# Water and Rust **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2003, 4:47pm UTC](https://boards.straightdope.com/t/water-and-rust/174725 "2003-05-12T16:47:26Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [May 12, 2003, 4:47pm UTC](https://boards.straightdope.com/t/water-and-rust/174725/1 "2003-05-12T16:47:26Z")
My brother and I were talking the other day and this came up. We had some unprimed metal lying about the shop and he mentioned how water acts as a catalyst for rust formation. It had never occurred to me that the rust (some sort of ferric/ferrous oxide) wasn’t being formed from the oxygen component of water itself. When he mentioned it of course it made sense: you don’t see little hydrogen bubbles coming up from some dissociation of water molecules. So how does water make rust? And will iron rust in dry air? Your attention in this matter is appreciated. ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [May 12, 2003, 5:11pm UTC](https://boards.straightdope.com/t/water-and-rust/174725/2 "2003-05-12T17:11:57Z")
Well, I think that’s true: wouldn’t take a lot of energy to break up the water molecules? And there is plenty of free oxygen floating around. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [May 12, 2003, 8:16pm UTC](https://boards.straightdope.com/t/water-and-rust/174725/3 "2003-05-12T20:16:15Z")
Iron rusts in water because water open to the air typically contains 8-9 ppm dissolved oxygen (by weight.) The reaction is electrochemical: iron dissolution proceeds by metallic iron transforming to ferrous ions and leaving the excess electrons on the iron metal, via: Fesub[/sub]—\>Fe[sup]2+[/sup]+2e The dissolved oxygen in the water reacts with the excess electrons on the metal surface, and with the water itself, to form hydroxide ions via: Osub[/sub]+2Hsub[/sub]O+4e—\>4 OH[sup]-[/sup] The ferrous ions and hydroxide ions in the solution will precipitate solid ferrous hydroxide, Fe(OH)[sub]2[/sub], if their concentrations become high enough, which is rust, more or less. And no, iron will not rust in dry air. Iron doesn’t really rust in oxygen-free water either, under ideal conditions. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [May 13, 2003, 1:44am UTC](https://boards.straightdope.com/t/water-and-rust/174725/4 "2003-05-13T01:44:29Z")
Ah, thank you, **matt**. A reply I can sink my teeth into. > [@](#): > > \*Originally posted by matt \* > \*\* > The reaction is electrochemical: iron dissolution proceeds by metallic iron transforming to ferrous ions and leaving the excess electrons on the iron metal… > \*\* One clarification, if you, or anyone else would. Is the metallic iron dissolution to ferrous ions happening because of the water molecules polarity? And if so can any polar solvent corrode iron? ---
**Author:** ![Flying\_Monk](https://avatars.discourse-cdn.com/v4/letter/f/48db29/32.png) [@Flying\_Monk](https://boards.straightdope.com/u/Flying_Monk)\ **Post date:** [May 13, 2003, 1:59am UTC](https://boards.straightdope.com/t/water-and-rust/174725/5 "2003-05-13T01:59:17Z")
What if an electric current is flowing through the metal? ---
**Author:** ![Sock\_Munkey](https://avatars.discourse-cdn.com/v4/letter/s/aeb1de/32.png) [@Sock\_Munkey](https://boards.straightdope.com/u/Sock_Munkey)\ **Post date:** [May 13, 2003, 5:03am UTC](https://boards.straightdope.com/t/water-and-rust/174725/6 "2003-05-13T05:03:03Z")
AFAIK the rusting does cause a very weak current to flow. ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [May 13, 2003, 1:10pm UTC](https://boards.straightdope.com/t/water-and-rust/174725/7 "2003-05-13T13:10:44Z")
> [@](#): > > \*Originally posted by Flying\_Monk \* > \*\*What if an electric current is flowing through the metal? \*\* This is the basis of cathodic protection systems which use a sacrificial anode and an electrical current to protect the cathode (the metal you don’t want to rust).