How does 904L compare with super duplex stainless steels?
For most applications, super duplex stainless steels match 904L on corrosion resistance while offering higher strength and lower cost, and Ferralium® 255 even outperforms 904L in sulphuric acid thanks to its copper content. 904L's real advantage is a wider operating temperature range, holding up with confidence below -50°C or above 250°C where super duplex is limited.
904L vs Ferralium® 255: the numbers
| Property | 904L | Ferralium® 255 |
|---|---|---|
| PREN | ~43 | >40 |
| Chromium | ~20% | 24.5-26.5% |
| Molybdenum | ~4.0% | 3.10-3.80% |
| Nickel | ~25% | 5.50-6.50% |
| Nitrogen | - | 0.20-0.25% |
| 0.2% proof strength | - | 586 N/mm² (85 ksi) |
| Tensile strength | - | 790 N/mm² (114.5 ksi) |
| General corrosion resistance | Equivalent to most super duplex grades across a wide range of applications | Equivalent overall; outperforms 904L specifically in sulphuric acid, thanks to its copper content |
| Pitting / CPT resistance | PREN ~43 | PREN >40; comparable to 904L |
| Typical applications | Low/high-temperature extremes (below -50°C or above 250°C); historically also sulphuric acid duty | General replacement for 904L across most projects; also strong in sulphuric acid duty thanks to its copper content, unique among super duplex grades |
Calculate PREN for your own composition with our PREN calculator.
What 904L is
904L (UNS N08904, 1.4539) is a highly alloyed austenitic stainless steel. Its approximate composition is 20% Cr, 25% Ni, 4.0% Mo plus 1.5% Cu, giving a pitting resistance equivalent number (PREN) of around 43. It was developed specifically for service in sulphuric acid, mostly in concentrations from 20% to 85%.
Corrosion resistance
The corrosion resistance of 904L is equivalent to most super duplex stainless steels across a wide range of applications. Ferralium® 255 provides superior performance in sulphuric acid, however, thanks to its 2% copper content - unique amongst super duplex stainless steels.
Temperature range
As an austenitic stainless steel, 904L has a wider operating temperature range and can be specified with confidence for applications <-50°C or >250°C, where super duplex grades are limited. At low temperatures it retains good impact toughness, and it is not at risk of forming deleterious intermetallic phases at higher temperatures.
Strength and cost
Super duplex stainless steels have higher strength, so thinner sections can carry the same structural load. The higher alloy content of 904L also makes it inherently more expensive, before the strength difference is even considered.
Which should you choose?
Choose Ferralium® 255 for most projects: it matches 904L's corrosion resistance, carries more load per section thanks to its higher strength, costs less, and even beats 904L in sulphuric acid duty thanks to its copper content. We have supplied Ferralium® 255 to numerous projects specifically as a 904L replacement. Reach for 904L instead only where the operating temperature falls outside super duplex's range - below -50°C or above 250°C - since that is where 904L genuinely has the edge.