Why Does Iron Rust?
― Aluminium Doesn't Skip Rusting, It Just Finishes in an Instant
A bicycle left out in the rain turns reddish-brown. Yet an aluminium window frame or a stainless-steel sink, sitting outside just the same, doesn't. People assume "aluminium is a metal that doesn't rust" — but it's actually the opposite. Aluminium rusts far more violently than iron. The difference is that it finishes rusting in an instant, and that rust then protects everything underneath. This single distinction explains almost all of how we choose metals in everyday life.
An old bicycle you no longer use, a tool left in the garden, a nail soaked by rain. Anything made of iron will, left alone, turn reddish-brown and start flaking apart at the surface.
Yet other things face the same wind and rain without falling apart. Aluminium window frames, stainless-steel railings, ¥10 coins. These may change colour, but they don't crumble.
The usual explanation is "iron rusts easily, aluminium and stainless steel don't." But that explanation is backwards.
Aluminium bonds with oxygen far more readily than iron does. The reason it doesn't fall apart is something else entirely.
Iron, water, and oxygen. Without any one of them, it can't proceed. So the fix is simply to cut off one of the three.
Iron's rust is full of gaps and flakes away, exposing fresh iron underneath. Aluminium's rust clings tightly with no gaps and stops right there.
In other words, the real question isn't "does it rust?" but "does the rusting stop?" Let's look at this step by step.
Rusting Is a Metal Returning to Its "Original Form"
It may surprise you, but a lump of metal is actually an unusual state to find in nature. Most of the iron on Earth exists as rock — iron ore — already bonded with oxygen.
The iron we use has been forcibly stripped of that oxygen using huge amounts of heat and energy. In other words, iron is like a spring held back under tension.
Rusting is simply that spring slowly releasing. The iron bonds with oxygen again, returning to the rock-like form it once had. Of course it can't be stopped — left alone, it will always proceed.
So the fix isn't "find a metal that doesn't rust." It's "halt it partway back."
but stopping it once it starts.
Why Does Only Iron Keep Crumbling?
Aluminium bonds with oxygen too — more readily than iron, in fact. The reason it doesn't fall apart is that what forms has a different shape.
It's far bulkier than the original iron, and full of gaps. So it lifts up and flakes away, exposing fresh iron beneath.
The film that forms adheres with no gaps at all. It's extraordinarily thin, but it blocks both oxygen and water. And there, it stops.
This film rebuilds itself instantly if scratched. Scrape aluminium, and a new film forms right where you cut it. What's doing the protecting isn't the metal itself, but the "rust film" formed on its surface.
Stainless steel works the same way. Iron is mixed with a metal called chromium, and it's the film that chromium forms that does the protecting. So stainless steel isn't "iron that doesn't rust" — it's "iron whose rusting stops."
Leave an iron can sitting on a stainless-steel sink, and that spot can turn red. Often this is just rust transferred from the can, but if left alone it can spread into the steel itself.
Also, leaving chlorine bleach sitting on the surface for a long time can break down the protective film, letting pinpoint rust spots appear. That's why it's said to be important to rinse it off thoroughly after use.
Stainless steel's strength isn't that it "never rusts" — it's that even if the film breaks, it can rebuild itself as long as oxygen is present. So the best care is simply to keep the surface free of grime and rust, exposed to air.
Another Option: Sacrifice Something Else to Protect It
There's a completely different way to protect metal. Coat it with a metal that dissolves before the iron does.
The surface of galvanised roofing or guardrails is coated with zinc. Zinc dissolves before iron does, so when it gets rained on, the zinc sacrifices itself and the iron survives.
What's interesting is that this method still works even if it gets scratched. With ordinary paint, rust starts wherever the coating peels. But with zinc, even if the iron is exposed, the surrounding zinc keeps dissolving first and keeps protecting it.
Ship hulls and underground pipes sometimes have a lump of metal deliberately attached just to dissolve. It's a consumable metal, meant to be replaced regularly. I think it's a good example that "protecting" doesn't necessarily mean being hard and tough.
- Wipe it dry when it gets wetOf the three ingredients, water is the easiest one to cut off. Just wiping down a bicycle after rain makes a clear difference to how long it lasts. Chains and moving parts especially should be wiped, then oiled.
- Rinse off salt with fresh waterAfter a trip to the seaside, or driving on roads treated with salt in winter. Salt greatly speeds up rusting. Rinse the underside with fresh water too. In regions that spread de-icing salt, this needs repeating all winter.
- If you find rust, remove it before covering itScrape off loose rust, let it dry, then cover it with paint or oil. Just painting over it lets the rust keep spreading underneath. On bolts or blades, rust can even affect strength.
Iron's rust is bulkier than the original iron. So when rebar inside concrete rusts, it swells and cracks the concrete from the inside. Water and oxygen then get further in through the cracks, and it accelerates.
Rust colour seeping from a crack in a building, a bulge at the base of a balcony railing, thinned legs on scaffolding or a stepladder — in places like these, the strength may have dropped more than it looks.
Don't judge it yourself — have a building manager or a professional take a look. Especially anywhere people stand, or anywhere a fall would be dangerous, it's best to stop using it and get advice.
