Aluminum oxide layer
A heat-resistant oxide film on aluminum surfaces that provides corrosion resistance but must be broken up or removed during welding due to its high melting point.
Aluminum always has a thin but very hard oxide layer in air. It melts much later than the aluminum itself and prevents the weld pool from properly fusing. It also insulates electrically and disturbs the arc. If you don't remove the layer, you'll get inclusions in the weld, the weld becomes porous and won't hold. Before welding, you must remove the oxide layer. Use a stainless steel brush used only for aluminum, and degrease the part afterwards. A regular steel brush is taboo, as it introduces rust. Brush right before welding, as the layer forms quickly. For TIG welding, use AC; the positive half-cycle cleans the oxide. For MIG welding, use pulsed arc for a cleaner pool.