Corrosion of Iron
Unless noted otherwise, the following discussion applies to deaerated water at room temperature
and approximately neutral pH. The affects of temperature, oxygen, and pH are discussed later
in this chapter.
The oxidation and reduction half-reactions in the corrosion of iron are as follows.
The overall reaction is the sum of these half-reactions.
The Fe ions readily combine with OH ions at the metal surface, first forming Fe(OH) , which
decomposes to FeO.
Ferrous oxide (FeO) then forms a layer on the surface of the metal. Below about 1000?F,
however, FeO is unstable and undergoes further oxidation.
Atomic hydrogen then reacts to form molecular hydrogen, as described previously, and a layer
of ferric oxide (Fe O ) builds up on the FeO layer. Between these two layers is another layer
that has the apparent composition Fe O . It is believed that Fe O is a distinct crystalline state
composed of O , Fe , and Fe in proportions so that the apparent composition is Fe O . These
three layers are illustrated in Figure 5.
Once the oxide film begins to form, the metal surface is no longer in direct contact with the
aqueous environment. For further corrosion to occur, the reactants must diffuse through the
oxide barrier. It is believed that the oxidation step, Equation (2-3), occurs at the metal-oxide
interface. The Fe ions and electrons then diffuse through the oxide layer toward the
oxide-water interface. Eventually, Fe 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 , OH , and H O into the layer.