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Thread: Black oxide experiment

  1. #1

    Default 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 at 03:04 PM.

  2. #2

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    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

  3. #3
    Join Date
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    Helsinki, Finland, Europe
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    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 at 06:02 PM.

  4. #4

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    Some other parts I covered with the bath:





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

  5. #5
    Join Date
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    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.................

  6. #6
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    san jose, ca. usa
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    looks good, where do you get an?

  7. #7
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    Quote 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
    Work hard play hard

  8. #8
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    san jose, ca. usa
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    Quote 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.

  9. #9

    Default

    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 at 05:06 AM.

  10. #10
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    Finland
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    Quote 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 at 05:15 AM.

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