Rust is a type of iron oxide, a reddish-brown substance that forms when iron reacts with oxygen in the presence of water or moisture in the air. Rust is made of hydrous iron(III) oxides (Fe₂O₃·nH₂O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)₃). It is often linked to the corrosion of refined iron.

If left in the presence of water and oxygen for a long time, any iron object will eventually turn completely into rust. Surface rust is usually flaky and easily broken apart, and it does not protect the iron underneath. This is different from metals like aluminum, copper, and tin, which form strong, protective oxide layers. Rusting is the term used for the corrosion of pure iron and its alloys, such as steel. Other metals also corrode, but their oxides are not called "rust."

There are several types of rust that can be seen and identified using special tools. Some types form in environments without oxygen, such as when iron reacts with chloride. For example, rebar in underwater concrete pillars can create green rust. While rusting is usually harmful, a type called "stable rust" forms a thin layer of rust on the surface. This layer can protect the iron underneath if it stays dry, though not as well as the protective oxides on metals like aluminum.

Chemical reactions

Rust is a name for a mix of oxides and hydroxides of iron. This happens when iron or alloys that contain iron are exposed to oxygen and moisture for a long time. Over time, oxygen combines with the metal to form new compounds called rust in a process called rusting. Rusting is an oxidation reaction that happens specifically with iron. Other metals also corrode in similar ways, but this type of damage is not called rusting.

Water is the main reason rusting happens. Even though iron or steel structures look solid, water can enter tiny holes and cracks in the metal. Water contains hydrogen atoms that can join with other elements to form acids. These acids can expose more metal over time. If chloride ions are present, like in saltwater, rusting happens faster. At the same time, oxygen atoms combine with metal atoms to create a brittle oxide compound. These iron compounds are weak and replace strong iron, making the object less strong.

When iron is in contact with water and oxygen, it rusts. If salt is present, such as in seawater or salt spray, iron rusts more quickly because of chemical reactions. Pure water or dry oxygen does not affect iron much. Like other metals, such as aluminum, a thin layer of oxide forms on iron's surface, protecting it from further damage. This protective layer can change into rust when oxygen and water act together.

Other harmful substances include sulfur dioxide and carbon dioxide in water. These substances cause iron hydroxide to form. Unlike iron oxides, hydroxides do not stick to the metal. As they flake off, more iron is exposed, and rusting continues until all the iron, oxygen, water, carbon dioxide, or sulfur dioxide is used up.

When iron rusts, the oxides take up more space than the original metal. This expansion can create strong forces that damage structures made of iron.

Rusting is an electrochemical process that starts when electrons move from iron to oxygen. Iron acts as the reducing agent (losing electrons), while oxygen acts as the oxidizing agent (gaining electrons). Water affects how fast rusting happens, and electrolytes, like road salt, speed up the process. The key reaction involves oxygen reducing to form hydroxide ions. This process is strongly influenced by the presence of acid. Similarly, most metals corrode faster in acidic conditions. The oxidation of iron provides electrons for this reaction.

Another important reaction happens in the presence of water and is crucial for rust formation.

In addition, several acid-base reactions and dehydration steps influence how rust forms:

From these reactions, it is clear that the type of rust depends on the availability of water and oxygen. With limited oxygen, iron(II)-based materials like FeO or magnetite (Fe₃O₄) form. With more oxygen, materials like Fe(OH)₃−ₓOₓ⁄₂ form. Rust changes over time because the reactions are slow.

These processes are also affected by other ions, such as calcium (Ca), which act as electrolytes that speed up rusting or combine with iron hydroxides and oxides to form new compounds.

In the laboratory, rusting can be detected using a solution called ferroxyl indicator. This solution shows the presence of Fe ions and hydroxyl ions. Fe ions appear as blue spots, and hydroxyl ions appear as pink spots.

Prevention

Rust is a serious problem for iron and steel products because it weakens their strength, affects how well they work, and damages their appearance. Preventing and controlling rust is important in many industries. This text summarizes some methods; for more details, refer to related articles.

Rust allows air and water to pass through it, so the iron underneath continues to corrode. To stop rust, coatings must be used to prevent rust from forming.

Stainless steel creates a protective layer of chromium oxide on its surface. Similar protective layers can form on materials like magnesium, titanium, zinc, aluminum, and certain conductive polymers.

Special alloys called "weathering steel," such as Cor-Ten, rust more slowly than regular steel. This happens because the rust forms a protective layer on the metal’s surface. Even so, designs using this material must avoid harsh conditions, as rusting still occurs slowly over time.

Galvanization is a method where a layer of zinc is applied to steel using hot-dip or electroplating techniques. Zinc is often used because it is inexpensive, sticks well to steel, and protects the steel if the zinc layer is damaged. In very corrosive environments, like salt water, cadmium plating is preferred instead of zinc. Over time, the zinc layer wears away, so galvanization only lasts for a limited time.

