Aluminum degassing requires nitrogen with at least 99.99% purity, moisture below 10 ppm, and oxygen below 5 ppm to support effective hydrogen removal [S1].
For most aluminum casting alloys, the recommended degassing temperature is 720–740°C, while common rotary treatment uses 8–12 minutes, 300–450 RPM, and 6–12 L/min for 300–800 kg ladles [S5].
Why Nitrogen Purity Controls Hydrogen Removal
Dissolved hydrogen is the principal gas-related defect mechanism in molten aluminum because its solubility falls sharply during solidification, leaving pores, pinholes, and internal cavities in the casting [S2][S5]. Hydrogen solubility is about 0.69 mL per 100 g at 660°C and about 0.04 mL per 100 g after solidification, an approximately 18-fold reduction [S5].
High-purity nitrogen entering the melt through a graphite or ceramic rotor is divided into fine bubbles. These bubbles create a hydrogen partial-pressure difference, allowing dissolved hydrogen to transfer from the aluminum into the bubbles and leave at the melt surface [S2]. Carrier-gas purity, bubble size, treatment time, gas flow, and rotor speed therefore act together, so nitrogen purity alone does not determine the final hydrogen level [S1].
For reference, a well-executed nitrogen or argon cycle can reduce hydrogen from a typical pretreatment level of 0.30–0.50 cm³/100 g to 0.08–0.15 cm³/100 g, depending on gas purity and operating conditions [S1]. The resulting degassing unit specification should cover both gas quality and treatment parameters rather than rely on purity alone.
Specify Purity, Moisture, and Oxygen Together
The recommended carrier-gas specification is at least 99.99% nitrogen purity, no more than 10 ppm moisture, and no more than 5 ppm oxygen [S1]. Moisture is tied directly to dew point, with the cited quality ceiling corresponding to a dew point below -60°C [S1]. Buying only on the stated nitrogen percentage leaves an important gap because moisture and oxygen can enter through supply, storage, and delivery systems.
A laboratory study on hydrogen-supersaturated liquid aluminum reports final hydrogen levels of 1.86, 1.73, 1.73, and 2.03 ppm after 20 minutes for four reported treatment cases [S6]. Those results show that treatment duration remains a measured variable, not a substitute for controlling incoming gas quality. A process trial should compare measured outlet hydrogen with the specified gas purity and operating recipe before the specification is released for production.
Cost calculations are covered separately in Nitrogen Degassing Cost per Ton of Aluminum. That commercial comparison belongs after the gas-quality floor is fixed, because a cheaper gas that cannot meet the metallurgical target creates a false economy.
Set the Process Window for Rotary Nitrogen Treatment
For 300–800 kg ladles, a starting process window of 8–12 minutes at 300–450 RPM and 6–12 L/min is recommended for nitrogen degassing [S5]. The same treatment can shear the carrier gas into bubbles approximately 1–3 mm in size, increasing gas-metal contact area and distributing the bubbles through the bath [S5].
Rotary treatment is more consistent than simple lance purging because the rotating dispersion head controls bubble generation and distribution. A large bubble offers less surface area and a shorter effective transfer path, while fine bubbles improve hydrogen pickup and can also lift some oxide inclusions toward the surface for skimming [S5]. The equipment comparison in Gas Diffuser vs Rotary Impeller for Aluminum Degassing provides a useful mechanical basis for that selection.
The 720–740°C range applies to most aluminum casting alloys requiring effective hydrogen removal and stable melt quality [S5]. Temperature must remain within the usable process range for the alloy and equipment; a purity-compliant gas cannot compensate for a treatment performed outside an appropriate thermal and hydrodynamic window.
Compare Nitrogen, Argon, and Membrane Duty
Nitrogen and argon both support rotary degassing, but their selection criteria differ. Nitrogen is commonly used for high-volume commercial casting because it costs 60%–80% less per cubic metre than argon, while argon is chemically inert and can outperform nitrogen in sensitive alloys requiring lower final hydrogen content, fewer inclusions, and better surface finish [S1].
For water treatment, the membrane comparison has a different process boundary. For X40-type units, the most efficient route to low dissolved oxygen is a combination of high-purity nitrogen and vacuum, rather than nitrogen alone [S4]. This distinction prevents a water-degassing specification from being copied directly into a molten-metal purchase order.
Match the Supply Method to the Process Load
Bulk liquid nitrogen, cylinders, and onsite membrane or pressure-swing nitrogen generation can all supply degassing gas, but their operating profiles differ. A stationary onsite generator accepts compressed air and delivers nitrogen through a dedicated outlet, removing routine cylinder and dewar handling while allowing production to match gas demand to the degassing cycle [S3].
Nitrogen demand is continuous during treatment, so the chosen supply must maintain the specified flow of 6–12 L/min used in the cited 300–800 kg ladle process window [S5]. An onsite generator does not remove the need for a purity specification: the measured gas at the degassing inlet must still meet 99.99% nitrogen, below 10 ppm moisture, and below 5 ppm oxygen [S1].
For foundries using large ladles or multiple treatment stations, Gas Diffuser vs Rotary Impeller for Aluminum Degassing and Set Degassing Gas Flow by Melt Volume and RPM provide the operating context for sizing the equipment. The casting ladle should also be checked for thermal capacity, refractory condition, and the working mass used in the treatment calculation.
Separate Hydrogen Removal from Inclusion Filtration
Nitrogen degassing removes dissolved gas, but it does not replace solid-inclusion filtration [S2]. The normal melt-treatment sequence is degassing followed by filtration, because fine nitrogen bubbles can assist in floating some oxide particles while ceramic filtration captures the remaining solid inclusions [S2].
Hydrogen and inclusions therefore require separate acceptance limits and measurement methods. The published hydrogen quality bands are below 0.10 cm³/100 g for excellent castings, 0.10–0.15 cm³/100 g for good structural castings, 0.15–0.20 cm³/100 g for acceptable non-critical parts, 0.20–0.30 cm³/100 g where visible microporosity becomes likely, and above 0.30 cm³/100 g where significant macro-porosity creates substantial rejection risk [S1].
Operationally, verify three things before production release: the nitrogen certificate at the supply boundary, the measured 99.99% purity, below 10 ppm moisture, and below 5 ppm oxygen limits, and the post-degassing hydrogen result against the casting quality target [S1]. The most trackable process signal is the reduction from the 0.30–0.50 cm³/100 g pretreatment range to the specified post-treatment target of 0.08–0.15 cm³/100 g [S1].