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### Theory:

Corrosion:

When you slice an apple and leave it for a while, the outside layer of the apple turns brown. Furthermore, iron bolts and nuts in metallic structures corrode in a similar fashion.
If you have any clue why this is happening?
It is due to a chemical reaction known as oxidation.

Corrosion on car

Oxidation:
The chemical reaction includes the addition of oxygen or removal of hydrogen or loss of electrons is called Oxidation.
Example:
$$2Mg$$ + $$O_2$$ → $$2MgO$$     (addition of oxygen)
$$CaH_2$$ → $$Ca$$ + $$H_2$$        (removal of hydrogen)
$$Fe^2{^+}$$ + → $$Fe^3{^+}$$ + $$e^−$$    (loss of electron)
Reduction:
The chemical reaction includes the addition of hydrogen or removal of oxygen or gain of electrons is called Reductions.
Example:
$$2Na$$ + $$H_2$$ → $$2NaH$$            (addition of hydrogen)
$$CuO$$ + $$H_2$$ → $$Cu$$ + $$H_2O$$   (removal of oxygen)
$$Fe^3{^+}$$ + $$e^−$$ → $$Fe^2{^+}$$          (gain of electron)
Redox reactions:
The oxidation and reduction happen in a similar reaction (simultaneously).
If one reactant gets oxidised, the other gets reduced. Such reactions are named as Oxidation-reduction reactions or Redox reactions.
Example:
$$2PbO$$ + $$C$$ → $$2Pb$$ + $$CO_2$$              (reduction)
$$Zn$$ + $$CuSO_4$$ → $$Cu$$ + $$ZnSO_4$$   (oxidation)