The electrochemical oxidation of iron results in the formation
of ferrous ions as the initial product.
The electrochemical corrosion of metals has been detailed in
a number of sources. In the case of iron, it has been shown that
in any electrochemical cell where iron establishes a metallic
couple in salt water with a more noble metal, such as copper or
silver, or even with another piece of iron or a different part
of the same iron object, the anodic and cathodic reactions are
the same (see Potter 1956:236-237; Evans 1963:28).
At the surface of the more noble metal (the
cathode), the following reaction occurs:
2H2O + 2e >> H2
+ 2(OH)-
The hydroxides combine with the sodium ion
in the solution to form sodium hydroxide as the cathodic product:
Na+ + OH-
>> NaOH
At the anode, the reaction is the production
of ferrous ions:
Fe+
- 2e >> F+2
which, in turn combine with chloride in the
salt water to form ferrous chloride as the anodic product:
Fe+2 + 2CI-
>> FeCl2
On exposure to air, or solutions containing
dissolved oxygen, the ferrous chloride oxidizes to ferric chloride
and ferric oxide. Ferrous chloride and ferric chloride and are
freely soluble and may yield ferrous hydroxide when they combine
with the cathodic product sodium hydroxide:
FeCl2 + 2NaOH >> Fe(OH)2 + 2NaCl
In solutions containing dissolved oxygen,
a secondary reaction oxidizes the ferrous hydroxide to a ferric
state. In the presence of hydroxyl ions in a neutral or slightly
alkaline solution, this hydrated ferric hydroxide (any form of
ferric oxide with internal water, i.e., common rust) is precipitated
on or around the electrodes of the cell. The sequence of reaction
at an iron anode in the presence of oxygen as stated by Potter
(1956:236) is:
| ferrous ion |
Fe - 2e >> Fe+2 |
| ferrous hydroxide |
Fe+2+
2OH-
>> Fe(OH)2 |
| hydrated ferric hydroxide
(red-brown rust) |
4Fe(OH)2
+ O2
>> 2H2O
+ 2Fe2O30H2O |
The primary anodic reaction of electrochemical
corrosion of iron is the production of ferrous ions. The secondary
stage, the oxidation of the ferrous ion compounds to a ferric
state, is modified in anaerobic environments. Intermediate oxidation
products of ferrous hydroxide, such as hydrated magnetite and
black magnetite, are formed (Potter 1956:236-237; Evans 1963:28-29,
75):
6Fe(OH)2 + O2
>> 4H2O
+ 2Fe3O40H2O
(green hydrated magnetite)
Fe3O4 0 H2O >> H2O
+ Fe3O4
(black magnetite)
Depending on the environment, the corrosion
products can take on a variety states of division and hydration,
as well as a variety of physical forms. It is common to find corroded
iron from marine sites with an outer layer of hydrated ferric
hydroxide (common rust), which has restricted the supply of oxygen
to the ferrous hydroxide briefly formed at the surface of the
metal. Laminated corrosion layers consisting of an inner layer
of black magnetite, a thin layer of hydrated magnetite, and an
outer layer of hydrated ferric hydroxide are formed:
Fe3O4/2Fe3O4 0 H2O or 2Fe2O3 0 H2O
It is easy to see how two different areas
of the same metal object can become anodic and cathodic to form
an electrolytic cell. Electrons flow from the anodic area to the
cathodic area causing the metal to corrode by forming soluble
positive ions at the anode. Millions of these cells over the surface
of the metal result in massive oxidation, which continues until
an equilibrium state is reached. The corrosion process is halted
at the cells when they come into equilibrium but may continue
at alternate anodic and cathodic positions on the object until
the bulk of the metal is oxidized.
