What makes stainless steel stainless?
Chromium. At around 11% or more, chromium in the alloy reacts with oxygen to form an extremely thin, transparent, self-repairing oxide layer that seals the surface — so stainless steel does not resist corrosion by being inert, it resists it by corroding instantly and then stopping.
The passive layer
Chromium oxidises faster than iron does. The result is a chromium-oxide film a few nanometres thick, tightly bonded, transparent, and — crucially — non-porous. Iron oxide, ordinary rust, is porous and flakes, which is why rust keeps going and this does not.
Why "self-repairing" is literal
Scratch stainless steel and you expose fresh metal, which oxidises within moments and reseals. That is the property that matters in practice: the protection is not a coating that can be worn through, it is a reaction that happens again every time the surface is damaged, as long as oxygen is available.
When it does stain
Deprive it of oxygen and the film cannot re-form. That is why stainless steel corrodes under a gasket, inside a crevice, or beneath a stuck-on deposit, and why chlorides — seawater, swimming pools, de-icing salt — are its classic enemy: chloride ions break the film locally and the damage concentrates into a pit.
Why there are so many grades
304 is the common austenitic grade. 316 adds molybdenum specifically to resist chloride pitting, which is why it is specified for marine and medical use. Ferritic grades like 430 are cheaper and magnetic. The grade is a corrosion-environment decision rather than a quality ranking.
The magnet test
Austenitic stainless (304, 316) is largely non-magnetic; ferritic and martensitic grades are magnetic. So a fridge magnet tells you something about the grade family — but not whether the steel is stainless, which is the thing people usually think they are testing.
Why it still needs cleaning
The passive film needs oxygen to re-form, so anything that sits on the surface and excludes air is a problem — grease, a stuck-on deposit, or ordinary carbon steel dust from a nearby grinder. That last one is the commonest cause of what looks like rusting stainless: the rust is the embedded steel particle rather than the stainless around it, and it keeps staining until the particle is removed.
Sources
British Stainless Steel Association — the role of chromium; ASM Handbook, Volume 13: Corrosion. Every claim above is drawn from these; where a claim is contested or unsettled the article says so rather than picking a side.
How this article was made
Written by a person. Reviewed by Editorial review, Leumas on 2026-09-01. 373 words.
Questions
What makes stainless steel stainless?
Chromium. At around 11% or more, chromium in the alloy reacts with oxygen to form an extremely thin, transparent, self-repairing oxide layer that seals the surface — so stainless steel does not resist corrosion by being inert, it resists it by corroding instantly and then stopping.
How do I know this is right?
The sources are listed on the page and a person reviewed it on 2026-09-01. Where a question is genuinely open, the article says so instead of resolving it.