316L or 904L: what the molybdenum and nickel actually buy you

904L costs several times what 316L does per kilogram. It earns that premium in chloride plus sulphuric acid service — and wastes it in clean water. Here is how to tell which case you are in.

Most stainless selection arguments in chemical plant come down to one question: is the chloride level high enough that 316L will pit, or is it only high enough to worry about? 904L is the usual answer to that worry, and it is an expensive answer, so it is worth knowing exactly what the extra alloy buys.

	316L	904L
UNS	S31603	N08904
Nickel	10.0 – 14.0%	23.0 – 28.0%
Chromium	16.0 – 18.0%	19.0 – 23.0%
Molybdenum	2.00 – 3.00%	4.00 – 5.00%
Copper	—	1.00 – 2.00%
Carbon	≤ 0.03%	≤ 0.02%
Pitting resistance (PREN)	about 24 – 26	about 34 – 36

What 904L does better

The higher molybdenum raises pitting and crevice corrosion resistance; the copper addition specifically helps in sulphuric acid; and the high nickel content makes the alloy far more resistant to chloride stress-corrosion cracking than the 300-series austenitics. That combination is why 904L appears in phosphoric and sulphuric acid circuits, scrubbers, and seawater-cooled exchangers where a duplex is not permitted by the specification.

What it does not do

904L is not a seawater alloy in the way 2507 or C276 are. It resists chlorides far better than 316L, but it is still an austenitic stainless and will suffer crevice attack in stagnant seawater, particularly under deposits. If the duty is continuous seawater with no flow, the answer is a super duplex or a nickel alloy, not 904L.

Nor does 904L solve high-temperature strength problems. It is a corrosion alloy; its mechanical properties are close to 316L's at temperature.

The commercial arithmetic

904L typically prices at three to five times 316L per kilogram, and it is harder to machine and weld — slower speeds, more careful procedure qualification, and a demand for low-heat-input welding. On a large tank that difference dwarfs the fabrication cost. Where the chloride is marginal and the temperature is moderate, 2205 duplex frequently delivers more pitting resistance than 316L at a fraction of the 904L premium, which is why so many projects now treat the choice as three-way.

Rule of thumb we use: if PREN above 30 is genuinely required and duplex is excluded by the specification, 904L is the answer. If the only problem is chloride cracking at moderate temperature, look at 2205 before you look at 904L's price.

What to send us

Medium, chloride concentration, temperature, and whether the surface will be stagnant or flowing. With those four numbers we can quote 316L, 2205 and 904L side by side and tell you which one we would put in our own plant.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.