passivation
/ pass-ih-VAY-shun /
Some metals ought to corrode furiously — aluminum and chromium have very active electrode potentials, more eager to react than iron — yet an aluminum ladder or a stainless spoon lasts for decades. The reason is passivation: the metal reacts with oxygen so quickly and so neatly that it wraps itself in an ultra-thin, dense, tightly-bonded oxide skin, and that skin then acts as a shield that all but stops any further attack. The metal, in effect, uses a first sip of corrosion to armor-plate itself against the rest.
It works because the film is thin (only a few nanometers), coherent, and pore-free, so it seals the metal off from water and oxygen and its own ions can barely cross it. The corrosion rate drops by a factor of thousands or more the moment the film forms — the metal switches from an 'active' state to a 'passive' state. Crucially the film is self-healing: scratch it, and the exposed fresh metal instantly re-oxidizes and reseals in air or aerated water. This is exactly why cutting a piece of stainless steel does not expose a rusting edge, and why aluminum can be shiny for years. Chromium is the magic ingredient in stainless steel; above about 11 percent chromium the alloy passivates, which is what 'stainless' really means.
The catch — and it is a big one — is that passivity is conditional, not permanent. The film needs the right environment (usually enough oxygen and a moderate pH) to stay whole, and certain aggressors, above all chloride ions, can punch through it locally and it may not re-heal. That is the doorway to pitting and crevice corrosion. So passivation is a superb but fragile defense: it makes reactive metals behave like noble ones, but only while the film survives, which is why stainless steel and aluminum still fail in salty, stagnant, or reducing conditions where the film cannot hold.
Aluminum is a striking case: its electrode potential (-1.66 V) is far more active than iron's, so bare aluminum 'should' corrode much faster than steel. In practice a fresh aluminum surface grows a hard, transparent alumina film in milliseconds, and that few-nanometer skin is why aluminum window frames and drink cans survive weather that rusts steel.
Passivation, not nobility, is why reactive aluminum and chromium resist corrosion in ordinary air.
Passivity is a state, not a permanent property: the very same stainless steel is highly corrosion-resistant in aerated water yet can corrode freely in a de-aerated, chloride-rich, or strongly acidic solution where the film cannot form or stay intact.