Iron

 ( Building Materials ) 

Fe + 2 HX → FeX₂ + H₂ (X = F, Cl, Br, I)

2 Fe + 3 X₂ → 2 FeX₃ (X = F, Cl, Br)

2 I⁻ + 2 Fe³⁺ → I₂ + 2 Fe²⁺ (E⁰ = +0.23 V)

Ferric iodide, a black solid, is not stable in ordinary conditions, but can be prepared through the reaction of iron pentacarbonyl with iodine and carbon monoxide in the presence of hexane and light at the temperature of −20 °C, with oxygen and water excluded. Complexes of ferric iodide with some soft bases are known to be stable compounds.Siegfried Pohl, Ulrich Bierbach, Wolfgang Saak; "FeI3SC(NMe2)2, a Neutral Thiourea Complex of Iron(III) Iodide" /ref>Nicholas A. Barnes, Stephen M.Godfrey, Nicholas Ho, Charles A.McAuliffe, Robin G.Pritchard; "Facile synthesis of a rare example of an iron(III) iodide complex, FeI3(AsMe3)2, from the reaction of Me3AsI2 with unactivated iron powder" /ref>

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E0 = −0.447 V

E0 = +0.77 V

E0 = +2.20 V

The red-purple tetrahedral ferrate(VI) anion is such a strong oxidizing agent that it oxidizes ammonia to nitrogen (N₂) and water to oxygen:

4 + 34 → 4 + 20 + 3 O₂

The pale-violet hexaquo complex is an acid such that above pH 0 it is fully hydrolyzed:

>

K = 10−3.05 mol dm−3

K = 10−3.26 mol dm−3

K = 10−2.91 mol dm−3

As pH rises above 0 the above yellow hydrolyzed species form and as it rises above 2–3, reddish-brown hydrous iron(III) oxide precipitates out of solution. Although Fe³⁺ has a d⁵ configuration, its absorption spectrum is not like that of Mn²⁺ with its weak, spin-forbidden d–d bands, because Fe³⁺ has higher positive charge and is more polarizing, lowering the energy of its ligand-to-metal charge transfer absorptions. Thus, all the above complexes are rather strongly colored, with the single exception of the hexaquo ion – and even that has a spectrum dominated by charge transfer in the near ultraviolet region. On the other hand, the pale green iron(II) hexaquo ion does not undergo appreciable hydrolysis. Carbon dioxide is not evolved when carbonate anions are added, which instead results in white iron(II) carbonate being precipitated out. In excess carbon dioxide this forms the slightly soluble bicarbonate, which occurs commonly in groundwater, but it oxidises quickly in air to form iron(III) oxide that accounts for the brown deposits present in a sizeable number of streams.

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Coordination compounds

Due to its electronic structure, iron has a very large coordination and organometallic chemistry. Many coordination compounds of iron are known. A typical six-coordinate anion is hexachloroferrate(III), FeCl₆³⁻, found in the mixed salt tetrakis(methylammonium) hexachloroferrate(III) chloride. Complexes with multiple bidentate ligands have . For example, the trans-chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II) complex is used as a starting material for compounds with the moiety.

(1977). 9780470132487 ISBN 9780470132487

The ferrioxalate ion with three oxalate ligands displays helical chirality with its two non-superposable geometries labelled Λ (lambda) for the left-handed screw axis and Δ (delta) for the right-handed screw axis, in line with IUPAC conventions. Potassium ferrioxalate is used in chemical actinometry and along with its sodium salt undergoes photoreduction applied in old-style photographic processes. The hydrate of iron(II) oxalate has a structure with co-planar oxalate ions bridging between iron centres with the water of crystallisation located forming the caps of each octahedron, as illustrated below.

Iron(III) complexes are quite similar to those of chromium(III) with the exception of iron(III)'s preference for O-donor instead of N-donor ligands. The latter tend to be rather more unstable than iron(II) complexes and often dissociate in water. Many Fe–O complexes show intense colors and are used as tests for or . For example, in the ferric chloride test, used to determine the presence of phenols, iron(III) chloride reacts with a phenol to form a deep violet complex:

3 ArOH + FeCl₃ → Fe(OAr)₃ + 3 HCl (Ar = aryl)

Among the halide and pseudohalide complexes, fluoro complexes of iron(III) are the most stable, with the colorless FeF₅(H₂O)²⁻ being the most stable in aqueous solution. Chloro complexes are less stable and favor tetrahedral coordination as in FeCl₄⁻; FeBr₄⁻ and FeI₄⁻ are reduced easily to iron(II). Thiocyanate is a common test for the presence of iron(III) as it forms the blood-red Fe(SCN)(H₂O)₅²⁺. Like manganese(II), most iron(III) complexes are high-spin, the exceptions being those with ligands that are high in the spectrochemical series such as cyanide. An example of a low-spin iron(III) complex is Fe(CN)₆³⁻. Iron shows a great variety of electronic spin states, including every possible spin quantum number value for a d-block element from 0 (diamagnetic) to (5 unpaired electrons). This value is always half the number of unpaired electrons. Complexes with zero to two unpaired electrons are considered low-spin and those with four or five are considered high-spin.

Iron(II) complexes are less stable than iron(III) complexes but the preference for O-donor ligands is less marked, so that for example is known while is not. They have a tendency to be oxidized to iron(III) but this can be moderated by low pH and the specific ligands used.

