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  • Black oxide experiment

    Perhaps this be interesting for some viewers here....

    I have been doing some development work on black oxide coating process to apply a visually appealing appearance to the machined steel parts. The classical approach for a hot blackening bath usually involves saturated solutions of lye and nitrate with some additives like nitrites. Although it works well and gives a durable finish I do not like the highly corrosive composition and hot temperatures involved in the process. Spill the contents of the hot bath and all hell shall break loose.
    So perhaps it can be done in milder conditions and with less danger of permanent chemical burn.....

    I propose the following bath composition:

    Ammonium nitrate : 10g
    Distilled water : 1000ml (1L)

    *AN content is not critical, bath remains operational from 1-15% AN content by mass (10-150g per liter), but higher AN content causes more instability and sidereactions.


    Optional accelerators for the bath:

    Option A- Potassium or sodium chlorate : 160-240mg /1L
    Option B- Potassium or sodium perchlorate: 400-800mg/1L

    The bath operates with ferrous alloys at boiling point of water or slightly lower (95-100C) due to the dilute solutions and has no caustic components that can cause chemical burn or damage the surroundings when spilled. A decent black coating is obtained within a timeframe of 5min (for simple carbon steels) up to 2 hours (for heavily alloyed tool steels). The coating time is individual for each alloy, but usually 30-40min shall guarantee a nice coverage for most ferrous alloys. The accelerators are optional and do not have to be included into bath composition. They do provide faster oxide coverage of some alloys and slightly impart on the coloration of the coating (bluish purple coatings can be obtained, but the effect is a bit vague). For simplicity sake they can be left out of the composition.....as when overdosed the acceleration effect is reversed and oxide formation is slowed significantly. That is especially true for "Option A", upon overdosing of chlorate the bath becomes inert.

    Regarding plating vessel material and storage of the plating solution:

    Stainless steel is best. Aluminium could theoretically be used, but I have not tried it. A steel vessel could be used, it will be coated on the inside with black oxide and become inert in terms of further involvement in the coating process. Galvanised steel and copper or zinc alloys must be avoided, they shall contaminate and render the bath inert. Also galvanised parts that have been stripped from the metal coating do not react well in the bath. Oxide coating is uneven and very slow to develop....mostly of matte appearance and porous. Also do not store the bath solution in the metallic container, cold AN solutions are corrosive towards metals (except to stainless). Pour the bath into a plastic or glass vessel for storage between use. PET and PE vessels are ok to use for that purpose. The ferrous oxides shall precipitate to the bottom of the storage vessel and next time one can decant the clear solution off the precipitate for another round of use. Or filter the sediment off separately. I never bothered as it shall form again upon every use of the solution.


    So lets test it out:


    Fig. 1 Degreased carbon steel parts- cutting tool holders and case hardened dead centers


    Fig. 2 Parts immersed into preheated near boiling bath of 10g/L AN and 400mg/L sodium perchlorate


    Fig. 3 The coating process begins with the solution turning brownish due to the formation of iron oxides


    Fig. 4 The parts after 40 minutes of simmering on the bath and being flushed off with water
    Last edited by markx; 11-15-2018, 03:04 PM.

  • #2

    Fig. 5 Dead centers before oiling


    Fig. 6 Dead center after oiling




    Fig. 7 All parts oiled with a light coat of mineral oil


    Fig. 8 In operation with a lathe dog

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    • #3
      I havent found easy source here in Finland/EU for ammonium nitrate but this might be of interest for US members
      https://www.homedepot.com/p/Penningt...1378/204279955

      edit: Sorry, found out you are also in piss...made of Urea, not ammonium nitrate
      Last edited by MattiJ; 11-14-2018, 06:02 PM.

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      • #4
        Some other parts I covered with the bath:





        Last edited by markx; 11-14-2018, 06:11 PM.

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        • #5
          Your parts look pretty good. Looking at your pictures I couldn't tell them from mine. Bluing a high polished mirror finished part would be a good test to see if your concoction etches a high polish finish.



          As far as the safety hazards of hot blue, I always do my stuff out side, so if I have a spill or anything drips no big deal.
          Haven't had any accidents yet..... I did get splashed a bit on my right hand one year, got a good blister from it.

          If you process doesn't give off an steam from boiling temps you should be able to do your parts inside, something I can't do with my hot blue set up.



          JL.................

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          • #6
            looks good, where do you get an?

