An alloy is a metal made up of a base metal together with one or more deliberately added elements, so that the whole gains properties the pure metal does not have: more strength, better corrosion resistance, better formability or a lower melting point.
Practically every metal you handle on a building site is an alloy. Pure iron is too soft to build with, pure copper too weak for structure. The additions determine what a product can and cannot do.
The alloys you meet in construction
- Steel — iron with carbon. The carbon content sets hardness and weldability: structural steel has little carbon and welds well, tool steel has much more and is hard but brittle.
- Stainless steel — steel with at least around 11% chromium, which forms a thin, self-healing oxide film in air. The common grade 304 (18% chromium, 8% nickel) is fine indoors and in ordinary outdoor air; 316 also contains molybdenum and resists chlorides markedly better — the choice by the coast, around swimming pools and where de-icing salt is used.
- Brass — copper with zinc. Easy to machine, attractive in colour, widely used for door furniture, taps and fittings.
- Bronze — copper with tin. Hard, wear-resistant and very durable outdoors; used in bearings, bells, sculpture and historic ironmongery.
- Aluminium alloys — aluminium with magnesium and silicon among others. Light, corrosion resistant and extrudable into sections; the basis of virtually all aluminium windows, curtain walling and glazing beads.
- Zinc and lead used in roofing are alloyed too, partly to resist deformation and creep.
Why it matters on site
- Galvanic corrosion. Two dissimilar metals in contact with moisture form a galvanic cell: the less noble metal is attacked faster. Copper or stainless in contact with zinc or galvanised steel is the classic problem — and run-off direction matters too: water dripping from a copper pipe onto a zinc gutter attacks that gutter. Separate metals with a plastic or bitumen layer, or choose compatible materials.
- Fixings must suit the material. Galvanised screws into stainless sheet or into cedar will fail. Use 304 or 316 stainless outdoors, and keep the screw at least as noble as the material it fixes.
- Weldability differs. Aluminium and stainless need different filler, different gases and a different heat input from mild steel.
- Thermal movement differs. Aluminium expands considerably more than steel. On long facade sections and roof edges that movement has to be taken up in the detailing.
When ordering
Always give the material designation, not just the name: S235 or S355 for structural steel, 304 or 316 (1.4301 and 1.4401) for stainless, and the alloy designation for aluminium sections. “Stainless steel” with no further specification tells you nothing about corrosion resistance — and that is usually exactly why it was chosen.
Alloy checks on site and when ordering
- Dissimilar metals are separated: Use a plastic or bitumen layer, or choose compatible materials; also check run-off, such as water dripping from copper onto a zinc gutter.
- Fixings suit the material: Use 304 or 316 stainless outdoors, and keep the screw at least as noble as the material it fixes.
- Thermal movement is taken up in the detailing: Aluminium expands considerably more than steel; allow for this on long facade sections and roof edges.
- Material designation is specified, not just the name: Structural steel: S235 or S355. Stainless: 304 or 316 (1.4301 and 1.4401). Aluminium sections: the alloy designation.
Related terms
- Corten steel
- Reinforcing steel
- Aluminium
- Bronze
- Formed zinc
- Cold-formed steel
- Corrosivity category
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.