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Iron and steel

Smelting is the extraction of iron $$(\ce{Fe})$$ from its ore using coke and limestone in a blast furnace.

Iron ore often exists as haematite $$(\ce{Fe2O3})$$. Coke (a fuel containing mainly carbon) provides carbon for reducing haematite.

When coke ($$\ce{C}$$) is heated in air, carbon monoxide $$(\ce{CO})$$ is produced.

$$$\ce{2C {(s)} + O2 {(g)} -> 2CO {(g)}}$$$

Carbon monoxide then reduces haematite to iron $$(\ce{Fe})$$.

$$$\ce{3CO {(g)} + Fe2O3 {(s)} -> 2Fe {(s)} + 3CO2 {(g)}}$$$

Limestone $$(\ce{CaCO3})$$ removes silicon dioxide $$(\ce{SiO2})$$ impurities present in haematite.

Limestone decomposes into calcium oxide $$(\ce{CaO})$$, which reacts with $$\ce{SiO2}$$ to form liquid calcium silicate $$(\ce{CaSiO3})$$.

$$$\ce{CaCO3 {(s)} -> CaO {(s)} + CO2 {(g)}}$$$ $$$\ce{CaO {(s)} + SiO2 {(s)} -> CaSiO3 {(l)}}$$$

$$\ce{CaSiO3}$$ is removed from the furnace as slag. Pure iron metal is the final product of smelting.

Steel is an alloy of iron ($$\ce{Fe}$$) and carbon ($$\ce{C}$$). Steel also contains small amounts of other elements (e.g. manganese, phosphorous and aluminium).

Steel is usually 0.2% to 2.1% carbon by weight. Properties of steel can be changed by varying the amount of carbon added.

Increasing the amount of carbon makes steel harder and more brittle.

When there is less carbon, the planes of iron cations can more easily slide over one another. With more carbon, the orderly arrangement of iron cations is disrupted, making the planes of iron less flexible.

Cast iron is an alloy of iron that has more than 2.1% carbon by weight. Cast iron is more brittle than steel.

Mild steel and stainless steel are two specialised forms of steel that are used widely in everyday life.

Mild steel contains less than 0.3% carbon. It is relatively weak but highly malleable because of its low amount of carbon. Car bodies and various types of machinery are often made of mild steel.

Mild steel is relatively inexpensive and is preferred when large quantities are needed (to help keep costs down).

Stainless steel contains the element chromium. It usually contains at least 10-11% chromium by mass. Stainless steel is used widely because it does not rust or corrode as easily as other types of steel.

Chemical plants, cutlery and surgical instruments often employ stainless steel.

Iron oxidises in the presence of oxygen to form rust. This reaction occurs much more quickly in the presence of water.

In water, iron loses electrons (oxidation) to form iron cations.

$$$\ce{2Fe {(s)} -> 2Fe^2+ {(aq)} + 4e^-}$$$

Oxygen is reduced by the electrons from iron. Oxygen thus forms hydroxide ions in water.

$$$\ce{O2 {(g)} + 4e^- + 2H2O {(l)} -> 4OH^- {(aq)}}$$$

The hydroxide ions combine with the iron ions to make iron hydroxide $$(\ce{Fe(OH)2})$$, which appears as rust on iron.

$$\ce{Fe(OH)2}$$ can react to form other compounds such as $$\ce{FeO}$$, which is also found in rust.

When iron is in a moist environment, a protective layer is usually applied to prevent iron from getting exposed to oxygen, therefore preventing rusting.

Paint, grease, plastic and zinc are used to coat iron to prevent rusting. The use of a zinc coat to prevent rusting is called galvanisation.

Sacrificial protection is a method of preventing rust.

This involves attaching a metal that is more reactive than iron. This more reactive metal corrodes first, thus protecting iron.

Zinc and magnesium are commonly used. These metals are more likely to be oxidised than iron.

When zinc or magnesium is oxidised, the electrons released reduce the iron ions $$(\ce{Fe^2+})$$ back to iron $$(\ce{Fe})$$. This prevents the formation of rust.

Sacrificial protection is usually employed to protect large iron objects.

Large blocks of magnesium are connected to underground pipes via a wire for rust prevention.

The layer of zinc in galvanised iron acts as a barrier against oxygen.

The zinc layer continues to protect the iron even if the layer is scratched. Zinc is more reactive than iron and will corrode first.

Silvery blocks of zinc seen on the hull of the ship prevent rusting.
Silvery blocks of zinc seen on the hull of the ship prevent rusting.