Which stainless steel grade for sports equipment: 304, 316L or superaustenitic

The grade is decided by molybdenum content: 316L contains 2.0 to 2.5 % of it under EN 10088, while 304 contains none. That gap is read in the PREN index, which measures resistance to pitting corrosion: 23.1 to 28.5 for 316L, 17.5 to 20.8 for 304. In a chloride-laden atmosphere – sea spray, seafront, ship deck, marine tropical climate – this is the figure that decides how long the installation lasts.

This page is the shared technical reference behind all our sector pages. It sets out the selection method, the values, and the cases where 316L is not the right answer, because such cases exist.

The PREN index, the only comparison that holds

Resistance to pitting corrosion is calculated as PREN = Cr + 3.3 Mo + 16 N, where Cr is chromium, Mo molybdenum and N nitrogen. Every percent of molybdenum is worth 3.3 points. The higher the index, the better the behaviour in a chloride environment.

These values are published by the British Stainless Steel Association from the composition ranges given in EN 10088. One reservation matters: they serve to rank grades against one another, they are not enough to qualify a material for a given application. PREN guides the choice, it does not prove it.

A note on the L in 316L: it denotes a reduced carbon content, which limits sensitisation of welded zones. On a welded structure that is a real advantage, independent of PREN.

Read the corrosivity of the site before choosing

ISO 9223 classifies atmospheres by corrosivity, from C1, very low, to CX, extreme, the latter covering specific marine and industrial environments. The clearest way to compare stainless steel with galvanised steel is the rate at which zinc is consumed.

A standard hot-dip galvanised coating carries a few tens of microns of zinc. In category CX the arithmetic is quickly done. Stainless steel is not consumed: its passive layer re-forms in the presence of oxygen, provided the film is not held under a deposit.

A point of vocabulary that carries weight with a design office: the category C5-M no longer exists. The 2017 edition of ISO 12944-2 replaced C5-I and C5-M with C5 for severe land-based exposure and CX for offshore and permanent sea spray. Seeing it quoted in a specification is a sign that the document has not been updated.

Which grade for which installation environment

The Light In Fitness stainless range comprises 98 references in 316L. Superaustenitic grades are produced on request and on the basis of a study, with a higher material cost and a longer procurement lead time.

The exception: stress corrosion cracking in an indoor pool hall

This is the only case where moving up from 316L is not a comfort but a necessity, and it deserves to be known.

On 9 May 1985 the suspended ceiling of the indoor swimming pool at Uster, Switzerland, collapsed during a training session, killing twelve people. The identified cause was chloride-induced stress corrosion cracking of type 304 stainless steel hangers, containing no molybdenum. The damage was almost invisible: the cracking progressed inside the metal.

The mechanism belongs to the covered building. Under a roof, the air loads with volatile chlorinated compounds which condense on unrinsed surfaces; the water film then concentrates chlorides and turns acidic. In an open-air pool the combination does not occur: without a roof these compounds do not accumulate and rain rinses the deposits away.

The practical consequence: in a covered hall, grades 304 and 316 are ruled out by the reference documents for load-bearing, suspended or safety-critical components that are not cleaned regularly. They remain acceptable for immersed components, for components rinsed regularly, or for components not under tensile stress. The subject is covered by the normative annex of EN 13451-1 dealing with the use of stainless steel in a load-bearing function in a swimming pool atmosphere, the version in force being EN 13451-1+A1 of August 2024.

What the grade does not settle

Choosing the right steel does not remove the three other checks in an outdoor equipment project.

• Anchoring: foundation block and base plate are sized for the site – Eurocode 8 in seismic zones, the applicable cyclonic wind rules in tropical regions, inertia loads on board a vessel. The steel grade changes none of this.
• Fire behaviour: on board a vessel, annex 2 of the 2010 FTP Code lists common metals and alloys among the products that may be installed without test or approval. That is an advantage of metal in general, not of 316L in particular; upholstered parts and surface finishes follow their own regime.
• Safety in use: equipment in free public access falls under EN 16630, which covers design, safety zones and user information, whatever the material.

Fabrication and maintenance

A correct grade badly executed behaves badly. Four points for the specification:

• Welds: pickling and passivation after welding, failing which the heat-affected zone becomes the starting point of corrosion.
• Fasteners and fittings: specify a grade at least equivalent to that of the structure. A lower-grade bolt downgrades the whole assembly.
• Contact with other metals: the galvanic couple between stainless steel and a carbon steel supporting structure is handled by isolation or by choosing a compatible material.
• No brushing with carbon steel tools: ferrous particles deposited on the surface rust and initiate corrosion. Tools dedicated to stainless steel are the rule.

Routine maintenance comes down to periodic rinsing with fresh water, which removes surface chloride deposits, and a visual check of the anchors. International good practice recommends inspecting stainless components in a chloride atmosphere at least twice a year, looking for brown discolouration.

Frequently asked questions

Is 316L always the right choice outdoors?

No. Inland, far from any source of chlorides, hot-dip galvanised steel does the job at a lower cost, and it is the material used for the majority of public space installations. 316L becomes relevant as soon as there is salt in the air: coastline, harbour, vessel, marine tropical climate. It becomes insufficient in one common case only, that of load-bearing components in an indoor pool hall.

What is the difference between 316 and 316L?

The maximum carbon content, which is lower in 316L. This limits sensitisation of the heat-affected zones of a weld and therefore the risk of intergranular corrosion on an assembled structure. The composition is otherwise identical, and both grades share the same PREN index.

Why the price gap with 304?

Molybdenum. It is the element that makes the difference in a chloride environment, and it is also the most expensive in the alloy. The relevant reasoning is not the purchase price but the cost over the service life: a 316L stainless structure carries a lifetime guarantee at Light In Fitness, a steel structure twenty years, and galvanised steel installed by the sea is replaced well before either.

Can a stainless structure rust?

It can show superficial rust staining, almost always of external origin: ferrous particles left by a tool, unsuitable brushing or a neighbouring steel structure. It is not the structure itself corroding. Suitable cleaning removes the staining. Persistent brown discolouration on a component in a chloride atmosphere does, however, warrant closer examination.

Do you supply superaustenitic grades?

On request and on the basis of a study, with a higher material cost and a longer procurement lead time than 316L. State the intended location of the station when you request a quotation: it is the location that determines the grade to be priced. The standard catalogue is in 316L.

Discussing your project

Tell us the installation environment, the exposure and the mode of use: the quotation comes back with the selected grade, the material documents and the technical items your design office will be asked for.

Réalisations documentées

• Functional Rig inox – installations sur trois continents
• iGreenMill, tapis de course exterieur – installations sur trois continents