Anodising guide - 2. Basics


2.1  Properties of the layer

Polished aluminum oxidizes in air very quickly but becomes covered with an air-impermeable colorless oxide (Al2O3). This very thin (typically 0.5-1 micron thick) layer protects the metal from further attacks of atmospheric oxygen. It is quite soft and unsightly and isn't acceptable for more advanced requirements because of its very low strength. However, using an aqueous electrolyte you can build up this layer much more giving it the following features:

2.2  Chemical reaction

The following main chemical reactions are running in the anodising bath:

There are two things apparent:

  1. The formation of the layer itself do not consume sulfuric acid. It serves as the electrolyte (this massively reduces the electrical resistance of the bath) and - this is the important part - to solve the layer locally, thus ensuring a continuous current flow and building of layer structure. Due to the small layer thickness a refreshing of sulfuric acid (recovery of initial concentration of 15-20%) is rarely necessary.
  2. At the cathode (negative terminal) gaseous hydrogen is generated, which in large quantities should be removed because of the explosion risk (hydrogen is odorless, tasteless and colorless!).

2.3  Structure of the layer

First in sulfuric acid a closed layer is electrolytically produced which grows further to fine capillary pores:

Anodised layer sectional view 1

The dye molecules now migrate into these pores:

Anodised layer sectional view 2

Finally, the pores are closed by sealing.

Anodised layer sectional view 3

So the dye is not applied on but is part of the anodized layer itself and thus protected very well from abrasion and chemical attack.