The rusting of iron requires oxygen and water. This process forms rust, or hydrated iron oxide, through a chemical reaction. Water acts as a medium for ions to move, enabling the oxidation of iron by oxygen.

Chemical Process

Rusting involves iron reacting with oxygen in water's presence, often as moisture or vapor. The simplified reaction is:
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ , which dehydrates to rust (Fe₂O₃·nH₂O).

Both substances are essential; oxygen alone or dry conditions prevent rusting.

Why Water?

Water provides hydrogen ions that speed up electron transfer from iron to oxygen. In humid areas, rust forms faster due to higher moisture.

Experiments confirm: iron stays rust-free in dry oxygen or oxygen-free water.

Prevention Tips

  • Coatings : Paint or oil blocks air and water.
  • Alloying : Stainless steel resists rust.
  • Sacrificial protection : Zinc corrodes instead of iron.

TL;DR: Water (or moisture). Information gathered from public forums or data available on the internet and portrayed here.