Features
Super austenitic stainless steel 904L (00Cr20Ni25Mo4.5Cu, UNS: N08904, EN: 1.4539) is a low carbon, high-alloy austenitic stainless steel with excellent corrosion resistance in dilute sulfuric acid. It is designed for harsh corrosion conditions.
904L super austenitic stainless steel is a low carbon, high nickel, molybdenum austenitic stainless steel, It has the high chromium content and plenty of nickel content, the addition of copper makes it have strong ability to resist acid, especially for chloride clearance corrosion and stress corrosion cracking are highly resistant, not easy to appear, erosion and fracture pitting resistance slightly better than other steel grade, good machinability and weldability, can be used for pressure vessels.
Applicable to all concentrations of sulfuric acid below 70℃, under normal pressure any concentration, any temperature of acetic acid, and formic acid and acetic acid in the mixture of corrosion resistance is also very good.
Chemical Composition (%)
| Element | Minimum percentage | Maximum percentage |
| C | – | 0.02 |
| Si | – | 1 |
| Mn | – | 2 |
| P | – | 0.045 |
| S | – | 0.035 |
| Cr | 19 | 23 |
| Mo | 4 | 5 |
| Ni | 23 | 28 |
| Cu | 1 | 2 |
Corrosion Resistance
The high content of alloying elements in 904L gives the alloy exceptionally good resistance to uniform corrosion.
904L was originally developed to withstand environments containing dilute sulfuric acid, and is one of the few stainless steels that, at temperatures up to 95°F (35°C), provides full resistance in such environments within the entire concentration range of 0 to 100%. 904L also offers good resistance to a number of other inorganic acids, such as phosphoric acid and most organic acids. However, acids and acid solutions containing halide ions can be very aggressive, and the corrosion resistance of 317L, 317LMN and 904L may be insufficient.
Fractional distillation of tall oil often needs better material than 316L or even the more frequently used 317LMN. In these hot concentrated caustic solutions, the corrosion resistance is mainly determined by the nickel content of the material. With a nickel content of 25%, 904L has proven to be a good alternative to most conventional stainless steels.
Conventional stainless steels such as 304L and 316L are susceptible to chloride stress corrosion cracking (SSC) under certain conditions. Resistance to SSC increases with the increased content of nickel and molybdenum. Therefore, the higher performance austenitic stainless steels such as 904L have very good resistance to SSC.
Uniform Corrosion in pickling acid* at 25°C
| Alloy | Corrosion Rate, mm/year |
| 316L | >6 |
| 904L | 0.47 |
| 254 SMO | 0.27 |
| 654 SMO | 0.06 |
Uniform Corrosion in wet process phosphoric acid at 60°C
| Alloy | Corrosion Rate, mm/year |
| 316L | >5 |
| 904L | 1.2 |
| 254 SMO | 0.05 |
Corrosion rates in fatty acid column for the distillation of tall oil at 253°C
| Alloy | Corrosion Rate, mm/year |
| 316L | 0.88 |
| 317LMN | 0.29 |
| 904L | 0.056 |
| 254 SMO | 0.01 |
