A brief discussion of the impact of impurity elements on the surface quality of aluminum profiles.

A brief discussion of the impact of impurity elements on the surface quality of aluminum profiles.

The most common harmful impurity in aluminum is iron. In the production process of architectural aluminum profiles, iron content exceeding 0.25% no longer produces a normal color. As the iron content increases, glossiness decreases, and the color becomes bluish, or even unsightly, a pale gray. The impact of iron is more pronounced when the silicon content in the aluminum profile is low. Higher silicon content can mitigate this detrimental effect to a certain extent, as iron and silicon form an AlFeSi intermetallic compound, consuming some of the excess silicon. The main reason iron affects coloration is that it forms spikes or rods with aluminum, ranging from a few microns to tens of microns in size. Its electrode potential differs from that of aluminum, affecting the uniformity and continuity of the oxidation coloration and reducing the gloss and transparency of the oxide film, thus affecting the coloring effect.

A small amount of copper improves the mechanical properties and surface brightness of the profile without compromising corrosion resistance. However, high copper content results in a darker oxide film, which is visible to the naked eye.

A small amount of manganese can partially mitigate the detrimental effects of the AlFeSi structure and reduce the occurrence of extrusion marks. However, high manganese content results in a yellowish oxide film, which gradually shifts to a brownish-yellow color as the manganese content increases, resulting in poorer coloring.

High zinc content increases extrusion difficulty, coarsens the profile grains, increases mold wear, and causes the oxide film to appear milky. This also leads to the accumulation of zinc ions in the alkaline etching solution, causing zinc to invert on the profile, resulting in shiny, pear-skin-like spots.

Titanium content exceeding 0.1% significantly affects both the hue and color variation of the profile coloring, due to the unevenness of titanium.

Therefore, to ensure the surface quality of the profile, the iron content should be controlled below 0.25%, and other impurities below 0.1%.