Will Duplex Stainless Steels Rust?

06 January 2025 | by HUYI STEEL GROUP

Duplex stainless steels are widely celebrated for their high strength and superior corrosion resistance. They are used in demanding industries like chemical processing, marine applications, and oil and gas extraction. But despite their reputation, a common question arises: Will duplex stainless steels rust?

In this article, we’ll answer this question by exploring the factors that contribute to the rusting of duplex stainless steels, how they compare to other materials, and how you can prevent rust from affecting their performance. By understanding these aspects, you’ll be better equipped to make informed decisions when selecting materials for your projects.

stainless steel flat bar

What Are Duplex Stainless Steels?

Duplex stainless steels are alloys that combine austenitic and ferritic microstructures in roughly equal proportions. This dual-phase structure gives them a unique balance of strength and corrosion resistance. The key elements in duplex stainless steels include chromium, molybdenum, and nitrogen, which contribute to their excellent performance in corrosive environments.

Common grades of duplex stainless steel, such as 2205 and 2507. are specifically engineered for demanding applications. While 2205 is suitable for a variety of industrial uses, 2507 is a super duplex alloy designed for even harsher environments like seawater.

These materials are particularly effective at resisting common types of corrosion, such as pitting, crevice corrosion, and stress corrosion cracking, making them ideal for high-risk environments.

The Corrosion Resistance of Duplex Stainless Steels

Duplex stainless steels are primarily valued for their corrosion resistance, which is significantly superior to that of austenitic steels like 304 or 316. The high chromium content (typically 22–25%) enables the formation of a passive oxide layer on the surface, protecting the material from corrosion. Molybdenum and nitrogen further enhance this protective layer, making duplex steels more resistant to pitting and crevice corrosion, especially in environments containing chlorides.

The dual-phase microstructure, consisting of both austenite and ferrite, also plays a key role in their corrosion resistance. While the austenitic phase provides excellent toughness, the ferritic phase contributes to strength and resistance to stress corrosion cracking. Together, these phases give duplex stainless steels outstanding resistance to various forms of degradation.

Can Duplex Stainless Steels Rust?

Although duplex stainless steels are known for their corrosion resistance, no material is entirely immune to rust. Under certain conditions, duplex stainless steel may rust or corrode, but this is relatively rare and usually occurs in extreme environments or when the material is not properly maintained.

Some factors that can cause rusting in duplex stainless steels include:

High-Chloride Environments: While duplex stainless steels are highly resistant to corrosion in chloride-rich environments, very high concentrations of chlorides, such as those found in seawater, can overwhelm the protective oxide layer and lead to pitting corrosion.

Lack of Passivation: Passivation is a process that further strengthens the oxide layer on stainless steel. If duplex stainless steel is not passivated after manufacturing or installation, its resistance to rust may be compromised.

Surface Contamination or Damage: Dirt, oil, or physical damage to the surface can degrade the oxide layer, exposing the underlying steel to the risk of rusting.

Extreme Environmental Conditions: Prolonged exposure to highly acidic or alkaline conditions, as well as high temperatures, can damage the protective oxide layer, leading to corrosion.

While these factors can contribute to rust, they are preventable with proper care and maintenance.

Why Duplex Stainless Steels Are More Resistant to Rust Than Other Materials

Duplex stainless steels offer significant advantages over other metals and even some grades of stainless steel when it comes to corrosion resistance. Here’s why:

Higher Chromium and Molybdenum Content: The elevated chromium (22–25%) and molybdenum (up to 5%) content in duplex steels provides superior resistance to corrosion. Chromium forms a protective oxide layer, while molybdenum helps prevent pitting and crevice corrosion, especially in chloride-rich environments.

Dual-Phase Structure: The combination of austenitic and ferritic phases in duplex stainless steel creates a material that is both strong and highly resistant to localized corrosion. The ferritic phase is particularly resistant to stress corrosion cracking, which can be a concern in environments where materials are subjected to both corrosion and mechanical stress.

