Rusting Revisited

Ever wonder why an old car can develop rust spots while the iron metal gets eaten away and gets holes in it somewhere completely different?  Rusting is a process which has parallels to what happens in a battery. The oxidation (anode reaction) may occur at a separate location from the reduction (cathode reaction).

  In the illustration,

Cathode Anode 1/2O2 + H2O + 2e- --> 2 OH- Fe(s) --> Fe2+ + 2e- Fe2+ + 2 OH- --> Fe(OH)2(s) Fe(OH)2 + O2 + H2O --> Fe2O3.H2O   (rust)

there is a piece of iron (gray) covered with green paint.   The paint has a scratch in it.  Water and oxygen from the air gain access to the iron through the scratch.   The iron surface under the paint becomes moist. This moisture can provide a pathway for ions to migrate from one place to another.

The anode can be anywhere on the moist iron surface.   It is typically at a place where there is some microscopic fault in the iron crystal structure.   The anode reaction is the oxidation of atoms of Fe.   Fe²⁺ ions are produced.

Electrons travel through the iron to another location of the damp surface on the iron that serves as a cathode.   At the same time, Fe²⁺ ions migrate to the same location through the water wetting the surface of the iron.

At the cathode, dissolved molecules of oxygen, O₂, are reduced in the presence of H₂O to form hydroxide anions, OH^-.    There are then two subsequent chemical events:

1. The Fe²⁺ ions produced at the anode and the OH^- ions produced at the cathode form ferrous hydroxide, Fe(OH)_2..  Fe(OH)_2. is an insoluble salt that precipitates at the cathode.
2. The solid Fe(OH)₂ is then further oxidized by dissolved oxygen to hydrated ferric oxide, commonly known as rust.   (Fe²⁺ is converted to Fe³⁺ ion and O₂ molecules are reduced to O^2- ions by direct contact.)

Why is rusting such a problem in the Caribbean?   There are two reasons.   First, the air is humid and that means that most iron surfaces stay damp.   Water is both a medium through which Fe²⁺ ions migrate (electrolytic conduction) and a prime reactant in the last step where ferrous hydroxide is oxidized to rust.   Second, most surface here will have a deposit of air-borne salt crystals that then dissolve in the damp surface (and in fact helps ensure that the surface stays damp).   Sodium chloride is an ionic substance and the sodium and chloride ions add to the total amount of charge that can be transferred, enabling the process to occur more rapidly.

One of the special projects in this section deals with a clever electrical system that combats the rusting process. It is called "cathodic protection".