An Experiment You Can Do in Your Kitchen
- Prepare four iron nails and line up four clear glasses
- ①Nothing added (just air) ②Half-filled with water ③Fill with water that's been boiled and cooled, then pour a thin layer of oil on top (to block air) ④Half-filled with salt water
- Leave for about three days and compare
- ②and ④ will rust. ④ should be faster. ① should show little change if it stays dry
- ③ resists rusting despite having water. That's because boiling reduced the oxygen, and the oil sealed it in
③ is the key to this experiment. It shows you can have water without rusting — proving for yourself that water alone isn't the culprit. The gap between ② and ④ shows the effect of salt. Salt itself doesn't react with the iron — it just makes the reaction proceed faster. Handle the nails carefully so you don't cut yourself, and when you're done, don't pour them down the sink — drain and dispose of them properly.
Summary
Iron rusts because it's trying to return to the rock it once was. It can't be stopped, but cutting off any one of iron, water, or oxygen will halt it. And the reason aluminium and stainless steel don't fall apart isn't that they don't rust — it's that the rust that forms clings with no gaps and blocks any further progress.
What's doing the protecting isn't the metal,
but the extremely thin rust film on its surface.
The same phenomenon — a metal reacting with something in the air — produces a green compound in the case of copper. You can read about that mechanism, and the story of how it was once called "poison," in our article on verdigris.
Want to know more? ― Terms, numbers, and how it connects to textbooksWe've labelled each part by level, from middle-school science up to topics still being researched
- MSCovered in middle-school science
- HSCovered in high-school "Basic Chemistry"
- HS+Covered in high-school "Chemistry," or treated as advanced/sidebar material in textbooks
- UnivNot covered in high school — content from a specialised university subject (corrosion engineering)
- ResearchNot even settled fact at university level — something researchers are actively investigating
MSTerms: The Vocabulary of Rust
- Oxidation: A change where a substance bonds with oxygen. Rusting and burning both belong to this family.
- Red rust: The rust iron forms in the presence of water and oxygen. Full of gaps, and it flakes off.
- Black rust (magnetite scale): A dense, low-gap rust formed at high temperature. This kind protects the iron. Seasoning a Chinese wok or an iron kettle is essentially creating this.
- Passivation (passive film): A dense film that forms on the surface, protecting what's underneath. This is what aluminium and stainless steel have.
- Sacrificial protection: A method where a metal that dissolves first is placed to spare the metal you want to protect. Galvanising is an example.
HSChecking with an Equation: Does Rusting Add Weight, or Remove It?
Because rusted iron flakes and falls apart, it feels like it's "disappearing." But if you write out the chemical equation, the opposite turns up.
4Fe + 3O₂ → 2Fe₂O₃
| Fe iron | 56 g per mole |
| O₂ oxygen | 32 g per mole |
| Fe₂O₃ iron oxide (red rust) | 160 g per mole |
Read the equation like this: 4 units of iron bond with 3 units of oxygen to make 2 units of iron oxide. Real red rust also contains water, but this is enough to get a sense of the quantities.
The key point: the oxygen doesn't vanish anywhere — it attaches to the iron and stays there. So the weight increases.
| Weight on the iron side | 4 × 56 = 224 g |
| Weight on the oxygen side | 3 × 32 = 96 g |
| Weight of red rust formed | 2 × 160 = 320 g |
| Check (mass is conserved) | 224 + 96 = 320 |
| Amount gained | 320 − 224 = 96 g |
| Percentage gained | (96 ÷ 224) × 100 ≒ 42.9% |
Rusting increases the weight by about 43%. It doesn't decrease it.
So why does it look like it's losing mass? Because the extra material doesn't stay put — it flakes off and falls away. It's not "getting lighter," it's "getting heavier, and then spilling off." A case where appearance and substance don't match.
It's not only the weight that increases by 43% — the volume grows even more. Red rust is said to reach roughly 2 to 6 times the volume of the original iron, because it contains so many gaps.
| Say a 0.1 mm layer of iron rusts | At 2× volume: 0.1 × 2 = 0.2 mm |
| Amount pushing outward | 0.2 − 0.1 = 0.1 mm |
| If a 0.5 mm layer rusts | 0.5 × 2 = 1.0 mm, pushing outward by 1.0 − 0.5 = 0.5 mm |
You might think 0.5 mm sounds trivial. But concrete is a material that's very weak against outward pressure. That's enough to crack it.
And once it cracks, water and oxygen get in even more through the crack. Rust progresses → it swells → it cracks → more rust follows. There's nothing anywhere in this cycle to stop it. This repeating cycle is said to be what determines the lifespan of reinforced concrete.
The film protecting aluminium and stainless steel is said to be roughly 1 to 3 nm (nanometres) thick. 1 nm is one millionth of a millimetre.
| Thickness of a human hair | ≈ 0.08 mm = 80,000 nm |
| Thickness of the protective film | ≈ 3 nm |
| What fraction of a hair is that? | 80,000 ÷ 3 ≒ 1/26,667 |
A film less than 1/20,000th the thickness of a hair is protecting the metal. Invisible to the eye, and you can't feel it with your finger.