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            • #7
              Originally posted by gambler View Post
              looks good, where do you get an?
              '

              If looks are all you're after, a black sharpie is also an option

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              • #8
                Originally posted by 3 Phase Lightbulb View Post
                '

                If looks are all you're after, a black sharpie is also an option
                I use sharpies to color my gray hair. washes off too easily.

                Comment


                • #9
                  That coat is a weeeee bit tougher than the one from a black sharpie! Good luck washing it off the parts....

                  As for any fumes during application....there are none, except water vapor. I do my coating inside on the kitchen gas stove, no problem at all. And if spilled, one can wipe the drips away with paper towel and no damage is left behind. Rust staining on porous surfaces or fabric might be an issue though as the bath collects a fine precipitate of iron oxides.

                  These parts have been banged around and used a fair bit:



                  Deformation from tightening of bolts to fix the tool into the lathe....oxide coat is still there where the metal to metal contact happens



                  Same thing on these bits...
                  Last edited by markx; 11-15-2018, 05:06 AM.

                  Comment


                  • #10
                    Originally posted by MattiJ View Post
                    I havent found easy source here in Finland/EU for ammonium nitrate but this might be of interest for US members
                    https://www.homedepot.com/p/Penningt...1378/204279955

                    edit: Sorry, found out you are also in piss...made of Urea, not ammonium nitrate
                    When we're talking the usual hot bluing usually applied to guns I've replaced ammonium nitrated with KNO3, potassium nitrate. I have not tried it in this process, as I am satisfied with the old fashioned methods. But if it can be swapped out there, perhaps also in this application.

                    Another method is fume bluing, suspend the clean degreased parts in a tight container with some acid (hydrochloric) and let them rust, then take them out and clean off the rust with a very fine steel brush, then boil the parts. For firearms this can be repeated for a deeper finish, for parts just one pass might be enough.
                    Last edited by DennisCA; 11-15-2018, 05:15 AM.

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                    • #11
                      Originally posted by DennisCA View Post
                      We're talking the usual hot bluing usually applied to guns now. In that case KNO3, potassium nitrate works as well. I've used it to blue firearms.

                      Another method is fume bluing, suspend the clean degreased parts in a tight container with some acid (hydrochloric) and let them rust, then take them out and clean off the rust with a very fine steel brush, then boil the parts. For firearms this can be repeated for a deeper finish, for parts just one pass might be enough.
                      Even potassium nitrate is difficult to get nowadays. And it requires? the strongly caustic&hot lye bath.. not something you want to cook in kitchen.
                      Fume bluing.. too much hassle unless you are into antique firearms and what I have seen it's not as durable as hot bluing.
                      "real" AN-based hot "blackening" is supposed to be more durable than KNO3+lye hot blue.

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                      • #12
                        I edited my post to clarify. I get KNO3 from my parents who are farmers.

                        Fume bluing is the same process as rust bluing, different application, it is considered more durable than hot caustic bluing.

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                        • #13
                          How do you dispose of the liquid waste? What is it, and how toxic is it?

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                          • #14
                            Originally posted by Glug View Post
                            How do you dispose of the liquid waste? What is it, and how toxic is it?
                            Should be really mild and easy to dispose compared to "normal" lye hot bluing bath.
                            Ammonium nitrate is usable as lawn fertilizer and some iron oxides won't matter in that use.
                            Hexavalent chromium would be the only potential skeleton in this closet.. no idea if it can form in the process or not.

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                            • #15
                              Originally posted by MattiJ View Post
                              Should be really mild and easy to dispose compared to "normal" lye hot bluing bath.
                              Ammonium nitrate is usable as lawn fertilizer and some iron oxides won't matter in that use.
                              Hexavalent chromium would be the only potential skeleton in this closet.. no idea if it can form in the process or not.
                              One does not need to dispose of the bath after using.....essentially the ingredients do not spoil or become depleted in the process (apart from water being evaporated), but rather work like a chemical pump to convert the surface layer of iron into magnetite. I've been using the same solution for dozens of times and it still works pretty much the same. If one does not add any accelerants to the bath then by all means it can be used to fertilize the lawn around the workshop. That's what the initial purpose of the main ingredient is

                              As for Cr(VI) I really doubt that it shall form or remain stable in this kind of environment. Hexavalent chromium is a strong oxydiser and there are too many reductive species in the bath that would react with it. First of all the iron compounds that are initially formed in the bath when metal surface is contacted with the solution are of ferrous nature :Fe(II)+2. These are known and effective hexavalent chromium reducers. E.g. ferrous compounds like ferrous sulfate are added to cement and cementitious mortars for chromium reducing purposes. I know because I formulate cementitious mortars as a day job

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