Page 1 of 4 123 ... LastLast
Results 1 to 10 of 35

Thread: Degassing confusion

  1. #1

    Degassing confusion

    I've had mixed results when trying to find how to degas without chlorine because of all the different names some people say washing soda which is sodium carbonate or Na2CO3 which as far as I know decomposes at really high temperatures
    So I don't see how it would degas anything. Then the other names are baking soda or soda ash or whathaveyou which is sodium bicarbonate or NaHCO3 which does decompose but into water and CO2 so the co2 would do some degassing but the water would add right back to it? I just want to avoid chlorine based stuff and being winter here pool supples are sorta hard to find and I just want some pretty looking fluxes and degassed aluminum for my first pour.

  2. #2
    How much metal are you looking to pour, here? If it's a reasonable amount (a #10 crucible for instance) you don't have to worry about gas too much. It takes time for hydrogen to be absorbed into the molten metal. The trick it to have all of your casting equipment ready, fire the furnace at a reasonable rate (having it oxidixing or "lean" helps too with gas ), add your metal and allow it to melt, give the metal an extra minute or two to get hot enough to pour without freezing, and pour your mold. The only time the aluminum has to absorb gas is while it's a liquid and you shouldn't leave it at liquid temperature very long at all.

    Fluxing is done just before you pour and can be done, in a pinch, with Lite Salt or a 50/50 mixture of standard table salt (sodium chloride) and "no sodium" table salt (potassium chloride). I simply mix them together quite well before dumping them in but most recommendations I've seen involve melting them together before hand and making small bits of salt tablet that you can throw in just before you're ready to pour. Stir lightly (don't stir too vigorously, you'll just whip metal into the dross!) into the top layer of metal and then scoop off that dross.

  3. #3
    Ok it's a small steel crucible that I think can hold a max of 8 pounds ill probably only ever put 5 in it. And so you don't plunge the flux to the bottom cuz that what's several other people do? And that also didn't tell me which compound is used for no chlorine degassing. And I already made a eutectic salt flux of KCl NaCl and cacl2 that melts about the same time aluminum does. And the evaporators waste oil fire is hard to run lean or oxidizing until everything's hot.
    Last edited by Metalmaster; 12-10-2013 at 10:08 AM.

  4. #4
    Senior Member Robert's Avatar
    Join Date
    Sep 2008
    Location
    Charlotte, Nascarolina
    Posts
    2,983
    I think there is a misunderstanding in that degassing and fluxing are two completely different things. Degassing can be done with chlorine producing compounds or inert gas lances. It removes dissolved hydrogen.
    Fluxing brings impurities to the surface. KCl NaCl is used for aluminum and is stirred in before skimming.
    R
    "Whether you think that you can, or that you can't, you are usually right."
    - Henry Ford (1863-1947)

    Forklift Project
    Sand Mixer

  5. #5
    No there isn't any confusion I know what flux is I'm saying I've seen people even professional companies plunge it to the bottom and stir it up. And myfordboy uses baking soda as a degasser and so do a few other people who I can't remember the names of right now I just wanted to know if its baking soda which is Nahco3 or washing soda which is na2co3 there two completely different compounds that do completely different things when hit with heat. I know most people use chlorine I even mentioned it in my first post I wanted to know how people use the sodium compounds to do it and which one do they use. Don't say I misunderstand when I mentioned everything you did. The fluxing question was just a curiosity because sivicdude mentioned it I've seen people drop it on and I've seen people stir it in I just wanted to know which was "right" .

  6. #6
    Administrator Site Admin Anon's Avatar
    Join Date
    Nov 2005
    Location
    Indianapolis, IN
    Posts
    8,146
    Blog Entries
    11
    A few notes:

    -Be careful about salt flux containing CaCl2--it's very hygroscopic, so you should always assume it needs to be thoroughly preheated before adding. (Seriously--leave it out overnight and it turns into liquid. Even in a well-sealed container it's hard to keep dry.)

