs. The initial investment in passivation pays off by reducing long-term expenses.

How to Passivate Stainless Steel?

1. Cleaning:

Start by removing any surface contaminants like grease, oil, and dirt. This step ensures the surface is clean for the passivation process.

2. Passivation:

Submerge the cleaned stainless steel in an acid solution, typically nitric or citric acid. A typical nitric acid bath contains 20-45% acid at 70-90°F for at least 30 minutes. In certain instances, sodium dichromate is included to accelerate the formation of the oxide layer. However, safer alternatives like passivation equipment with citric acid are also used to enhance the process.

3. Neutralization and Rinsing:

After passivation, neutralize the parts using a sodium hydroxide bath. Afterward, rinse with fresh water and dry completely. This step ensures all acid residues are removed.

4. Testing:

Test the passivated surface to confirm its effectiveness. Common tests include humidity exposure, heat, or salt spray to check for rust and corrosion resistance.

This process removes surface iron, restores the oxide layer, and cleans off any welding byproducts or contaminants.

What to Pay Attention to in Stainless Steel Passivation

1. Flash Attack (Uncontrolled Corrosion):
   Passivation can lead to uncontrolled corrosion if not properly managed. The flash attack results in dark, etched surfaces, which is the opposite of what the passivation process should achieve.
2. Contaminated Acid Solution:
   Keeping the acid solution free from contaminants is crucial to avoid flash attacks. Regularly replacing the acid bath with a fresh solution prevents contamination buildup.
3. Water Quality:
   Use high-grade water, such as reverse osmosis or deionized water, with lower chloride levels than tap water. This reduces the risk of flash attacks and other corrosion issues.
4. Mixing Different Grades of Stainless Steel:
   Avoid passivating different stainless steel grades, like the 300 and 400 series, together in the same bath. This can cause galvanic corrosion, where the less noble metal corrodes faster.

By following these precautions, the passivation process will be more effective and avoid potential issues.

How to Test if Stainless Steel is Passivated

Testing is essential to ensure that stainless steel has been properly passivated. Several methods can check for the quality and presence of the passivation layer.

1. Water Immersion Test

Submerge the stainless steel in water. No rust or discoloration after a certain time indicates proper passivation.

2. Salt Spray Test

Expose the steel to a salt spray environment. A passivated surface should show no rust for an extended period.

3. Copper Sulfate Test

Apply a copper sulfate solution to the steel. If no copper deposits form, the surface is properly passivated.

4. Humidity Test

Place the stainless steel in a high-humidity environment. No rust or corrosion confirms the passivation layer is effective.

5. Oxidation Test

Use an oxidizing agent on the surface. Resistance to oxidation indicates successful passivation.

6. Blue Dot Test

Apply blue dot solution to a dry stainless steel surface. If no blue spots appear within 30 seconds, the passivation layer is good.

7. Chemical Composition Analysis

Analyze surface elements like iron, chromium, and nickel to confirm successful passivation. Reduced iron and sufficient chromium indicate a proper protective oxide layer, ensuring the material meets corrosion resistance standards.

These tests ensure the stainless steel is ready for use and has a strong passivation layer for corrosion resistance.

What Passivating Stainless Steel Cannot Do

• No Electrolysis:
  Passivation is a chemical treatment, not an electrochemical process. It does not involve applying electrical currents to the metal surface.
• Cannot Remove Scale:
  Passivation does not remove heavy-scale or oxide layers formed from heat treatment or welding. Pre-cleaning is required for heavily scaled surfaces.
• Not a Paint Layer:
  Passivation is not a coating like paint. It forms a thin, invisible oxide layer, but does not add any physical layer to the surface.
• Does Not Completely Prevent Corrosion:
  While passivation enhances corrosion resistance, it does not make the stainless steel completely immune to corrosion, especially in harsh environments.
• Cannot Replace Other Rust Prevention Methods:
  Passivation should not be seen as a replacement for other rust prevention measures, such as applying protective coatings or regular maintenance.
• Does Not Fix Surface Defects:
  Surface imperfections like scratches or pits are not repaired through passivation. The process only enhances the surface’s resistance, but does not smooth out defects.

