What If? Lab
What if metal never rusted?
Old bikes and gates can turn orange and flaky. That is rust! What if metal never rusted? Let's imagine it with Olly.
Rust forms when iron and steel react with oxygen and water. It slowly eats away bikes, gates and even giant bridges, so people fight it with paint, zinc and special steels. Imagine a world where metal never rusted and follow what might change.
Rusting is the corrosion of iron and its alloys: an electrochemical reaction with oxygen and moisture, sped up by salt, that forms a porous oxide which lets the attack continue. This thought experiment removes rust and follows the consequences for bridges, for the chemistry of iron and for stainless steel.
The big question
Have you seen an old, orange, flaky gate? That orange stuff is rust.
Rust slowly eats away metal. What if metal never rusted at all?
Leave an iron nail out in the rain and, before long, it grows a crust of orange-brown rust. Rust is not just dirt sitting on top. The metal itself is changing, and the steel left behind gets thinner and weaker.
Rust means a lot of costly repair work, from small tools to giant bridges. So what if metal never rusted? This lab is a thought experiment: we can't switch rust off, but we can follow the clues.
Corrosion is the wearing away of a material, often a refined metal, by chemical reactions, mainly oxidation. Rusting is the name for the corrosion of iron and its alloys, including steel. Britannica notes that corrosion of infrastructure, such as industrial plants and transportation systems, demands continual and costly maintenance, repair and replacement.
This lab asks what would change if iron never rusted, and uses the question to explore why it does, and how engineers already slow it down.
Explore the idea
Make a prediction
What does iron need to rust?
Which two things does iron need to rust?
Which two substances must reach an iron surface for rusting to occur?
- Air and water
- Sunlight and sand
- Heat and darkness
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Established fact
Iron rusts when air and water touch it.
Iron and steel rust when exposed to oxygen and water; salt and warmth speed it up.
Rusting is electrochemical corrosion of iron in moist air, accelerated by salts, acids and warmth (Britannica).
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Established fact
Paint, zinc and stainless steel help stop rust.
People slow rust with paint, with zinc that corrodes first, and with stainless steel's thin chromium-rich film.
Coatings, sacrificial anodes, galvanizing and passive-film alloys such as stainless steel all limit corrosion (Britannica; Scientific American).
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An idea scientists discuss
A world with no rust is pretend. We imagine it carefully.
Metal will keep rusting. What a rust-free world would be like is an idea built from what rust does today.
All consequences of removing rust are reasoned from current corrosion and maintenance practice, not observed.
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Established fact
Rust happens when iron meets air and water.
Iron and steel rust when they are exposed to oxygen in the air and to water.
Rusting is the corrosion of iron and its alloys, driven by moisture and oxygen at an exposed surface (Britannica).
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Established fact
Rust makes steel thinner and weaker.
Rust eats into steel, making it slightly smaller and weaker than before.
Corrosion erodes steel, leaving it slightly smaller and weaker than before (Golden Gate Bridge, Highway and Transportation District).
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Established fact
Salt and warmth make rust happen faster.
Rusting speeds up when it is warm and when salt or acids are around.
Corrosion is accelerated by warm temperatures and by acids and salts (Britannica).
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Established fact
Rust is crumbly, so it does not protect the metal.
Rust is a porous layer, full of tiny holes, so it lets the attack keep going. Aluminum, copper and zinc make tight layers that slow it down.
Iron forms rust, a porous oxide that promotes further corrosion, whereas aluminum, copper and zinc form compact, adherent oxide layers that limit attack (Britannica).
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An idea scientists discuss
A nail that never rusts would stay strong.
If iron never rusted, old nails, tools and railings might stay strong and shiny for much longer.
Iron that did not corrode would keep its full section indefinitely in damp air; this is a reasoned idea, since real iron cannot avoid the reaction.
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Established fact
Paint keeps salty air off the bridge's steel.
The Golden Gate Bridge's paint protects its steel from the salty air. Painting it is an ongoing job, never finished.
The bridge district calls painting an ongoing task and a primary maintenance job; the paint protects the steel from salt in the air.
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Established fact
The bridge's paint has zinc in it. Zinc protects the steel.
The first coat of paint on the bridge contains zinc. Zinc corrodes more easily than steel, so it gets used up first and protects the steel.
Today's primer contains zinc instead of lead; zinc acts as a sacrificial metal, corroding in place of the steel. The steel sections were also made oversized in anticipation of corrosion.
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An idea scientists discuss
Without rust, painters might paint bridges less often.
Without rust, bridges might need less painting and repair, and they might be built with thinner steel.
Removing corrosion might cut maintenance and the extra steel added to allow for section loss; how much is speculation, since paint also has other jobs.
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Established fact
Stainless steel has chromium in it.
Stainless steel usually contains between 10 and 30 percent chromium, which gives it remarkable resistance to rust.
Stainless steels usually contain 10–30 percent chromium; the common 18/8 (304) grade has 18 percent chromium and 8 percent nickel (Britannica).
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Established fact
Its thin skin is too thin to see.
Stainless steel's protective film is much thinner than a wavelength of light, so you can't see it, but it keeps water and air off the metal.
The passive film is only a few atomic layers thick, thinner than a wavelength of light, and limits oxygen and water access (Michael Free, University of Utah, in Scientific American).
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An idea scientists discuss
Why not make everything from stainless steel?
Stainless steel already gives us metal that hardly rusts, but it is not used for everything; engineers choose metals for their cost, strength and other jobs too.
Stainless steel shows what a rust-resistant world might look like; why ordinary steel is still widely used involves cost and other properties, which this lab does not measure.
How rust happens
Iron needs two things to rust: air and water.
Rain or wet air touches the metal. Slowly, the iron turns into orange rust.
Iron rusts when it meets two things at once: oxygen from the air and water. Even the dampness in the air can be enough, because a thin film of moisture forms on the metal's surface. Iron atoms in that wet film react with oxygen, and the iron slowly turns into rust.
Salt makes it happen faster, and so does warmth. Sea air carries both water and tiny bits of salt, so steel by the sea, like the Golden Gate Bridge, needs extra care.
Corrosion of metals is electrochemical. A thin surface film of moisture, with dissolved oxygen and pollutants, acts as an electrolyte. Some areas of the metal become anodes, where iron atoms lose electrons and leave as ions; elsewhere, dissolved oxygen accepts those electrons. The products build up as rust.
The reaction speeds up with warmth and with acids and salts. Rust is porous, so it does not seal the metal; it promotes further corrosion. The iron returns toward an oxide, the kind of form in which metals occur naturally as ores.
Switch rust off
Now pretend rust is gone. Your bike stays shiny in the rain.
Bridges might not need so much painting. Old nails would stay strong.
Imagine iron that never rusted. A bike left in the rain would stay shiny. Steel bridges might not need painters working on them so often, and engineers might not need to make steel parts extra thick to allow for rust.
But metal might still wear down in other ways, and paint might still be wanted for color. These are ideas, worked out from what rust does today. In the real world, people can't stop rust completely, but they can slow it down a lot.
Without rusting, iron and steel structures would avoid the slow loss of section that weakens them; the Golden Gate Bridge's steel members were made oversized in anticipation of corrosion, and painting is its primary maintenance job. A rust-free world might allow lighter structures and need far less upkeep.
Those outcomes are reasoned speculation. Corrosion is only one way metal structures age, and real refined iron is chemically unstable in moist air, so no real iron behaves this way.
Why iron rusts and others don't
Some metals make a thin, tough skin. The skin keeps air and water out.
Iron's rust is crumbly. It lets air and water get to the metal underneath.
Lots of metals react with air, but they don't all fall apart. Aluminum, copper and zinc grow a thin, tight layer on their surface that sticks well and slows any more damage.
Iron's rust is different. It is crumbly and full of tiny holes, so it doesn't protect anything. Air and water seep through and keep attacking the metal underneath, which is why rust keeps spreading.
Corrosion products differ. Aluminum, copper and zinc form compact, adherent oxide layers that limit further attack; silver forms a thin tarnish that generally does not harm the metal beneath. Iron forms rust, a porous oxide layer that promotes further corrosion.
Refined iron is unstable in moist air: corrosion converts it to more thermodynamically stable compounds, typically oxides and hydroxides. Preventing rust therefore means interrupting the reaction, by keeping water and oxygen out or by changing the electrochemistry.
Fighting rust for real
People paint metal to keep water out. Some paint has zinc in it. The zinc wears away first to protect the steel.
Stainless steel has extra chromium. It makes a clear, thin skin that stops rust.
People fight rust in clever ways. Paint keeps water and air off the metal. Zinc helps too: because zinc corrodes more easily than steel, it gets eaten away first and protects the steel. Dipping steel in melted zinc is called galvanizing.
Stainless steel is the third trick. It contains at least 10 percent chromium, which reacts with oxygen to form a film so thin you can't see it. The film blocks water and air, so the steel stays shiny. Spoons, sinks and bank vaults use it.
Britannica groups the defenses: protective coatings such as paint; cathodic protection with sacrificial anodes of zinc or magnesium, or an impressed current; choosing resistant materials such as stainless steel, aluminum or titanium alloys; and galvanizing, dipping steel in molten zinc, which gives both a barrier and sacrificial protection.
Stainless steels contain at least about 10 percent chromium. Metallurgist Michael Free of the University of Utah explains that their elements react with oxygen to form a stable film only a few atomic layers thick, which limits oxygen and water access; ordinary steel instead forms an unstable oxide that keeps growing.
Amazing facts
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The Golden Gate Bridge is painted all the time.
The Golden Gate Bridge is not painted end to end once a year: it is painted continuously, wherever inspections find rust advancing.
The bridge district says the span is painted continuously, with priorities set by inspections of where corrosion is advancing.
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The bridge's first paint had lots of lead in it.
