Which metal for which environment: a selection matrix

At a glance

Metal and its caveat	Dry interior	Hands, bars	Wet room, spa	Outside, inland	Coastal	Pool hall
Brass and bronze	Suits	Care	Care	Care	Care	Expert
Name the alloy: some brasses lose zinc when wet. Both change with touch; lacquer wears where hands land. Basis: trade practice.
Copper	Suits	Care	Care	Suits	Care	Expert
Brown then green, faster near the sea (Zahner); run-off stains neighbours; tarnishes under hands unless waxed. Basis: fabricator.
Mild steel, coated or sealed	Suits	Care	No	Care	Expert	Expert
No protective coating needed in dry heated interiors (steelconstruction.info); elsewhere the coating protects, and fails at edges.
Weathering steel	Care	No	No	Suits	No	No
Needs wet-dry cycles; not in damp shelter or standing water, nor within 2 km of coastal water (ArcelorMittal). Rusts onto hands.
Stainless 304	Suits	Care	Care	Suits	Care	No
Finish and cleaning matter as much as grade; no chloride cleaners. Coastal only if the IMOA score is 2 or less. See TL-MET-01.
Stainless 316	Suits	Suits	Suits	Suits	Suits	Expert
Resists chloride pitting better than 304, not immune. Pool hall: washed parts only; suspended parts need an engineer. TL-MET-01.
Duplex stainless	Suits	Suits	Suits	Suits	Suits	Expert
Higher resistance than 316 (IMOA), stronger, magnetic, less formable. Not for load-bearing parts in indoor pool air (Euro Inox).
Aluminium, anodised or coated	Suits	Care	Care	Care	Care	Expert
The coating or anodic layer is the protection; edges and scratches matter. Acid cleaners for stainless attack it (Euro Inox).

Key. Suits: normally suitable. Care: suits only with the finish, coating, detail or upkeep in the caveat, or if the client accepts visible change. No: not recommended. Expert: a specialist decides; do not choose from this table. Coastal is about 30 m to 1.6 km from the sea.

The six environments, and a common scale

Dry interior: heated buildings with clean air. ISO 12944-2 gives offices, shops, schools and hotels as its examples of category C1.
Hands, bars: interior surfaces that are touched all day, wiped with detergents or sanitiser, or splashed, such as door furniture, handrails, bar tops and kitchens.
Wet room, spa: fresh water, steam and condensation. Chlorinated or salt water belongs in the pool-hall column.
Outside, inland: rain-washed. ISO 12944-2 puts mostly rural air at C2, and urban or industrial air with moderate sulphur dioxide at C3.
Coastal: salt-laden air, from about 30 m to 1.6 km from the sea on IMOA's bands; closer, or splashed, is a specialist case (see TL-MET-01). Coastal air with low salinity is C3 and high salinity C5.
Pool hall: an indoor chlorinated pool atmosphere. ISO 12944-2 lists swimming pools under C4; Euro Inox reports pool atmospheres generally at C4, in some cases at C2 and in others beyond C5 (zinc measurements).

The drawing places the six on the ISO 12944-2 corrosivity scale, the vocabulary a coating specification uses. The scale is defined by how fast low-carbon steel loses thickness in the first year, so it describes steel best; Euro Inox warns that it may not predict how stainless steels behave. Our placing of the six environments on it is indicative.

Six environments on the ISO 12944-2 scale (examples and steel losses from steelconstruction.info); dashed: pool halls vary (Euro Inox). Original drawing; the placing is indicative.

Why each row says what it says

Each caveat in the matrix names where it comes from. In more detail:

Mild steel. steelconstruction.info states that in a dry heated interior the risk of corrosion is insignificant and no protective coating is necessary, and that steel in an aggressive environment needs a high-performance treatment. Its tables list swimming pools as a category C4 interior. A finish is still needed in practice for handling, rust staining and looks, and coastal or pool-hall steelwork needs a coating system designed by a specialist.
Weathering steel. ArcelorMittal, a producer, says its weathering steel needs alternating wet and dry cycles; must not be used in sheltered damp locations, in permanent contact with water, or in soil or under vegetation without paint; and, citing EN ISO 9223, should not be used within two kilometres of coastal water unless airborne chlorides stay at or below its S2 class (under 300 mg/m² a day). It advises avoiding contact with road de-icing salts. Indoors, rust particles can come off on hands; a clear varnish stops that, and also stops the patina evolving. It advises against clear varnish outdoors. The trade name Corten is a trademark: the generic term is weathering steel.
Stainless 304, 316 and duplex. The ratings follow our sheet TL-MET-01 (304 or 316 by environment), which rests on IMOA and Euro Inox. Euro Inox says duplex 2205 is not recommended for load-bearing parts in an indoor pool atmosphere, though it lists it among grades suitable for parts that are cleaned regularly.
Copper. Zahner, a fabricator, describes copper weathering from reddish-orange to brown to green, with the aging "somewhat faster when near the sea", and notes that copper roofs exposed for a century or more develop a thick green patina. In dry climates the green patina may never develop.
Brass and bronze. They also keep ageing, whether pre-patinated or not (Zahner). Which brass matters: some lose their zinc in wet conditions. Our own journal guide on metal finishes notes that unlacquered brass in a kitchen or bathroom will darken quickly and unevenly, and that the choice between a living and a lacquered finish is the client's to make in writing.
Aluminium. Euro Inox warns that acid cleaners made for stainless steel can damage aluminium and galvanised steel, so protect them when cleaning nearby. The rest of the aluminium row is trade practice: the coating or anodic layer does the protecting, so scratches and cut edges matter, and a coastal site needs a coastal-grade system. It is listed for partner confirmation.

