ELECTROCHEMICAL THEORY OF RUSTING

ELECTROCHEMICAL THEORY OF RUSTING
According to this theory, the impure surface behaves like a small electrochemical cell in presence of water containing dissolved oxygen or carbon dioxide. Moisture having dissolved oxygen or carbon dioxide in it constitutes electrolytic solution.
At anode , oxidation of Fe atoms takes place. Thus, Fe atoms pass into solutions as Fe 2+ ions leaving behind electrons in the metal which are pushedinto the cathode area.
Fe → Fe 2+ + 2 e − : E Θ OX = + 0.44 V
At cathode the electrons are picked up by H + ions which are produced either from H 2 O or from H 2 CO 3 ( formed bydissolution of CO 2 in moisture).
CO 2 + H 2 O → H + + HCO 3 −
2 H + (aq) + 2 e − → 2 H
The H atoms, thus formed reduce the dissolved oxygen as :
2 H + ½ O 2 → H 2 O
The net reduction process at the cathodic area is :
2 H + + ½ O 2 + 2 e − → H 2 O : E Θ red = 1.23 V
The net reaction of of corrosion cell can be put as :
Fe + 2 H + + ½ O 2 → Fe 2+ + H 2 O : E Θ cell =1.67 V
The ferrous ions so formed move through water and come at the surface of iron object where these are further oxidised to ferric state by atmospheric oxygen and constitute rust which is hydrated iron(III) oxide.
2 Fe 2+ + ½ O 2 + 2 H 2 O → Fe 2 O 3 + 4 H +
Fe 2 O 3 + x H 2 O → Fe 2 O 3 . x H 2 O
( rust )
2 Fe 2+ + ½ O 2 + (2 + x ) H 2 O → Fe 2 O 3 . x H 2 O + 4 H +