Passivation of Stainless Steel – Process, Benefits, and Alternatives

The passivation process is widely used in industries where stainless steel products must withstand harsh conditions, such as aerospace, medical technology, and food processing. While it effectively improves corrosion resistance, passivation does not enhance wear resistance, which is crucial in high-friction applications. In such cases, advanced surface treatments like BORINOX® hardening offer a more durable solution.

Why is passivation important for stainless steel?

Stainless steel alloys are naturally corrosion-resistant due to their high chromium content, which reacts with oxygen to form a passive layer. However, contamination from manufacturing processes, machining, or environmental exposure can introduce surface iron particles that lead to rust and localized corrosion.

By undergoing the passivation process, stainless steel products regain and strengthen their corrosion resistance, ensuring long-term durability and reducing maintenance costs.

How passivation works

Passivation is a non-electrolytic chemical process that removes free iron and other contaminants from the surface layer of stainless steel alloys. The process involves several key steps:

• Cleaning – The surface layer is thoroughly cleaned to remove dirt, grease, and machining residues.
• Acid treatment – The component is immersed in a passivating solution, typically nitric acid, citric acid, or phosphoric acid, which dissolves surface iron and enhances the protective layer.
• Neutralization – In some cases, a neutralization step using sodium hydroxide follows to eliminate any residual acid.
• Rinsing and drying – The treated component is rinsed with clean water and dried to prevent contamination.
• Testing – The effectiveness of the process is verified using industry-standard tests such as the copper sulfate test or salt spray test.

By accelerating natural chemical reactions, the passivation process ensures a more uniform and corrosion-resistant protective layer.

Benefits of passivation

The passivation process provides several advantages, making it an essential treatment for stainless steel products used in medical technology, food processing, and aerospace applications.

• Enhanced corrosion resistance – The process strengthens the passive layer, protecting stainless steel alloys from rust and oxidation.
• Removal of surface contaminants – Eliminates free iron and other impurities that can lead to localized corrosion.
• Extended lifespan of components – Reduces the likelihood of corrosion-related failures and extends the service life of stainless steel products.
• Preservation of dimensional stability – Since passivation does not involve coatings or structural alterations, it maintains the original dimensions of the component.

Nitric acid vs. citric acid vs. phosphoric acid passivation

There are three primary chemical approaches to passivation: nitric acid passivation, citric acid passivation, and phosphoric acid passivation.

• Nitric acid passivation – This traditional method is highly effective at removing surface iron and contaminants. It sometimes includes sodium dichromate for additional passivation effects. However, nitric acid is hazardous to handle and produces toxic fumes, requiring strict safety measures.
• Citric acid passivation – A more environmentally friendly alternative, citric acid passivation achieves similar results without the risks associated with nitric acid. It is biodegradable and easier to dispose of, making it a preferred choice in industries with strict environmental regulations.
• Phosphoric acid passivation – Less common but useful in specific applications, phosphoric acid passivation provides an effective protective layer against certain types of chemical reactions and environmental conditions.

All three methods enhance the passive layer, and the choice depends on safety requirements, industry standards, and regulatory compliance.

Challenges and limitations of passivation

Although the passivation process significantly improves corrosion resistance, it has limitations that make it unsuitable for certain applications.

• Limited wear resistance – Passivation does not increase the hardness or wear resistance of stainless steel products, making it less effective in high-friction or abrasive environments.
• Not effective for all stainless steel alloys – Some alloys, such as martensitic stainless steel, require special treatment as they are difficult to passivate.
• Potential for incomplete passivation – Improper cleaning or exposure to contaminants after treatment can lead to an insufficient passive layer, resulting in localized corrosion.

Alternatives to Passivation: Superior Surface Treatments from BorTec

While passivation enhances corrosion resistance, it does not improve wear resistance or provide long-term protection in extreme environments. BorTec offers advanced surface treatments that go beyond passivation by enhancing both corrosion and mechanical resistance.

BorTec Alternatives to Passivation

• Borinox® Hardening
• • Increases surface hardness while maintaining corrosion resistance
  • Prevents cold welding and fretting
  • Ideal for components exposed to both friction and corrosion
• Nickelcoat® Electroless Nickel Plating
• • Forms a low-porosity, protective layer superior to the passive layer
  • Resists chemical reactions, oxidation, and aggressive environments
  • Recommended for oil & gas, marine, and chemical processing applications
• Borocoat® Boronizing
• • Creates an ultra-hard boride surface layer for extreme wear resistance
  • Outperforms passivation by protecting against abrasion and adhesion wear
  • Ensures longer component lifespan and reduced maintenance

Comparison of Passivation vs. BorTec Surface Treatments

Conclusion

The passivation process is an effective method for enhancing the corrosion resistance of stainless steel products by removing surface iron and reinforcing the protective layer. However, it does not improve wear resistance, making it less suitable for applications exposed to high mechanical stress or aggressive environments.

For industries requiring both corrosion and wear resistance, BorTec’s advanced surface treatments offer superior alternatives. BORINOX® provides exceptional hardness while maintaining corrosion protection, NICKELCOAT® offers a chemically resistant surface layer ideal for acidic conditions, and BOROCOAT® ensures extreme wear resistance in high-friction environments.

Compared to passivation, these processes significantly extend component lifespan, reduce maintenance costs, and enhance performance in demanding applications. Choosing the right surface treatment depends on the specific challenges a component will face.

For expert guidance on the best solution for your stainless steel alloys, contact BorTec today.