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?
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:
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.
Ah, thank you, matt. A reply I can sink my teeth into.
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?
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).