Corrosion of Iron
The oxidation and reduction half-reactions in the corrosion of iron are as
Fe ---> Fe2+ + 2e- (1) oxidation
H3O+ + e- ---> H + H2O
The overall reaction is the sum of these half-reactions.
Fe + 2H3O+ --->
Fe2+ + 2H + 2H2O
The Fe+2 ions readily combine with OH- ions at the metal
surface, first forming Fe(OH)2 , which
decomposes to FeO.
Fe2+ + 2OH-
---> Fe(OH)2 + H2O
Ferrous oxide (FeO) then forms a layer on the surface of the metal. Below about
1000°F, however, FeO is unstable and undergoes further
2FeO + H2O ---> Fe2O3
Atomic hydrogen then reacts to form molecular hydrogen, as described previously,
and a layer of ferric oxide (Fe2O3)
builds up on the Fe3O4 layer. Between these two layers is another layer that has the apparent
It is believed that Fe3O4
is a distinct crystalline state composed of O2-,
Fe2+, and Fe3+
in proportions so that the apparent composition is Fe3O4.
These thhree layers are illustrated in figure below. Once the oxide film begins to form, the metal
surface is no longer in direct contact with the aqueous environment. For further
iron corrosion to occur, the reactants must diffuse through the oxide barrier.
It is believed that the oxidation step, Equation (1), occurs at the
metal-oxide interface. The Fe+2 ions and electrons then diffuse through the oxide layer toward the oxide-water
interface. Eventually, Fe+2 ions encounter OH- ions and form FeO. The electrons
participate in the reduction reaction with hydronium ions. These latter
reactions are believed to take place predominately at the oxide-water interface,
but some reaction may occur within the oxide layer by the diffusion of H+,
and H2O into the layer.
Simplified Schematic Diagram of Oxide Corrosion Film on
the Surface of a Metal
Regardless of the exact diffusion mechanism, the oxide
layer represents a barrier to continued corrosion and tends to slow the corrosion rate.
The exact effect of this layer on the corrosion rate depends on
the uniformity and tenacity of the film.
If the film is loosely attached,
develops defects, or is removed, the metal surface is again exposed to the
environment and corrosion occurs more readily.
The effects of temperature on Iron Corrosion,
the effect of oxygen on corrosion , and effect of pH on corrosion are discussed in other pages.
The source of this page is :"Department of Energy
Fundamentals Handbook" CHEMISTRY Module 2 Corrosion DOE-HDBK-1015/1-93