Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of anodising. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
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.
Common misuse
Ignoring the dimensional growth on threads. A 1/4-20 thread that is within tolerance before hard anodising may not accept a nut afterwards.