If the oxide film is required to be colorless and transparent after oxidation, 5xxx and 6xxx series aluminum alloys are better, and they can also be colored after oxidation. If it is only required to be anodized to form a dense anodized film, and if there is no requirement for color, most aluminum alloys can be oxidized. Before selecting the oxidation processes, it is necessary to have an understanding of aluminum or aluminum alloy materials. because the quality of the materials and the difference in the compositions will directly affect the quality of the aluminum products after anodization. For example, if there are bubbles, scratches, peeling, roughness and other defects on the aluminum surface, all defects will still be revealed after anodizing. The alloy composition also has a direct effect on the surface appearance after anodization. The influence of copper, silicon, iron and other impurities in aluminum alloys on the surface appearance of the oxide film is as follows: Copper will make the oxide film red, damage the quality of the electrolyte, and increase oxidation defects; silicon will make the oxide film gray, especially when the content exceeds 4.5%, the effect is more obvious; iron will exist in the form of black spots after anodization due to its own characteristics.
In addition, there is the influence of other impurities in the alloy on the appearance of the oxide film: 1-2% manganese aluminum alloy, brownish blue after oxidation, with the increase of manganese content in the aluminum, the surface color after oxidation changes from brownish blue to dark brown conversion. Aluminum alloy with 0.6-1.5% silicon content will be gray after oxidation, and white-gray when silicon content is 3-6%. Zinc-containing products have a milky color, chromium-containing products have an uneven tone of golden to gray, and nickel-containing products are light yellow. Generally speaking, only aluminum alloys containing more than 5% magnesium and titanium can obtain a colorless, transparent, bright and smooth appearance after oxidation.
It should be noted that some aluminum profiles have different colors in appearance. These colors are not oxidized, but are formed by dyeing or electrolytic coloring after the aluminum is anodized. There are basically any colors for dyeing, but electrolytic coloring is relatively rare. It can be done in black, bronze, champagne, golden, and imitation stainless steel.
The most common harmful impurity in aluminum is iron. In the production process of aluminum profiles in the construction industry, when the iron content is greater than 0.25%, a normal color tone cannot be obtained. With the increase of iron content, the glossiness decreases and the color tone tends to be cyan, light gray, which is ugly. When the silicon content in aluminum alloys is low, the effect of iron is more obvious. When the silicon is high, the harmful effect of iron can be reduced to a certain extent. At this time, iron and silicon form an AlFeSi intermetallic compound, which also consumes part of excess silicon. The main reason why iron affects coloring is that iron and aluminum form a sharp or rod-like structure, ranging from several microns to tens of microns, and its electrode potential is different from aluminum, so it affects the uniformity and continuity of oxidation coloring, and It also reduces the gloss and transparency of the oxide film and affects the coloring effect.
A small amount of copper is beneficial to the mechanical properties and surface brightness of aluminum profiles without reducing corrosion resistance. However, when the copper content is high, the oxide film becomes black, which can be seen by the naked eyes.
A small amount of manganese will eliminate the harmful effects of AlFeSi structure to a certain extent and reduce the production of extrusion lines. However, when the manganese content is high, the oxide film becomes yellowish, and gradually develops to brownish yellow with the increase of manganese content, and the coloring effect is worse.
When the zinc content is high, the difficulty of aluminum extrusion is increased, the profile grains are coarse, the die loss is also large, the oxide film is opalescent, and leads to the accumulation of zinc ions in the alkaline etching solution, and the zinc is inverted on the profile, resulting in a shiny pear-like spots.
When the titanium content is greater than 0.1%, it has a great influence on the color tone and color difference of the aluminum profile, which is caused by the inhomogeneity of titanium.
Therefore, from the perspective of ensuring the surface quality of aluminum profiles, the iron content should be controlled below 0.25%, and the content of other impurities should be less than 0.1%.
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