Modern coatings add aluminum to zinc, creating a mixture called zinc-alume. Aluminum moves to cover scratches, offering longer protection. These coatings rely on aluminum and zinc oxides to protect the surface rather than acting as a sacrificial anode, as in traditional galvanization. In highly corrosive environments or for long-term use, both zinc and additional coatings may be applied for better protection.

For steel products exposed to normal outdoor conditions, a hot-dipped zinc coating of 85 micrometers is typical. Under normal weather, this coating wears away at a rate of 1 micrometer per year, providing about 85 years of protection.

Cathodic protection stops corrosion by using an electrical charge to block the chemical reactions that cause rust. One simple method uses a sacrificial anode made of zinc, aluminum, or magnesium, which corrodes instead of the steel. The anode must be replaced when it wears away. Another method uses an electrical current, called impressed current cathodic protection (ICCP), to provide protection.

Rust can be controlled with coatings like paint, lacquer, varnish, or wax. Large structures, such as ships and cars, often use wax-based products inside enclosed sections to prevent rust. These treatments may also include rust inhibitors. Covering steel with concrete can help protect it because the alkaline environment at the steel-concrete interface slows rusting. However, rust can still damage concrete by expanding and causing cracks.

In 1898, iron clamps were used to join marble blocks during a restoration of the Parthenon in Greece. These clamps caused damage to the marble because the iron rusted and expanded. Ancient Greek builders had used a similar method but added molten lead over the iron joints to protect against corrosion and seismic shocks. This method worked for thousands of years, but temporary repairs later caused the structure to risk collapse. For short-term protection, a thin layer of oil, grease, or a special mixture like Cosmoline can be applied to iron surfaces. This is often used when storing or transporting equipment for long periods.

Anti-seize lubricants, which mix grease with copper, zinc, or aluminum powder, are used on metal threads and precision parts to prevent rust.

Bluing is a method to slightly resist rust on small steel items, like firearms. It works best when a water-displacing oil is applied to the blued surface.

Corrosion inhibitors, such as gas-phase or volatile inhibitors, can stop rust inside sealed systems. They are ineffective if air circulation spreads them and introduces fresh oxygen and moisture.

Rust can be avoided by controlling humidity in the air. For example, silica gel packets are used to reduce moisture in equipment shipped by sea.

Treatment

Rust can be removed from small iron or steel objects using a process called electrolysis in a home workshop. This method uses simple materials, including a plastic bucket filled with an electrolyte made by dissolving washing soda in tap water. A piece of rebar is placed vertically in the bucket to act as the anode, and another rebar is laid across the top of the bucket to hold the object being treated. Baling wire is used to suspend the object in the solution from the horizontal rebar. A battery charger provides the power, with the positive terminal connected to the anode and the negative terminal connected to the object, which becomes the cathode. During this process, hydrogen gas forms at the cathode, and oxygen gas forms at the anode. These gases can be flammable or explosive, so care must be taken. Hydrogen can also weaken metals over time. Overvoltage may create small amounts of ozone, a highly toxic gas, making a low-voltage phone charger a safer option for providing direct current. Recent studies have also examined the impact of hydrogen on global warming.

Rust can also be treated with commercial products called rust converters, which contain tannic acid or phosphoric acid. These substances react with rust to change its appearance. Rust can be removed using organic acids like citric acid or vinegar, or with chelating agents found in some commercial products. Molasses solutions are sometimes used as well.

Economic effect

Rust causes damage to tools and structures made of iron. Rust takes up more space than the original iron, which can push apart nearby parts. This process, called "rust jacking," can lead to failures. For example, the Mianus River Bridge collapsed in 1983 because rust inside the bridge's supports pushed a section of the road off its base.

In 1967, the Silver Bridge in West Virginia collapsed quickly, killing 46 people. The bridge was a steel suspension bridge. In 2003, the Kinzua Bridge in Pennsylvania was destroyed by a tornado because the bolts holding it to the ground had rusted away, leaving it supported only by gravity.

Reinforced concrete can also be damaged by rust. When rust forms inside concrete covering steel or iron, it expands and creates pressure. This pressure can cause the concrete to crack and break off, leading to serious structural problems. This is a common reason for failures in reinforced concrete bridges and buildings.

• Structural failures caused by rust
• The collapsed Silver Bridge, viewed from Ohio
• The Kinzua Bridge after it fell
• Rusted and damaged supports of the 70-year-old Nandu River Iron Bridge
• Rusting steel bars inside concrete have caused the surface to crack and break away.

Cultural symbolism

Rust is often used as a symbol for slow damage caused by not taking care of things, because it slowly changes strong iron and steel into a soft, broken powder. Many areas in the American Midwest and Northeast, which were once home to steel factories, car companies, and other industries, have faced big economic losses. This has led people to call the area the "Rust Belt."

In music, books, and art, rust is connected to ideas of old glory, being forgotten, falling apart, and destruction.