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Letter 21083
Hard Anodize Colors
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My question is regarding the reason in which different alloys of
aluminum when hard anodized appear different in color. I do
understand the fact that the impurities from different elements in
the aluminum cause the different colors, but I was wondering if
someone could give me a more detailed explanation.
Christopher Pierre
- Ajax, Ontario, Canada
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You anodize "aluminum" not the alloying ingredients: copper,
silicon, magnesium, manganese, etc and each alloying impurity forms a
different inorganic salt which therefore has a different color to
impart to the realtively clear aluminum oxide.
The best example is in the architectural anodizing, whereby
manganese is at a higher concentration in the aluminum recipe.
Manganese forms two oxides, one yellow, one purple. Under special
conditions the metal comes right out of the tank yellow to brown to
black, depending on anodizing parameters. These "doped" colors are
light fast, never fade.
Silicon Oxide (sand) is dull gray, so that is the color that is
mixed in with the clear aluminum when you process castings in the 360
and 380 groups.
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