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Organometallic compounds

Organoiron chemistry is the study of organometallic compounds of iron, where carbon atoms are covalently bound to the metal atom. They are many and varied, including cyanometallate, , sandwich and half-sandwich compounds.

Prussian blue or "ferric ferrocyanide", Fe₄Fe(CN)₆₃, is an old and well-known iron-cyanide complex, extensively used as pigment and in several other applications. Its formation can be used as a simple wet chemistry test to distinguish between aqueous solutions of Fe²⁺ and Fe³⁺ as they react (respectively) with potassium ferricyanide and potassium ferrocyanide to form Prussian blue.

Another old example of an organoiron compound is iron pentacarbonyl, Fe(CO)₅, in which a neutral iron atom is bound to the carbon atoms of five carbon monoxide molecules. The compound can be used to make carbonyl iron powder, a highly reactive form of metallic iron. Thermolysis of iron pentacarbonyl gives triiron dodecacarbonyl, , a complex with a cluster of three iron atoms at its core. Collman's reagent, disodium tetracarbonylferrate, is a useful reagent for organic chemistry; it contains iron in the −2 oxidation state. Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1 oxidation state..

A landmark in this field was the discovery in 1951 of the remarkably stable sandwich compound ferrocene , by Pauson and Kealy and independently by Miller and colleagues, whose surprising molecular structure was determined only a year later by Woodward and Wilkinson and Fischer. Ferrocene is still one of the most important tools and models in this class.

Iron-centered organometallic species are used as . The Knölker complex, for example, is a transfer hydrogenation catalyst for .

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Industrial uses

The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO₄·7H₂O) and iron(III) chloride (FeCl₃). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt (). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.

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History

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Development of iron metallurgy

Iron is one of the elements undoubtedly known to the ancient world. It has been worked, or wrought iron, for millennia. However, iron artefacts of great age are much rarer than objects made of gold or silver due to the ease with which iron corrodes. The technology developed slowly, and even after the discovery of smelting it took many centuries for iron to replace bronze as the metal of choice for tools and weapons.

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Meteoritic iron

Beads made from meteoric iron in 3500 BC or earlier were found in Gerzeh culture, Egypt by G. A. Wainwright. The beads contain 7.5% nickel, which is a signature of meteoric origin since iron found in the Earth's crust generally has only minuscule nickel impurities.

Meteoric iron was highly regarded due to its origin in the heavens and was often used to forge weapons and tools. For example, a dagger made of meteoric iron was found in the tomb of Tutankhamun, containing similar proportions of iron, cobalt, and nickel to a meteorite discovered in the area, deposited by an ancient meteor shower. Items that were likely made of iron by Egyptians date from 3000 to 2500 BC.

Meteoritic iron is comparably soft and ductile and easily cold forging but may get brittle when heated because of the nickel content.

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Wrought iron

The first iron production started in the Middle Bronze Age, but it took several centuries before iron displaced bronze. Samples of smelting iron from Asmar, Mesopotamia and Tall Chagar Bazaar in northern Syria were made sometime between 3000 and 2700 BC. The Hittites established an empire in north-central Anatolia around 1600 BC. They appear to be the first to understand the production of iron from its ores and regard it highly in their society.McNutt, Paula (1990 1). The Forging of Israel: Iron Technology, Symbolism and Tradition in Ancient Society. A&C Black. The Hittites began to smelt iron between 1500 and 1200 BC and the practice spread to the rest of the Near East after their empire fell in 1180 BC. The subsequent period is called the Iron Age.

Artifacts of smelted iron are found in India dating from 1800 to 1200 BC, and in the Levant from about 1500 BC (suggesting smelting in Anatolia or the Caucasus). Alleged references (compare history of metallurgy in South Asia) to iron in the Indian Vedas have been used for claims of a very early usage of iron in India respectively to date the texts as such. The rigveda term ayas (metal) refers to copper, while iron which is called as śyāma ayas, literally "black copper", first is mentioned in the post-rigvedic Atharvaveda.Michael Witzel (2001), "Autochthonous Aryans? The Evidence from Old Indian and Iranian Texts", in Electronic Journal of Vedic Studies (EJVS) 7-3, pp. 1–93

Some archaeological evidence suggests iron was smelted in Zimbabwe and southeast Africa as early as the eighth century BC.Weeks, p. 33, quoting Cline, Walter (1937) "Mining and Metallurgy in Negro Africa", George Banta Publishing Co., Menasha, Wis., pp. 17–23. Iron working was introduced to Ancient Greece in the late 11th century BC, from which it spread quickly throughout Europe.Riederer, Josef; Wartke, Ralf-B. (2009) "Iron", Cancik, Hubert; Schneider, Helmuth (eds.): Brill's New Pauly, Brill.

The spread of ironworking in Central and Western Europe is associated with Celts expansion. According to Pliny the Elder, iron use was common in the Ancient Rome era. In the lands of what is now considered China, iron appears approximately 700–500 BC.Sawyer, Ralph D. and Sawyer, Mei-chün (1993). The Seven Military Classics of Ancient China. Boulder: Westview. . p. 10. Iron smelting may have been introduced into China through Central Asia.Pigott, Vincent C. (1999). The Archaeometallurgy of the Asian Old World. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. , p. 8. The earliest evidence of the use of a blast furnace in China dates to the 1st century AD,

Peter J. Golas (1999). 9780521580007, Cambridge University Press. ISBN 9780521580007

and cupola furnaces were used as early as the Warring States period (403–221 BC).Pigott, Vincent C. (1999). The Archaeometallurgy of the Asian Old World. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. , p. 191. Usage of the blast and cupola furnace remained widespread during the Tang dynasty and Song dynasty dynasties.

