Aluminium has a high strength-to-weight ratio, and its patina is the same colour as the metal. Therefore, many industries use aluminium, in place of steel or plastic, for strong, lightweight parts that resist corrosion. However, the aluminium surface does oxidise, and oxidation weakens the metal. Scratching and incising the softened, oxidised layer creates undulation in the patina, but the surface colour will not vary, unless you rub pigment over the incisions. Aluminium oxidation hastens with an abundance of ions washed over the aluminium surface.
Mix equal parts salt and water to create a saline slurry. The water partially dissolves the salt, allowing the slurry to stick to the aluminium.
Rub the slurry over the aluminium surface and allow it to set for one week.
Clean the aluminium surface with water and a wire brush. Remove all of the crusted salt.
Rub the slurry over the aluminium surface again. Do not disturb the surface for one additional week.
Rinse the aluminium surface with water to dissolve the remaining salt. Allow the aluminium to dry. A patina, from oxidation, will have formed on the aluminium surface, although there will not be a change in the colour or finish of the surface.
This method of oxidising aluminium is not the same as anodising aluminium. Anodising is a process of artificial oxidation using an acid and electricity to create a strong surface on the metal. Dye can be added to the acid to change the surface colour of anodised aluminium. Anodising is dangerous, because of the introduction of acid and electricity.