Why does water cause rust?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

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?

Ken replies:

Rust is rust. Specifically, when water comes in contact with iron, you create a galvanic cell, like a little battery. When H20 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?

Send questions to Cecil via cecil@straightdope.com.

STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL'S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU'D BETTER KEEP YOUR FINGERS CROSSED.

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