    -You cannot effectively degas with CO2 or CO2 producing compounds. To my knowledge chlorine compounds are the only solid degassers (the other option is a tank of dry nitrogen or other inert gas). Note that solid degassers also must be kept dry--if you end up bubbling steam through your melt, you'll add more gas than you remove.

    -I'm increasingly recommending, if you have clean scrap and don't degas, don't flux either (or flux very lightly). Most of my gas problems seem to be caused by flux. Either way: flux goes on top, degasser gets bubbled through the melt.

    -If you're running rich, you'll have a lot more trouble with gas than if you're running neutral or lean.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

    Foundry Tutorial
    My Website <<Now at prometheus-foundry.com

  7. #7
    Is running lean just giving it enough air that you don't have flames escaping the furnace? That would make sense to me since there are only uncombustiable exhaust gases shooting out not excess hydrocarbons and whatnot? And I know CaCl2 absorbs water like mad I did chemistry for years before I even knew about home casting so while I might seem dumb about foundry work chemistry is something I got down pretty dang well not to toot my own horn or anything. I just never did much or nothing with metals just compounds. And my scrap is dirty so I do need to do both but I can't get chlorine compounds easily in winter. And the flux will be set by the exhaust in a stainless steel tin then dump onto the aluminum. And if you degas after fluxing your stir a lot while degassing so you would be stirring the flux into the melt a lot?

  8. #8
    The easiest way to solve the issue of trying to clean up dirty scrap is to melt it twice. Do one melt where you melt it down to clean it (degas and flux away, it won't be included in the final pour you want anyway).

    Then pour that off into ingots taking extreme care not to pour any of trash into the aluminum. Once you have CLEAN aluminum ingots, prepare your mold and remelt the ingots to obtain clean casting aluminum.

    Degassing and fluxing seem to be MUCH more required of bulk casting operations where the time needed to melt the aluminum is great enough that hydrogen may naturally diffuse into the molten metal.

  9. #9
    Okay that is what I'll do when in doubt I have a few pounds of aluminum muffins but after figuring the furnace out I think that'll go fast. So my answer was sorta answered by anon but I still don't know which co2 containing compound is used to degas( ok i know there not as good as chlorine but these are decorative castings that I just want to be devoid of surface bubbles I don't care what the inside looks like) and I have no clue if I can get a chlorine compound like calcium hypochlorite and I can't afford the chlorine neutralizing mask so unless its safe to do outside without a mask I'm screwed there to.

  10. #10
    Administrator Site Admin Anon's Avatar
    Join Date
    Nov 2005
    Location
    Indianapolis, IN
    Posts
    8,146
    Blog Entries
    11
    You can identify the mixture of an oilburner by holding a piece of sheet steel (cold) in the exhaust. If it soots up, it's rich. If the exhaust cleans soot off the steel, it's lean. For a propane burner, it's easier to look at the flame characteristics, because propane doesn't make soot unless it's very rich.

    As far as I know CO2 reacts with molten aluminum, so trying to degas with it will just make dross.

    The best way to "stir your flux in" is to degas, yes. Typically you don't want to agitate the melt (i.e. when you aren't degassing) because the more you break the oxide skin and expose metal surface to the furnace atmosphere, the more chance it has to dissolve hydrogen. In degassing you're doing the exact opposite (exposing the metal to a gas that's both inert and is depleted in hydrogen) so you want as much surface area and agitation as possible. But if you flux and don't degas, flux on the surface will do fine. Aluminum oxide floats in aluminum, after all.

    Note that if you have a severely gassed melt (you'll be able to tell if you see little pops of hydrogen flame when you disturb it) then it can be beneficial to stir it in open air, to at least get the worst of the gas out. But normally, bubbling air (or CO2) would not be useful, because you form an oxide skin almost instantly, and end up making a bunch of dross without extracting that much hydrogen. You need an inert gas, or an active gas like Cl2, instead.

    FYI: an OV/AG/P100 combination cartridge is about $20 a set (plus maybe $15-$20 for the facepiece if you don't have a respirator) so it's not that much of an investment. Certainly cheaper than the consequences of breathing chlorine!
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

    Foundry Tutorial
    My Website <<Now at prometheus-foundry.com

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •