Updated 2026 — The anodic layer is specified by thickness class (AA10/AA15/AA20/AA25, per ISO 7599): a porous, hexagonal-cell oxide structure that accepts dye and is then hydrothermally sealed. Harsher ISO 9223 corrosivity categories call for thicker classes on solar mounting profiles.
Why aluminum anodizing dyes so well? It has to do with the microscopic structure of the anodizing layer.
The oxide or “anodic” layer formed in the anodizing process consists of microscopic hexagonal oxide columns with holes or “pores” which extend nearly the entire length of each column. It is these pores which hold the dye.
The anodic layer is extremely hard- usually between 60-70 Rockwell C. The layer is also an excellent insulator, with a .002″hard coat layer having a breakdown voltage of 1500-2000 volts.
The characteristic hexagonal structure of anodizing (hypothetically about 40,000x magnification). Note the hexagonal oxide columns have been sliced through to show the structure. An actual layer would be much thicker, with the height of the columns 200 or more times higher than their diameter.
The anodic layer grows outward from the aluminum surface as a part is anodized. As such, very sharp inside or outside corners on a part can cause gaps or voids to develop- especially with thick anodic layers such as Type III hard coat anodizing. Avoid very sharp inside or outside corners when possible, and allow a radius of at least.015″(1/64th”).
In outdoor environment, need to seal the holes to have better weather proof performance.