Superior Strength and Toughness: Duplex stainless steels are stronger than most austenitic stainless steels, making them less likely to suffer from mechanical failure or surface damage that could lead to rusting.

As a result, duplex stainless steels are ideal for use in industries such as oil and gas, marine, and chemical processing, where materials face harsh environmental conditions.

Common Applications of Duplex Stainless Steels

Thanks to their excellent corrosion resistance and strength, duplex stainless steels are used in various industries. Some common applications include:

Chemical Processing: Duplex stainless steels are commonly used for manufacturing equipment such as storage tanks, heat exchangers, and piping systems in the chemical industry. Their ability to withstand aggressive chemicals, such as acids and alkalis, makes them highly suitable for this sector.

Marine Environments: The marine industry benefits from duplex stainless steels due to their resistance to corrosion from seawater. Applications include offshore oil rigs, shipbuilding, and seawater desalination plants, where exposure to harsh saltwater can quickly corrode other materials.

Oil and Gas: In the oil and gas industry, duplex stainless steels are used for critical components like pipelines, risers, and valves. Their resistance to corrosion in high-pressure and high-temperature environments makes them a preferred choice for offshore drilling and subsea applications.

Power Generation: Duplex stainless steels are also used in power generation plants, where equipment must withstand both high temperatures and corrosive elements. Their ability to perform under these extreme conditions makes them valuable in maintaining the integrity of key infrastructure.

How to Prevent Duplex Stainless Steels from Rusting

To maximize the longevity and corrosion resistance of duplex stainless steels, proper care is essential. Here are some best practices for preventing rust:

Regular Cleaning and Passivation: Regular cleaning with non-abrasive cleaners helps maintain the protective oxide layer. Passivating the material, particularly after manufacturing or installation, ensures that the oxide layer is intact and fully functional.

Avoid Exposure to Harsh Chemicals: Avoid exposing duplex stainless steel to strong acids or other corrosive chemicals. If exposure is unavoidable, rinse the steel thoroughly with fresh water to remove any residues that could damage the oxide layer.

Use Protective Coatings: In some cases, applying protective coatings can enhance the rust resistance of duplex stainless steel. These coatings act as an additional barrier against environmental stressors.

Periodic Inspections: Regularly inspect duplex stainless steel components for signs of damage or contamination. If any issues are detected, prompt cleaning and re-passivation can help restore the material's corrosion resistance.

By following these steps, you can ensure that duplex stainless steel retains its excellent corrosion resistance and prevents rust from compromising its performance.


Duplex stainless steels are highly resistant to rust and corrosion, making them ideal for use in industries where materials are exposed to harsh environments. While they are not entirely immune to rust, proper care and maintenance can significantly reduce the risk of corrosion. By understanding the conditions under which rusting can occur and taking proactive measures to prevent it, you can ensure that duplex stainless steels perform optimally throughout their service life.

Contact us today for expert guidance, premium stainless steel products, and customized solutions to tackle chloride exposure challenges!

FAQs

Q: Can duplex stainless steel rust in seawater?

A: Duplex stainless steel is highly resistant to seawater corrosion. However, extreme chloride concentrations can cause pitting or crevice corrosion if the material is not maintained properly. Regular cleaning and passivation are essential.

Q: Is duplex stainless steel better than regular stainless steel for rust resistance?

A: Yes, duplex stainless steel offers superior corrosion resistance compared to grades like 304 or 316 stainless steel, thanks to its higher chromium, molybdenum, and nitrogen content, as well as its dual-phase structure.

Q: How do you maintain duplex stainless steel to prevent rust?

A: To maintain duplex stainless steel, clean it regularly with mild cleaners, passivate it after installation, and avoid exposure to harsh chemicals. Periodic inspections help identify and address any issues before they lead to rust.

Q: What is the difference between duplex and super duplex stainless steel?

A: Super duplex stainless steel has higher levels of chromium, molybdenum, and nitrogen than standard duplex steels, making it even more resistant to corrosion in highly aggressive environments, such as seawater and chemical processing.

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