Choosing between two super-austenitic grades involves more than comparing price. Alloy 28 Sheet and 904L Sheet contain higher alloy levels than standard stainless steel and are often selected when 316L or 317L cannot withstand the process of chemistry. The key consideration is not simply which grade contains more nickel or molybdenum, but which corrosion mechanism the equipment must resist, such as uniform acid corrosion, weld-seam pitting, or cracking in chloride-rich crevices.
Alloy 28 Sheet and 904L Sheet: What Is the Difference?
Alloy 28, identified as UNS N08028 and EN 1.4563, is a high-alloy austenitic steel built around a chromium-nickel-molybdenum balance with added copper. This alloy is used in mixed acid service where attack is by sulfuric and phosphoric acids in the presence of chlorides, e.g. fertilizer plants and flue-gas desulfurisation units.
904L carries the designation UNS N08904 and EN 1.4539, a super-austenitic grade built on high nickel, molybdenum, and copper with low carbon to limit weld sensitization. Refineries and seawater systems use 904L where standard 316L falls short on chloride pitting resistance. Both grades share an austenitic structure, keeping them non-magnetic and ductile at low temperatures.
Chemical Composition Comparison
Composition ranges vary slightly by specification, so the figures below reflect EN 1.4563 for Alloy 28 chemical composition and standard UNS N08904 references for 904L.
| Element | Alloy 28 | 904L |
| Chromium | 26-28% | 19-23% |
| Nickel | 30-32% | 23-28% |
| Molybdenum | 3-4% | 4-5% |
| Copper | 0.7-1.5% | 1-2% |
| Carbon | 0.02% max | 0.02% max |
| UNS | N08028 | N08904 |
Chromium builds the passive oxide film that shields the surface from oxidizing acids. Molybdenum interrupts pit and crevice initiation at weld lines. Nickel and copper push resistance higher in reducing acid conditions, which is why Alloy 28’s nickel content matters in sulfuric acid service while 904L’s molybdenum edge helps in seawater.
Alloy 28 vs 904L: Corrosion Resistance Comparison
Alloy 28 corrosion resistance shows up most clearly in dilute-to-moderate sulfuric acid and phosphoric acid production, where the nickel-copper balance limits general wall loss. Technical references describe this grade as well suited to pitting, crevice corrosion, and stress-corrosion cracking resistance in chloride-bearing acid mixtures.
904L Sheet corrosion resistance holds up strongly in sulfuric acid, phosphoric acid, and chloride environments too, with its molybdenum content giving it an edge against localized pitting in near-neutral chloride solutions such as seawater. That resistance narrows as acid concentration and chloride level climb together.
Neither grade is superior across all corrosion mechanisms. 904L also tends to perform comparably in chloride-dominant, lower-acidity streams. Alloy 28 also tends to pull away in mixed acid attacks with chlorides present. Service life still depends on temperature, concentration, and flow velocity at the point of attack.
When Should You Choose Alloy 28 Sheet Over 904L Sheet?
Specify Alloy 28 when the process stream combines sulfuric or phosphoric acid with chlorides, and when localized corrosion at welds or crevices is a documented concern from prior equipment history. Match actual concentration, temperature, and flow conditions against the grade’s known performance envelope rather than assuming alloy content alone guarantees service life.
Alloy 28 Sheet applications include chemical processing vessels, acid-duty heat exchangers, oil and gas components exposed to sour chloride-laden fluids, and flue-gas desulfurization systems.
Procurement teams sourcing this grade can review full specifications on the Alloy 28 Sheet product page before finalizing a material selection.
When Can 904L Sheets Be the Better Choice?
904L remains a proven, widely stocked option for sulfuric acid, phosphoric acid, and chloride-heavy service, particularly where pitting resistance matters more than mixed-acid attack. Seawater systems and chemical handling lines often run on 904L without the added alloy cost that Alloy 28 carries.
Selection should track the actual chemistry of the stream. Buyers evaluating 904L stainless steel Sheet against Alloy 28 can find detailed specifications on the 904L Stainless Steel Sheet page.
Alloy 28 Sheet vs 904L Sheet: Final Verdict
Alloy 28 is used where mixed acids and chlorides attack together, and where pitting or crevice corrosion has caused prior failures. 904L continues to serve well across acid and chloride-handling equipment generally, backed by decades of field history. Final selection rests on process chemistry data.
Contact Nandini Steel to match your process conditions against the Alloy 28 Sheet specification before placing an order.