During the Industrial Revolution in Britain, Henry Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative production systems. In 1783 he patented the puddling process for refining iron ore. It was later improved by others, including Joseph Hall.

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Cast iron

Cast iron was first produced in China during 5th century BC, originally published in but was hardly in Europe until the medieval period.Giannichedda, Enrico (2007): "Metal production in Late Antiquity", in Technology in Transition AD 300–650 Lavan, L.; Zanini, E. and Sarantis, A.(eds.), Brill, Leiden; , p. 200. The earliest cast iron artifacts were discovered by archaeologists in what is now modern Luhe County, Jiangsu in China. Cast iron was used in ancient China for warfare, agriculture, and architecture.

Donald B. Wagner (1993). 9789004096325, Brill. ISBN 9789004096325

During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using . For all these processes, charcoal was required as fuel. Medieval blast furnaces were about tall and made of fireproof brick; forced air was usually provided by hand-operated bellows. Modern blast furnaces have grown much bigger, with hearths fourteen meters in diameter that allow them to produce thousands of tons of iron each day, but essentially operate in much the same way as they did during medieval times.

In 1709, Abraham Darby I established a coke-fired blast furnace to produce cast iron, replacing charcoal, although continuing to use blast furnaces. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution. Toward the end of the 18th century, cast iron began to replace wrought iron for certain purposes, because it was cheaper. Carbon content in iron was not implicated as the reason for the differences in properties of wrought iron, cast iron, and steel until the 18th century.

Since iron was becoming cheaper and more plentiful, it also became a major structural material following the building of the innovative first iron bridge in 1778. This bridge still stands today as a monument to the role iron played in the Industrial Revolution. Following this, iron was used in rails, boats, ships, aqueducts, and buildings, as well as in iron cylinders in . Railways have been central to the formation of modernity and ideas of progressSchivelbusch, G. (1986) The Railway Journey: Industrialization and Perception of Time and Space in the 19th Century. Oxford: Berg. and various languages refer to railways as iron road (e.g. French chemin de fer, German Eisenbahn, Turkish demiryolu, Russian железная дорога, CJK 鐵道, Vietnamese đường sắt).

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Steel

Steel (with smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity by using a bloomery. Blacksmiths in Luristan in western Persia were making good steel by 1000 BC. Then improved versions, Wootz steel by India and Damascus steel were developed around 300 BC and AD 500 respectively. These methods were specialized, and so steel did not become a major commodity until the 1850s.Spoerl, Joseph S. A Brief History of Iron and Steel Production . Saint Anselm College

New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This made steel much more economical, thereby leading to wrought iron no longer being produced in large quantities.

(2008). 9783527612345, John Wiley & Sons. ISBN 9783527612345

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Foundations of modern chemistry

In 1774, Antoine Lavoisier used the reaction of water steam with metallic iron inside an incandescent iron tube to produce hydrogen in his experiments leading to the demonstration of the conservation of mass, which was instrumental in changing chemistry from a qualitative science to a quantitative one.

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Symbolic role

Iron plays a certain role in mythology and has found various usage as a metaphor and in folklore. The Greeks poet Hesiod's Works and Days (lines 109–201) lists different ages of man named after metals like gold, silver, bronze and iron to account for successive ages of humanity. The Iron Age was closely related with Rome, and in Ovid's Metamorphoses

An example of the importance of iron's symbolic role may be found in the German Campaign of 1813. Frederick William III commissioned then the first Iron Cross as military decoration. Berlin iron jewellery reached its peak production between 1813 and 1815, when the Prussian royal family urged citizens to donate gold and silver jewellery for military funding. The inscription Ich gab Gold für Eisen (I gave gold for iron) was used as well in later war efforts.Schmidt, Eva (1981) Der preußische Eisenkunstguss. (Art of Prussian cast iron) Technik, Geschichte, Werke, Künstler. Verlag Mann, Berlin,

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Laboratory routes

For a few limited purposes when it is needed, pure iron is produced in the laboratory in small quantities by reducing the pure oxide or hydroxide with hydrogen, or forming iron pentacarbonyl and heating it to 250 °C so that it decomposes to form pure iron powder. Another method is electrolysis of ferrous chloride onto an iron cathode.Lux, H. (1963) "Metallic Iron" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. G. Brauer (ed.), Academic Press, NY. Vol. 2. pp. 1490–91.

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Main industrial route

+Iron production 2009 (million tonnes) target="_blank" rel="nofollow"> Steel Statistical Yearbook 2010. World Steel Association !Country!!Iron ore!!Pig iron!!Direct iron!!Steel
573.6
5.2
26.5
87.5
63.5
60.0
29.9
48.6
32.7

Nowadays, the industrial production of iron or steel consists of two main stages. In the first stage, iron ore is redox with coke in a blast furnace, and the molten metal is separated from gross impurities such as . This stage yields an alloy – pig iron – that contains relatively large amounts of carbon. In the second stage, the amount of carbon in the pig iron is lowered by oxidation to yield wrought iron, steel, or cast iron. Other metals can be added at this stage to form .

