How does the process take place?

The oxide that is formed is not due to material input but to a transformation of the original surface of the object. The aluminum surface undergoing anodizing is progressively oxidized from the outside of the surface progressing inward. The ‘oxide that is formed has a larger volume than the original metal (aluminum) so that apparently, from a dimensional point of view, it seems away is a material contribution, in reality the thickness of oxide partly “penetrates” into the metal partly “grows” outward. In practice for a thickness of e.g. 10 microns, 5 microns penetrated and 5 microns expanded so the geometric increase amounts to 5 microns. Note that, on a diameter, the 10-micron thickness produces an increase in diameter of 10 microns, not 20.

Oxide also tends to form fairly uniformly in recesses, undercuts, and inside pipes. The structure of the oxide is very similar to the “honeycomb” structure with vertical tubules with respect to the thickness formed. These tubules are much less than a micron in diameter, but particular dyes can penetrate them, being able to impart a wide variety of colorations to the oxide layer. Following the formation of the oxide layer and, eventual staining, a pore-closing operation is performed in order to ensure compactness and corrosion resistance. This operation is called fixing and can be done cold or hot. in any case, the pores are closed; however, it turns out to be safer to heat fix especially for colored oxide layers.