Stainless steel for a pool and chlorinated water
A pool needs stainless steel with molybdenum: 316L or duplex 2205. Ordinary 304 is not suitable in chlorinated water — chlorine destroys the passive film, and the metal becomes covered with pitting in stagnant zones, under gaskets and along weld seams.
Handrails, ladders, steps, fasteners and basin elements are taken from 316L, and after welding the seams are cleaned and passivated.
Why ordinary 304 is not suitable
304 holds up to water, steam and food environments, but not chlorides. Chlorine in water works as an oxidiser and destroys the chromium oxide film locally, in micro-areas where the metal composition or the state of the surface differs from the rest of the area.
Then a chain works:
- at the point where the film is destroyed, pitting begins, which goes deep into the metal;
- in gaps under gaskets, fasteners and in joints, crevice corrosion develops;
- residual stresses after welding and bending give stress corrosion cracking;
- warm water accelerates all three processes.
The first to suffer are not sheets and not pipes, but seams, fasteners and hidden cavities. Therefore an assembly made of 304 may look intact on the open surface and at the same time have a through hole in the seam or a crack at the flange.
The quality of the surface has a separate influence. Coarse grinding, scale after welding and deposits from water create a relief in which chlorine and dirt linger longer. Even the right grade with poor surface preparation loses resistance.
| Grade | Resistance to chlorides | Where it is used in a pool | Limitations |
|---|---|---|---|
| AISI 201 | Low | Not used | Not for a wet chloride environment |
| AISI 304 | Low | Not used in water and the wet zone | Pitting, crevice corrosion, cracking |
| AISI 316L | High | Handrails, ladders, steps, fasteners, the basin | Requires passivation of seams |
| Duplex 2205 | High | Loaded structures, fasteners, assemblies with a risk of cracking | More difficult to weld, more expensive |
| AISI 904L | Very high | Individual assemblies with aggressive water chemistry | More expensive and less available |
316 and duplex
Molybdenum in the composition of 316 increases resistance to pitting and crevice corrosion. For water with chlorine this is enough: 316L works on handrails, ladders, steps, fasteners, overflow channels and embedded parts.
The letter L in the grade means low carbon. For welded structures this is important: in ordinary 316 during welding chromium binds into carbides along the grain boundaries, and the seam loses resistance. 316L has little carbon, so a welded assembly does not require annealing. For a pool, where almost all elements are welded, this is the main option.
Duplex 2205 is taken where strength is added to resistance: long handrails, load-bearing consoles, fasteners of large sections, assemblies with high stresses that in an austenitic grade would give cracking. Duplex is stronger, so a part can be made thinner, but it is more difficult to weld: control of heat input and the right filler materials are needed.
For fasteners the rule is simple: they must be from the same group of steels as the structure. Galvanised or aluminium fasteners paired with stainless steel form a galvanic pair, and corrosion will proceed along the less resistant element. For connections immersed in water, A4 fasteners are taken.
Welding and passivation
Welding is the most vulnerable point of a pool element. In the seam zone the metal is heated, chromium partially goes into carbides, and scale remains on the surface. Under it lies a chromium-depleted layer that does not restore the passive film.
What to do in order:
- weld in a shielding gas atmosphere, without overheating and without spatter on the surface;
- clean the seam and the heat-affected zone, remove scale and heat tint;
- pickle the seam if the scale is deep;
- passivate the surface and wash it with clean water;
- check the result with a free iron test.
The tool for stainless steel must be separate. A brush that has been used to clean carbon steel will leave particles of ordinary iron on the surface, and they will give rusty spots long before pitting from chlorine begins.
Machining also leaves traces: cutting, bending and grinding introduce contamination and create microrelief. After installation the assembly is inspected once more and the places that were not reached in production are treated.
Care
Stainless steel in a pool does not require special servicing, but it does require regularity.
- Wash the surface with clean water after the season and as deposits appear.
- Remove deposits and salt film with a soft brush, without abrasives and without chlorine-containing concentrates.
- Do not leave chlorine tablets, acid products and fertilisers on handrails and steps.
- Inspect seams, joints and fasteners for the first points of rust.
- Keep the acidity and the chlorine content of the water under control: the more aggressive the water, the higher the requirements for the grade.
- If points of corrosion appear, clean the area and repeat passivation, rather than painting over it.
A common mistake is washing stainless steel elements with the same products as tiles, with concentrated acid or chlorine. A short-term exposure to concentrate is more dangerous than constant contact with pool water.
Frequently asked questions
Which stainless steel is suitable for a pool
316L or duplex 2205. For fasteners and immersed parts, A4 fasteners from the same group of steels are taken.
Why does 304 rust in a pool
Chlorine destroys the passive film, and pitting begins in stagnant zones. The first to corrode are seams, joints and places under gaskets.
What is better for a pool: 316 or 304
- Molybdenum in its composition increases resistance to chlorides, while 304 has no such protection.
Is passivation of handrails and steps needed
Yes, after welding and installation. A seam without treatment becomes a centre of corrosion, even if the grade is chosen correctly.
Can 201 be used for pool elements
No. This grade is suitable for dry rooms and decor; in a chloride environment it loses resistance faster than 304.
Do stainless handrails rust indoors in a pool building
They can, if splashes of chlorinated water, concentrated cleaning products or particles of carbon steel from tools get on them.
What to do about the rust spots that have appeared
Clean the area, wash it and passivate it. If the spots follow the seam, the quality of the welding and of the seam treatment is checked.
What to look at next
- AISI 304 or 316: how they differ and what to choose — comparison of grades by environments.
- Passivation and pickling of stainless steel — the order of treating seams.
- Stainless profile pipe — square and rectangular for frames and handrails.
- AISI 316 — catalogue items in a chloride-resistant grade.
Place an order
Choose the grade, profile and surface in the stainless steel products catalogue and place an order with the required shipment week. The shipment week is an ISO week, and it must match exactly.