Mill Scale versus Rust

[Adams_Mitch]

I recently took a workshop on ring making during which we made rings out of steel, masonry nails. The nails were covered with a black oxide that the instructor call "mill scale". What is the difference (chemically and functionally) between the black, iron oxide that is called "mill scale" and regular, old iron oxide that is reddish brown and called "rust"?

Mitch Adams

[Steelybone]

 What is the difference (chemically and functionally) between the
black, iron oxide that is called "mill scale" and regular, old iron
oxide that is reddish brown and called "rust"?

My guess it that it's the valence state of the iron. Standard rust
is Fe2O3 (where iron is +3); another oxide is FeO (where iron is
+2), and I believe there's another that's Fe3O4, in which iron is
partly +2 and partly +3 ( a non-stoichiometric compound as I recall
from geochem class years ago). There are more oxides or iron that
are also hydrated - iron's one of those changelings that really gets
around, being different things every time.

Tas
http://www.earthlywealth.com/

[David_Popham1]

The Handbook of Chemistry and Physics records Fe(II) oxide as black
(wuestite) and Fe(III) oxide as red-brown (hematite) in support of of
Tas.

David

[Alden_Glenda_Blood]

    My guess it that it's the valence state of the iron.  Standard
rust is Fe2O3 (where iron is +3); another oxide is FeO (where iron
is +2), and I believe there's another that's Fe3O4, in which iron
is partly +2 and partly +3 ( a non-stoichiometric compound as I
recall from geochem class years ago).  There are more oxides or
iron that are also hydrated - iron's one of those changelings that
really gets around, being different things every time.

Mill scale is ferric ferrous oxide (Fe3O4 or FeO-Fe2O3) and rust is a
poorly understood mixture of hydrated ferric oxide (Fe2O3-XH2O) and
ferric oxyhydroxide [FeO(OH)]. (I am having to use a - where a dot
would normally be.) Mill scale (called fire scale by the blacksmith)
is produced by surface oxidation of hot worked iron or mild steel.
Rust is produced by a poorly understood chemical process involving
iron, water, oxygen and carbon dioxide. It is ironic that in an age
when rocks on another planet are being analyzed we do not completely
understand the nature of rust or the process that produces it.

Ferrous oxide (FeO) is the fourth oxide of iron. The powdered ferrous
oxide of commerce is easily ignited and burns rapidly. This is a
characteristic that has caused explosions.

Captain Blood
"Marlinespike Seamanship in Precious Metals"
@Alden_Glenda_Blood