Question

Write the chemical equation for all the steps involved in the rusting of iron. Explain why does alkaline medium inhibits rusting of iron?

Accepted Answer

The formation of rust can be explained by the electrochemical theory of corrosion as follows :-

(i) Iron surface gets covered with the deposition of moisture $(H_{2} O)$ and carbon dioxide $({CO}_{2})$ from the atmosphere to form carbonic acid $(H_{2} {CO}_{3})$ , which further acts as an electrolyte and gets dissociated as :

$H_{2} O + {CO}_{2} \to H_{2} {CO}_{3} . . . (1) H_{2} {CO}_{3} ⇌ 2 H^{+} + {CO}_{3}^{2 -} . . . (2)$

(ii) Iron reacts with dissolved oxygen in presence of $H^{+}$ ions to undergo oxidation and form ferric ions at anode : $Fe \to {Fe}^{2 +} + 2 e^{-} (E °_{ox} = 0.44 V) . . . (3)$

(iii) Reduction of the dissolved oxygen in the presence of $H^{+}$ ions at the cathodic surface occurs as :

$O_{2} + 4 H^{+} + 4 e^{-} \to 2 H_{2} O (E °_{red} = 1.23 V) . . . (4)$

Adding reaction $(3)$ and $(4)$ : $2 Fe (s) + O_{2} (g) + 4 H^{+} (aq) \to 2 {Fe}^{2 +} (aq) + 2 H_{2} O (l)$

(iv) Ferrous ions formed react with dissolved oxygen with further production of $H^{+}$ ions as :

$4 {Fe}^{2 +} + O_{2} + 4 H_{2} O \to 2 {Fe}_{2} O_{3} + 8 H^{+} (aq)$

(v) Ferric oxide then undergoes hydration to form hydrated ferric oxide or rust as follows :-

${Fe}_{2} O_{3} + {xH}_{2} O \to {Fe}_{2} O_{3} . {xH}_{2} O$

In equation $(4)$ , $H^{+}$ ions are involved in the reduction of $O_{2}$ . As the concentration of $H^{+}$ ions is lowered, $pH$ increases and hence, the reduction of $O_{2}$ becomes less favourable, thereby resulting in the inhibition of the iron rusting process in an alkaline medium $(9 - 10 pH)$ .

Group-A	Group-B
(i) Corrosion	(a) Lanthanides
(ii) Gold sol	(b) Used in treatment of cancer
(iii) Radon	(c) Purple of cassius
(iv) f-block elements	(d) Barbituric acid
(v) Tranquilizers	(e) Hydrated ferric oxide.

Write the chemical equation for all the steps involved in the rusting of iron. Explain why does alkaline medium inhibits rusting of iron?

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