Should you specify duplex or 304 stainless steel? Specify 304 for mild, low-chloride, general-purpose service where cost and easy fabrication matter most. Specify duplex (2205) when the environment is chloride-bearing, corrosive, or demands higher strength, because 304 has the lowest corrosion resistance of the common grades and can pit, corrode, or crack where duplex performs reliably. Duplex costs more per kilogram but lasts far longer in aggressive service.
Why 304 and duplex are near-opposite ends of the range
304 is the most common and economical stainless steel: excellent for general use, easy to fabricate, and inexpensive, but with the lowest corrosion resistance of the mainstream grades and no molybdenum. Duplex 2205 sits at the high-performance end, with high chromium and molybdenum, roughly double the strength, and strong chloride resistance. Comparing them is really a question of whether the service is benign enough for 304 or aggressive enough to justify duplex. In many aggressive cases the honest answer is that neither is ideal, and 316 or duplex is the real contest, so 304 should be ruled out early.
For plants in the Netherlands and near the North Sea coast, where chlorides are common, 304 is frequently not enough, which is exactly why this comparison arises. GreyTec helps EU clients avoid both under-specifying with 304 and over-specifying elsewhere.
How do duplex and 304 compare on corrosion resistance?
Answer: Duplex 2205 vastly outperforms 304 on corrosion. Duplex has a PREN of roughly 34 to 38 versus about 18 to 20 for 304, and contains molybdenum that 304 lacks, giving far better pitting, crevice, and chloride stress corrosion cracking resistance. In chloride or corrosive service, 304 can fail where duplex performs reliably.
304 is designed for mild environments. Introduce chlorides, acids, or elevated temperature and its limited resistance shows quickly as pitting or, in the right conditions, stress corrosion cracking. Duplex was built to handle exactly these conditions. The corrosion gap between them is wider than between duplex and 316, because 304 lacks molybdenum entirely.
How do they compare on strength and cost?
Answer: Duplex has roughly double the yield strength of 304, allowing thinner, lighter sections, but it costs more per kilogram. 304 is the cheaper, easier-to-fabricate option with adequate strength for general service. In aggressive service, duplex’s longer life usually makes it the lower lifecycle cost despite the higher purchase price.
Cost is where this decision is won or lost. For a mild-service handrail, tank, or general fabrication, 304’s low cost and easy welding are decisive. For a chloride-exposed process vessel, 304’s low purchase price is a false economy if it corrodes and needs early replacement, while duplex’s strength can also cut material weight.
Duplex 2205 vs 304: head to head
| Factor | 304 | Duplex 2205 |
|---|---|---|
| Molybdenum content | None | Yes |
| Indicative PREN | 18 to 20 | 34 to 38 |
| Chloride pitting resistance | Low | High |
| Stress corrosion cracking resistance | Low | High |
| Yield strength | Baseline | About double |
| Fabrication and welding ease | Easiest | Needs controlled procedure |
| Cost per kilogram | Lowest | Higher |
| Best-fit service | Mild, general, low chloride | Chloride, corrosive, high strength |
Values are indicative. Confirm against certified composition and the specific service environment.
Numbered decision guide: duplex or 304?
- Check for chlorides or corrosives. Any meaningful presence rules out 304 in most cases.
- If the service is genuinely mild, 304 is often the economical answer.
- If it is aggressive, compare duplex against 316, not 304.
- Assess strength needs. A weight or thickness benefit favours duplex.
- Assess fabrication capability. 304 is simplest; duplex needs procedure control.
- Compare lifecycle cost. Include corrosion-driven replacement and downtime.
- Confirm availability and lead time for the chosen grade.
When is 304 the right choice?
Answer: 304 is the right choice for mild, dry, or low-chloride environments, general fabrication, food and beverage equipment in non-aggressive service, and applications where low cost and easy welding matter and corrosion demands are modest. Where the environment is aggressive, 304 is usually the wrong starting point.
304 remains one of the most useful materials in industry for the right duty. The mistake is using it in chloride or corrosive service to save money, where it becomes the most expensive option through early failure.
Checklist: making the duplex vs 304 decision
- ☐ Chloride and corrosive exposure of the service is defined.
- ☐ 304 confirmed suitable only if the environment is genuinely mild.
- ☐ For aggressive service, duplex compared against 316 as well.
- ☐ Strength and weight benefits of duplex quantified.
- ☐ Fabrication and welding capability confirmed.
- ☐ Lifecycle cost compared, not just purchase price.
- ☐ Availability and lead time confirmed.
Myth: 304 stainless steel does not corrode
Is 304 stainless steel corrosion-proof? No. 304 resists corrosion well in mild environments, but it has the lowest corrosion resistance of the common grades and contains no molybdenum, so it pits and can crack in chloride service. The word stainless does not mean corrosion-proof. In aggressive conditions, 304 corrodes where duplex does not.
This misconception causes real failures. Specifying 304 in a coastal, chloride, or chemical environment because it is stainless steel often leads to pitting and premature replacement.
More questions (FAQ)
Is duplex much stronger than 304? Yes, duplex has roughly double the yield strength of 304, which can allow thinner, lighter equipment for the same duty.
