What anodizing is
Anodizing is an electrolytic process that converts the surface of aluminium into a controlled aluminium oxide layer. The oxide grows out of the substrate rather than being deposited on it, which is the defining property: because it is integral, it cannot delaminate, and a knife edge that flexes the metal will flex the coating with it instead of flaking it.
The same property works against dimensional control. The layer grows outward from each surface — roughly half the coating thickness per face — so a 25 µm coating adds about 12 µm per surface. On a bore, a fit, a threaded hole or a sealing face, that has to be designed in or masked.
- An integral aluminium oxide — grown from the substrate, not deposited on it
- Type II: 5–25 µm, appearance and corrosion protection
- Type III: 25–50 µm and above, hard and wear-resistant
- Aluminium only — there is no equivalent on steel or stainless
- Worked to MIL-PRF-8625, with the type and class declared
Type II and Type III: the decision that matters
Type II is the thin, functional class: 5–25 µm, mostly for appearance and atmospheric corrosion, the default for consumer and enclosure work. Type III is the thick, hard class — 25–50 µm and above — and it is what gives anodizing its wear resistance. Type III is a harder, more abrasive surface and a visibly heavier layer, and it costs more in material, time and masking.
State the type and the sealing on the drawing. An unsealed anodic layer is porous, which is fine for appearance but not the right choice where the surface will be wiped, wetted or handled; a sealed layer closes the pores and hardens the surface further. A drawing that says "anodized" without type and sealing does not fully specify the part.
| Criterion | Type II | Type III |
|---|---|---|
| Thickness | 5–25 µm | 25–50 µm and above |
| Primary role | Appearance and atmospheric corrosion | Wear resistance and serviceability |
| Hardness | Standard | Hard, abrasive-resistant |
| Dimensional growth | ~half the coating per face | ~half the coating per face — larger absolute effect |
| Typical use | Enclosures, panels, general appearance | Wearing surfaces, high-cycle parts |
| Sealing | State it — unsealed is porous | Normally sealed |
Dimensional change, and what to mask
Because the layer grows out of the metal, anodizing closes features. On a 25 µm coating that is roughly 12 µm per surface — enough to change a press fit, to seize a mating thread, or to stop a sealing face sealing. Three options exist, and which one applies is a drawing decision: allow for the growth in the design, mask the feature, or specify a thinner class.
Masking is routine on the features that must stay bare: bores that hold a bearing, threaded holes that will be tapped afterwards, ground datum faces, and any surface that makes electrical contact. Masking is specified, not assumed — a masked area is a hole in the coating that had to be made on purpose.
- Allow roughly half the coating thickness per face on fits and bores
- Mask threads that will be tapped after finishing
- Mask sealing faces, bearing bores and ground datums
- State type, thickness class and whether the coating is sealed
Anodizing against the alternatives
Anodizing competes with powder coating on aluminium and with plating on aluminium, and on any steel part it does not compete at all. The real comparison is usually Type II versus Type III, and then the question becomes whether an integrated finish is right for the part at all — a powder coat carries colour that anodic coatings cannot, and it is a deposited film, so it can be repaired in a way anodizing cannot.
Against passivation: passivation is a conversion treatment that restores and stabilises the surface for corrosion performance and leaves the part visually unchanged, with no measurable thickness. Where the requirement is corrosion rather than appearance or wear, passivation is often the more proportionate answer, and the two are frequently specified together — anodize for appearance, passivate for the corrosion system.
How WERIX specifies and executes it
Anodizing is partner-executed, coordinated under our specification, inspection and delivery control. We choose the class, declare the type and thickness on the drawing, and inspect the result — which is why these pages carry reference values from the specification rather than a certificate claim. ISO 9001 is the only management-system certification held; MIL-PRF-8625 is the specification the coating is worked to, not a certification WERIX holds.
The engineering question we settle at DFM review is usually the dimensional one. An anodized aluminium part whose holes, bores and threads were drawn without allowance for oxide growth is a part that will not assemble, and the rework happens after the coating rather than before it.