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DEGASSING PRINCIPLES
Hydrogen degassing of aluminum works on the principle that the hydrogen
gas will move from an area of high pressure (in the melt) to an area of low pressure (the
inert gas). Hydrogen gas disperses in the molten metal as it would if it were released in
the air. It will maintain an even pressure throughout the melt. When you
introduce the inert gas, the hydrogen in the metal will flow into the inert gas. As
Hydrogen gas is removed, it will equalize its pressure throughout the melt. The
ability of hydrogen gas to move through and equalize its pressure in liquid metal is
almost as fast as it is in air. It is therefore unnecessary to bring all of the
metal in contact with the inert gas. Two factors, the transfer rate across the
metal/gas interface and the surface area of gas exposed to the metal determine how well
and how fast a heat of aluminum can be degassed.
Gas bubbling hydrogen degassing systems work on the principle of using
specialty gasses (chlorine, Freon or SF6) to speed up the hydrogen transfer across the
metal/gas interface to large bubbles of gas in the metal. There is a practical limit to
the hydrogen removal on humid days because the large bubbles brake the surface and expose
metal to the humid atmosphere where more hydrogen was picked up. Chlorine was the
original gas of choice because chlorine combines with hydrogen to produce hydrochloric
acid so the bubbles never became saturated with hydrogen. Because of the hazardous
nature of chlorine gas and the hydrochloric acid vapor byproduct most foundries switched
to other gasses. What most foundries have not considered are the hazardous materials
released by the breakdown of some of the specialty gases currently in use.
Rotary degassing works on the principle of increasing the surface area of an
inert gas exposed to the metal. The greater the surface area the faster the
degassing. For a given volume of gas the smaller the bubble size the greater the
surface area and the faster the degassing. For example a 1" square bubble of
gas has a surface area of 6 square inches. If you divide this bubble into 1/16"
square bubbles, the total surface area increases to 96 square inches. So for the
same volume of gas the surface area and therefore the hydrogen transferred to the gas
bubble has been increased 16 times. Additionally small bubbles do not disturb the
surface of the molten metal so there is very little hydrogen pickup from the atmosphere.
We have only used our rotary degassing equipment with Dry Nitrogen or Argon
gas and we do not guarantee compatibility with specialty gases or gas mixes. With
the exception of the graphite lance all internal parts exposed to the degassing gasses are
type 304 stainless steel and the seal's Buena-N. Experience has shown that dry
nitrogen used in a rotary degasser will degas 400 lbs. of aluminum in four minutes or
less. There is no practical reason to use a specialty gas that may cost $500.00 per
cylinder when dry nitrogen can be purchased for $25.00 per cylinder. In addition there are
no toxic emissions from nitrogen gas.
If you are interested in further information the American Foundrymen's Society has compiled all the AFS
technical papers on gas porosity in a binder. To order this information contact the
AFS 800/537-4237 847/824-0181 and ask the compiled literature for "Gas Porosity in
Aluminum Castings", publication number LS0003. At this time the material is under
review and there are only a few copies remaining. The literature is very technical but
quite detailed and informative. If you have any further questions, please
contact us.
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