why does anodized aluminum discolor?

by:AAG     2020-11-03
The addition of a thick layer of extra oxide to the surface of the anode alumina helps to improve its corrosion resistance and some mechanical properties, including ultimate strength and yield strength, or should be deformed.
Some parts such as the piston may last longer when the anode is oxidized.
However, despite these advantages, anodized aluminum sometimes changes color.
There may be several different culprits behind the discoloration.
Weathering flowering is the first possible cause of discoloration.
Usually, during the manufacturing process, the surface of the anode alumina is sealed with hot water.
If the seal is not sufficient, the surface may be etched later, especially in long-term contact with an acidic environment.
Another similar problem is chemical corrosion, which occurs when an acid or alkaline solution is in contact with the product for a long time.
When the discoloration becomes visible, it is basically irreversible and parts may need to be replaced.
Powder is another possible cause of discoloration.
If the anode of the product is not properly oxidized--
For example, if the temperature in the oxidation process is too high-
The resulting oxide coating may be soft.
When exposed to air, this soft coating will begin to crack and its behavior will reflect the changes in the current humidity.
The cracked layer may eventually fall off, reveal the aluminum below and degrade it in the same way.
The need to grind off the surface coating for welding aluminum after the anode oxidation leaves confusion and is not recommended.
However, the oxidation of the anode after welding also poses special challenges.
It is important to use the lowest possible heat, because if the nearby metal is disturbed by the heat, it may change color after the anode is oxidized.
The welder must also use the correct alloy welding wiring;
Using the wrong wire can produce a gray or black color on the product that is anodized.
Rainbow color or purple color on aluminum coating is another more unusual color change problem.
If the aluminum after Anode Oxidation is sealed at a high pH, it may form an alumina crystal or \"seal black powder disease \";
These crystals produce a faint pattern of an unusual color, just like the pattern on the oil drop.
If the product is black, the color will be more obvious.
The use of a high bath temperature to anoze can also lead to rainbow colors in the product.
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