SUS321 Stainless Steel Vs. Duplex Stainless Steel: Corrosion Resistance

SUS321 Stainless Steel vs. Duplex Stainless Steel: Comprehensive Corrosion Resistance Comparison

When selecting corrosion-resistant stainless steel for harsh environments, SUS321 austenitic stainless steel and duplex stainless steels such as 2205 and 2507 are the first choice. This article provides a detailed comparison of four key corrosion resistance properties: intergranular corrosion, pitting/crevice corrosion, stress corrosion cracking, and high temperature oxidation resistance to support your project selection.

1. SUS321 Stainless Steel： Intergranular Corrosion Resistance

SUS321 Stainless Steel: Utilizes titanium (Ti) stabilization to prevent chromium depletion and Cr₂₃C₆ carbide precipitation at grain boundaries, significantly improving resistance to intergranular corrosion in weld-affected zones. Ideal for high-temperature (450–850°C) service and welded structures.

Duplex Stainless Steel: With a dual-phase microstructure (approximately 50% austenite + 50% ferrite) and low carbon content, duplex grades inherently resist intergranular corrosion. They typically require no post-weld stabilization treatment and offer excellent corrosion resistance even after welding.

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2. SUS321 Stainless Steel：Pitting and Crevice Corrosion Resistance

SUS321: Exhibits a pitting potential (E_b) of around +0.35V (vs SCE), indicating moderate resistance in neutral chloride environments. Suitable for chloride concentrations up to ~300 ppm. In marine or aggressive chloride-rich settings, additional surface treatment or cathodic protection is recommended.

Duplex Stainless Steel (e.g., 2205, 2507): With high molybdenum (Mo) and nitrogen (N) content, these grades offer pitting potentials as high as +0.5V to +0.75V. They provide superior protection against pitting and crevice corrosion, making them ideal for offshore structures, pulp and paper equipment, and desalination systems.

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3. SUS321 Stainless Steel：Stress Corrosion Cracking (SCC) Resistance

SUS321 Stainless Steel: Like other austenitic steels, SUS321 is susceptible to SCC in chloride-containing environments under tensile stress-particularly above 60°C-such as in heat exchangers.

Duplex Stainless Steel: Ferrite in the dual-phase structure inhibits SCC propagation, offering far superior resistance compared to conventional austenitic stainless steels. Suitable for long-term use in chloride-rich environments at elevated temperatures.

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4. SUS321 Stainless Steel：High-Temperature Corrosion and Oxidation Resistance

SUS321: Forms a stable and dense Cr₂O₃ oxide layer when exposed to temperatures between 600°C and 850°C, providing strong oxidation resistance. Commonly used in high-temperature applications like furnace tubes and heat treatment fixtures.

Duplex Stainless Steel: Although highly corrosion-resistant at moderate temperatures, duplex grades are prone to phase transformation (σ-phase precipitation) at prolonged exposure above ~300°C, which degrades both mechanical and corrosion performance. Not recommended for high-temperature service.

 

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Pros and Cons of SUS321 vs. Duplex Stainless Steel

Property	SUS321 Stainless Steel	Duplex Stainless Steel (e.g., 2205)
Intergranular Corrosion	Strong (Ti-stabilized)	Excellent (inherent two-phase design)
Pitting/Crevice Corrosion	Moderate	Superior (high Mo, N content)
Stress Corrosion Cracking	Moderate	Excellent
High-Temperature Oxidation	Good (up to 850°C)	Moderate (not suitable for >300°C)