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Blast furnace processing

The blast furnace is loaded with iron ores, usually hematite or magnetite , along with coke (coal that has been separately baked to remove volatile components) and flux (limestone or Dolomites). "Blasts" of air pre-heated to 900 °C (sometimes with oxygen enrichment) is blown through the mixture, in sufficient amount to turn the carbon into carbon monoxide:

This reaction raises the temperature to about 2000 °C. The carbon monoxide reduces the iron ore to metallic iron:

Some iron in the high-temperature lower region of the furnace reacts directly with the coke:

The flux removes silicaceous minerals in the ore, which would otherwise clog the furnace: The heat of the furnace decomposes the carbonates to calcium oxide, which reacts with any excess silica to form a slag composed of calcium silicate or other products. At the furnace's temperature, the metal and the slag are both molten. They collect at the bottom as two immiscible liquid layers (with the slag on top), that are then easily separated. The slag can be used as a material in road construction or to improve mineral-poor soils for agriculture.

Steelmaking thus remains one of the largest industrial contributors of CO₂ emissions in the world.

File:Chinese Fining and Blast Furnace.jpg|17th century Chinese illustration of workers at a blast furnace, making wrought iron from pig ironSong Yingxing (1637): The Tiangong Kaiwu encyclopedia. File:Iron-Making.jpg|How iron was extracted in the 19th century File:Geography of Ohio - DPLA - aaba7b3295ff6973b6fd1e23e33cde14 (page 111) (cropped).jpg|Iron furnace in Columbus, Ohio, 1922

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Steelmaking

The pig iron produced by the blast furnace process contains up to 4–5% carbon (by mass), with small amounts of other impurities like sulfur, magnesium, phosphorus, and manganese. This high level of carbon makes it relatively weak and brittle. Reducing the amount of carbon to 0.002–2.1% produces steel, which may be up to 1000 times harder than pure iron. A great variety of steel articles can then be made by cold working, hot rolling, forging, machining, etc. Removing the impurities from pig iron, but leaving 2–4% carbon, results in cast iron, which is cast by foundry into articles such as stoves, pipes, radiators, lamp-posts, and rails.

Steel products often undergo various after they are forged to shape. Annealing consists of heating them to 700–800 °C for several hours and then gradual cooling. It makes the steel softer and more workable.Verhoeven, J.D. (1975) Fundamentals of Physical Metallurgy, Wiley, New York, p. 326

File:LightningVolt Iron Ore Pellets.jpg|This heap of iron ore pellets will be used in steel production. File:Melted raw-iron.jpg|A pot of molten iron being used to make steel

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Direct iron reduction

Owing to environmental concerns, alternative methods of processing iron have been developed. "Direct iron reduction" reduces iron ore to a ferrous lump called sponge iron or "direct" iron that is suitable for steelmaking. Two main reactions comprise the direct reduction process:

Natural gas is partially oxidized (with heat and a catalyst):

Iron ore is then treated with these gases in a furnace, producing solid sponge iron:

Silica is removed by adding a limestone flux as described above.

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Thermite process

Ignition of a mixture of aluminium powder and iron oxide yields metallic iron via the thermite reaction:

Alternatively pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron). Various processes have been used for this, including , puddling furnaces, Bessemer converters, open hearth furnaces, basic oxygen furnaces, and electric arc furnaces. In all cases, the objective is to oxidize some or all of the carbon, together with other impurities. On the other hand, other metals may be added to make alloy steels.

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Molten oxide electrolysis

Molten oxide electrolysis (MOE) uses electrolysis of molten iron oxide to yield metallic iron. It is studied in laboratory-scale experiments and is proposed as a method for industrial iron production that has no direct emissions of carbon dioxide. It uses a liquid iron cathode, an anode formed from an alloy of chromium, aluminium and iron, and the electrolyte is a mixture of molten metal oxides into which iron ore is dissolved. The current keeps the electrolyte molten and reduces the iron oxide. Oxygen gas is produced in addition to liquid iron. The only carbon dioxide emissions come from any fossil fuel-generated electricity used to heat and reduce the metal.

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Applications

+ Characteristic values of tensile strength (TS) and Brinell hardness (BH) of various forms of iron. Walter H. Kohl (1995). 9781563963872, Springer. ISBN 9781563963872 (2025). 9780871703897, ASM International. . ISBN 9780871703897 !Material !TS (MPa) !BH (Brinell scale)
Iron whiskers	11000
Ausformed (hardened) steel	2930	850–1200
Martensite	2070	600
Bainite	1380	400
Pearlite	1200	350
cold forming iron	690	200
Small-grain iron	340	100
Carbon-containing iron	140	40
Pure, single-crystal iron	10	3

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As structural material

Iron is the most widely used of all the metals, accounting for over 90% of worldwide metal production. Its low cost and high strength often make it the material of choice to withstand stress or transmit forces, such as the construction of machinery and , rails, , ship hulls, rebar, and the load-carrying framework of buildings. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.

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Mechanical properties

The mechanical properties of iron and its alloys are extremely relevant to their structural applications. Those properties can be evaluated in various ways, including the Brinell scale, the Rockwell scale and the Vickers hardness test.

The properties of pure iron are often used to calibrate measurements or to compare tests. However, the mechanical properties of iron are significantly affected by the sample's purity: pure, single crystals of iron are actually softer than aluminium, and the purest industrially produced iron (99.99%) has a hardness of 20–30 Brinell. The pure iron (99.9%～99.999%), especially called electrolytic iron, is industrially produced by electrolytic refining.

