There're different types of coating.
Anodizing is one of the popular process for aluminum. It is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called anodizing because the part to be treated forms the anode electrode of an electrolytic cell.
Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light.
Purpose of Anodizing Aluminum
The purpose of anodizing aluminum is to prevent corrosion, improve appearance unity, enhance wear resistance as well as to go for many other reasons, such as complete adhesive bonding, better paint coating effect, and stronger pressure tolerance in industrial applications.
Aluminum is fully anodized when it is put in an acid tank that is electrified by direct current electricity. The electricity turns some of the water in the acid pool into positively charged hydrogen and negatively charged oxygen, while the oxygen is physically attached to the positively charged aluminum surface, where aluminum hydroxide and related compounds arise after such chemical reaction.
On the other hand, cathode may cause hydrogen gas bubbles gradually evaporate into the air when hydrogen is bound to negatively charged electrode.
Although acid serves to conduct the electricity, it wears away the anodized film, a process that helps to creates a porous coating covered with tiny and invisible pits. As a result of this destructive chemical reaction, there are various properties coming about on the anodized coating in terms of strength and density of aluminum surface.
Typical Anodizing Sequence:
Clean->Rinse->Etch->Rinse->Desmut->Rinse->Anodize->Rinse->Neutralize->Rinse->Dye->Rinse->Seal->Rinse
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