Correction to: Why does water cause rust?

Someone has probably already pointed this out, but I’ll say it anyway:

Water does not cause rust through disassociation into H2 and O2, as stated by SDSTAFF Ken in the article. Neither water nor oxygen “cause” rust - a piece of iron in water will not rust if there is no oxygen present, and iron in oxygen-saturated oil won’t rust either.

Instead, water and oxygen combine with iron to form an electrochemical cell. At the cathode, oxygen, free electrons, and aqueous H+ ions are reduced to water. At the anode, iron is oxidized to the ferrous iron ion (Fe2+) and free elctrons, which travel to the cathode. The ferrous iron ions are further oxidized to ferric iron ions (Fe3+), which react with oxygen in the presence of water to form hydrated iron (III) oxide (Fe2O3*xH2O) or rust.

So, water doesn’t cause rust so much as it facilitates the formation of rust. If you want a graphic example, look at a shovel that’s been stuck in the ground and left for a while. The part under the soil (where the water is) isn’t rusted so much as it is pitted (from the removal of iron). The part just above the soil (where the oxygen is) will be “rusted.”

I bet you used to be ironwolf until you figured out about the rust part. :smiley: Great comment. Thanks.

A link to the Staff Report is appreciated. Why does water cause rust?

What’s the other electrode in this cell? Is the iron the cathode or the anode?

It’s both. The area in contact with water is the anode, and the area in contact with gaseous oxygen is the cathode.

The voltaic mechanism explains why the underbody of your vehicle corrodes faster in winter, when the roads are salted: the dissolved salt provides positive ions that are needed to complete the cathode reaction.

For those who are interested, my reference was:

Brown, Lemay, and Bursten. Chemistry: The Central Science. Prentice Hall. (1997)