Iron Corrosion (Pilling Bedworth Ratio) 1

Hi Everyone.

I'm wondering why the oxide layers such as FeO and Fe2O3 etc dont protect iron by forming a pacifying layer once the oxide layer is thick enough to stop ion diffusion. I'm confused because the pilling bedworth ratio for iron is between 1 and 2 (1.77) which is described as a 'protective' layer.

Thanks in advance,

bmgri

 

[kenvlach]

The problem for high temperature oxidation is because iron has 2 common valences, +2 and +3. If only Fe⁺³, then Fe₂O₃ could be stable in contact with the metal, and maybe very protective, as are Al₂O₃ and Cr₂O₃.
However, during high T oxidation, the Fe₂O₃ surface layer is very thin. It keeps getting reduced by the mixed valence Fe₃O₄ which is continually getting reduced by FeO. FeO isn't protective because it has high ionic diffusivity stemming from its non-stoichiometric composition range, written as either Fe_1-xO or FeO_1+y.

At ambient temperatures, the FeO phase isn't stable, but H₂O is usually present, so form non-protective rust FeOOH.