How to remove a heavy, mixed oxide film on aluminum

Working on the materials for the first and only online course about anodizing - the Introduction to Anodizing and thought I would like to share some information about how the natural formed oxide layer can be difficult to remove in our etching tank.

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Aluminum reacts immediately in contact with air forming a very thin oxide film - normally not more than 1 - 3 nm in thickness.

If you want to read more about this you will find an earlier blog post about 
The natural formed oxide layer by clicking the link

When aluminum alloys are heat treated this natural formed oxide layer thickness up to 10 times of the thickness of the air formed. The oxide layers formed at high temperature are more difficult to remove in the etching solution than the air formed ones.

The reason for this is the alloying elements - these alloying elements create an oxide with uneven thickness, as you can see here in the drawing.

The major alloying element incorporated within the film is magnesium. Magnesium is the primary alloying element in the 5000 series - aluminum magnesium alloy and is the major alloying element together with silicon in the 6000 series aluminum magnesium silicon alloys.

The mobility of magnesium atoms at temperatures above 340𝇈C are much higher than for aluminum atoms.

Magnesium atoms will diffuse from the bulk of the alloy through the surface and oxidize up to 5 - 10 times faster than the aluminum atoms.

Leaving 5000 and 6000 series aluminum alloy with a heavier and mixed oxide - containing aluminum oxide - the alumina and magnesium oxide - the magnesia.

Because the magnesium oxide is significantly more resistant to alkaline etching than aluminum oxide a patchy appearance can be found after etching.

The patchy appearance is due to dissolution of the aluminum oxide creating a surface with areas with a film of magnesium oxide and by this localized attack can roughens the surface.

A deoxidizing before etching can be used to remove this film, or some of the new types of acid etch - another topic which is covered in an earlier blog post - Acid etch - the hot topic.

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