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HRS Mill Scale

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  • HRS Mill Scale

    Prepping some steel for paint. Phosphoric acid takes forever to eat through mill scale. What is it exactly? Is it important to get it off? Don't worry about for interior stuff but building some farm stuff that will always be out in the weather and want the best protection I can get.

  • #2
    It's rust. Iron oxide takes two forms; the red kind is not fully oxidized and is therefor unstable. The blue/black kind is fully oxidized and is very stable, it will not turn back into red rust. The heat of the rolling operation allows the full oxidation. I'm sure Evan will be along anytime with a link to the chemical formulas. Authentic gun blueing is a refined method of producing this stable oxide.

    For what you're doing you should wire brush the metal, wipe off residual oil, prime and paint. The scale will actualy help as long as it doesn't interfear with the paint bonding to the surface.

    Dave

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    • #3
      Red rust=FeO

      Black=Fe3O4

      [This message has been edited by Evan (edited 08-08-2004).]
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        Thanks Evan, I figured you'd either know that off the top of your head or have a link.

        I belive those rust stabilizers convert red to black.

        Dave


        Ooops, I thought one of them was Fe2O3. http://www.micronmetals.com/iron_oxide_red.htm

        [This message has been edited by Dave Opincarne (edited 08-08-2004).]

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        • #5
          Initial oxidation of iron produces FeO, iron oxide(II). It further oxidises to Fe2O3 red iron oxide(III),

          http://www.reade.com/Products/Catego...ron_oxide.html


          [This message has been edited by Evan (edited 08-08-2004).]
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            I have painted over mill scale for years and have not had any problems.

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