Anodizing is a popular electrochemical process to change the surface properties of a metal by growing a protective metal-oxide layer.
This layer has ceramic properties, like electrical insulation, transparency, and resistance to wear and corrosion. Because of its porous structure, it is possible to color the passivation layer by implanting pigments or dyes into the pores, thus making the coloring permanent and resistant.
Thin oxide films even allow for interference effects resulting in various color for cosmetic reasons.
Most anodic films are generally stronger, more adherent to the metal substrate and longer lasting than organic paints or metal plating, but are brittle and can crack in forming processes or from thermal stress.
Although some metals like aluminium, titanium or magnesium naturally form a thin (few nanometer) oxide layer, when exposed to the atmosphere, it is not a strong protection against corrosive media.
It needs a series of electrochemical (galvanic) process steps to grow a passivation oxide-layer that is strong enough to serve as an effective shield against corrosion and wear.
The first step of the anodizing process involves a cleaning step to remove any organic residuals on the surface and the natural passivation layer.
After a rinsing step, the aluminium part will undergo the main reaction in a bath of sulphuric acid.
A direct current is applied so, that the metal acts as an anode and slowly grows a porous oxide layer as a result of the oxidation reaction.
These pores need to be sealed off to improve the wear resistance of the oxide, by placing the metal part in boiling water for a period of time.
Typically anodizing is done in a batch process, lowering metal parts or sheet panels in different reaction baths, but it is also possible to create thin anodizing films via roll-to-roll processes.