Question

Give a brief account of the mechanism of corrosion.

Explain the mechanism of rusting of iron.

Answer 1

1. One particular instance of the corrosion phenomenon is the rusting of iron. By using the example of iron rusting, it is simple to understand the mechanism of corrosion as it is described by electrochemical theory.
2. Iron rusts or corrodes when it comes into contact with an environment that contains carbon dioxide, water, and dissolved oxygen.
3. The electrochemical theory states that when water with dissolved carbon dioxide and oxygen is present, the chemically irregular portions of the iron surface act like tiny galvanic cells.
4. A portion of the surface serves as the cathode and another as the anode. As a result, electrons start moving from the anodic to the cathodic region.
5. In the anodic area, oxidation occurs, and iron atoms get oxidised to Fe²⁺ ions.
   In the anodic area:
   \[\ce{Fe_{(s)} -> Fe{^{2+}_{(aq)}} + 2e-}\]; \[\ce{E^{\circ}_{{Cu^{2+}/{Cu}}} = -0.44 V}\] 
6. The resulting electrons travel to the cathodic region through the metal. When H⁺ ions are present, electrons in the cathodic region decrease the oxygen. The dissociation of H₂CO₃, which results from the breakdown of CO₂ in water, produces H⁺ ions in the water film. Thus,
   In the water film:
   \[\ce{H2O_{(l)} + CO2_{(g)} -> H2CO3_{(aq)}}\]
   \[\ce{H2CO3_{(aq)} -> H{^{+}_{(aq)}} + HCO^{-}_{3}_{(aq)}}\]
   In the cathodic area:
   \[\ce{O2_{(g)} + 4H{^+_{(aq)}} + 4e- -> 2H2O_{(l)}}\]; E° = 1.23 V
7. Thus, the overall cell reaction that takes place in a miniature galvanic cell on the surface of iron can be obtained as follows.
   At cathode: \[\ce{Fe_{(s)} -> Fe{^{2+}_{(aq)}} + 2e-}\] [× 2]
   At cathode: \[\ce{O2_{(g)} + 4H{^+_{(aq)}} + 4e- -> 2H2O_{(l)}}\]
   Overall reaction in a miniature cell: \[\ce{2Fe_{(s)} + O2_{(g)} + 4H{^{+}_{(aq)}} -> 2Fe{^{2+}_{(aq)}} + 2H2O_{(l)}}\]; \[\ce{E^{\circ}_{cell}}\] = 1.67 V