Types of steel for interiors and architectural metalwork

At a glance

Austenitic grades are slightly magnetic if cold-worked. PRE (pitting resistance equivalent) ranks pitting resistance from a grade's chromium, molybdenum and nitrogen; bands are our calculation from the Euro Inox composition limits, not guaranteed values.

Three kinds of steel, and what keeps each from rusting

What separates the groups is how each is kept from rusting. Stainless steel carries chromium in the metal itself, so it protects itself. Mild steel carries no protection and relies on a coating, wax or blackening. Weathering steel is allowed to rust once, into a stable skin, which only forms outdoors in wet and dry cycles. The tree shows the grades in each group.

Stainless steel: families and grade table

Stainless steels are grouped by their internal structure, and the group tells you three things at once: whether a magnet will stick, how well the sheet forms, and roughly how well it resists salt.

• Austenitic (iron, chromium and nickel): 304 and 316. Non-magnetic as supplied and very good for forming. They dominate architectural use.
• Ferritic (iron and chromium, little or no nickel): 430 and 444. Magnetic, with limited formability.
• Duplex (iron, chromium and an intermediate amount of nickel): 2304 and 2205. Magnetic, higher strength than the other two, intermediate formability.

The table is the asset to keep. Composition limits, thermal movement and magnetic behaviour are taken from the Euro Inox tables (which reproduce EN 10088-1); the family descriptions are from the International Molybdenum Association (IMOA).

Cr is chromium, Ni nickel, Mo molybdenum. Expansion is the mean coefficient between 20 and 200 °C. Nitrogen, the other element in the PRE sum, is up to 0.11% in 304 and 316, 0.05 to 0.20% in 2304 and 0.10 to 0.22% in 2205. *Austenitic grades are slightly magnetic if cold-worked (Euro Inox footnote), so a magnet cannot tell 304 from 316.

Pitting resistance ladder

Pitting resistance equivalent (PRE) is a single number worked out from the chromium, molybdenum and nitrogen in the metal. It ranks grades against each other: the higher the number, the better the grade resists pitting by salt and chlorides. The ladder shows where each grade sits.

Two things stand out. Molybdenum is what lifts 316 and 444 clear of 304. And finish, design and cleaning count as much as grade: IMOA treats a surface of Ra 0.5 micrometres or smoother as smooth for corrosion purposes, and says vertical grain is washed more easily than horizontal. For how to choose between 304 and 316 by environment, see 304 vs 316 stainless steel; for the finish codes, see stainless steel finish codes.

Mild and carbon steel: how to name it

"Mild steel" is a trade word, not a grade. In a specification, name the product standard and grade. For plate, bar and sections that is normally BS EN 10025-2, with a designation such as S275JR or S355J2. The number after the S is the minimum yield strength in N/mm² for material up to 16 mm thick. The letters JR, J0 and J2 are impact-test sub-grades (27 J at 20 °C, 0 °C and −20 °C). For interiors the finish and the weld quality usually matter more than strength, and any structural capacity is the engineer's call, not a catalogue figure.

For sheet and strip, say which route made it. Hot-rolled low-carbon strip for cold forming is supplied to EN 10111 (grades DD11 to DD14, up to 11 mm) as non-pickled, pickled, or pickled and oiled. Cold-reduced sheet for forming is supplied to EN 10130 (grades DC01 to DC06). The surface condition decides how the steel takes a blackened, painted or powder-coated finish, so write it into the schedule.

• It rusts. Bare mild steel has no protection of its own. The coating, wax or blackening is the protection, so it belongs in the specification as a line item with a sample.
• It moves less than stainless. Structural steel expands about 12 x 10^-6 per °C, against 16.5 for 304 and 316 and 10.0 for 430, which matters where different metals are fixed together.
• It is magnetic, as is every other steel on this page except the austenitic stainless grades.

Weathering steel: outdoors and in

Weathering steel is specified to BS EN 10025-5. Plate in S355J0W, S355J2W and S355K2W is readily available in the UK, and S355J2W is the grade most used for bridgework. Its chemistry is close to ordinary structural steel with small additions of chromium (0.40 to 0.80%) and copper (0.25 to 0.55%) in one UK mill's datasheet. Those additions let it form a stable rust skin, the patina, that slows further corrosion.

• It needs wet and dry cycles. The patina forms as the surface wets and dries. Where the steel is continuously wet, buried, or covered by plants, it corrodes at about the rate of ordinary steel and must be painted.
• It takes years to settle. It starts orange-brown and, in bridge guidance, usually reaches its characteristic dark brown within 2 to 5 years. During the first years rain washes rust from it and that run-off stains what lies below. Drip details and drainage that carry water away from stone, timber and glass are part of the design.
• Coast and salt. UK bridge guidance generally avoids weathering steel within about 2 km of coastal waters unless airborne chloride is low. It is a guide, not a fixed limit.
• Indoors it is a different product. Weathering steel is supplied un-weathered. One producer describes a pre-treatment (degrease, sandblast, then accelerated wet and dry cycles for about two months) that gives a red-orange patina, but says it does not stabilise the rust layer. For indoor use rust can transfer to hands and clothing, and a clear varnish over the pre-weathered surface stops the patina developing further and protects users and neighbouring finishes. The same producer advises against clear coatings outdoors, where they stop the patina from forming.

