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Iron Corrosion (Pilling Bedworth Ratio)

Iron Corrosion (Pilling Bedworth Ratio)

Iron Corrosion (Pilling Bedworth Ratio)

(OP)
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

RE: Iron Corrosion (Pilling Bedworth Ratio)

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 Fe1-xO or FeO1+y.

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

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