It works despite being this thin because it has no gaps. Which also means even the tiniest scratch or smudge becomes a weak point. That's why stainless steel sometimes shows pinpoint rust spots.
It's not that it protects because it's thick and tough — it protects because it has no gaps. The red rust we saw in ③, despite being 2 to 6 times bulkier, doesn't protect anything. Thickness and protective power turned out to be separate things.
HS+Rusting Iron Is a Tiny Battery
Rusting isn't simply oxygen attaching to something. It's a reaction that involves an electric current.
On the surface of wet iron, iron releases electrons and dissolves at one spot, while oxygen picks up those electrons at another spot. The electrons travel through the iron to get there. In effect, a battery has formed on a single sheet of iron.
This explains several things.
- Why salt water speeds things up. Salt makes it easier to carry electric current, so more flow happens as a battery. The salt itself doesn't react.
- Why gaps and scratches get hit hardest. Areas where oxygen has trouble reaching become the dissolving side, so the attack concentrates at one spot.
- Why one of two metals in contact wears out faster. Because of how the battery behaves, the more easily dissolved metal is sacrificed one-sidedly. Galvanising's sacrificial protection deliberately uses this.
Conversely, tinplate (iron coated with tin) has the iron dissolve first if it's scratched. That's because tin is harder to dissolve than iron. Even though both are "plating," zinc and tin behave in opposite ways once scratched.
UnivWhere It Dissolves Can Be Mapped by Potential and pH
Whether a metal dissolves, forms a protective film, or stays stable as-is can be roughly mapped using a combination of electrical potential and acidity (pH). Diagrams of this relationship are used to predict "under these conditions a film forms and it's protected" or "under these conditions it dissolves."
However, this diagram shows "which direction it goes," not "how fast." Real-world design needs the speed measured separately. That direction and speed are separate questions is a distinction that comes up throughout chemistry, not just in corrosion.
What causes trouble in practice is often pitting corrosion. The surface looks fine overall, but one single point eats deep. Estimating lifespan from an average wear rate makes it look far longer-lasting than it really is. Pipe leaks usually happen this way.
ResearchWe Still Can't Predict When a Hole Will Form
- Where and when pitting corrosion begins can only be treated statistically. Which single point in the film fails depends on tiny irregularities in the material. We can measure an average corrosion rate, but predicting when any given part will actually develop a hole remains difficult.
- Predicting atmospheric corrosion rates involves too many variables. Time spent wet, salt content, airborne pollutants, temperature swings, sunlight. Because these all interact, the standard method is still leaving test pieces in place for years and measuring in each actual environment.
- Some corrosion involves microorganisms. In places where certain bacteria are involved, corrosion is known to proceed far faster than predicted. The very fact that living organisms can be involved in metal corrosion was only recognised relatively recently. Working out the mechanism is still ongoing.
- How the rust layer forms on steel deliberately left to rust for protection. Some steels, used in bridges, are designed to form a stable rust layer on the surface. Done well, this removes the need for paint, but in places with high salt content that layer doesn't stay stable, so judging the right conditions is difficult.
Rust is one of the most familiar chemical changes in everyday life. And yet, even now, we can't precisely answer "how many more years does this bridge have?" I think it's a good example of how being well understood and being precisely predictable are two different things.
Connections to Textbooks (by Level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science - Chemical Change and Oxidation | That rusting is a change involving bonding with oxygen |
| HS | Basic Chemistry - Chemical Equations and Quantitative Relations | Mass calculations from 4Fe + 3O₂ → 2Fe₂O₃ |
| HS | Basic Chemistry - Oxidation and Reduction | Rusting as an exchange of electrons |
| HS+ | Chemistry - Batteries and Ionisation Tendency | Local cells, sacrificial protection, passivation |
| Univ | Corrosion Engineering / Materials Engineering | Mapping by potential and pH, pitting corrosion |
| Research | Corrosion Science (unresolved) | Predicting pitting, microbial corrosion, stable rust layers |
| ― | Everyday Life / Maintenance | Wiping dry, rinsing off salt, rust on structures |
- Explanatory material on corrosion and corrosion prevention by the Corrosion Protection Division Committee of the Society of Materials Science, Japan (日本材料学会・腐食防食部門委員会).
- Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions (the original source for the potential/pH mapping).
- Guidelines on rebar corrosion and concrete cracking by the Japan Society of Civil Engineers and the Japan Concrete Institute (土木学会・日本コンクリート工学会).
- Little, B. J. & Lee, J. S., Microbiologically Influenced Corrosion.
- Explanatory material on weathering steel and plated steel sheet by The Japan Iron and Steel Federation (日本鉄鋼連盟).
※ Film thickness, volume increase, and corrosion rate vary greatly depending on the material and environment. This article presents commonly cited approximate figures.
※This article is a general-audience science explainer. For rust on buildings, bridges, scaffolding, stepladders, or any structure where people's safety is at stake, don't judge it yourself — please consult a building manager or a qualified professional. The figures given here are approximations meant to help explain the underlying mechanism.