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Rust is an , a usually reddish-brown formed by the reaction of and in the presence of or . Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the of .

Given sufficient time, any iron mass, in the presence of water and oxygen, could eventually convert entirely to rust. Surface rust is commonly flaky and , and provides no passivational protection to the underlying iron, unlike the formation of on copper surfaces. Rusting is the common term for of elemental iron and such as . Many other undergo similar corrosion, but the resulting oxides are not commonly called "rust".

Several forms of rust are distinguishable both visually and by , and form under different circumstances. Other forms of rust include the result of reactions between iron and in an environment deprived of oxygen. used in underwater , which generates , is an example. Although rusting is generally a negative aspect of iron, a particular form of rusting, known as stable rust, causes the object to have a thin coating of rust over the top. If kept in low relative humidity, it makes the "stable" layer protective to the iron below, but not to the extent of other oxides such as on .

Chemical reactions
Rust is a general name for a complex of oxides and hydroxides of iron,
(1980). 9780675075695, C.E. Merrill. .
which occur when iron or some alloys that contain iron are exposed to oxygen and moisture for a long period of time. Over time, the oxygen combines with the metal forming new compounds collectively called rust. Although rust may generally be termed as "oxidation", that term is much more general and describes a vast number of processes involving the loss of or increased oxidation state, as part of a reaction. The best-known of these reactions involve , hence the name "oxidation". The terms "rust" and "rusting" only mean oxidation of iron and its resulting products. Many other oxidation reactions exist which do not involve iron or produce rust. But only iron or alloys that contain iron can rust. However, other can corrode in similar ways.

The main catalyst for the rusting process is water. Iron or steel structures might appear to be solid, but water molecules can penetrate the microscopic pits and cracks in any exposed metal. The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed. If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly. Meanwhile, the oxygen atoms combine with metallic atoms to form the destructive oxide compound. As the atoms combine, they weaken the metal, making the structure brittle and crumbly.

Oxidation of iron
When iron is in contact with water and oxygen, it rusts. If is present, for example in or , the iron tends to rust more quickly, as a result of chemical reactions. Iron metal is relatively unaffected by pure water or by dry oxygen. As with other metals, like aluminium, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation. The conversion of the passivating layer to rust results from the combined action of two agents, usually oxygen and water.

Other degrading solutions are in water and in water. Under these corrosive conditions, species are formed. Unlike ferrous oxides, the hydroxides do not adhere to the bulk metal. As they form and flake off from the surface, fresh iron is exposed, and the corrosion process continues until either all of the iron is consumed or all of the oxygen, water, carbon dioxide or sulfur dioxide in the system are removed or consumed.

(2022). 9780123526519, Academic Press.

When iron rusts, the oxides take up more volume than the original metal; this expansion can generate enormous forces, damaging structures made with iron. See economic effect for more details.

Associated reactions
The rusting of iron is an electrochemical process that begins with the transfer of from iron to oxygen. The iron is the reducing agent (gives up electrons) while the oxygen is the oxidizing agent (gains electrons). The rate of corrosion is affected by water and accelerated by , as illustrated by the effects of on the corrosion of automobiles. The key reaction is the reduction of oxygen:
O2 + 4  + 2  → 4 
Because it forms , this process is strongly affected by the presence of acid. Likewise, the corrosion of most metals by oxygen is accelerated at low pH. Providing the electrons for the above reaction is the oxidation of iron that may be described as follows:
Fe → Fe2+ + 2 

The following also occurs in the presence of water and is crucial to the formation of rust:

4 Fe2+ + O2 → 4 Fe3+ + 2 O2−

In addition, the following multistep acid–base reactions affect the course of rust formation:

Fe2+ + 2 H2O ⇌ Fe(OH)2 + 2 
Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 

as do the following dehydration equilibria:

(OH)2 ⇌ FeO +
(OH)3 ⇌ FeO(OH) +
2 FeO(OH) ⇌ Fe2O3 +

From the above equations, it is also seen that the corrosion products are dictated by the availability of water and oxygen. With limited dissolved oxygen, iron(II)-containing materials are favoured, including FeO and black or (Fe3O4). High oxygen concentrations favour materials with the nominal formulae Fe(OH)3− xO. The nature of rust changes with time, reflecting the slow rates of the reactions of solids.