An increase in the carbon content will cause a significant increase in the hardness and tensile strength of iron. Maximum hardness of Rockwell scale is achieved with a 0.6% carbon content, although the alloy has low tensile strength.

V. Raghavan (2025). 9788120324558, PHI Learning Pvt. Ltd.. ISBN 9788120324558

Because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium.

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Types of steels and alloys

α-Iron is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).

John Wilson Martin (2025). 9780080451275, Elsevier. ISBN 9780080451275

Austenite (γ-iron) is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.

Commercially available iron is classified based on purity and the abundance of additives. Pig iron has 3.5–4.5% carbon

(2025). 9781147644234, Carnegie Steel Company. ISBN 9781147644234

and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of manganese. Pig iron has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.

"White" cast irons contain their carbon in the form of cementite, or iron carbide (Fe₃C). This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron carbide, a very pale, silvery, shiny material, hence the appellation. Cooling a mixture of iron with 0.8% carbon slowly below 723 °C to room temperature results in separate, alternating layers of cementite and α-iron, which is soft and malleable and is called pearlite for its appearance. Rapid cooling, on the other hand, does not allow time for this separation and creates hard and brittle martensite. The steel can then be tempered by reheating to a temperature in between, changing the proportions of pearlite and martensite. The end product below 0.8% carbon content is a pearlite-αFe mixture, and that above 0.8% carbon content is a pearlite-cementite mixture.

In gray iron the carbon exists as separate, fine flakes of graphite, and also renders the material brittle due to the sharp edged flakes of graphite that produce stress concentration sites within the material. A newer variant of gray iron, referred to as ductile iron, is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.

Wrought iron contains less than 0.25% carbon but large amounts of slag that give it a fibrous characteristic. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel, which corrodes more readily than wrought iron, but is cheaper and more widely available. Carbon steel contains 2.0% carbon or less, with small amounts of manganese, sulfur, phosphorus, and silicon. contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. Their alloy content raises their cost, and so they are usually only employed for specialist uses. One common alloy steel, though, is stainless steel. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA steel' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.

Alloys with high purity elemental makeups (such as alloys of electrolytic iron) have specifically enhanced properties such as ductility, tensile strength, toughness, fatigue limit, heat resistance, and corrosion resistance.

Apart from traditional applications, iron is also used for protection from ionizing radiation. Although it is lighter than another traditional protection material, lead, it is much stronger mechanically.

The main disadvantage of iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way, a cost amounting to over 1% of the world's economy. , galvanization, passivation, plastic coating and bluing are all used to protect iron from rust by excluding water and oxygen or by cathodic protection. The mechanism of the rusting of iron is as follows:

Cathode: 3 O₂ + 6 H₂O + 12 e⁻ → 12 OH⁻

Anode: 4 Fe → 4 Fe²⁺ + 8 e⁻; 4 Fe²⁺ → 4 Fe³⁺ + 4 e⁻

Overall: 4 Fe + 3 O₂ + 6 H₂O → 4 Fe³⁺ + 12 OH⁻ → 4 Fe(OH)₃ or 4 FeO(OH) + 4 H₂O

The electrolyte is usually iron(II) sulfate in urban areas (formed when atmospheric sulfur dioxide attacks iron), and salt particles in the atmosphere in seaside areas.

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Catalysts and reagents

Because Fe is inexpensive and nontoxic, much effort has been devoted to the development of Fe-based catalysts and . Iron is however less common as a catalyst in commercial processes than more expensive metals. In biology, Fe-containing enzymes are pervasive.

Iron catalysts are traditionally used in the Haber–Bosch process for the production of ammonia and the Fischer–Tropsch process for conversion of carbon monoxide to for fuels and lubricants.

Kurt W. Kolasinski (2025). 9780471492443, John Wiley and Sons. ISBN 9780471492443

Powdered iron in an acidic medium is used in the Bechamp reduction, the conversion of nitrobenzene to aniline.

John J. McKetta (1989). 9780824724818, CRC Press. ISBN 9780824724818

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Iron compounds

Iron(III) oxide mixed with aluminium powder can be ignited to create a thermite, used in welding large iron parts (like railroad) and purifying ores. Iron(III) oxide and oxyhydroxide are used as reddish and ocher .

Iron(III) chloride finds use in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an etchant for copper in the manufacture of printed circuit boards. It can also be dissolved in alcohol to form tincture of iron, which is used as a medicine to stop bleeding in Domestic canary.

Robert Stroud (2025). 9781446546567, Canary Publishers Company. ISBN 9781446546567

Iron(II) sulfate is used as a precursor to other iron compounds. It is also used to redox chromate in cement. It is used to fortify foods and treat iron deficiency anemia. Iron(III) sulfate is used in settling minute sewage particles in tank water. Iron(II) chloride is used as a reducing flocculating agent, in the formation of iron complexes and magnetic iron oxides, and as a reducing agent in organic synthesis.

Sodium nitroprusside is a drug used as a vasodilator. It is on the World Health Organization's List of Essential Medicines.

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Biological and pathological role

Iron is required for life.

(2025). 9789400755604, Springer. ISBN 9789400755604

electronic-book

(2025). 9783319124148, Springer. ISBN 9783319124148

The iron–sulfur clusters are pervasive and include nitrogenase, the enzymes responsible for biological nitrogen fixation. Iron-containing proteins participate in transport, storage and use of oxygen. Iron proteins are involved in electron transfer.

