Why does water cause rust?

Dear Straight Dope: I left my hatchet outside last night. It sprinkled a bit. In the morning, when I found it, the portions of the steel that were in contact with the drops of water were rusty. The rest of the hatchet was not. I know that rust is caused by simple oxidation. Certainly steel rusts over time, when in contact with the air, but what is it about good old H2O that causes such a increased rate of oxidation? jslater

SDStaff Ken replies:

Rust is rust. Specifically, when water comes in contact with iron, you create a galvanic cell, like a little battery. When H₂O comes into contact with iron (Fe), the water will dissociate (break into its constituent elements, hydrogen and oxygen), so you now have free O₂ running around, which will bond with the Fe, creating FeO₂, iron oxide, commonly called rust. Anyhow, that’s what my resident metallurgist says.

So, you often walk around outside at night with a hatchet?

SDStaff Ken, Straight Dope Science Advisory Board

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Why does water cause rust?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD