In basic terms, Why does water make iron rust faster than air?

Question

In basic terms, Why does water make iron rust faster than air?

In: Chemistry

Answer 1

At an ELI5 level:

Rusting requires separate metal regions on the surface to react with each other, and [the reaction involves ions and electrons moving from one region to the other](https://chem.libretexts.org/@api/deki/files/107739/28f7e127eabcc9de2916eefc451f6e033.jpg?revision=1). The rate is limited by how fast the iron ions can move from one site to the other. Water is a lot better conductor of ions than air is, so it lets the reaction happen much faster.

This is also why salt water makes metal rust even faster than fresh water – it’s even more conductive.

At a highschool level:

Rusting is a redox reaction [*shudder]*, where there is iron getting oxidized and oxygen getting reduced. But these happen at different sites, and electrons moving from one site to the other is the rate limiting step. Anything that speeds up this electron transport (such as an easy path through water) lets the whole reaction go faster.