General metal question

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This is more of a general metal question, not specific to welding but not far enough off-topic to go there.I was cutting some c-channel for a basement wall repair which ended up taking a few weeks to finish. While outside, the cut steel rusted a bit, but less so on the corner that discolored under the heat of cutting. I cut it with a 14" cutoff saw. I'm curious if anyone knows why the blue steel rusted less than the shiny silver steel? I thought that heated steel would contain more carbon and thus rust faster.
Reply:I am by no means a professional but if you look real close at the blued area it looks just as affected as the rest it the iron. I opened the photo you provided and then enlarged it and the only difference I see is the blue coloring. The rust is just as prevalent Sent from my Desire HD using Tapatalk 2
Reply:More carbon would mean less Iron Oxide (rust).
Reply:Burnt steel or iron is already an oxide much like mill scale. It just isn't necessarily the red rust you're used to seeing."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:Different forms of iron oxide exist. The black film in the lower corner looks like Fe3O4, commonly called Smithy Scales, while the orange color is due to a different, hydrated form that formed from the steel later. Different compounds have different stability wrt oxidation or any other type of reaction, and that first type, which is similar to the old bluing purposely applied to firearms, happens to be more resistant to change than the steel is. The orange rust there probably formed underneath the thin black film, under its edges and through cracks in it..Last edited by Oldiron2; 05-06-2012 at 01:04 PM.
Reply:Thanks for everyone's responses. While it is true there are rust spots on the blue section, there are far fewer of them then on the shiny steel. The explanation that it is already oxidized and thus forms a protective layer of sorts makes sense. Thanks again.