Alloy family
When 316L is not enough but a nickel alloy is overkill, the answer is usually here.
This group costs far less than nickel alloys and performs far better than standard stainless.
The number that decides the choice is PREN, the pitting resistance equivalent number, calculated from chromium, molybdenum and nitrogen content. 316L sits around 25; super duplex 2507 is above 42. Grades above PREN 40 are classed as super duplex.
904L (1.4539) is a high-nickel austenitic grade used in sulphuric and phosphoric acid service and wherever stress corrosion cracking is a risk. Readily welded and formed.
254 SMO (1.4547) is super austenitic, with 6% molybdenum and a PREN around 43. Seawater cooling systems, desalination plants, pulp bleaching lines. Its chloride resistance approaches that of nickel alloys.
Duplex 2205 (1.4462) is the workhorse of the family. Its roughly equal austenite and ferrite phases give it close to twice the strength of 316L, which means the same pressure rating at a thinner wall — less material for the same job. Storage tanks, pressure vessels, process piping.
Super Duplex 2507 (1.4410) is the next step up in chloride and pitting resistance. Offshore platforms, seawater systems, chemical tankers, sour service. Covered by NACE MR0175.
Zeron 100 (1.4501) is a tungsten and copper bearing super duplex giving extra margin against crevice corrosion, preferred where seawater and reducing acids occur together.
Where duplex stops. Duplex grades are not used above roughly 250–300 °C. Held at that temperature they form embrittling phases and lose toughness. If the job is a hot one, look at austenitic or nickel-based grades instead. They are also sensitive to welding and heat treatment; if the phase balance is lost, so is the corrosion resistance.
| Grade | W.Nr | UNS | PREN (approx.) | Commonly requested standards |
|---|---|---|---|---|
| 904L | 1.4539 | N08904 | 34 | ASTM A240, B625, B649 |
| 254 SMO | 1.4547 | S31254 | 43 | ASTM A240, A182, A312 |
| Duplex 2205 | 1.4462 | S32205 | 35 | ASTM A240, A182, A790, A789 |
| Super Duplex 2507 | 1.4410 | S32750 | 42 | ASTM A240, A182, A790, NACE MR0175 |
| Zeron 100 | 1.4501 | S32760 | 41 | ASTM A240, A182 F55, A790 |
PREN values are approximate, calculated from typical composition; the actual value depends on the analysis on the certificate.
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