If we look at the requirement of colorless and transparent oxide film after oxidation, 5 and 6 series aluminum plain sheet is better and can also be colored after oxidation. If it is only required to be able to anodize and form a dense anodic oxide film, and there is no requirement for color, most aluminum plates can be oxidized. Before selecting the oxidation process, should have an understanding of the aluminum or aluminum plate material, because, the quality of the material, the different ingredients contained, is will directly affect the quality of the aluminum plate after anodic oxidation. For example, if the surface of aluminum has bubbles, scratches, peeling, roughness and other defects, all defects will still be revealed after anodizing. The alloy composition, on the surface appearance after anodizing, also has a direct impact.
Impurities such as copper, silicon and iron in the aluminum plate have the following effects on the surface appearance of the oxide film: copper will make the oxide film red, destroy the quality of the electrolyte and increase the 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 anodizing because of its own characteristics.
In addition, there are other impurity components in the aluminum plate on the appearance of the oxide film: 1-2% manganese aluminum alloy, brown-blue after oxidation, with the increase of manganese content in aluminum, the surface color after oxidation from brown-blue to dark brown transformation. Aluminum alloy containing 0.6-1.5% silicon is gray after oxidation, and when it contains 3-6% silicon, it is white-gray. Those containing zinc have a milky color, those containing chromium have a golden to gray uneven hue, and those containing nickel have a light yellow color. Generally speaking, only the aluminum containing magnesium and titanium containing more than 5% gold, after oxidation can get colorless and transparent and bright, glossy appearance.
It should be noted that: some aluminum plate appearance to make different colors, these colors are not oxidation up, but after anodic oxidation, dyeing or electrolytic coloring formed. Dyeing basically has any color, while electrolytic coloring is less, can do, black, bronze, champagne, gold, imitation stainless steel color.
The most common harmful impurity in aluminum is iron, in the production process of building industrial aluminum profiles, when the iron content is greater than 0.25% has not been able to get a very normal tone, as the iron content rises, the gloss decreases, the tone is greenish, light gray light gray is very difficult to see. When the silicon content is low, the influence of iron is more obvious, silicon can be high to a certain extent to reduce the harmful effects of iron, when iron and silicon to form AlFeSi intermetallic compounds, while also consuming part of the excess silicon. Iron affects the coloring mainly because iron and aluminum form a sharp or rod-like organization, ranging from a few microns to tens of microns, and its electrode potential is different from aluminum, so it affects the uniformity and continuity of oxidation coloring, and also reduces the gloss and transparency of the oxide film, affecting the coloring effect.
A small amount of copper is beneficial to the mechanical properties and surface brightness of the aluminum plate, without reducing the corrosion resistance. However, when the copper content is more oxide film is black, the naked eye can see.
A small amount of manganese will eliminate the harmful effects of AlFeSi organization to some extent and reduce the generation of extrusion patterns. However, the oxide film is yellowish when the content of manganese is high, and gradually develops to brownish yellow with the increase of manganese content, and the coloring effect is even worse.
When the zinc content is high, it increases the difficulty of aluminum extrusion, the profile grain is coarse, the die loss is also large, the oxide film is milky color, and leads to the accumulation of zinc ions in the alkali etching solution, and the zinc is inverted on the profile, producing shiny pear skin-like spots.
Titanium content greater than 0.1% has a greater impact on both the hue and color difference of aluminum plate coloring, which is caused by the inhomogeneity of titanium.
Therefore, from the protection of the surface quality of the aluminum plate, the iron content should be controlled below 0.25%, and the content of other impurities should be lower than 0.1%.