Why do some metals rust faster?

Rust is specifically the corrosion of iron, which reacts with oxygen and water to form flaky iron oxide, but all metals corrode at different rates depending on their reactivity. Iron and ordinary steel rust quickly because the oxide layer they form is porous and flakes away, exposing fresh metal underneath to keep reacting. Metals like aluminium also react with oxygen, but they form a thin, tightly bonded oxide layer that seals the surface and stops further corrosion. Salt, humidity, and acids all speed up rusting by helping carry the electric charge that drives the chemical reaction. This is why cars rust faster in coastal or road-salted regions, and why stainless steel, which contains chromium, resists rust so well.