Iron is happiest as iron ore — the rock we dig it out of. We spend a lot of energy turning ore into shiny metal, and the metal would rather drift back. Give it oxygen from the air and a bit of water, and iron slowly combines with the oxygen to make a crumbly orange stuff: rust. The mean trick is that rust flakes off instead of forming a tight skin, so it keeps exposing clean metal underneath and eats deeper and deeper. Paint or a zinc coat seals the iron away from air and water and stops the whole thing. Try it in the simulator: add moisture and oxygen and watch the bar corrode, then seal it.
Most people think rust is simply the metal being attacked. In fact it is iron relaxing back to the ore it came from, and it needs water to go electrochemical — which is why salt and damp speed it and a dry desert barely touches ironwork.
What's actually happening
Smelting iron is a fight against nature. In the ground iron sits contentedly as oxide, ore, and it takes a blast furnace and a great deal of energy to strip the oxygen away and leave bright metal. Thermodynamically that metal is sitting at the top of a hill it would love to roll back down. Rust is simply iron rolling: recombining with oxygen to return, almost exactly, to the oxide it started as. Corrosion isn’t the metal being attacked so much as the metal relaxing.
It can’t relax without help, though, and the helper is water. Pure dry air barely touches iron; add moisture and the process turns electrochemical. A film of water lets one patch of the iron surface give up electrons (it dissolves) while another patch hands those electrons to oxygen — a tiny battery wired through the metal itself. That’s why iron rusts fastest where it’s damp, why salt (a brilliant electrical go-between) makes seaside and road-gritted steel rot so quickly, and why a dry desert can leave old ironwork almost untouched for decades.
Now the detail that makes iron special and unlucky. When aluminium meets air it instantly grows a microscopically thin, dense oxide skin that bonds tight and seals the metal — the corrosion stops itself in a day. Iron oxide does the opposite: it’s bulky and flaky, it doesn’t stick, and it crumbles away to expose fresh metal underneath. So rusting never self-limits; left alone it works inward until the iron is gone. Every rust defence is a way of cheating the triangle of iron, oxygen and water: paint and grease are physical barriers; galvanising coats the steel in zinc that corrodes in its place (a sacrifice); and stainless steel is alloyed with chromium, which forms that same self-sealing oxide skin aluminium has, lending it to the iron.
Rust never self-limits because it flakes off and bares fresh metal, unlike aluminium's tight oxide skin, so every defence just seals iron away from oxygen and water.
- 1Put a plain steel nail in a little water, a second nail in salty water, and a third coated in petroleum jelly or nail varnish into water too.
- 2Leave them a few days where you can see them. The salty-water nail rusts fastest, the plain-water one slower, and the sealed one barely at all.
- 3You’ve shown all three levers at once: water enables it, salt speeds it, and a barrier stops it.
Common questions
Water turns the process electrochemical, letting one patch of iron give up electrons while another hands them to oxygen — a tiny battery in the metal. That is why salt, a brilliant electrical go-between, makes seaside and gritted steel rot so fast.
Aluminium instantly grows a thin, dense oxide skin that bonds tight and seals the metal, so corrosion stops itself. Iron's oxide is bulky and flaky, crumbling off to expose fresh metal underneath.
By cheating the iron-oxygen-water triangle: paint and grease are barriers; galvanising coats steel in zinc that corrodes in its place; and stainless steel is alloyed with chromium, which forms a self-sealing oxide skin like aluminium's.