Furthermore, these complex processes are affected by the presence of other ions, such as , which serve as electrolytes which accelerate rust formation, or combine with the and of iron to precipitate a variety of Ca, Fe, O, OH species.

The onset of rusting can also be detected in the laboratory with the use of ferroxyl indicator solution. The solution detects both Fe2+ ions and hydroxyl ions. Formation of Fe2+ ions and hydroxyl ions are indicated by blue and pink patches respectively.

Because of the widespread use and importance of iron and steel products, the prevention or slowing of rust is the basis of major economic activities in a number of specialized technologies. A brief overview of methods is presented here; for detailed coverage, see the cross-referenced articles.

Rust is to air and water, therefore the interior metallic iron beneath a rust layer continues to corrode. Rust prevention thus requires coatings that preclude rust formation.

Rust-resistant alloys
Stainless steel forms a passivation layer of chromium(III) oxide.
(2014). 9781420071092, CRC Press. .
Similar passivation behavior occurs with , , , , , , and other electroactive conductive polymers.

Special "" alloys such as Cor-Ten rust at a much slower rate than normal, because the rust adheres to the surface of the metal in a protective layer. Designs using this material must include measures that avoid worst-case exposures since the material still continues to rust slowly even under near-ideal conditions.

Galvanization consists of an application on the object to be protected of a layer of metallic by either hot-dip galvanizing or . Zinc is traditionally used because it is cheap, adheres well to steel, and provides cathodic protection to the steel surface in case of damage of the zinc layer. In more corrosive environments (such as salt water), plating is preferred. Galvanization often fails at seams, holes, and joints where there are gaps in the coating. In these cases, the coating still provides some partial cathodic protection to iron, by acting as a and corroding itself instead of the underlying protected metal. The protective zinc layer is consumed by this action, and thus galvanization provides protection only for a limited period of time.

More modern coatings add aluminium to the coating as zinc-alume; aluminium will migrate to cover scratches and thus provide protection for a longer period. These approaches rely on the aluminium and zinc oxides protecting a once-scratched surface, rather than oxidizing as a sacrificial anode as in traditional galvanized coatings. In some cases, such as very aggressive environments or long design life, both zinc and a are applied to provide enhanced corrosion protection.

Typical galvanization of steel products that are to be subjected to normal day-to-day weathering in an outside environment consists of a hot-dipped 85 µm zinc coating. Under normal weather conditions, this will deteriorate at a rate of 1 µm per year, giving approximately 85 years of protection.

Cathodic protection
Cathodic protection is a technique used to inhibit corrosion on buried or immersed structures by supplying an electrical charge that suppresses the electrochemical reaction. If correctly applied, corrosion can be stopped completely. In its simplest form, it is achieved by attaching a sacrificial anode, thereby making the iron or steel the cathode in the cell formed. The sacrificial anode must be made from something with a more negative electrode potential than the iron or steel, commonly zinc, aluminium, or magnesium. The sacrificial anode will eventually corrode away, ceasing its protective action unless it is replaced in a timely manner.

Cathodic protection can also be provided by using an applied electrical current. This would then be known as ICCP Impressed Current Cathodic Protection.

Coatings and painting
Rust formation can be controlled with coatings, such as , , , or wax tapes that isolate the iron from the environment.
(2022). 9780849382468, CRC Press. .
Large structures with enclosed box sections, such as ships and modern automobiles, often have a wax-based product (technically a "slushing oil") injected into these sections. Such treatments usually also contain rust inhibitors. Covering steel with concrete can provide some protection to steel because of the pH environment at the steel–concrete interface. However, rusting of steel in concrete can still be a problem, as expanding rust can fracture concrete from within.