So an interior "weathered" panel is a coloured, sealed finish and will not heal itself. Ask the partner workshop for an approved sample of the colour and the sealer, and do not promise a colour or a timescale.

What to write instead of the trade word

Trade words leave the grade, the standard and the surface open. Replace them with a line the workshop can price and the contractor can check.

Five checks before you fix a grade

• A magnet proves little. Annealed 304 and 316 are non-magnetic, but cold-worked austenitic can be slightly magnetic, and 304 and 316 behave the same. To prove a grade, ask for the inspection certificate (type 3.1) for the batch.
• Welds in thick sections. For welded sections heavier than about 6 mm, IMOA advises the low-carbon "L" grade (304L, 316L) to keep corrosion resistance at the weld.
• Sulphur and passivation. IMOA says a sulphur content not above 0.005% improves pitting resistance, and that if sulphur is higher the finished part should be passivated with phosphoric or nitric acid after fabrication. The EN tables recommend 0.008 to 0.030% for weldability, so pitting resistance and weldability pull in opposite directions: ask for passivation after welding rather than a sulphur limit.
• Keep iron off stainless. IMOA warns that contamination from carbon steel tools, abrasives, wire brushes or airborne steel particles causes rapid corrosion. Ask whether the workshop separates stainless from carbon steel work.
• Mixing metals. Different metals touching in damp conditions corrode each other, and the area ratio decides how badly. A small stainless fixing in a large steel plate is usually fine; the other way round is not. One producer advises avoiding galvanised bolts on weathering steel.

Stainless steel to BS EN 10088-2, grade [1.4301 (304) interior / 1.4404 (316L) wet or exposed], [2.0] mm, finish [2B] with code and grain direction [vertical] as drawing, exposed faces [Ra 0.5 micrometres] or smoother. Welded sections over about 6 mm in the L grade. Certificate [type 3.1] with each batch. Worked with tools not used on carbon steel; welds blended to the finish and passivated. Finish sample approved before production. Site corrosion risk assessed by [specifier / corrosion expert] before the grade is confirmed.

Mild steel to BS EN 10025-2, grade [S275JR], hot rolled, surface [mill scale retained for blackening / pickled for paint or powder]. Finish [blackened and sealed with wax / powder coated, RAL ref], sample [ref] approved. Welds ground [flush] before finishing. Certificate [type 3.1].

Weathering steel to BS EN 10025-5, grade [S355J2W]. Interior: pre-weathered to approved sample [ref] and sealed with [clear coating agreed with the workshop]. Exterior: no clear coating; drips and drainage keep run-off off [adjoining finishes]. No colour or timescale guaranteed.

Standards and sources

• BS EN 10088, parts 1, 2 and 3 (stainless steels): named; the standard was not read for this sheet.
• Euro Inox, Stainless Steel: Tables of Technical Properties, 2nd edition 2007: composition, expansion, magnetism.
• International Molybdenum Association (IMOA): Which Stainless Steel Should Be Specified for Exterior Applications?: families, PRE, L grade, sulphur, roughness.
• steelconstruction.info: Steel material properties: S275 and S355 designation, BS EN 10025-2:2019.
• steelconstruction.info: Weathering steel: BS EN 10025-5 grades, patina, run-off (for bridges).
• BS EN 10025-5, 10111 and 10130 (weathering steel; steel for forming): grades and limits from producer documents.
• British Stainless Steel Association (BSSA): Technical help: advisory service and technical library.

Questions we are asked

What are the main types of stainless steel?

Three families matter in architecture. Austenitic (304 and 316) is non-magnetic and the usual choice. Ferritic (430 and 444) is magnetic and has chromium but little or no nickel. Duplex (2304 and 2205) is magnetic and stronger. IMOA notes that 304 and 316 dominate architectural use.

What is the difference between 304 and 316 stainless steel?

316 contains molybdenum (2.0 to 2.5% in the EN limits) and 304 does not, which gives 316 better resistance to salt and pitting. Our 304 vs 316 sheet explains how to choose by environment.

Is stainless steel magnetic, and can a magnet tell 304 from 316?

Austenitic grades (304 and 316) are non-magnetic as supplied, though cold working can make them slightly magnetic. Ferritic and duplex grades are magnetic. A magnet therefore separates austenitic from ferritic or duplex, but cannot tell 304 from 316. Ask for the inspection certificate.

What does S275 or S355 mean?

The number is the minimum yield strength in N/mm² for material up to 16 mm thick, so S275 is 275 and S355 is 355. The letters that follow (JR, J0, J2) give the impact-test sub-grade. They come from BS EN 10025-2.

What is mild steel, and does it rust?

Mild steel is ordinary low-carbon steel. It has no protection of its own and rusts, so it relies on paint, powder coating, wax or a blackened finish. In a specification, name the grade, such as S275JR to BS EN 10025-2, rather than the trade word.

What is weathering steel, and can I use it indoors?

Weathering steel (often sold under trade names such as Corten) is a low-alloy steel to BS EN 10025-5 that forms a protective rust skin outdoors in wet and dry cycles. Indoors that cycle does not happen, so panels are pre-weathered and then sealed with a clear coating. The result is a finished colour, not a self-healing surface.