Why is duplex more corrosion resistant than 304? Duplex has higher chromium, contains molybdenum (which 304 lacks), added nitrogen, and a two-phase microstructure, all of which improve chloride and stress corrosion resistance.
Is 304 ever a better choice than duplex? Yes, in mild, low-chloride, general service where cost and easy fabrication matter and corrosion demands are modest.
Should I compare 304 or 316 with duplex for aggressive service? For aggressive chloride service, compare duplex against 316, because 304 is usually unsuitable and 316 is the realistic austenitic alternative.
Does duplex cost more than 304? Yes per kilogram, but its longer life in aggressive service and its higher strength can make it the lower total cost.
Why does the word stainless mislead buyers?
Answer: The word stainless suggests a material that never corrodes, but stainless steels resist corrosion only within the conditions each grade is designed for. 304, the most common grade, has the lowest corrosion resistance of the mainstream stainless steels and no molybdenum, so in chloride or corrosive service it pits and can crack. Stainless means corrosion-resistant in the right environment, not corrosion-proof everywhere.
This misunderstanding drives real specification errors. A buyer choosing 304 because it is stainless steel, without checking the chloride level or chemistry of the service, may install equipment that corrodes within a fraction of its intended life. The lesson is to specify by the actual environment and the grade’s resistance data, not by the reassuring name. In aggressive conditions the honest choice is often 316 or duplex, not 304.
Where does each grade genuinely fit?
Answer: 304 fits mild, dry, and low-chloride general service: structural stainless, non-aggressive tanks, and general fabrication where cost and easy welding matter. Duplex 2205 fits chloride-bearing, corrosive, and high-strength duties. For aggressive service, the realistic comparison is usually duplex against 316, with 304 ruled out early because it lacks the molybdenum needed for chloride resistance.
Practical examples make the boundary clear. Indoor structural supports, non-corrosive general equipment, and mild-service fabrication are comfortable 304 territory. A chloride-exposed process vessel, a coastal or seawater-adjacent installation, or a warm chemical duty moves the decision to 316 or duplex. Using 304 to save money in those aggressive settings is the classic false economy this comparison is meant to prevent.
More expert questions
Does 304L or 316L change the comparison? The low-carbon L grades improve weldability and resistance to sensitisation but do not fundamentally change the corrosion ranking: 304 and 304L remain the least chloride-resistant, with duplex far ahead.
Is 304 ever suitable near the coast? Coastal and marine atmospheres carry chlorides that can pit 304, so 316 or duplex is usually preferred for coastal and near-seawater service.
How much stronger is duplex than 304? Duplex has roughly double the yield strength of 304, which can allow thinner, lighter sections for the same duty.
If 304 is too weak, should I jump straight to duplex? Not always. For moderately aggressive service, 316 may be the appropriate step up from 304, with duplex reserved for high-chloride, high-strength, or stress-corrosion-prone duties.
How should you make the final grade decision?
Answer: Make the decision by defining the service first: the chemistry, chloride level, temperature, and strength needs. If the environment is genuinely mild, 304 is often the economical answer. If it is aggressive, rule 304 out early and compare 316 against duplex on corrosion life, strength, fabrication, availability, and lifecycle cost. Specifying by real conditions, not by habit or headline price, is what prevents both failure and waste.
This disciplined approach avoids the two classic errors: under-specifying with 304 in a corrosive setting, and reflexively over-specifying duplex where 316 or even 304 would serve. A short, honest assessment of the actual duty resolves most cases quickly, and where it is genuinely borderline, a materials review with a knowledgeable supplier is time well spent given the cost of getting it wrong.
Even more questions
Is 304 acceptable for indoor, dry structural use? Yes. In dry, low-chloride indoor environments, 304 performs well and is economical, which is a large part of why it is so widely used.
Does duplex require different design codes than 304? Both are covered by the relevant pressure and equipment codes, but duplex brings specific fabrication and welding requirements that must be followed to preserve its properties.
Why is molybdenum so important in this comparison? Molybdenum sharply improves resistance to chloride pitting and crevice corrosion. 304 contains none, which is the core reason it trails both 316 and duplex in chloride service.
Key facts at a glance
- 304 has the lowest corrosion resistance of the common grades and contains no molybdenum.
- Duplex 2205 has a much higher PREN and far better chloride resistance.
- Duplex has roughly double the yield strength, allowing lighter sections.
- 304 is cheaper and easier to fabricate and suits mild, general service.
- For aggressive service, the real comparison is usually duplex versus 316, not 304 (Source: alloy data per IMOA and the Nickel Institute; confirm from certified composition).
Specify the right grade and avoid false economy
Choosing 304 to save money in an aggressive environment is one of the most common and costly materials mistakes. GreyTec helps chemical and petrochemical clients across the Netherlands and the EU specify the correct grade, 304, 316, or duplex, for the real service, and supplies the corrosion-resistant equipment to match.
See our engineering solutions for petrochemical plants, or call for a grade-selection review.
Related reading: Duplex stainless steel vs 316: which to specify? and duplex stainless steel corrosion resistance explained.