Examples of iron-containing proteins in higher organisms include hemoglobin, cytochrome (see high-valent iron), and catalase.

(1994). 9780935702736, University Science Books. ISBN 9780935702736

The average adult human contains about 0.005% body weight of iron, or about four grams, of which three quarters is in hemoglobin—a level that remains constant despite only about one milligram of iron being absorbed each day, because the human body recycles its hemoglobin for the iron content.

Microbial growth may be assisted by oxidation of iron(II) or by reduction of iron(III).

(2025). 9783110589771, De Gruyter. ISBN 9783110589771

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Biochemistry

Iron acquisition poses a problem for aerobic organisms because ferric iron is poorly soluble near neutral pH. Thus, these organisms have developed means to absorb iron as complexes, sometimes taking up ferrous iron before oxidising it back to ferric iron. In particular, bacteria have evolved very high-affinity agents called .

After uptake in human cells, iron storage is precisely regulated. A major component of this regulation is the protein transferrin, which binds iron ions absorbed from the duodenum and carries it in the bloodstream to cells. Transferrin contains Fe³⁺ in the middle of a distorted octahedron, bonded to one nitrogen, three oxygens and a chelating carbonate anion that traps the Fe³⁺ ion: it has such a high stability constant that it is very effective at taking up Fe³⁺ ions even from the most stable complexes. At the bone marrow, transferrin is reduced from Fe³⁺ to Fe²⁺ and stored as ferritin to be incorporated into hemoglobin.

The most commonly known and studied bioinorganic iron compounds (biological iron molecules) are the heme proteins: examples are hemoglobin, myoglobin, and cytochrome P450. These compounds participate in transporting gases, building enzymes, and transferring . Metalloproteins are a group of proteins with metal ion cofactors. Some examples of iron metalloproteins are ferritin and rubredoxin. Many enzymes vital to life contain iron, such as catalase, lipoxygenases, and IRE-BP.

Hemoglobin is an oxygen carrier that occurs in red blood cells and contributes their color, transporting oxygen in the arteries from the lungs to the muscles where it is transferred to myoglobin, which stores it until it is needed for the metabolic oxidation of glucose, generating energy. Here the hemoglobin binds to carbon dioxide, produced when glucose is oxidized, which is transported through the veins by hemoglobin (predominantly as bicarbonate anions) back to the lungs where it is exhaled. In hemoglobin, the iron is in one of four heme groups and has six possible coordination sites; four are occupied by nitrogen atoms in a porphyrin ring, the fifth by an imidazole nitrogen in a histidine residue of one of the protein chains attached to the heme group, and the sixth is reserved for the oxygen molecule it can reversibly bind to. When hemoglobin is not attached to oxygen (and is then called deoxyhemoglobin), the Fe²⁺ ion at the center of the heme group (in the hydrophobic protein interior) is in a high-spin configuration. It is thus too large to fit inside the porphyrin ring, which bends instead into a dome with the Fe²⁺ ion about 55 picometers above it. In this configuration, the sixth coordination site reserved for the oxygen is blocked by another histidine residue.

When deoxyhemoglobin picks up an oxygen molecule, this histidine residue moves away and returns once the oxygen is securely attached to form a hydrogen bond with it. This results in the Fe²⁺ ion switching to a low-spin configuration, resulting in a 20% decrease in ionic radius so that now it can fit into the porphyrin ring, which becomes planar. Additionally, this hydrogen bonding results in the tilting of the oxygen molecule, resulting in a Fe–O–O bond angle of around 120° that avoids the formation of Fe–O–Fe or Fe–O₂–Fe bridges that would lead to electron transfer, the oxidation of Fe²⁺ to Fe³⁺, and the destruction of hemoglobin. This results in a movement of all the protein chains that leads to the other subunits of hemoglobin changing shape to a form with larger oxygen affinity. Thus, when deoxyhemoglobin takes up oxygen, its affinity for more oxygen increases, and vice versa. Myoglobin, on the other hand, contains only one heme group and hence this cooperative effect cannot occur. Thus, while hemoglobin is almost saturated with oxygen in the high partial pressures of oxygen found in the lungs, its affinity for oxygen is much lower than that of myoglobin, which oxygenates even at low partial pressures of oxygen found in muscle tissue. As described by the Bohr effect (named after Christian Bohr, the father of Niels Bohr), the oxygen affinity of hemoglobin diminishes in the presence of carbon dioxide.

Carbon monoxide and phosphorus trifluoride are poisonous to humans because they bind to hemoglobin similarly to oxygen, but with much more strength, so that oxygen can no longer be transported throughout the body. Hemoglobin bound to carbon monoxide is known as carboxyhemoglobin. This effect also plays a minor role in the toxicity of cyanide, but there the major effect is by far its interference with the proper functioning of the electron transport protein cytochrome a. The cytochrome proteins also involve heme groups and are involved in the metabolic oxidation of glucose by oxygen. The sixth coordination site is then occupied by either another imidazole nitrogen or a methionine sulfur, so that these proteins are largely inert to oxygen—with the exception of cytochrome a, which bonds directly to oxygen and thus is very easily poisoned by cyanide. Here, the electron transfer takes place as the iron remains in low spin but changes between the +2 and +3 oxidation states. Since the reduction potential of each step is slightly greater than the previous one, the energy is released step-by-step and can thus be stored in adenosine triphosphate. Cytochrome a is slightly distinct, as it occurs at the mitochondrial membrane, binds directly to oxygen, and transports protons as well as electrons, as follows:

