An electrochemical process that thickens the natural oxide layer on aluminium.
Anodising grows the aluminium oxide layer from the base metal rather than depositing anything on it, so it cannot chip or peel. It gives useful wear and corrosion resistance and takes dyes for colour.
It also adds to the part's dimensions — typically 5 to 25 µm per surface in the standard sulfuric process, more for hard anodising. That matters on threads and close-tolerance fits, and it is why anodised threads are specified with allowance.
Ignoring the dimensional growth on threads. A 1/4-20 thread that is within tolerance before hard anodising may not accept a nut afterwards.
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