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  for Metal Finishing since 1989
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Hard Anodize Colors

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
2003

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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 relatively 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.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003