Stainless steel
Steel with enough chromium to repair its own oxide film — the material that made corrosion a design choice rather than a certainty.
Stainless steel is steel carrying at least about a tenth chromium by weight, and the threshold is not arbitrary. Below it, the oxide that forms on the surface is porous iron oxide and rust spreads underneath. Above it, chromium forms a film a few atoms thick that is dense, transparent and — critically — reforms instantly when scratched.
That self-repair is the whole property. Stainless steel is not unreactive; it is reactive in a way that protects itself, which is why it fails in conditions that stop the film reforming. Chloride attack in seawater and crevice corrosion under gaskets are both cases of the film being denied the oxygen it needs.
Why it behaves as it does
Stainless steel resists corrosion because chromium oxidises faster than iron does, and produces a better oxide when it goes.
With at least roughly a tenth chromium by weight, oxygen reaching the surface is taken up by chromium rather than iron, forming a chromium oxide layer a few atoms thick. That layer is dense and continuous, where iron oxide is porous and flaky — rust does not seal a surface, it lifts off it and exposes fresh metal underneath. The chromium film seals, and because it is transparent at that thickness the steel still looks like steel.
The part that matters most is that the film reforms. Scratch it and the exposed chromium oxidises again within moments, provided oxygen can reach it. That proviso is also the failure mode: stainless steel corrodes in crevices, under gaskets and in stagnant seawater, precisely where the film is denied the oxygen it needs to repair itself. It is not an unreactive metal; it is a metal that is reactive in a useful direction.
Processing
The alloying is straightforward; the difficulty is keeping carbon and chromium apart. Heating stainless steel through roughly 450 to 850 degrees lets chromium carbides precipitate at the grain boundaries, stripping chromium from the metal beside them and leaving a path for corrosion to run along.
That is sensitisation, and welding causes it in the zone either side of the weld. The industry solves it in three ways: low-carbon grades that have little carbon to precipitate, stabilised grades containing titanium or niobium that grab the carbon first, and heat treatment afterwards to redissolve the carbides.
Uses
Food and pharmaceutical equipment, where the surface must not react or harbour bacteria and must survive repeated aggressive cleaning. Chemical plant and pressure vessels. Cutlery, sinks and architectural cladding.
Surgical instruments and implants use specific grades, and the choice matters more than it appears: an implant sits in a warm chloride solution for decades, which is close to the worst case for the passive film. Nickel-free grades exist because nickel sensitisation is common enough to be a real clinical constraint.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
is an alternative to
- Steel — alloy · chosen where corrosion resistance is worth several times the cost
- Titanium — element · where seawater or the body is involved: titanium resists both far better and weighs less, at several times the price and with machining costs to match
- Nickel superalloy — alloy · in hot service, where stainless has lost most of its strength by 600 °C and a superalloy has not; the cost difference is a factor of ten and the temperature difference is what buys it
- Weathering steel — alloy · both avoid painting and by opposite means — one lets a protective rust form, the other prevents rust entirely — and weathering steel costs a fraction as much and is wrong anywhere near chlorides
is used as
- Corrosion protection — application · by a self-repairing chromium oxide film rather than by a coating
- Structural engineering — application · where the structure is exposed and repainting it is not an option, at several times the cost of ordinary steel
- Tableware and vessels — application · cookware, cutlery and sinks, chosen because it neither reacts with food nor breaks when dropped
is used in
- Construction — industry · cladding, fixings and structural elements exposed to weather
- Medical devices — industry · surgical instruments and temporary fixation, and specific low-nickel grades exist because nickel sensitisation is a real clinical constraint
is produced by
- Alloying and melting — process · chromium into steel, at the proportion where the oxide film becomes continuous
is an input to
- Welding — process · and the heat-affected zone is where it corrodes: held between 450 and 850 °C, chromium carbides precipitate and strip the chromium from the metal beside them
is commonly confused with
- Weathering steel — alloy · both are 'steel that does not need painting', and one of them is covered in rust on purpose
is produced at
- Sheffield — place · discovered here in 1913 by Harry Brearley, investigating gun barrel erosion, and found on a scrap heap because it had not rusted
Sources
- Material WorldOur own writing