Spangle on etch aluminum EN AW 6063

The spangle effect on the aluminum EN AW 6063 alloy has been known for many years.

There are still a lot of unanswered questions but more recently explanations have thrown light on some of the issues. 

The spangling effect on aluminum can be divided into three categories: grainy, galvanizing and sparkling.

The type where the grains are visible by the naked eye after etching is called grainy. Galvanizing is used when only few of the grains are shiny, and the sparkling is when the whole surface is shiny.

The first picture shows a EN AW 6063 aluminum alloy directly from extrusion - not a very beautiful appearance and with a lot of die lines - looking closer you can maybe see some of the grainy structure already. 

The next picture shows the surface after etching and anodizing. Here a very grainy structure is seen leaving to a very unattractive surface - especially if it should fit together with areas which are not having this surface appearance.

If you as a designer want to create something special - this could be a very interesting surface appearance to work with.

Well, back to the subject - why do we see this spangle effect on anodized aluminum.

There are two reason to look for - the first one is the content of zinc (Zn) in the etching tank and the second is the content of Zn in the aluminum alloy.

The grainy appearance is caused by the chemical composition, whereas the two more shiny appearance are due to the content of Zn in the solution and grain orientation.

The orientation of the grains has an influence on how much they are etched in the alkaline solution. Zinc is more noble than aluminum so in the alkaline solution there will be a selective dissolution of aluminum. This will lead to a higher concentration of Zn in the grains with a certain orientation creating small galvanic elements from grain to grain.

If at the same time there is zinc in the alkaline solution, even as little as 5 ppm can cause spangle under the right conditions.

Addition of a small amount of sodium sulphide can be used to precipitate the zinc decreasing the spangling effect of the aluminum alloy.

The EN AW 6063 itself should not contain more than 0.03% zinc.

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