In coastal or seaside projects, the most problematic category is "installing 304 but expecting it to perform like 316L". The two materials look identical and differ in price by about thirty percent, with only one elemental difference:316L contains 2–3% molybdenum (Mo). Molybdenum does not contribute to strength and does not change workability; it does only one thing —resist pitting and crevice corrosion caused by chlorides.
One-sentence verdict
For long-term contact with chloride-containing media (seawater, salt spray, brine, sodium hypochlorite, salty sauces) or where temperatures are elevated and chloride concentrations are not low, use 316L or duplex steel; for pure water, indoor, dry atmosphere, and general food contact, 304 is sufficient.In coastal areas, a further distinction must be made between "indoor" and "open-air exposed to sea breeze": for components exposed to sea breeze outdoors, even decorative ones, 304 often develops rust spots within one to two years.
Criterion: calculate PREN first, then consider temperature
To assess pitting resistance, the industry generally uses a pitting resistance equivalent number (PREN):
PREN = %Cr + 3.3 × %Mo + 16 × %N
- 304 / 304L: no molybdenum, PREN approximately 18–19;
- 316L: contains 2–3% molybdenum, PREN approximately 24–26;
- 2205 duplex steel: PREN approximately 34–36; 2507 super duplex is higher, approximately above 40.
For every increase of about 10 in PREN, the critical pitting temperature (CPT) roughly rises by one step — so "316L is a bit better than 304" is not psychological comfort but a calculable difference. What really determines selection is the combination of three quantities:chloride concentration, medium temperature, and whether crevices or deposits exist. When the three are superimposed, the risk is not additive but multiplicative: the same 200 ppm chloride may cause no problem for years in circulating water at 25℃, yet perforate within months in a stagnant crevice at 60℃.
A commonly cited rule of thumb in the industry is that "when chloride exceeds approximately 200 ppm and the temperature is above 60℃, 304 is no longer suitable". It is only an empirical value, not a mandatory standard clause; in practice, medium analysis reports and actual measurements under similar operating conditions should prevail — but it can help you quickly rule out clearly unsuitable options: seawater chloride is approximately 19000 ppm, two orders of magnitude above this line, so seawater service should not start with 304.
Two easily overlooked factors
① Crevices are more dangerous than surfaces.Under gaskets, between bolts and plates, at the junction of weld spatter and base metal, and in pits where deposits are not cleaned, all are dead corners for chloride enrichment. For the same material, components with crevices often perforate from within the crevice first.
② Sensitization of the weld heat-affected zone.When 304 stays in the 450–850℃ range (welding, hot working), chromium carbides precipitate and deplete chromium at grain boundaries, leading to intergranular corrosion in chloride or nitric acid environments. Therefore, for welded parts, low-carbon 304L / 316L should be preferred over ordinary 304.
Surface rust spots do not necessarily mean the wrong material was selected
A considerable portion of the "304 rusting" commonly seen on coastal projects is not pitting butiron contamination: carbon steel tooling, ordinary wire brushes, lifting chains, and cutting tables shared with carbon steel parts all embed iron particles into the stainless steel surface, which rust first upon moisture exposure. The treatment for this type of problem is completely opposite to material selection — not switching to 316L, butacid pickling and passivation + process isolation: dedicated stainless steel tools, dedicated grinding wheels, areas not shared with carbon steel, post-weld acid pickling to remove scale and free iron, then passivation to restore the passive film.
The distinction is simple: if rust spots are concentrated on "foreign traces" such as scratches, welds, and lifting contact points, it is mostly contamination; if rust spots are scattered on flat surfaces and appear as deep pits, that is pitting.
Three things we can confirm for you
Material selection advice must start from the medium, so before quoting we usually ask four things:medium composition (especially chloride), operating temperature, whether crevices/deposits exist, and the applicable standard. With all four provided, we can give defensible material recommendations rather than "try 304 first". On delivery, what we can ensure is: mill test certificates provided with each batch, packaging treated for moisture protection for sea freight, and slitting and leveling performed to your dimensional tolerances — all of which are inspectable and traceable.
Common pitfalls
- Treating "coastal area" uniformly as "must use 316L": for decorative parts in dry indoor environments, 304 is entirely sufficient; blind upgrading only costs more;
- Conversely, using 304 for structural components exposed to sea breeze outdoors to save money — the cost of rework two years later far exceeds the initial price difference;
- Looking only at the grade and not the condition: for the same 316L, corrosion resistance differs greatly between the solution-annealed and non-solution-annealed conditions; check the heat treatment condition on the quality certificate;
- Neglecting cleaning: 316L will also fail at locations with long-term salt and deposit accumulation if not cleaned; regular rinsing is the cheapest anti-corrosion measure.
If you have specific operating conditions (medium, temperature, concentration, standard), send us the parameters and we will provide material selection advice and a corresponding quotation within 24 hours.
