Why is anodised coating on aluminum porous

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It is said that anodic coating is a kind of porous coating. what is the source of this porosity?I mean what are the agents which cause this porosity.

Muhammad Bashir
GIKI - Pakistan

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The solution that the piece is anodized in - typically sulfuric acid - dissolves the coating while it is being applied. The result is a thicker and more porous coating than would otherwise be possible.

Jim Gorsich
Accurate Anodizing Inc.

Compton, CA, USA