Corrosion and passivation

In detail

Several metals that behave as though they are unreactive are in fact highly reactive metals carrying a protective skin. Aluminium and stainless steel both form a thin surface oxide within moments of exposure to air, and that passive film — not the bulk metal — is what resists corrosion. The film is self-repairing while oxygen is available, which is why a scratch in stainless steel does not usually lead to rust. Two things defeat it. Chloride ions can break the passive film down at localised points, producing pitting corrosion: small deep pits rather than general rusting, which is why chloride-containing residues such as bleach or salt left in contact with stainless steel are a genuine risk. And strong alkalis dissolve the oxide on aluminium outright, since aluminium oxide is amphoteric, exposing the reactive metal beneath. Both mechanisms share a practical conclusion: what matters is not only which cleaner is used but how completely it is removed afterwards.

Why

• Seasoned cast iron is explained by Corrosion and passivation

  By contrast with stainless steel: the protection here is an applied organic layer, not a self-repairing oxide film.

• Sodium hypochlorite (chlorine bleach) is explained by Corrosion and passivation

  Its chloride content is what threatens stainless steel — a separate property from its oxidising power.

• Rust is explained by Corrosion and passivation
• Stainless steel is explained by Corrosion and passivation

  Stainless steel is not a metal that does not rust. It is a metal that repairs its own oxide film, and chlorides are what stop it doing so.