904L vs 316 Stainless Steel: Composition, Corrosion and Cost Compared

Introduction: Two Austenitic Grades, Two Price Tiers

Type 316 and type 904L are both austenitic stainless steels, yet they are separated by a wide gap in both performance and price. Type 316, with 16-18% chromium, 10-14% nickel and 2-3% molybdenum, is the general-purpose molybdenum-bearing grade used for chemical tanks, food processing equipment and architectural components. Type 904L, sold under UNS N08904, is a super-austenitic grade that raises nickel to 23-28%, chromium to 19-23% and molybdenum to 4-5%, and adds 1-2% copper to survive sulfuric acid, phosphoric acid and concentrated chloride service.

Because 904L can cost three to four times as much as 316 and is less readily available, the selection question is rarely about which alloy is "better". It is about how aggressive the operating environment really is, and whether a longer service life justifies the material premium. The comparison below covers composition, corrosion behaviour, mechanical properties, fabrication and lifecycle cost so that buyers and fabricators can make that call with data rather than habit.

Chemical Composition Comparison

Composition is the root cause of every other difference between the two grades. The table below lists representative specification limits for the most common product forms.

Element	316	316L	904L (N08904)
Carbon (C)	0.08% max	0.030% max	0.020% max
Chromium (Cr)	16.0-18.0%	16.0-18.0%	19.0-23.0%
Nickel (Ni)	10.0-14.0%	10.0-14.0%	23.0-28.0%
Molybdenum (Mo)	2.00-3.00%	2.00-3.00%	4.00-5.00%
Copper (Cu)	-	-	1.00-2.00%
Manganese (Mn)	2.00% max	2.00% max	2.00% max
Silicon (Si)	0.75% max	0.75% max	1.00% max
Nitrogen (N)	0.10% max	0.10% max	0.10% max

The engineering index used for chloride service is the Pitting Resistance Equivalent Number, calculated as PREN = %Cr + 3.3 x %Mo + 16 x %N. Using mid-range compositions, type 316 returns roughly 25 to 26, while 904L returns roughly 34 to 36. A ten-point difference on this scale is a very large gain in resistance to pitting and crevice attack, which is exactly why 904L appears in acid and chloride service where 316 fails.

Corrosion Resistance in Aggressive Media

Type 316 performs well in neutral chlorides, saltwater spray and many organic acids, but it has two recognised limits: pitting in warm, stagnant chloride solutions, and chloride stress corrosion cracking once temperature and tensile stress rise together. 904L pushes both limits outward.

Sulfuric acid: 904L handles dilute through moderately concentrated sulfuric acid over a wide temperature band; type 316 is generally restricted to dilute, cool solutions.

Phosphoric acid: 904L is a standard choice for phosphoric acid evaporators, agitators and slurry lines where 316 corrodes too quickly.

Pitting and crevice corrosion: 4-5% molybdenum plus 1-2% copper raises the critical pitting temperature of 904L far above that of type 316, so flanges, gaskets and weld crevices remain sound.

Seawater and brine: 904L resists flowing and low-velocity seawater, while 316 commonly suffers crevice attack at flange faces and weld toes in the same service.

Intergranular corrosion: both 316L and 904L resist sensitisation after welding because of their restricted carbon content, and 904L retains that advantage after prolonged elevated-temperature exposure.

Reducing acids: the copper addition in 904L improves performance in reducing environments such as sulfuric and phosphoric acid, a mechanism that nickel and molybdenum alone cannot deliver.

Chloride stress corrosion cracking, however, should not be treated as fully solved by either grade. Above roughly 60 °C in concentrated chlorides, even 904L benefits from stress relief of fabricated components and from design details that avoid stagnant pockets.

Mechanical Properties, Welding and Forming

Strength is not the deciding factor between these grades; fabricability usually is.

Property	316 / 316L	904L
Tensile strength, annealed	485-550 MPa	490-600 MPa
Yield strength, 0.2% offset	170-310 MPa	220-240 MPa
Elongation in 50 mm	40% min	35% min
Hardness	95 HRB max	90 HRB max
Solution annealing	1040 °C min, water quench	1090-1170 °C, rapid quench
Weldability	Excellent	Good, tighter heat input control
Machinability	Moderate	Low, work hardens quickly

Both grades are readily cold formed, but 904L work hardens faster, so bend radii must be generous and machining needs lower surface speeds with heavy, continuous coolant flow. Welding is routine when matching austenitic fillers are used: ER316L for 316 and ER385 for 904L, with controlled heat input and interpass temperature to preserve the corrosion-resistant microstructure. Heat tint must be removed by pickling or grinding, because an oxidised weld surface becomes a preferred pitting site in chloride service.

Cost, Availability and Selection Guidance

904L is typically three to four times the price of 316 per tonne because of its nickel, molybdenum and copper content, and mill availability is narrower, so lead times are longer. The decision framework is straightforward.

Stay with 316 or 316L for atmospheric exposure, potable water, food and beverage equipment, dilute neutral chlorides, and marine hardware that stays above the waterline.

Upgrade to 904L for sulfuric and phosphoric acid circuits, chloride-rich process streams, offshore topsides, flue-gas desulphurisation components, and pharmaceutical vessels that must avoid metallic contamination of the product.

Evaluate lifecycle cost, not purchase price. In continuous chemical plants, one unplanned shutdown for corrosion repair usually exceeds the material premium on the entire order.

Do not over-specify. A 316L system with correct drainage, clean welds and controlled chloride levels often outperforms a poorly fabricated 904L system in the same duty.

Frequently Asked Questions

Q: Is 904L always better than 316 stainless steel?
No. 904L resists more aggressive chemistry, but 316 is stronger per unit of cost, easier to weld and machine, and far more widely available. Use 904L only where 316 has a demonstrated corrosion risk.

Q: What is the difference in chemical composition between 904L and 316?
904L carries 19-23% chromium, 23-28% nickel, 4-5% molybdenum and 1-2% copper, whereas 316 carries 16-18% chromium, 10-14% nickel and 2-3% molybdenum with no deliberate copper addition.

Q: Can 904L replace 316 in seawater piping?
Yes for flowing and low-velocity seawater, where 904L resists pitting and crevice corrosion far better. In stagnant, hot, high-chloride conditions even 904L can pit, so drainage and velocity design still matter.

Q: Does 904L cost more to fabricate than 316?
It does. 904L work hardens rapidly, so machining rates are lower and forming pressures higher. The extra fabrication cost should be included in any comparison with 316.

Q: Can 316 and 904L be welded to each other?
Yes. A matching austenitic filler such as ER385 or ER316L is normally used, and the corrosion requirements of the 904L side govern the weld procedure and post-weld cleaning.

Q: Which grade should be used for sulfuric acid storage tanks?
904L is the usual choice because its high nickel, molybdenum and copper content resists dilute and moderate sulfuric acid. Type 316 is limited to dilute, low-temperature acid service.