Alu from Scrap: Brief introduction to the metallurgical fundamentals

Aluminum exhibits strong reactivity with various gases, which can lower metal yield and degrade the quality of secondary alloys. The most significant reactions occur with oxygen, hydrogen, and nitrogen, all of which influence the purity and properties of the recycled material.

Oxidation behavior

Aluminum readily reacts with oxygen, forming a stable oxide layer that plays a crucial role in recycling processes. This oxide layer cannot be reduced during conventional remelting, meaning that oxidation losses are largely irreversible. Although oxygen gas itself is nearly insoluble in liquid aluminum, the oxidation process intensifies above approximately 727 °C. A dense and adherent aluminum oxide film rapidly forms on fresh metallic surfaces when exposed to air, protecting the underlying metal from further oxidation.

Oxidation can also proceed through reactions with components of combustion gases such as carbon dioxide or water vapor. Additionally, aluminum can reduce metal oxides adhering to tools or furnace linings, such as iron oxide or silicon dioxide, producing aluminum oxide and metallic impurities. Other compounds, such as aluminum carbide or nitride formed in the melt, may subsequently oxidize to form additional alumina.

Under ambient conditions, aluminum forms an amorphous oxide film 2–10 nm thick in dry air, which thickens more rapidly in humid air due to the growth of a porous hydrous layer. Upon heating above the melting point, this amorphous layer transforms into γ‑Al₂O₃, and at higher temperatures (above 700 °C), into α‑Al₂O₃ (corundum). This phase transformation involves volume contraction and cracking, allowing oxygen to penetrate deeper and accelerating oxidation—particularly above 780 °C. To minimize oxidation losses, overheating caused by burning dross, organic residues, or poorly adjusted burners must be avoided.

Influence of alloying elements

During melting, oxidation may occur selectively depending on the oxygen affinity of the alloying elements. Elements such as magnesium, strontium, and calcium, which have higher oxygen affinity than aluminum, oxidize preferentially and form separate oxide phases. In contrast, elements like copper, iron, and zinc typically undergo non-selective oxidation, where their oxides integrate into the alumina lattice and affect the structure and properties of the oxide layer.

In wrought alloys, oxide inclusions, non-metallic particles, and intermetallic compounds impair mechanical properties and corrosion resistance, causing problems during surface finishing operations such as pickling, anodizing, and polishing. In cast alloys, similar inclusions can affect surface quality, pressure tightness, weldability, corrosion resistance, and machinability. Oxidized melts become more viscous, exhibit poorer mold filling, and accelerate mold wear during casting.

Minimizing oxidation during remelting

Efficient recycling requires minimizing oxygen reactions throughout scrap handling, storage, melting, and casting. Oxidation depends on temperature, exposure time, melt composition, and impurity content. To achieve high metal yields, several principles must be followed:

• Prevent oxidation and moisture absorption of scrap by ensuring dry storage and rapid processing.

• Use the lowest practical melting temperature and minimize holding times.

• Avoid turbulence in the melt to prevent rupture of the protective oxide film.

• Submerge fine scrap quickly into the molten metal or salt bath.

• Keep the melt surface area small to limit contact with air.

• Use optimized fluxes to remove oxide layers and enable aluminum droplets to coalesce.

Oxide layers also contribute to dross formation—a mixture of aluminum, oxides, and various impurities. Excessive dross formation traps liquid aluminum within it, reducing yield. Dross formation typically follows several steps: formation and rupture of the oxide skin, movement and aggregation of oxide particles, metallic entrapment by capillary forces, and continued oxidation of dispersed aluminum if dross is not cooled or treated promptly.

Hydrogen solubility and effects

Hydrogen dissolves readily in aluminum, and its solubility increases with temperature and hydrogen partial pressure. During solidification, the decreasing solubility leads to pore and bubble formation, which negatively affects mechanical properties. High cooling rates, as used in continuous or die casting, can prevent such defects by suppressing hydrogen precipitation. However, during later heat treatments or hot forming, the release of trapped hydrogen can cause porosity or intergranular defects.

Hydrogen absorption originates primarily from moisture in the atmosphere or from wet scrap, dross, and melting salts. Furnaces, crucibles, and tools that are newly lined or inadequately dried can contribute significantly. Hydrogen uptake is further promoted by turbulence, organic contaminants, or incomplete combustion of surface residues. These reactions not only increase hydrogen levels but also promote the formation of aluminum oxide, further decreasing yield.

Certain alloying elements influence hydrogen solubility: magnesium, titanium, sodium, strontium, and calcium tend to increase it, while copper, manganese, nickel, silicon, zinc, and tin reduce it. The recommended measures to reduce hydrogen contamination include using dry, clean scrap, shortening melting and holding times, employing dry fluxes and gases, preventing overheating, and thoroughly drying furnace linings and handling tools.

Nitrogen behavior and other solid reactions

Nitrogen can react with aluminum to form aluminum nitride, which, like alumina, forms a thin, dense layer on molten metal surfaces that provides some protective effect. Aluminum nitride may also form in dross or salts, particularly during gas purging with nitrogen or under local overheating. When aluminum nitride comes into contact with moisture, it reacts to release ammonia gas. Without suitable melt treatment, this can compromise product quality.

Finally, due to the high reactivity of molten aluminum and its alloying elements, it can also react with solid oxides found in refractories, master alloys, or surface coatings on scrap. Such reactions introduce impurities into the melt and reduce yield by producing additional aluminum oxide. Effective fluxing, temperature control, and melt refining are therefore essential to minimize these undesirable interactions and ensure the production of high-quality secondary aluminum.