Every plant engineer has watched a 316L pipe fail in a duty that the datasheet said should work. The pitting starts at a weld, or under a gasket, or at a stagnant chloride spot, and within a season the line is leaking. The standard response is to move up one rung - and for a large share of those duties, the correct next rung is not a 6% molybdenum grade and it is not a duplex.
It is Alloy 904L, the 4.5% molybdenum super austenitic stainless that sits between the 316 family and the 6Mo alloys, and that carries the sulfuric acid, wet phosphoric acid, and warm seawater duties that defeat the standard austenitics.
This page documents Alloy 904L (UNS N08904, W.Nr 1.4539) - the chemistry, the mechanical properties, the corrosion envelope, and the manufacturing standards that apply to 904L pipe and plate.
What 904L is, and where it sits in the stainless family
Alloy 904L is a low-carbon, high-alloy austenitic stainless steel with a chemistry that puts it in the "super austenitic" category: roughly 23-28% nickel, 19-23% chromium, 4-5% molybdenum, and 1-2% copper.
The copper is the signature element. In combination with the molybdenum, copper gives 904L its unusual resistance to sulfuric acid, because copper suppresses the active dissolution of the steel in reducing acid media.
The high nickel content - more than double that of 316L - gives the alloy resistance to chloride stress-corrosion cracking that 316L simply does not have. The result is a stainless grade that handles a broad corrosive envelope without the price of the 6% molybdenum alloys or the fabrication restrictions of duplex grades.
Chemical composition per ASTM B677
The heat-analysis limits below follow ASTM B677 for 904L seamless pipe (the ASTM B625 specification covers plate, and ASTM B673 covers welded pipe). Nickel is 23.0-28.0%, and the balance is iron.
| Element | Composition limit, % |
| Ni | 23.0 - 28.0 |
| Cr | 19.0 - 23.0 |
| Mo | 4.0 - 5.0 |
| Cu | 1.0 - 2.0 |
| C | 0.020 max |
| Mn | 2.0 max |
| Si | 1.0 max |
| P | 0.045 max |
| S | 0.035 max |
| N | 0.10 max |
Three rows deserve attention. The carbon maximum of 0.020% means the alloy is supplied in the low-carbon condition, so it does not sensitize during welding and the heat-affected zone keeps its corrosion resistance without a solution-anneal after welding - a decisive advantage in field-fabricated piping.
The copper row at 1.0-2.0% is the element that makes 904L excel in sulfuric acid, because copper is the classic reducer of corrosion rates in reducing acid environments. The nickel row at 23.0-28.0% is what pushes the alloy's chloride SCC resistance far above the 316 family: as nickel content rises above roughly 20%, the austenitic matrix becomes increasingly immune to chloride stress-corrosion cracking.
Mechanical properties
ASTM B677 requires the following minimum tensile properties for 904L seamless pipe in the solution-annealed condition, which is the standard delivery condition:
| Property | Minimum value (ASTM B677) |
| Tensile strength | 490 MPa (71 ksi) |
| Yield strength, 0.2% offset | 216 MPa (31 ksi) |
| Elongation | 35% |
904L is an austenitic alloy, which means it shares the austenitic virtues: it is fully non-magnetic, it retains ductility and toughness down to cryogenic temperatures, and it is weldable and formable with ordinary austenitic stainless techniques. Its yield strength is comparable to 316L, so the alloy is not bought for strength - it is bought for corrosion resistance, and the design wall is set by the corrosion allowance and the pitting resistance rather than by the pressure calculation.
Physical properties
The density of 904L is 8.0 g/cm³, and the melting range is approximately 1300-1390°C. The alloy has a modulus of elasticity near 185 GPa at room temperature, slightly higher than 316L because of the increased nickel and molybdenum content.
Its thermal conductivity is somewhat lower than the standard austenitics - about 12.4 W/m·K at 100°C - which matters in heat-exchanger design where the tube wall contributes to the heat-transfer resistance.
The corrosion envelope: where 904L wins the job
The commercial case for 904L rests on three corrosion duties, and each one is a documented failure mode of the 316 family:
| Duty | 904L behavior | Why |
| Sulfuric acid, dilute to moderate concentration | Excellent | Copper + molybdenum suppress active dissolution |
| Wet-process phosphoric acid | Excellent | The standard material class for fertilizer plant duty |
| Chloride pitting and crevice corrosion | Very good (PREN ~ 34) | 4-5% Mo lifts the pitting resistance above 316L |
| Chloride stress-corrosion cracking | Good resistance | 23-28% Ni moves the alloy above the SCC threshold |
| Seawater, ambient to moderate temperature | Good with monitoring | Better than 316L; 6Mo grades still superior in warm seawater |
| Nitric acid and strong oxidizing media | Moderate | Not the alloy's primary duty |
For the buyer the practical line is this: where 316L has failed or is marginal in sulfuric or phosphoric acid service, in warm chloride duty, or in SCC-sensitive environments, 904L is the cost-effective step up.
