Sciency question: iron oxide, black rust

[Bob W]

This may sound like a dumb question coming from someone who's been collecting knives for 45 years, I just never thought about it before.

Ok, so an old carbon steel knife gets some black spots, or in a more severe case might have pitting filled with a black coating. It often remains on the steel after cleaning the active red rust from a neglected blade.

This black stuff is iron oxide, aka black rust? Or it's something else?

A person recently told me this was carbon...

 

[Eli Chaps]

Black and red rust are both iron oxides, just with different quantities of their elements (Fe and O).

 

[cabdmd]

https://en.m.wikipedia.org/wiki/Bluing_(steel)
Here is a link about bluing with more information.

 

[Bob W]

So just to clarify, both in terminology and understanding, this old blade of mine (from a post that's nearly as old) is covered in black.

https://www.bladeforums.com/threads/stacked-leather-handles.768100/#post-8578077

The black stuff is a form of ferrous oxide, aka black rust, and is essentially the same chemical reaction process as blueing.
And this is also commonly referred to as "patina."

Does that all sound right?

 

[danbot]

So just to clarify, both in terminology and understanding, this old blade of mine (from a post that's nearly as old) is covered in black.

https://www.bladeforums.com/threads/stacked-leather-handles.768100/#post-8578077

The black stuff is a form of ferrous oxide, aka black rust, and is essentially the same chemical reaction process as blueing.
And this is also commonly referred to as "patina."

Does that all sound right?

It's also called black oxide. It's from oxidizing in a low oxygen environment.