304 or 316 stainless steel: which should you choose?

What molybdenum changes, and what it doesn't

Chromium gives stainless its passive film: an oxide layer a few nanometres thick that reforms spontaneously the moment it is scratched. That film resists water, air and most neutral media. It has one weakness: the chloride ion. Chloride adsorbs locally, breaches the film at a point, and corrosion starts there without spreading. This is pitting — a localised attack that goes through a wall while the surface around it stays sound.

Molybdenum stabilises the film against that attack and helps it repassivate. That is its main function. It also brings better resistance in reducing media — sulphuric or phosphoric acid — and against crevice corrosion. It does not, however, change weldability or appearance, and its effect on room-temperature mechanical strength is marginal.

Hence the practical conclusion: molybdenum is only worth paying for when it works. On a workshop structure, a fresh water network, a support or non-chlorinated process piping, a 304L does exactly the same job as a 316L.

The threshold that separates them

The working figure is around 200 ppm of chlorides at ambient temperature for 304L. It is not a standard's limit but an engineering marker, and it moves with three factors.

Temperature. It lowers the admissible threshold sharply. Water that passes in winter does not necessarily pass in summer, and an insulated run works hotter than the surrounding air.

Stagnation. In flowing water, chlorides do not concentrate. In a low point, a dead leg or under a deposit, they concentrate locally and the real threshold falls well below the theoretical one.

Confined spaces. Under a gasket, in a flange crevice, beneath a patch of deposit, the attack takes a different form — crevice corrosion — and it starts at far lower concentrations than open-surface pitting.

Which is why water reported at 150 ppm does not guarantee that a 304L will hold: what matters is the local concentration, not the average.

The L suffix, which is not the point but does matter

Under the EN standard (1.4301/1.4307, 1.4401/1.4404), 304 and 304L, 316 and 316L differ only in carbon: 0.07% maximum against 0.030% — ASTM sets 0.08% maximum for the non-L versions (304, 316). At high temperature, carbon precipitates as chromium carbides at grain boundaries and locally depletes the matrix in chromium — the passive film no longer forms there. This phenomenon, sensitisation, occurs precisely in the temperature band the zone beside a weld passes through as it cools.

On piping welded on site, with no possibility of heat treatment after assembly, it is the L version you need, in both grades. That is why the European market supplies 304L and 316L by default, and why this guide compares them in that form.

The atmosphere case

A frequent mistake is to look for chlorides only in the fluid. They are often in the air.

By the sea, spray deposits chlorides on external surfaces: a 304 handrail pits within a few seasons where a 316 holds. In an indoor swimming pool, the chlorinated atmosphere attacks structural components, and the subject has been the object of specific guidance following fixing failures. In polluted urban or industrial settings, deposits combined with humidity produce the same effect.

In those cases the service medium is not what flows inside the tube — it is what surrounds it.

And the price difference, concretely?

On our internal index, base 100 for 304L, 316L sits at 130. The gap comes from molybdenum and higher nickel, two elements whose prices move; the figure is therefore not constant over time, though its order of magnitude is.

Across a whole installation, the real gap is calculated on the full bill of materials rather than on tube alone, since fittings and flanges follow the same rule.

When neither will do

If the medium is seawater, the 304/316 question does not arise: both fail. A PREN of 25 is far below the 40 commonly required for that service. You need to change family — duplex 2205, super duplex 2507, super austenitic 254 SMO, copper-nickel or titanium depending on flow velocity, temperature and budget.

If chloride stress corrosion cracking is the main risk — the textbook case being an insulated vessel in a chlorinated atmosphere — no austenitic is the right choice, 316L included: a ferritic 444 or a duplex 2304 is markedly more resistant.

Frequently asked questions

Is 316 "better" than 304?

It is better on one front: chloride resistance. On every other, they are equivalent. Presenting it as a higher quality leads to systematic over-specification.

How do I know whether my medium contains chlorides?

A water analysis will tell you. Failing that, some indicators: proximity to the sea, chlorinated cleaning products, borehole water, process brine, insulation that has taken up moisture. Where genuine doubt persists on a new installation, 316L is the reasonable insurance — but that is a decision, not a reflex.

Can 304L and 316L be mixed on one network?

Metallurgically yes, with no serious consequence. In practice it complicates the certificate trail and creates a heterogeneity that is hard to trace during later work. On an existing 316L network, dropping to 304L for one section returns little and costs in traceability.

Is 304 magnetic?

In the annealed condition, no: it is austenitic. Slight magnetism can appear after cold work — bending, forming — without indicating anything wrong with the grade.