It delivers most of the corrosion benefit of the 6% molybdenum alloys at a significantly lower alloy surcharge, and it does so in a fully austenitic structure that welds and forms like standard stainless. The boundary case is continuous warm seawater: 904L performs but is not the best choice there, and the design engineer should step up to a 6Mo grade or a nickel alloy for that duty.
Manufacturing standards and product forms
904L is supplied against the following specifications, selected by product form:
- ASTM B677 / ASME SB-677 - seamless pipe
- ASTM B673 / ASME SB-673 - welded pipe
- ASTM B625 / ASME SB-625 - plate, sheet, and strip
- ASTM B649 / ASME SB-649 - bar and forgings
- EN 10216-5 / EN 10217-7 - European equivalents (1.4539)
Because 904L is a fully austenitic alloy, the manufacturing route mirrors 316L: the pipe is hot pierced or rolled, then solution annealed and quenched, with the anneal temperature typically in the 1090-1175°C range.
The low carbon content is maintained through the melt, and the finished product is supplied in the annealed, pickled, and passivated condition. The ASTM B677 seamless pipe is available in the full range of standard and custom dimensions, and the plate per B625 is supplied in thicknesses from thin sheet to heavy plate for vessel construction.
Typical applications
Alloy 904L is specified in the following services:
- Sulfuric acid production and handling lines, including dilution and storage systems
- Wet-process phosphoric acid plants - evaporators, transfer lines, and reactor internals
- Seawater cooling systems and heat exchangers where 316L is marginal
- Chemical process plants handling mixed chlorides and reducing acids
- Pulp and paper bleach plants
- Pharmaceutical and fine-chemical equipment where product purity drives material selection
In each service the specification is driven by a documented failure of a lower grade, and 904L is selected because it closes the gap without the fabrication penalties of duplex or the cost of 6Mo and nickel alloys. For vessel and tank construction, 904L plate is a standard specification for sulfuric acid storage, and for piping the seamless B677 product is the default for process lines.
Welding and fabrication
904L welds with the same shop practices as 316L: GTAW and GMAW with matching filler, no preheat required, and no post-weld heat treatment normally required because the low carbon content prevents sensitization.
Matching filler metal to AWS A5.9 ERNiCrMo-3 (625-type filler) or the specific 904L filler classification is commonly used to match corrosion resistance in the weld metal, and where the application demands, the weld is made with a nickel-based filler to guarantee the pitting resistance of the joint.
The alloy is fully austenitic, so it is sensitive to hot cracking if the weld is made with excessive heat input - the standard austenitic practice of controlled interpass temperature and low heat input applies. Cold forming is straightforward with standard stainless tooling, and the alloy's work-hardening rate is moderate.
What to specify on the order
To get a clean 904L delivery, the enquiry should state the following:
- Standard and form: "seamless pipe to ASTM B677", "plate to ASTM B625", UNS N08904 / 1.4539
- Size in the correct system: NPS + schedule for pipe, OD x wall or thickness x width x length for plate
- Condition: solution annealed, pickled, passivated
- Certificate: EN 10204 3.1, with PMI and the required examination
FAQ
Q: Is 904L a duplex stainless steel?
A: No. 904L is fully austenitic with a face-centred-cubic structure, like 316L. It carries higher nickel and molybdenum than 316L, but it is not a duplex grade and does not have the two-phase structure of 2205 or 2507.
Q: What is 904L used for?
A: Primarily for sulfuric acid, wet-process phosphoric acid, and warm chloride service where 316L fails by pitting, crevice corrosion, or chloride stress-corrosion cracking. It is common in fertilizer plants, acid handling, and seawater cooling systems.
Q: What is the PREN of 904L?
A: Approximately 34, calculated from 19-23% chromium, 4-5% molybdenum, and up to 0.10% nitrogen. This is well above 316L (about 24) and below the 6Mo grades (about 41-44).
Q: Does 904L resist chloride stress-corrosion cracking?
A: Yes, far better than 316L. The 23-28% nickel content puts the austenitic matrix above the SCC susceptibility threshold for most chloride environments, although it is not immune in the most severe conditions.
Q: Can 904L be welded without post-weld heat treatment?
A: Yes. The 0.020% maximum carbon content prevents carbide sensitization in the heat-affected zone, so the as-welded joint retains its corrosion resistance. Matching or nickel-based filler is used to match weld-metal performance.
Q: What is the difference between 904L and 254SMO?
A: 254SMO is a 6% molybdenum grade (UNS S31254) with higher pitting resistance and higher strength, intended for warm seawater and aggressive chlorides. 904L (4-5% Mo) is less expensive and excels in sulfuric and phosphoric acid duty. The choice is made on the medium: acid points to 904L, warm seawater points to 6Mo.
