Corrosion of Iron

Unless noted otherwise, the following discussion applies to deaerated water at room temperature and approximately neutral pH.

The effects of temperature on Corrosion, the effect of oxygen on corrosion , and effect of pH on corrosion are discussed in other pages.

The oxidation and reduction half-reactions in the corrosion of iron are as follows.

                              Fe --->  Fe²⁺ + 2e^-   (1) oxidation

                        H₃O+   +  e^-  ---> H  + H₂O   (2) reduction

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 oxidation.

                      2FeO + H₂O --->  Fe₂O₃  +  2H

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 composition Fe₃O₄.

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 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⁺, OH^- 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 source of this page is :"Department of Energy Fundamentals Handbook" CHEMISTRY Module 2 Corrosion DOE-HDBK-1015/1-93