GENERAL CORROSION   DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 12 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. (2-3) (2-4) The overall reaction is the sum of these half-reactions. (2-8) The Fe    ions readily combine with OH  ions at the metal surface, first forming Fe(OH) , which +2 - 2 decomposes to FeO. (2-9) 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. (2-10) 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 2 3 that has the apparent composition Fe O .  It is believed that Fe O  is a distinct crystalline state 3 4 3 4 composed of O  , Fe   , and Fe    in proportions so that the apparent composition is Fe O .  These -2 +2 +3 3 4 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 +2 oxide-water interface.  Eventually, Fe    ions encounter OH  ions and form FeO.  The electrons +2 - 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. + - 2