4 Cytc²⁺ + O₂ + 8H → 4 Cytc³⁺ + 2 H₂O + 4H

Although the heme proteins are the most important class of iron-containing proteins, the iron–sulfur proteins are also very important, being involved in electron transfer, which is possible since iron can exist stably in either the +2 or +3 oxidation states. These have one, two, four, or eight iron atoms that are each approximately tetrahedrally coordinated to four sulfur atoms; because of this tetrahedral coordination, they always have high-spin iron. The simplest of such compounds is rubredoxin, which has only one iron atom coordinated to four sulfur atoms from cysteine residues in the surrounding peptide chains. Another important class of iron–sulfur proteins is the , which have multiple iron atoms. Transferrin does not belong to either of these classes.

The ability of sea to maintain their grip on rocks in the ocean is facilitated by their use of organometallic iron-based bonds in their protein-rich . Based on synthetic replicas, the presence of iron in these structures increased elastic modulus 770 times, tensile strength 58 times, and toughness 92 times. The amount of stress required to permanently damage them increased 76 times.

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Nutrition

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Diet

Iron is pervasive, but particularly rich sources of dietary iron include red meat, , , poultry, fish, , watercress, tofu, and blackstrap molasses. Bread and are sometimes specifically fortified with iron. Food Standards Agency – Eat well, be well – Iron deficiency . Eatwell.gov.uk (5 March 2012). Retrieved on 27 June 2012.

Iron provided by dietary supplements is often found as iron(II) fumarate, although iron(II) sulfate is cheaper and is absorbed equally well. Elemental iron, or reduced iron, despite being absorbed at only one-third to two-thirds the efficiency (relative to iron sulfate), is often added to foods such as breakfast cereals or enriched wheat flour. Iron is most available to the body when Chelation to amino acids and is also available for use as a common iron supplement. Glycine, the least expensive amino acid, is most often used to produce iron glycinate supplements.

H. DeWayne Ashmead (1989). 087983501X, Keats Publishing. 087983501X

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Dietary recommendations

The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iron in 2001. The current EAR for iron for women ages 1418 is 7.9 mg/day, 8.1 mg/day for ages 1950 and 5.0 mg/day thereafter (postmenopause). For men, the EAR is 6.0 mg/day for ages 19 and up. The RDA is 15.0 mg/day for women ages 1518, 18.0 mg/day for ages 1950 and 8.0 mg/day thereafter. For men, 8.0 mg/day for ages 19 and up. RDAs are higher than EARs so as to identify amounts that will cover people with higher-than-average requirements. RDA for pregnancy is 27 mg/day and, for lactation, 9 mg/day. For children ages 13 years 7 mg/day, 10 mg/day for ages 4–8 and 8 mg/day for ages 913. As for safety, the IOM also sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of iron, the UL is set at 45 mg/day. Collectively the EARs, RDAs and ULs are referred to as Dietary Reference Intakes.

(2025). 9780309072793, National Academy Press. ISBN 9780309072793

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For women the PRI is 13 mg/day ages 1517 years, 16 mg/day for women ages 18 and up who are premenopausal and 11 mg/day postmenopausal. For pregnancy and lactation, 16 mg/day. For men the PRI is 11 mg/day ages 15 and older. For children ages 1 to 14, the PRI increases from 7 to 11 mg/day. The PRIs are higher than the U.S. RDAs, with the exception of pregnancy. The EFSA reviewed the same safety question did not establish a UL.

Infants may require iron supplements if they are bottle-fed cow's milk. Frequent Blood donation are at risk of low iron levels and are often advised to supplement their iron intake.

For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For iron labeling purposes, 100% of the Daily Value was 18 mg, and remained unchanged at 18 mg. A table of the old and new adult daily values is provided at Reference Daily Intake.

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Deficiency

Iron deficiency is the most common nutritional deficiency in the world.

(2025). 9789400774995, Springer. ISBN 9789400774995

(2025). 9789400755604, Springer. ISBN 9789400755604

electronic-book When loss of iron is not adequately compensated by adequate dietary iron intake, a state of latent iron deficiency occurs, which over time leads to iron-deficiency anemia if left untreated, which is characterised by an insufficient number of red blood cells and an insufficient amount of hemoglobin. Children, pre-menopausal women (women of child-bearing age), and people with poor diet are most susceptible to the disease. Most cases of iron-deficiency anemia are mild, but if not treated can cause problems like fast or irregular heartbeat, complications during pregnancy, and delayed growth in infants and children.

The brain is resistant to acute iron deficiency due to the slow transport of iron through the blood brain barrier. Acute fluctuations in iron status (marked by serum ferritin levels) do not reflect brain iron status, but prolonged nutritional iron deficiency is suspected to reduce brain iron concentrations over time. In the brain, iron plays a role in oxygen transport, myelin synthesis, mitochondrial respiration, and as a cofactor for neurotransmitter synthesis and metabolism. Animal models of nutritional iron deficiency report biomolecular changes resembling those seen in Parkinson's and Huntington's disease. However, age-related accumulation of iron in the brain has also been linked to the development of Parkinson's.

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Excess

Iron uptake is tightly regulated by the human body, which has no regulated physiological means of excreting iron. Only small amounts of iron are lost daily due to mucosal and skin epithelial cell sloughing, so control of iron levels is primarily accomplished by regulating uptake.

(1999). 9780721673356, Saunders. ISBN 9780721673356

Regulation of iron uptake is impaired in some people as a result of a Genetic disorder that maps to the HLA-H gene region on chromosome 6 and leads to abnormally low levels of hepcidin, a key regulator of the entry of iron into the circulatory system in mammals. In these people, excessive iron intake can result in iron overload disorders, known medically as hemochromatosis. Many people have an undiagnosed genetic susceptibility to iron overload, and are not aware of a family history of the problem. For this reason, people should not take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Hemochromatosis is estimated to be the cause of 0.3–0.8% of all metabolic diseases of Caucasians.

Overdoses of ingested iron can cause excessive levels of free iron in the blood. High blood levels of free ferrous iron react with to produce highly reactive that can damage DNA, proteins, lipids, and other cellular components. Iron toxicity occurs when the cell contains free iron, which generally occurs when iron levels exceed the availability of transferrin to bind the iron. Damage to the cells of the gastrointestinal tract can also prevent them from regulating iron absorption, leading to further increases in blood levels. Iron typically damages cells in the heart, liver and elsewhere, causing adverse effects that include coma, metabolic acidosis, shock, liver failure, coagulopathy, long-term organ damage, and even death. Humans experience iron toxicity when the iron exceeds 20 milligrams for every kilogram of body mass; 60 milligrams per kilogram is considered a lethal dose. Overconsumption of iron, often the result of children eating large quantities of ferrous sulfate tablets intended for adult consumption, is one of the most common toxicological causes of death in children under six. The Dietary Reference Intake (DRI) sets the Tolerable Upper Intake Level (UL) for adults at 45 mg/day. For children under fourteen years old the UL is 40 mg/day.

The medical management of iron toxicity is complicated, and can include use of a specific chelation agent called deferoxamine to bind and expel excess iron from the body.

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ADHD

Some research has suggested that low Thalamus iron levels may play a role in the pathophysiology of ADHD. Some researchers have found that iron supplementation can be effective especially in the inattentive subtype of the disorder.

Some researchers in the 2000s suggested a link between low levels of iron in the blood and ADHD. A 2012 study found no such correlation.

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Cancer

The role of iron in cancer defense can be described as a "double-edged sword" because of its pervasive presence in non-pathological processes. People having chemotherapy may develop iron deficiency and anemia, for which intravenous iron therapy is used to restore iron levels. Iron overload, which may occur from high consumption of red meat, may initiate tumor growth and increase susceptibility to cancer onset, particularly for colorectal cancer.

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Marine systems

Iron plays an essential role in marine systems and can act as a limiting nutrient for planktonic activity. Because of this, too much of a decrease in iron may lead to a decrease in growth rates in phytoplanktonic organisms such as diatoms. Iron can also be oxidized by marine microbes under conditions that are high in iron and low in oxygen.

Iron can enter marine systems through adjoining rivers and directly from the atmosphere. Once iron enters the ocean, it can be distributed throughout the water column through ocean mixing and through recycling on the cellular level. In the arctic, sea ice plays a major role in the store and distribution of iron in the ocean, depleting oceanic iron as it freezes in the winter and releasing it back into the water when thawing occurs in the summer. The iron cycle can fluctuate the forms of iron from aqueous to particle forms altering the availability of iron to primary producers. Increased light and warmth increases the amount of iron that is in forms that are usable by primary producers.

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See also

• Economically important iron deposits include:
• Carajás Mine in the state of Pará, Brazil, is thought to be the largest iron deposit in the world.
• El Mutún in Bolivia, where 10% of the world's accessible iron ore is located.
• Hamersley Basin is the largest iron ore deposit in Australia.
• Kiirunavaara in Sweden, where one of the world's largest deposits of iron ore is located
• The Mesabi Range is the chief iron ore mining district in the United States.
• Iron and steel industry
• Iron cycle
• Iron nanoparticle
• Iron–platinum nanoparticle
• Iron fertilization – proposed fertilization of oceans to stimulate phytoplankton growth
• Iron-oxidizing bacteria
• List of countries by iron production
• Pelletizing – process of creation of iron ore pellets
• Rustproof iron
• Steel

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Bibliography

• (1968). 9780766138728, Journal of Chemical Education. . ISBN 9780766138728

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Further reading

• H.R. Schubert, History of the British Iron and Steel Industry ... to 1775 AD (Routledge, London, 1957)
• R.F. Tylecote, History of Metallurgy (Institute of Materials, London 1992).
• R.F. Tylecote, "Iron in the Industrial Revolution" in J. Day and R.F. Tylecote, The Industrial Revolution in Metals (Institute of Materials 1991), 200–60.

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External links

• It's Elemental – Iron
• Iron at The Periodic Table of Videos (University of Nottingham)
• Metallurgy for the non-Metallurgist
• Iron by J. B. Calvert

Categories: Building Materials, Chemical Elements With Body-centered Cubic Structure, Chemical Elements, Cubic Minerals, Dietary Minerals, Ferromagnetic Materials, Minerals In Space Group 225, Minerals In Space Group 229, Native Element Minerals, Pyrotechnic Fuels, Transition Metals