Stainless Steel Grades for Passivation

Stainless Steel Grades that Require Passivation

Stainless steel that undergoes welding or is exposed to extremely corrosive environments must be passivated to ensure corrosion resistance and long-term durability. This includes various types of stainless steel, such as austenitic (passivated 18-8 stainless steel), ferritic, martensitic, duplex, precipitation-hardened, and medical-grade stainless steel.

Stainless Steel Types That Rarely Require Passivation

Certain stainless steel grades have high natural corrosion resistance and therefore passivation is rarely required.

High-Chromium Stainless Steel

Such as 446 forms a strong oxide layer that usually provides enough protection in most environments.

High-Nickel Alloy Stainless Steel

Such as 904L has excellent corrosion resistance due to its nickel, chromium, and molybdenum content, so it rarely needs passivation.

While these types don’t often require passivation, it can still be useful in specific, demanding environments.

What Other Metals Can Be Passivated Besides Stainless Steel?

Passivation can also be applied to metals like iron, aluminum, copper, and certain transition metals such as molybdenum, nickel, tantalum, niobium, and tungsten. However, some metals, like lead and zinc-aluminum alloys, do not undergo passivation because they cannot form a stable oxide layer.

Aluminum:

Passivation Method: Immersion in chromate or phosphate solution.
Effect: Creates a protective oxide coating that boosts corrosion resistance and preps the surface for additional treatments like painting or anodizing.

Titanium:

Passivation Method: Nitric acid treatment to remove surface impurities.
Effect: Strengthens the natural oxide layer, improving corrosion resistance, especially in medical applications.

Copper and Copper Alloys:

Passivation Method: Treated with sodium carbonate or benzotriazole solutions.
Effect: Forms a stable protective film that reduces tarnishing and environmental corrosion.

Zinc:

Passivation Method: Chromate-based solution treatment.
Effect: A thin protective film is formed, which enhances corrosion resistance, especially in galvanized zinc.

Carbon Steel:

Passivation Method: Phosphate solution treatment.
Effect: Forms a protective phosphate layer, increasing corrosion resistance and improving paint adhesion.

What Happens If Stainless Steel Is Not Passivated?

If stainless steel isn’t passivated, it becomes more susceptible to corrosion, particularly in harsh conditions. Contaminants like free iron can remain on the surface, leading to rust and localized corrosion over time.

How Long Does Passivation Last On Stainless Steel?

Passivation can last for several years, but the exact duration depends on the environment and exposure conditions. In highly corrosive environments, it may need to be reapplied more frequently.

Does 316 Stainless Steel Need To Be Passivated?

Yes, 316 stainless steel benefits from passivation, especially if it has been welded or exposed to contaminants. Passivation enhances its corrosion resistance, making it more durable in harsh conditions.

Can You Passivate Stainless Steel With Vinegar?

Vinegar (acetic acid) is not typically used for passivating stainless steel because it is not as effective as stronger acids like nitric or citric acid. It may clean the surface but won’t form a protective oxide layer.

How Do You Remove Passivation From Stainless Steel?

Passivation can be removed by using abrasive methods like sandblasting or chemical treatments such as acid pickling. These processes strip the protective oxide layer from the surface.

Is Passivated Stainless Steel Conductive?

Yes, passivated stainless steel is still conductive. The passivation process forms a thin, non-conductive oxide layer, but it does not significantly affect the material’s overall electrical conductivity.

Secure Your Passivated Stainless Steel Today

Passivation improves stainless steel by removing contaminants and enhancing corrosion resistance. It increases durability, reduces maintenance, and is especially useful after welding or exposure to harsh environments. While not always necessary, it is highly beneficial for many industrial, marine, and medical applications.

SteelPRO Group offers a large selection of passivated stainless steel, as well as popular passivated stainless steel bars and other products. For more information on other processes, please visit our blog page. If you’re interested in stainless steel surface treatments, please check out our comprehensive guide to stainless steel surface finishing.

Contact our experts now for the best quote!