Rust is not simply “iron + oxygen”
On a wet iron surface, different microscopic regions can play different electrochemical roles. At an anodic region, iron atoms lose electrons:
The electrons move through the metal. Elsewhere, often where dissolved oxygen is available, a cathodic region consumes them:
Iron ions and hydroxide ions then take part in further reactions that lead to hydrated iron oxides and related compounds—the material we call rust.
Why a little salt can make a big difference
Pure water conducts poorly. Dissolved ions make the water film a better electrolyte, so charge can be transported more easily between anodic and cathodic regions. That is why road salt and sea spray can accelerate corrosion so dramatically.
Corrosion rates also depend on oxygen access, acidity, temperature, surface condition and contact with another metal. The electrochemical circuit can change even when the underlying metal is the same.