Effect of Other Impurities in Aluminum Alloy on Surface Treatment of Aluminum Profiles

West Aluminum - West Aluminum Industry Telecom:  If the oxide film is colorless and transparent after oxidation, 5 and 6 series of aluminium alloys are better, and can also be colored after oxidation. Most aluminium alloys can be oxidized if only a dense layer of anodic oxide film is required, and there is no requirement for color. Before choosing the oxidation process, we should know something about the material quality of aluminium or aluminium alloy, because the quality of the material and the different composition of the material will directly affect the quality of aluminium products after anodizing. For example, if there are air bubbles, scratches, peeling and rough defects on the surface of aluminium material, all defects will still be exposed after anodizing. The alloy composition also has a direct impact on the surface appearance after anodizing. The effects of impurities such as copper, silicon and iron on the surface appearance of the oxide film are as follows: copper can make the oxide film reddish, destroy the quality of electrolyte and increase oxidation defects; silicon can make the oxide film grey, especially when the content is more than 4.5%; iron will exist in the form of black spots after anodic oxidation due to its own characteristics.

Aluminum Profile Surface Oxidation Workshop

In addition, there are other impurities in the alloy on the appearance of the oxide film: 1-2% manganese aluminium alloy oxidized brown-blue, with the increase of manganese content in aluminium, the surface color after oxidation changes from brown-blue to dark-brown. Aluminum alloys containing 0.6-1.5% silicon show gray after oxidation and white-gray when containing 3-6% silicon. Zinc is opaque, chromium is golden to gray, and nickel is pale yellow. Generally speaking, only aluminium containing more than 5% magnesium and titanium contains gold, which can obtain a colorless, transparent and bright appearance after oxidation.

It should be noted that some aluminium profiles have different colours. These colours are not oxidized, but formed by anodizing, dyeing or electrolytic coloring of aluminium profiles. Dyeing basically has any color, and electrolytic coloring is less, can be done, black, bronze, champagne, golden, imitation stainless steel color.

The common harmful impurity in aluminium is iron. In the production process of aluminium profiles in construction industry, when the iron content is more than 0.25%, the normal tone can not be obtained. With the increase of iron content, the gloss decreases, the tone is blue, and the light grey is very ugly. When the content of silicon in gold is low, the effect of iron is more obvious. When the content of silicon is high, the harmful effect of iron can be alleviated to a certain extent. At this time, iron and silicon form AlFeSi intermetallic compound and consume part of excess silicon. The main reason why iron affects coloring is that iron and aluminium form sharp or rod-like structure, ranging from several microns to tens of microns. The electrode potential is different from that of aluminium, so it affects the uniformity and continuity of oxidation coloring, and also reduces the gloss and transparency of oxide film and affects the coloring effect.

A small amount of copper is beneficial to the mechanical properties and surface brightness of aluminium profiles, but does not reduce the corrosion resistance. But when the copper content is high, the oxide film is black, which can be seen with the naked eye.

A small amount of manganese can eliminate the harmful effect of AlFeSi structure to a certain extent and reduce the generation of extrusion lines.  However, when the content of manganese is high, the oxide film is yellowish. With the increase of the content of manganese, it gradually develops to brown yellow, and the colouring effect is worse.

When zinc content is high, it increases the difficulty of aluminium extrusion, the grain size of profile is large, the die loss is large, and the oxide film is opaque, which leads to the accumulation of zinc ions in the alkali etching solution, and the inversion of zinc on the profile results in bright pear skin spots.

When the content of titanium is more than 0.1%, it has a great influence on the color tone and color difference of aluminium profile, which is caused by the inhomogeneity of titanium.

Therefore, in order to guarantee the surface quality of aluminium profiles, the content of iron should be controlled below 0.25% and the content of other impurities should be below 0.1%.

(Chongqing Xialu Xinda Aluminum Industry Co., Ltd. Publicity Department collates contributions)

Source: Network

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