Question

What is the definition of rusting?

Answer 1

Hint : Rusting is an example of corrosion, also called iron rust. The red or orange coating that forms on the surface of iron when exposed to air and moisture, consisting chiefly of ferric hydroxide and ferric oxide formed by oxidation.

Complete Step By Step Answer:
Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron $(III)$ oxide. Due to the presence of water in the equation, rust is more likely to occur on metal that has not been treated or protected against moisture. Salt also helps to speed up the rusting process. This reaction is not instantaneous, it generally proceeds over a considerably large time frame. The oxygen atom bonds with iron atoms, resulting in the formation of iron oxides. This weakens the bonds between iron atoms and in the object/structure.
The reaction of the rusting of iron involves an increase in the oxidation state of iron, accompanied by a loss of electrons. Rust is mostly made up of two different oxides of iron that vary in the oxidation state of the iron atom. These oxides are:
$\bullet$ Iron $(II)$ oxide or ferrous oxide. The oxidation state of iron in this compound is $FeO(OH) \rightleftharpoons F{e_2}{O_3} + {H_2}O$ and its chemical formula is $FeO$ .
$\bullet$ Iron $(III)$ oxide or ferric oxide, where the iron atom exhibits an oxidation state of $+ 3$ . The chemical formula of this is $F{e_2}{O_3}$ .
The key reaction is the reduction of oxygen because it forms hydroxide ions, this process is strongly affected by the presence of acid.
${O_2} + 4{e^ - } + 2{H_2}O \to 4O{H^ - }$
Providing the electrons for the above reaction is the oxidation of iron that may be described as follow
$Fe \to F{e^{2 + }} + 2{e^ - }$
The following redox reaction also occur in the presence of water and is crucial to the formation of rust:
$4F{e^{2 + }} + {O_2} \to 4F{e^{3 + }} + 2{O^{2 - }}$
Now, the following acid-base reactions occur between the iron cations and the water molecule.
$F{e^{2 + }} + 2{H_2}O \rightleftharpoons Fe{(OH)_2} + 2{H^ + }$
$F{e^{3 + }} + 3{H_2}O \rightleftharpoons Fe{(OH)_3} + 3{H^ + }$
The resulting hydroxides of iron now undergo dehydration to yield the iron oxides that constitute rust. This process involves many chemical reactions, some of which are listed below.
$Fe{(OH)_2} \rightleftharpoons FeO + {H_2}O$
$Fe{(OH)_3} \rightleftharpoons FeO(OH) + {H_2}O$
$2FeO(OH) \rightleftharpoons F{e_2}{O_3} + {H_2}O$
Preventing rusting:
There are several ways to prevent iron and steel rusting. Some work because they stop oxygen or water reaching the surface of the metal.
$\bullet$ Oiling- for example, bicycle chains
$\bullet$ Greasing- for example, nut and bolts
$\bullet$ Painting- for example, car body panels
$\bullet$ Coating with a thin layer of plastic.
$\bullet$ Galvanizing- it is a method of rust prevention. The iron and steel object is coated in a thin layer of zinc. This stops oxygen and water reaching the metal underneath- but the zinc also acts as a sacrificial metal. Zinc is more reactive than iron, so it oxidizes rather than the iron object.

Note :
Magnesium and zinc are often used as sacrificial metal. They are more reactive than iron and lose their electrons in preference of iron. This prevents iron from losing its electron and becoming oxidized.
Presence of acid also affects the rusting of iron as if the pH of the environment surrounding the metal is low, the rusting of iron speeds up when it is exposed to acid rains. Higher the pH inhibits the corrosion of iron.