Stainless steel is ordinary steel with one decisive addition. Iron and carbon make steel; add chromium, at least about a tenth of the alloy by weight, and the surface changes its behaviour. Exposed to air, the chromium reacts with oxygen and forms a layer of chromium oxide a few atoms thick. That layer is transparent, it is tightly bonded to the metal, and if you scratch through it, it grows back on its own. Carbon steel rusts because its oxide is loose and porous and keeps flaking away; stainless does not, because its oxide seals the metal off.
So “stainless” describes a surface, not the whole bar. The metal underneath is still steel: heavy, stiff, slower to conduct heat than carbon steel, and workable with the right tools. Everything we do on our floor — laser cutting, bending, V-grooving, perforation, PVD coating — is a way of working that steel while keeping its skin intact.
A family, not a grade
There is no single stainless steel. The name covers a family of alloys, grouped by the crystal structure the alloying gives the metal, and the group decides how the material behaves on the bench.
- Austenitic — chromium and nickel together. Non-magnetic in the annealed condition, easy to form and weld, and the group the everyday grades belong to: 304 and 316 are both austenitic. Most decorative sheet, most kitchen work and most architectural stainless is in this group.
- Ferritic — chromium without nickel. Magnetic, leaner in alloy, and harder to weld in thick sections. 430 is the common ferritic grade; you meet it in appliance panels and trim.
- Martensitic — chromium with more carbon, so it can be hardened by heat treatment. Knife blades and shafts, not cladding. 410 and 420 are the usual grades.
- Duplex — a mixed structure, roughly half austenite and half ferrite, with higher strength and better resistance to chloride cracking than the austenitic grades. 2205 is the usual grade, chosen for process plant and marine work rather than interiors.
What the chromium layer does not do
The passive layer is thin and it needs oxygen to repair itself. Anything that starves the surface of oxygen, or attacks the layer faster than it can re-form, will let the steel corrode: chloride-laden water sitting in a crevice, a smear of carbon-steel dust left by a grinding wheel, an unwashed heat tint beside a weld. Stainless is not corrosion-proof; it is corrosion-resistant, to a degree that depends on the grade and on how the surface is treated after fabrication. That is why a 316 grade exists at all, and why the cleaning a fabricator does after welding matters as much as the grade stamped on the sheet.
What it means for a sheet on the bench
- It is heavier than it looks. The austenitic grades carry a density of 8000 kg/m³ in our material database, a little above carbon steel, so a large sheet is a two-person lift long before it is a large sheet.
- It work-hardens. Every cut, fold and roll makes the metal at that spot harder, which is why a second bend on the same line cracks and why the laser is the clean way to cut it.
- It shows its surface. The finish — mill, brushed, mirror, bead-blasted — is part of the material, and every operation after it has to leave it untouched. That is the whole reason V-grooving exists.
- It takes colour by PVD. Because the surface is a stable oxide, a vapour-deposited film bonds to it; that is how gold, bronze and champagne stainless are made.
If you are choosing a grade, the two you will meet first are 304 and 316, and the difference between them is one element.