Where two metals meet

Most failures are at junctions, not in the metal itself. Three rules from the sources we read:

Fasteners: galvanised fasteners corrode quickly against stainless steel and leave rust marks on it; use stainless fasteners on stainless (Euro Inox).
Run-off: do not let run-off from carbon steel, weathering steel, or chloride-bearing cement, mastic or sealant fall on stainless steel (Euro Inox). Weathering steel also stains the stone or paving beneath it while it settles.
Cleaning: a cleaner chosen for one metal may attack its neighbour. Name the cleaning product for each metal in the maintenance schedule.

A galvanic-compatibility table, which depends on the environment, is a separate sheet that is not yet written.

Finishes: three cautions that apply across the matrix

Paint does not rescue the wrong grade. For stainless steel in pool environments Euro Inox says painting gives no additional protection, because paint can spall and form crevices, and that an organic coating cannot justify choosing a lower-alloyed grade.
Coloured and coated stainless cannot be repaired on site. Euro Inox notes that electrolytically coloured stainless steel has no possibility of rectification if it is scratched, and that painted stainless usually needs shorter cleaning intervals and gentler methods than bare metal.
Sealed or living is a decision. A lacquer on brass, or varnish on weathering steel, changes how the metal ages and how it must be maintained. Ask the client to approve a sample of the finish as it will look in use, not only new.

A finishes-by-environment matrix (PVD, powder, anodising, plating, lacquer, patina) needs partner data on each finish and is planned as a separate sheet.

Specification wording

Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.

Environment statement

Exposure for [element]: [dry heated interior, ISO 12944-2 category C1] [interior damp or washed, C2 to C3] [exterior inland, C2 to C3] [coastal, C3 to C5] [indoor pool hall, C4 or higher]. Metal and finish are chosen for this exposure. Any change of metal, finish or exposure to be referred back for approval. ISO 12944-2 edition [to be confirmed].

Metal and finish record

Element [ ]: metal [grade or alloy, for example EN 1.4404 or CW508L], finish [name, code and Ra or sample reference], environment [as above], living finish accepted by the client [yes, sample approved on [date] / no, sealed with [ ]]. Finish sample approved [date].

Dissimilar metals and run-off

Dissimilar metals not to touch where moisture can reach the joint unless isolated by [gasket, sleeve or coating]. Fasteners [stainless steel / the same metal]. Run-off from [carbon steel, weathering steel, copper] not to fall on [stainless steel, aluminium, stone]. Cleaning product for each metal named in the maintenance schedule.

Terms used on this page

Corrosivity category (C1 to C5, CX): A scale from ISO 12944-2 and ISO 9223 for how aggressive an atmosphere is, from first-year loss of low-carbon steel. C1 is a heated clean interior, C5 very aggressive, CX extreme.
Weathering steel: Low-alloy steel that forms a stable rust-coloured skin outdoors. It needs wet and dry cycles and fails in damp shelter or near the coast. Corten is a trademark.
Galvanic corrosion: Faster corrosion of one metal where it touches a different metal in the presence of moisture.

All metalwork terms

Standards and sources

Questions we are asked

Which metal is best for a bathroom or wet room?

316 stainless steel is the safest all-round choice where metal is rinsed and wiped; 304 can work with a smooth finish and no chloride cleaners. Brass, bronze and copper suit it if the client accepts them changing with moisture and touch. Avoid mild steel and weathering steel, which rust in permanently damp conditions.

Can I use Corten near the coast?

Usually not. ArcelorMittal, a weathering steel producer, says that under EN ISO 9223 weathering steel should not be used within two kilometres of coastal waters unless airborne chlorides stay at or below class S2. Corten is a trade name; the generic term is weathering steel.

Is mild steel suitable for interiors?

Yes in a dry, heated interior: steelconstruction.info says the corrosion risk there is insignificant and no protective coating is needed for that purpose. You will still want a finish for handling and looks. It is not suitable for wet rooms or pool halls without a specialist coating system.

Will brass tarnish in a bathroom?

Unlacquered brass changes with moisture and touch, quickly and unevenly in damp rooms. A lacquer slows it but wears where hands land and has to be stripped and reapplied. Decide which the client wants, in writing, and show an aged sample.

Which metal is safe in an indoor pool hall?

None from this table is a safe default. Chloramines in the air cause stress corrosion cracking of stressed stainless parts, so suspended and load-bearing items need a structural engineer to name the alloy. See TL-MET-01 for the detail.

Download the factsheet

A two-page A4 PDF with the matrix and its key, the corrosivity drawing and specification wording, to keep with your project file.

Factsheet TL-MET-02 v1.0 (PDF)