Question

How do you explain the electrochemical theory of corrosion?

Answer 1

Hint: Corrosion is the process of the deterioration of the metals due to some external factors and hence, the life of the metal objects gets reduced. It indicates the refined metal is converted into oxides, hydroxides of the metal. The atmospheric moisture, oxygen, temperature, pH of the medium, etc affect the corrosion of the metal.

Complete step by step answer:
To explain the electrochemical theory of corrosion uses the example of the correction of the iron.
In the first step of the corrosion iron in presence of the water gets oxidized and the reaction is as follows:
${\text{Fe}}\left( s \right)\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}}\,\left( l \right) \to \,\,{\text{FeO}}\left( s \right)\,{\text{ + }}\,\,2{{\text{H}}^ + }\left( {aq} \right) + \,{\text{2}}{{\text{e}}^ - }$ …………………..(i)
${\text{2Fe}}\left( s \right)\,{\text{ + }}\,{\text{3}}\,{{\text{H}}_{\text{2}}}{\text{O}}\,\left( l \right) \to \,\,{\text{F}}{{\text{e}}_2}{{\text{O}}_3}\left( s \right)\,{\text{ + }}\,\,6{{\text{H}}^ + }\left( {aq} \right) + \,6{{\text{e}}^ - }$ …………………...(ii)
Here, we can see that the iron metal reacts with the water around it or is present in the moisture and leads to form two iron oxides.
The oxygen in the air gets oxidized in water as follows:
$\dfrac{1}{2}{{\text{O}}_{\text{2}}}\left( g \right)\,{\text{ + }}\,\,2{{\text{H}}^ + }\left( {aq} \right) + \,{\text{2}}{{\text{e}}^ - } \to {{\text{H}}_{\text{2}}}{\text{O}}\left( l \right)$ …………………...(iii)
Multiply equation (iii) by 4.
$2{{\text{O}}_{\text{2}}}\left( g \right)\,{\text{ + }}\,\,8{{\text{H}}^ + }\left( {aq} \right) + \,8{{\text{e}}^ - } \to 4{{\text{H}}_{\text{2}}}{\text{O}}\left( l \right)$ ………………….(iv)

The combining result of all these is obtained by adding all (i), (ii), and (iv) as follows:
\[{\text{Fe}}\left( s \right)\, + {\text{2Fe}}\left( s \right)\, + 2{{\text{O}}_{\text{2}}}\left( g \right)\, \to \,\,{\text{FeO}}\left( s \right)\, + {\text{F}}{{\text{e}}_2}{{\text{O}}_3}\left( s \right)\]
This is the net chemical equation obtained.
Here, the compound formed \[{\text{F}}{{\text{e}}_2}{{\text{O}}_3}\] is called rust. And the whole process is known as rusting or corrosion of the iron metal.
In this way, the electrochemical theory explains the corrosion of metals.

Note: In the case of the corrosion process, the electron from the metal is removed by the electron acceptor species, and oxidized metal is formed.
- Here, oxygen, cations of the less active metals, acids act as acceptors and the water present around the metal acts as the iron transport medium.
- If all these conditions are favorable then corrosion of the metal takes place.
- As the corrosion leads to loss of the metal it is important to prevent the corrosion by using methods like galvanization, coating of metal by non-metal, by using corrosion inhibitors, etc.