What is surface scale actually made of?

[Nsled]

Steel as made and unless surface ground has that hard black scale on the surface, what is this stuff? What is its actual composition?

I guess it would be determined by the actual composition of the steel, I'm most interested in stainless and high alloy steels.

 

[polish avenger]

Mill scale is mostly iron oxide. Where are you buying stainless that has mill scale?
-Mark

 

[Alex Topfer]

basically, it's various oxides of the stuff in the alloy
for low alloy steels it will be Fe3O4. from a quick look it seems that dichromium trioxide is the most stable chromium oxide, so stainless steel scale will probably have some Cr2O3 in there as well. anything else will probably also form various oxides. even high alloy steels are mostly iron (Larrin's magnacut is 80% iron), although the carbon at the surface would have probably burnt off?

i've gotten some mild stainless that has scale on it. it's going to form at some point in the manufacturing, so if the stainless doesn't have scale it's been removed.