Anodizing

When welding aluminum, the natural oxide layer is the main problem. It only melts at much higher temperatures than the aluminum itself and prevents the weld pool from flowing together properly. That's why you have to mechanically remove the oxide layer before welding, for example with a stainless steel wire brush that you only use for aluminum. In TIG welding, alternating current does the cleaning by breaking up the oxide layer. If you don't remove the oxide layer, you'll get lack of fusion and inclusions in the weld.