What the number means
An anodic film grows both inward and outward from the original surface, and the class number is the average total film thickness in micrometres measured on significant surfaces. AA10 is roughly 10 microns, AA20 roughly 20. Dimensional allowance matters: a 20 micron film changes the fit of a close-tolerance assembly by a few microns per surface.
The film is not a coating laid on top — it is converted substrate. It will not peel, and it is harder than the aluminium underneath. What it will do is absorb dye unevenly if the alloy or the bath is inconsistent, which is why the class and the alloy specification belong in the same order.
Which class for which environment
- AA5: interior, dry, low traffic. Trim, decorative parts, indoor hardware.
- AA10: protected exterior and light interior duty. Window frames in sheltered positions, general fabrication.
- AA15: the usual architectural minimum for exterior building product. Curtain wall, window and door systems in normal urban exposure.
- AA20: aggressive exposure — coastal, industrial, high UV, or where cleaning access is difficult.
- AA25: severe marine or heavy industrial exposure, and specifiers who prefer margin over cost.
Thickness is the durability lever, not the appearance lever. If the problem you are solving is colour consistency, thickness helps but the single-heat and master-sample controls do the real work.
Sealing is half the specification
An unsealed anodic film is porous and will hold dirt and stains. Sealing closes the pores, and sealed versus unsealed is the difference between a facade that weathers cleanly and one that greys unevenly in two years.
Sealing quality is measured by mass loss in an acid dissolution test, ISO 3210 being the common method. Ask for the result rather than the word "sealed". Because sealing is a process step rather than a dimensional property, it is also the step most likely to be skipped when a job is rushed.
Alloy matters as much as class
6063 is the architectural anodising alloy, and its silicon-to-magnesium ratio is what produces a clean, bright, consistent film. 6061 anodises acceptably but darker and less uniformly, which is visible the moment the two sit next to each other on the same facade. 5xxx alloys can go grey at thicker films, and high-silicon casting alloys anodise poorly.
If the assembly mixes extrusions, sheet and castings and all of it is anodised, expect colour differences that no film thickness specification will fix. The answer is to design the finish around the alloy, or to use a paint system instead.
Comparison table
| Class | Average film | Environment |
|---|---|---|
| AA5 | 5 µm | Interior, dry, decorative |
| AA10 | 10 µm | Sheltered exterior, light duty |
| AA15 | 15 µm | Standard architectural exterior |
| AA20 | 20 µm | Coastal, industrial, high UV |
| AA25 | 25 µm | Severe marine or industrial |
Name the class, name the sealing standard, and ask whether the quantity can be anodised from a single heat. Those three lines prevent most of the disputes that anodised aluminium generates.