As a closely related example, iron clamps were used to join blocks during a restoration attempt of the in Athens, Greece, in 1898, but caused extensive damage to the marble by the rusting and swelling of unprotected iron. The ancient Greek builders had used a similar fastening system for the marble blocks during construction, however, they also poured molten over the iron joints for protection from seismic shocks as well as from corrosion. This method was successful for the 2500-year-old structure, but in less than a century the crude repairs were in imminent danger of collapse. When only temporary protection is needed for storage or transport, a thin layer of oil, grease or a special mixture such as can be applied to an iron surface. Such treatments are extensively used when "" a steel ship, automobile, or other equipment for long-term storage.

Special anti-seize lubricant mixtures are available and are applied to metallic threads and other precision machined surfaces to protect them from rust. These compounds usually contain grease mixed with copper, zinc, or aluminium powder, and other proprietary ingredients.

Bluing is a technique that can provide limited resistance to rusting for small steel items, such as firearms; for it to be successful, a water-displacing oil is rubbed onto the blued steel and other steel.

Corrosion inhibitors, such as gas-phase or volatile inhibitors, can be used to prevent corrosion inside sealed systems. They are not effective when air circulation disperses them, and brings in fresh oxygen and moisture.

Humidity control
Rust can be avoided by controlling the moisture in the atmosphere.
9788131756591, Pearson Education India. .
An example of this is the use of packets to control humidity in equipment shipped by sea.

Rust removal from small iron or steel objects by can be done in a home workshop using simple materials such as a plastic bucket filled with an electrolyte consisting of dissolved in , a length of suspended vertically in the solution to act as an , another laid across the top of the bucket to act as a support for suspending the object, to suspend the object in the solution from the horizontal rebar, and a as a power source in which the positive terminal is clamped to the anode and the negative terminal is clamped to the object to be treated which becomes the .

Rust may be treated with commercial products known as which contain or which combines with rust; removed with organic acids like and or the stronger hydrochloric acid; or removed with chelating agents as in some commercial formulations or even a solution of .

Economic effect
Rust is associated with the degradation of iron-based tools and structures. As rust has a much higher volume than the originating mass of iron, its buildup can also cause failure by forcing apart adjacent parts — a phenomenon sometimes known as "rust packing". It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support.

Rust was an important factor in the disaster of 1967 in , when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time. The in was blown down by a in 2003, largely because the central base bolts holding the structure to the ground had rusted away, leaving the bridge anchored by gravity alone.

Reinforced concrete is also vulnerable to rust damage. Internal pressure caused by expanding corrosion of concrete-covered steel and iron can cause the concrete to , creating severe structural problems. It is one of the most common failure modes of reinforced concrete and buildings.

File:Silver Bridge collapsed, Ohio side.jpg|The collapsed , as seen from the side File:Collapsed Kinzua Bridge.jpg|The after it collapsed File:Nandu River Iron Bridge corrosion - 03.jpg|Rusted and pitted struts of the 70-year-old Nandu River Iron Bridge

Cultural symbolism
Rust is a commonly used for slow decay due to neglect, since it gradually converts robust iron and steel metal into a soft crumbling powder. A wide section of the industrialized and American Northeast, once dominated by , the automotive industry, and other manufacturers, has experienced harsh economic cutbacks that have caused the region to be dubbed the "".

In music, literature, and art, rust is associated with images of faded glory, neglect, decay, and ruin.

See also
  • Corrosion engineering

Further reading
  • (2015): Rust - the longest war. Simon & Schuster, New York.
  • (2022). 9780849382444, CRC Press. .
  • (1996). 085404731X, Royal Society of Chemistry, Information Services. 085404731X
  • Corrosion - 2nd Edition ( Volume 1and 2; Editor: L L Shreir

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