In basic terms, Why does water make iron rust faster than air?
In: Chemistry
At an ELI5 level:
Rusting requires separate metal regions on the surface to react with each other, and [the reaction involves ions and electrons moving from one region to the other](https://chem.libretexts.org/@api/deki/files/107739/28f7e127eabcc9de2916eefc451f6e033.jpg?revision=1). The rate is limited by how fast the iron ions can move from one site to the other. Water is a lot better conductor of ions than air is, so it lets the reaction happen much faster.
This is also why salt water makes metal rust even faster than fresh water – it’s even more conductive.
At a highschool level:
Rusting is a redox reaction [*shudder]*, where there is iron getting oxidized and oxygen getting reduced. But these happen at different sites, and electrons moving from one site to the other is the rate limiting step. Anything that speeds up this electron transport (such as an easy path through water) lets the whole reaction go faster.
Because water has more water than air does (yes, that’s a bit obvious), so a reaction that happens when water interacts with iron and oxygen (rusting) happens faster with more water. The only real reason iron in air rusts is because the air has *some* water to it.
(and even faster with ions, like salt, in the water)
tl;dr Rusting is a process by which iron reacts with oxygen in the air to form iron oxide. This will not happen deep underwater, and it happens pretty slowly in just air. It’s pretty unlikely for oxygen to stick to iron randomly. Water adds a few steps in between “iron” and “rust” and each individual step is much easier than going the whole way in one event. The exact process is detailed below.
Full: It is specifically where water, iron, and oxygen *meet* that the rusting takes place. In water alone, a bit of the iron dissolves into the water, and this leaves some electrons behind in the remaining metal. Now, there are electrons separated from their atoms and an electrical imbalance is created. A voltage. This voltage pushes back against the dissolving, so *very* little iron is able to dissolve before the process grinds to a halt from the charge imbalance.
If there is also air present in or around the water, those electrons will join up with the oxygen in the air and yank hydrogen from nearby water molecules to produce hydroxide ions (an oxygen paired to a hydrogen, these are charged negatively by the extra electron). These hydroxide ions will pair up with the iron dissolved in the water, once again balancing the electric charge. This is all happening constantly, so the charge stays balanced and as long as the charge stays balanced more iron keeps dissolving. The paired iron+hydroxide forms a solid substance that mostly clings to the iron surface and when it dries the hydrogen and oxygen clump up, form water, and all that’s left is the iron and oxygen.
Rust is iron bonded to Oxygen. To put it simply, it’s easier to separate 2 hydrogen atoms from H20 molecules than it is to separate the 2 oxygen atoms in the O2 that makes up our atmospheric oxygen molecules.
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