Aluminum is a metal used mainly for its strength due to its low weight. However, when aluminum is processed, impurities get into its structure, weakening its resistance. There are several causes, but the most common is the presence of hydrogen.
Hydrogen gas is soluble in liquid aluminum and can pass through it as easily as in the air. As the liquid metal cools and its structure hardens, hydrogen passes from the high pressure areas to the low pressure ones. Hydrogen atoms fuse together and create small gas bags that, when the aluminum is solidified, become impurities that weaken the metal structure.
Grubbing with nitrogen is the procedure by which hydrogen is removed from the molten aluminum.
How does hydrogen come in aluminum?
By combustion in processing furnaces, through various tools, fluxes and alloy additives. However, the most common cause is the penetration of hydrogen into melted aluminum due to atmospheric humidity. Dissolving hydrogen grows in the same time with atmospheric humidity increasing.
The demand for superior quality aluminum is growing, especially in high-standard industries such as aeronautics. Consequently, the need to reduce the presence of hydrogen in aluminum increases steadily. A common procedure for removing hydrogen involves the introduction of gaseous nitrogen bubbles into liquid aluminum (bubbling with nitrogen).
Hydrogen is attracted to nitrogen bubbles, transported through liquid aluminum and released to the surface. Argon is also effective for this procedure, but because of the extremely high cost, nitrogen is widely used.
The gaseous nitrogen is introduced into the molten aluminum either by means of a rotary propeller or by a static conduit. The last one is a more economical but less efficient than the rotating propeller, especially in high humidity environments.
The static pipe is a pipe inserted in the molten aluminum container, where the nitrogen bubbles are released directly into the metal. Exposed to ambient humidity, nitrogen bubbles pass the surface of molten metal and attract more moisture decomposed into hydrogen and oxygen, and hydrogen is subsequently re-absorbed into aluminum.
The rotary jet – acts by increasing the surface through which the nitrogen gas is introduced into the molten metal. Nitrogen bubbles distributed through the rotating propeller cover a larger surface of molten aluminum and collect more hydrogen. The small surface through which nitrogen is distributed in the metal allows less hydrogen to be reintroduced into the aluminum due to atmospheric humidity.
Nitrogen required for bubbling procedures is available from two sources: nitrogen stored in pressure cylinders or generated from a local source (nitrogen generator).
The nitrogen delivered in pressurized bottles at 200 bar is produced by fractional distillation and has many drawbacks.
Local nitrogen generation is an efficient and sustainable production alternative, with energy costs being significantly reduced – details in Parker White Paper.
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