Metal Profile: Iron
Iron's use by humans dates back about 5,000 years. It is the second most abundant metal element in the Earth's crust and is primarily used to produce steel, one of the most important structural materials in the world.
Before getting too deep into the history and modern uses for iron, let's review the basics:
- Atomic symbol: Fe
- Atomic number: 26
- Element category: Transition metal
- Density: 7.874g/cm3
- Melting point: 2800°F (1538°C)
- Boiling point: 5182°F (2862°C)
- Moh's hardness: 4
Pure iron is a silver-colored metal that conducts heat and electricity well. Iron is too reactive to exist alone, so it only occurs naturally in the Earth's crust as iron ores, such as hematite, magnetite, and siderite.
One of iron's identifying characteristics is that it is strongly magnetic. Exposed to a strong magnetic field, any piece of iron can be magnetized. Scientists believe that the Earth's core is made up of about 90% iron. The magnetic force produced by this iron is what creates the magnetic north and south poles.
Iron was likely originally discovered and extracted as a result of wood burning on top of iron-containing ores. The carbon within the wood would have reacted with the oxygen in the ore, leaving behind a soft, malleable iron metal. Iron smelting and the use of iron to make tools and weapons began in Mesopotamia (present-day Iraq) between 2700 and 3000 BCE. Over the following 2,000 years, iron smelting knowledge spread eastward into Europe and Africa during a period known as the Iron Age.
From the 17th century, until an efficient method to produce steel was discovered in the mid-19th century, iron was increasingly used as a structural material to make ships, bridges, and buildings. The Eiffel Tower, constructed in 1889, was made using over 7 million kilograms of wrought iron.
Iron's most troublesome characteristic is its tendency to form rust. Rust (or ferric oxide) is a brown, crumbly compound that is produced when the iron is exposed to oxygen. The oxygen gas that is contained in water speeds up the process of corrosion. The rate of rust—how quickly iron turns into ferric oxide—is determined by the oxygen content of the water and the surface area of the iron. Salt water contains more oxygen than fresh water, which is why salt water rusts iron faster than fresh water.
Rust can be prevented by coating iron with other metals that are more chemically attractive to oxygen, such as zinc (the process of coating iron with zinc is referred to as "galvanizing"). However, the most effective method of protecting against rust is the use of steel.
Steel is an alloy of iron and various other metals, which are used to enhance the properties (strength, resistance to corrosion, tolerance of heat, etc.) of iron. Changing the type and amount of the elements alloyed with iron can produce different types of steel.
The most common steels are:
- Carbon steels, which contain between 0.5% and 1.5% carbon: This is the most common type of steel, used for auto bodies, ship hulls, knives, machinery, and all types of structural supports.
- Low alloy steels, which contain 1-5% other metals (often nickel or tungsten): Nickel steel can withstand high levels of tension and is, thus, often used in the construction of bridges and for making bicycle chains. Tungsten steels keep their shape and strength in high-temperature environments, and they're used in impact, rotary applications, such as drill bits.
- High alloy steels, which contain 12-18% other metals: This kind of steel is only used in specialty applications due to its high cost. One example of high alloy steel is stainless steel, which often contains chromium and nickel, but it can be alloyed with various other metals as well. Stainless steel is very strong and highly resistant to corrosion.
Most iron is produced from ores found near the Earth's surface. Modern extraction techniques use blast furnaces, which are characterized by their tall stacks (chimney-like structures). The iron is poured into the stacks along with coke (carbon-rich coal) and limestone (calcium carbonate). Nowadays, the iron ore normally goes through a process of sintering before entering the stack. The sintering process forms pieces of ore that are 10-25mm, and these pieces are then mixed with coke and limestone.
The sintered ore, coke, and limestone are then poured into the stack where it burns at 1,800 degrees Celsius. Coke burns as a source of heat and, along with oxygen that is shot into the furnace, helps to form the reducing gas carbon monoxide. The limestone mixes with impurities in the iron to form slag. Slag is lighter than molten iron ore, so it rises to the surface and can easily be removed. The hot iron is then poured into molds to produce pig iron or directly prepared for steel production.
Pig iron still contains between 3.5% and 4.5% carbon, along with other impurities, and it's brittle and difficult to work with. Various processes are used to lower the phosphorus and sulfur impurities in pig iron and produce cast iron. Wrought iron, which contains less than 0.25% carbon, is tough, malleable and easily welded, but it's much more laborious and costly to produce than low carbon steel.
In 2010, global iron ore production was around 2.4 billion tonnes. China, the largest producer, accounted for about 37.5% of all production, while other major producing countries include Australia, Brazil, India, and Russia. The U.S. Geological Survey estimates that 95% of all metal tonnage produced in the world is either iron or steel.
Iron was once the primary structural material, but it has since been replaced by steel in most applications. Nevertheless, cast iron is still used in pipes and automotive parts like cylinder heads, cylinder blocks, and gearbox cases. Wrought iron is still used to produce home decor items, such as wine racks, candle holders, and curtain rods.
Street, Arthur & Alexander, W. O. 1944. "Metals in the Service of Man" 11th Edition (1998).
International Iron Metallics Association. "Pig Iron Overview." Nov. 12, 2019
U.S. Geological Survey. "Iron and Steel Statistics and Information." Nov. 12, 2019.