As far as "iron oxide" goes, there are 3 types of oxidation, or rust, that
depend on how many iron and oxygen atoms are involved.  The simplistic way to
think about it is red, brown and black rust.  Red is the surface stuff,
easily removed.  Brown is the nasty, deep stuff that requires physical
removal or chemical treatment.  Black rust is fairly inert -- it isn't going
anywhere.  An interesting side note:  Brown rust gains volume as it rusts
(like double or triple), that's why it bubbles.  Black rust is about the same
size as an equal amount of iron, so no bubbling.

All you chemical engineer types, feel free to correct this, but I think it is
conceptually correct.

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL