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Best way to remove black oxide skin from hot roll

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  • #16
    I remove mill scale with a solution of 1 cup hydrochloric acid to one gallon of water. In about an hour, the scale has fallen off to the bottom of the bucket. More concentrated solutions will remove the scale faster. After the scale is removed, soak your piece in a baking soda/water solution for about 10 minutes to neutralize any residue acid. Hydrochloric acid will also remove a black oxide finish.


    • #17
      On 10/31/17 ,I posted EASY WAY ON THIS FORUM. uSE CHEAPO HARBOR FREIGHT DIAMOND FACE WHEEL. Pay no attention to the know it alls that wring their hands and cry THOU SHALT NOT USE DIAMONDS ON STEEL.Edwin Dirnbeck


      • #18
        Originally posted by markx View Post
        Some hotrolled material can have quite a scale on it and it can be quite hard, even to the point where a flap wheel on an angle grinder really can not clean it up. It just skids over and makes the surface shiny, but the layer of scale remains. But as far as machining it off I've never perceived it to be a huge problem in terms of tool wear. Cutting tools wear inevitably and have to be resharpened as they do. Soaking the material in etchants for prolonged periods vs. sharpening the cutting tool a few more times....I personally opt for sharpening. Seems like too much of a hassle to get rid of the scale by chemical means.
        I ran into that as well. Just polished the scale instead of removed it.

        In my searching I found that on YT has a video on a better style of abrasive aimed at removing that nasty stuff with abrasive on a high speed grinder.

        (114) Four Ways to Remove MILL SCALE - YouTube

        I've got no idea what the final diamond coated wire brush would cost but I'm going to guess that it would not be cheap. But the silicon carbide flap wheels seems like a reasonable cost option for a mechanical method.

        Machining it off is one of the classic methods for sure. And the big one that I keep seeing suggested for extending the tool life has always been to make the first cut deep enough that the tip of the cutter is below the mill scale and it gets chipped and carried away by the steel chip from below. It doesn't need to be much but the idea is to get in there by .02 or around that so the chip lifts the mill scale away instead of rubbing and wearing the point of the cutter.

        If I were the sort that used insert tooling a lot and I mostly used 80° "C" style rhombic cutters I think I'd buy or make a holder that allowed me to use the 100° corners for roughing off the mill scale and other nasty jobs. One for the lathe and another for a reasonable size flycutter or face mill on the milling machine.

        Chilliwack BC, Canada


        • #19
          Thanks for all of your suggestions, since I have about a dozen chipped or damaged .500" carbide end mills. I will start off by giving those a try.


          • #20
            Originally posted by psomero View Post
            mill scale or black oxide?

            one is a byproduct of the hot rolling process, one is a plating process. two different things with different removal methods. vinegar isn't going to do anything to mill scale except make it smell like vinegar. it is kind of like welding slag, in that it's the impurities that accumulate on the surface during the hot work, but it's a lot more tenacious.

            even with black oxide, you'd just be removing the sealing oil with a weak acid, not the actual iron oxide.

            yes mill scale can be kind of tough to machine through, but honestly isn't that big of a deal. if you're really worried about it, and have the stock allowance overhead for it, get an angle grinder with an abrasive-embedded nylon bristle stripping brush or a flapwheel, and cut the scale off. You need someyhing hard like aluminum oxide to chew through it. you'll know when you're down on good bright metal below the "crust"
            I assume you haven't actually tried the vinegar pickle?

            It works but it's not the fastest process, might take day or two especially if the solution is on cold side. And as mentioned adding salt seems to make it faster.
            Location: Helsinki, Finland, Europe


            • #21
              Originally posted by LKeithR View Post

              Can't say I've ever seen P & O bar stock or structural shapes but it's quite common in thinner plates and sheet.
              A lot of laser cutting shops won't use anything but P & O material because it's so much easier to get clean cuts.

              We had one project a few years ago where we used about 125 tons of 3/16 P & O plate for laser cut profiles...
              didn't know some laser guys prefer it, its never come up with when we get things laser cut, but we mostly plasma cut in house. Sort of don't get the concern and effort put in to removing it as the only situation I see done is coils that will be use for stamping. When machining, just take enough on the first first pass to get under it.
              in Toronto Ontario - where are you?


              • #22
                I treat mill scale the same as the surface of a sand casting. Whenever possible make the first cut deep enough to get completely under the scale. I've found the greatest tool wear comes from a light pass with the tool coming into and out of contact with an irregular surface. As long as the tip of the tool is under that scale just plow away. Normally I'm using HSS too, as I'm just taking the top layer off, even if there is some tool wear it's just a quick touch up on the grinder and then back to cutting. Quick and easy to touch up HSS.


                • #23
                  Diluted muriatic acit works a treat for me.


                  • #24
                    Oxy-acetylene flame will remove mill scale. Direct the flame at a low angle and watch the scale pop off.
                    Harris Calorific (now Lincoln) made special multi-flame tips for removing mill scale.
                    Last edited by Illinoyance; 01-08-2021, 06:57 PM.


                    • #25
                      I use The Force for removing hot rolled material mill scale.
                      The schwartz when it is particularly heavy. The schwartz should only be used in extreme conditions.


                      • #26
                        Black Oxide and mill scale are both oxide coatings, one from heat, the other from chemical action. Phosphoric or dilute muriatic acid will remove either one, and are typically faster than vinegar or other weaker acids.

                        For just a little more, you can do it yourself!


                        • #27
                          Originally posted by Bented View Post
                          The schwartz should only be used in extreme conditions.
                          ...and you should take extra care not to get it twisted; I hate it when I get my Swartz twisted!


                          • #28
                            Can't say I ever worry about the thin layer of black scale on steel. It comes off with the first pass.

                            All of the gear, no idea...


                            • #29
                              Originally posted by Ian B View Post
                              Can't say I ever worry about the thin layer of black scale on steel. It comes off with the first pass.

                              I don’t mind machining trough the mill scale but TIG welding is definetely more enjoyable with pickled parts.
                              Location: Helsinki, Finland, Europe


                              • #30
                                Originally posted by Ian B View Post
                                Can't say I ever worry about the thin layer of black scale on steel. It comes off with the first pass.

                                It's "thin", but not always "that" thin.... Not a "thin film" that almost rubs off.

                                Reasons to remove it include welding, non-machined surfaces of a part where it is just ugly, etc. And, if not removed, it seems to encourage rusting under paint. seems like it might be protective, but apparently not very much.
                                CNC machines only go through the motions.

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