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Sciency question: iron oxide, black rust

Discussion in 'General Knife Discussion' started by Bob W, Jun 6, 2019.

  1. Bob W

    Bob W

    Dec 31, 2000
    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...
  2. Eli Chaps

    Eli Chaps Basic Member Basic Member

    Apr 20, 2018
    Black and red rust are both iron oxides, just with different quantities of their elements (Fe and O).
  3. cabdmd

    cabdmd Basic Member Basic Member

    Dec 17, 2010
  4. Bob W

    Bob W

    Dec 31, 2000
    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.


    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?
  5. danbot

    danbot Gold Member Gold Member

    Oct 31, 2009
    It's also called black oxide. It's from oxidizing in a low oxygen environment.
    Bob W likes this.

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