Why Rust Never Stops

An old gate that nobody paints goes orange, then flaky, then thin, and one day a bit of it comes away in your hand. Left long enough, an iron gate will rust until there is no gate.

Now picture a drinks can dropped in the same ditch. A year later it is dull, scratched, muddy, and still a can. It has not gone flaky and it has not lost any thickness.

Aluminium is a more reactive metal than iron. It grabs oxygen more eagerly, not less. So the difference between the gate and the can is not that one metal reacts and the other does not. Both react, immediately. Something else decides what happens next.

Rust is not iron gone bad. It is iron plus something

Rust is a new substance. It is made of the iron that used to be there, joined to oxygen out of the air, with water taking part as well.

That means rust is not iron wearing away or getting dirty. Every atom of iron in a patch of rust is still there. It has simply been joined to oxygen atoms and rearranged into something soft and orange that behaves nothing like metal.

Here is the check that catches most people out. A rusted nail weighs more than it did when it was clean. All that oxygen came from somewhere and stayed. The thing that looks like it is falling apart has quietly got heavier.

Water is needed too, and that is why a bike in a shed survives a winter that finishes a bike left in the garden. Dry iron in dry air barely rusts at all. It takes both.

The bit that makes it never stop

Rust needs more room than the iron it was made from. Several times more.

Think about what that means. A thin layer of iron at the surface turns into a much thicker layer of rust, sitting exactly where the thin layer was. It does not fit. It bulges, it cracks, and eventually it lifts off and falls away.

Underneath is bare iron that has never met the air. So the rusting starts again, on a fresh surface, from nothing.

That is the whole mechanism, and it is why rust works inwards instead of forming a skin and stopping. Every layer of rust destroys its own hiding place. Rust is not a coating. It is a coating that keeps throwing itself away and exposing the next bit down.

What aluminium does instead

Expose fresh aluminium to air and it reacts within seconds, faster than iron ever does. It forms aluminium oxide.

But aluminium oxide is tight, even, and sticks fast to the metal underneath. It fits in the space available. It is also astonishingly thin, far too thin to see, which is why a freshly cut piece of aluminium still looks like bare metal a moment later.

That invisible skin seals the surface. Air cannot get past it to reach the metal, so the reaction stops almost as soon as it starts. The can in the ditch is not unreactive. It is a can wearing a coat of its own oxide that it put on within seconds of being made, and has worn ever since.

So the real difference is this. Iron makes a bulky, cracking product that falls off. Aluminium makes a tight, thin one that stays put. One reaction opens the metal up. The other closes it.

Two of those stay clean for the same reason, and it is not the reason you might expect. The nail in the dry jar has plenty of air and no water. The bolt in oil has neither reaching it. Rusting needs both ingredients, so removing either one is enough.

Watch it happen

The first nail rusts, and the worst of it appears right at the waterline, where air and water meet on the metal at the same time. The second stays clean. The third gets a little rust, from the air that was already dissolved in the water before you sealed it, and then almost stops.

Write the results as two rules and you have the whole thing: no water, no rust. No air, no rust.

Stopping it, then, comes down to two choices

You can keep the ingredients away. Paint, oil, grease and plastic coatings all do the same job, and all of them share the same weakness: one deep scratch and the rusting begins there, under the paint, out of sight. That is why a chipped car door is worth mentioning to somebody.

Or you can change what the metal produces when it does react. Mix chromium into steel and the chromium reacts with oxygen first, forming a tight invisible film exactly like aluminium's. That is stainless steel, and it is why a stainless sink can sit in water every day of its life. It is not that it refuses to react. It is that it reacts once and seals itself.

What to try next

Go and find rust on purpose. A drain cover, a skip, a bolt on a gate, a bracket under a bench. Look for where it started rather than where it is worst, and you will almost always find the same story: a scratch, a chip, a joint that holds water, or a low corner where rain sits and does not dry.

Then look at what is beside it and has not rusted, and work out which of the two rules is keeping it safe.