In the 1930s, the Golden Gate Bridge's first coat of paint was two-thirds lead. Lead is harmful to people and the environment, so today's first coat uses zinc instead.
The original 1930s primer was two-thirds lead by weight; lead resists rust but harms people and the environment, so today's primer uses zinc (Golden Gate Bridge District).
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Stainless steel is used for spoons and sinks.
Because it lasts and looks good, stainless steel is used for eating utensils, kitchen sinks and even bank vaults.
Its durability and appearance put stainless steel in eating utensils, bank vaults and kitchen sinks; titanium and aluminum also rely on passive films (Scientific American).
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There are many kinds of stainless steel.
There are more than 100 grades of stainless steel, sorted into five main families.
More than 100 grades exist, mostly in five groups: austenitic, ferritic, martensitic, duplex and precipitation-hardening (Britannica).
Words to know
- rust
- the crumbly orange-brown material that forms when iron or steel reacts with oxygen and water
- corrosion
- the slow wearing away of a material, often a metal, by chemical reactions
- oxygen
- a gas in the air that reacts with many materials, including iron
- alloy
- a metal made by mixing a metal with other elements; steel is mostly iron
- galvanizing
- coating steel with zinc by dipping it into melted zinc
- stainless steel
- steel with at least about 10 percent chromium, which forms a thin film that resists rust
Discovery challenge
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What does iron need to rust?
Which two things must reach iron for it to rust?
Rusting needs which two substances at the iron surface?
- Air and water
- Sand and sunlight
- Heat and darkness
Show the answer
Air and water
Air and water.
Oxygen from the air and water. Salt and warmth make it faster.
Oxygen and moisture; salts, acids and warmth accelerate it.
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Why does rust keep spreading?
Why doesn't a layer of rust protect the iron underneath?
Why does iron's corrosion product fail to protect the metal?
- It is crumbly and full of tiny holes
- It is too heavy
- It turns back into iron
Show the answer
It is crumbly and full of tiny holes
Rust is crumbly, so air and water get through.
Rust is porous, so air and water keep reaching the iron.
Rust is a porous oxide that promotes further corrosion, unlike the adherent oxides of aluminum or zinc.
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What metal in the bridge's paint protects the steel?
Which metal in the Golden Gate Bridge's first coat of paint protects the steel?
Which sacrificial metal is in the Golden Gate Bridge's present primer?
- Gold
- Zinc
- Silver
Show the answer
Zinc
Zinc. It wears away first to protect the steel.
Zinc, which corrodes more easily than steel, so it is used up first.
Zinc, which replaced the original lead and corrodes preferentially to the steel.
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What does stainless steel have extra of?
Which element gives stainless steel its rust-resisting film?
Which alloying element dominates stainless steel's passive film?
- Carbon
- Chromium
- Oxygen
Show the answer
Chromium
Chromium.
Chromium, at least 10 percent, reacts with oxygen to form a thin protective film.
Chromium plays the dominant role in forming the stable surface film (Scientific American).
Experiments to try
- Watch What If? Lab What if glass were bendy? Glass is made by melting sand and cooling it quickly. When it's cool, it's hard and brittle, so it cracks instead of bending. But hot glass is soft, and glass thinner than paper can bend. Imagine glass that was bendy and follow what might change.
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- Watch What If? Lab What if metal melted in your hand? Every metal melts at its own temperature. Iron needs more than 1,500 °C, but a metal called gallium melts at just under 30 °C, cooler than your body. Imagine if all metals melted that easily and follow what might change.
Explore next
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Parthenon
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For parents & teachers
What children will learn
- Iron and steel rust when exposed to oxygen and water; salt and warmth speed it up.
- Rust is porous, so unlike some other metals' oxide layers it does not protect the metal beneath.
- Paint, zinc coatings and galvanizing are common ways to protect steel.
- Stainless steel resists rust because its chromium forms a very thin protective film.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- Where have you seen rust? What do those places or things have in common?
- Why do you think metal near the sea rusts faster than metal in a dry desert?
- If metal never rusted, which jobs might change, and which would still be needed?
Try at home or in class
- With a grown-up, put one plain steel nail (not shiny stainless or galvanized) in a jar of tap water and another in a dry, closed jar. Look at them every day for a week. Which one rusts?
- Go on a rust hunt around your home or school with a grown-up. Make a list of rusty things and stainless or painted things. What protects the shiny ones?
Key vocabulary: rust, corrosion, oxygen, alloy, galvanizing, stainless steel
Where these facts come from
- Corrosion , Encyclopaedia Britannica
- Stainless steel , Encyclopaedia Britannica
- Why doesn't stainless steel rust? , Scientific American
- Fog, Steel, Salt, Rust, and Paint , Golden Gate Bridge, Highway and Transportation District
- Painting the Bridge , Golden Gate Bridge, Highway and Transportation District
Checked, not yet human-reviewed. Last updated October 27, 2026. Spotted a mistake? Tell us.