Duplex vs Super Duplex Stainless Steel: Selecting the Right Grade for Chloride-Rich Environments

Duplex vs Super Duplex Stainless Steel: Selecting the Right Grade for Chloride-Rich Environments

When piping systems handle chlorides at elevated temperatures and under mechanical load, standard 304 and 316 stainless steels often fall short. Chloride stress corrosion cracking (SCC) and pitting can lead to unexpected, costly failures in chemical plants, offshore platforms and desalination facilities.

Duplex and super duplex stainless steels were developed to solve exactly this problem. Their two-phase ferritic–austenitic microstructure delivers roughly twice the yield strength of standard austenitic grades while offering markedly better resistance to chloride attack. Choosing between the two, however, depends on how aggressive the service environment really is.

Understanding PREN: The Key Selection Metric

The Pitting Resistance Equivalent Number (PREN) is the industry standard for comparing chloride resistance between grades:

PREN = %Cr + 3.3 * %Mo + 16 * %N

A higher PREN means better resistance to pitting and crevice corrosion. This single number explains most duplex selection decisions.

Duplex 2205 — The Workhorse Grade

UNS S31803 / S32205 (commonly known as 2205) is the most widely used duplex grade. It offers an excellent balance of strength, corrosion resistance and cost, making it the default choice for many chloride-bearing applications.

• Suitable for moderately aggressive chloride environments
• Excellent stress corrosion cracking resistance versus 316L
• Commonly used in chemical processing, pulp & paper, and water treatment

Super Duplex 2507 — For Severe Service

UNS S32750 (2507) and the related S32760 deliver the highest practical resistance to chloride pitting and SCC among duplex grades. They are specified where 2205 would be pushed to its limits.

• Ideal for offshore oil & gas, seawater systems and desalination
• Superior resistance to pitting, crevice corrosion and SCC
• Higher strength allows thinner walls and weight savings

How to Choose

Use these practical guidelines when specifying duplex grades:

1. Chloride level and temperature — the higher both are, the more likely you need super duplex.
2. PREN requirement — confirm the minimum PREN demanded by the project specification.
3. Mechanical loads — both grades offer high strength, but 2507 provides extra margin for high-pressure service.
4. Code compliance — ensure the grade meets NORSOK, NACE MR0175 / ISO 15156 or client specifications.

Kasugai Shanghai: Your Duplex Supply Partner

Kasugai Shanghai Co., Ltd. supplies duplex and super duplex pipes, flanges and fittings manufactured to international standards including ASTM A182, A815, A790 and A928. Every shipment is backed by full quality control, including chemical composition analysis, mechanical testing, dimensional inspection and material test certificates (EN 10204 3.1) for complete traceability.

Whether your project calls for duplex 2205 or super duplex 2507, our team can help you select the right grade and standard for your chloride-rich application. For more information about our stainless steel pipe, flange and fitting solutions, please contact us:

Email: zhangying@kasugai-group.co.jp