904L Stainless Steel: Super Austenitic for Extreme Chemicals

What Is 904L Stainless Steel

904L, UNS N08904, is a super austenitic stainless steel with 19 to 23 percent chromium, 23 to 28 percent nickel, 4 to 5 percent molybdenum, 1 to 2 percent copper, and carbon 0.02 percent maximum. The very high nickel and molybdenum contents plus the copper addition give it an exceptionally high pitting resistance equivalent number around 34, enhancing its resistance to harsh chemicals and chloride environments.

Corrosion Performance

904L outperforms most stainless steels in highly aggressive settings, including sulfuric acid, phosphoric acid, and chloride solutions. It resists pitting, crevice corrosion, and stress corrosion cracking, with the copper content providing particular resistance to reducing acids such as dilute and medium-concentration sulfuric acid, which is the application for which the grade was originally developed.

Mechanical Properties

The grade has a tensile strength of about 550 MPa with good ductility and elongation up to 40 percent in the annealed condition. It is weldable and formable using standard techniques, though it work-hardens more than 316L and may require occasional annealing for complex shapes.

Cost and Value

904L is significantly more expensive than 316L, often several times the cost, because of its high alloy content. It is justified in extreme environments where 316L would fail, reducing downtime and replacement cost over the service life, such as in sulfuric acid production and offshore chloride service.

Applications

904L is used in pharmaceutical processing equipment, sulfuric acid storage tanks, offshore oil rig components, desalination plants, chemical reactors handling highly corrosive fluids, and pollution control equipment. It is also specified for pulp and paper bleach plants and heat exchangers in chloride-bearing service.

Frequently Asked Questions

Q: What is the composition of 904L?

A: 19 to 23 percent chromium, 23 to 28 percent nickel, 4 to 5 percent molybdenum, 1 to 2 percent copper.

Q: What is 904L equivalent to?

A: UNS N08904 and EN 1.4539.

Q: Why was 904L developed?

A: For sulfuric acid service, where copper improves resistance to reducing acids.

Q: How does 904L compare with 316L?

A: Far higher corrosion resistance in aggressive media, at much higher cost.

Q: What applications use 904L?

A: Sulfuric acid storage, pharmaceutical, offshore, and desalination equipment.

Q: Is 904L weldable?

A: Yes, with standard techniques using matching or over-alloyed filler.

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