Aluminium is corrosion resistant on its own: the moment it meets air, a thin transparent oxide layer forms on the surface and protects the metal. But that natural layer is very thin, does not resist wear and offers no control over colour. Surface treatment exists to close exactly those three gaps — thicken the protection, add wear resistance and define the appearance.
Three routes come up most often in practice. They do not do the same job; which one is right depends on where the part will work.
What each method does
Anodising
Anodising is not a coating — it adds nothing on top, it grows the existing oxide layer electrochemically. The resulting layer is converted from the metal itself, so it does not peel; even scratched, it does not separate from the metal beneath. It gives a hard, wear resistant surface that keeps a metallic look. Colour is applied by dyeing the porous layer before it is sealed.
Powder coating
Powder coating is a film held on electrostatically and fused in an oven. It is thick, opaque and the colour range is effectively unlimited. Outdoor durability and impact resistance are good, and its ability to hide small surface defects in a casting is far higher than anodising's.
Chromating (conversion coating)
Chromating forms a very thin conversion layer on the surface. It is not a decorative finish in its own right; it does two jobs: it provides light corrosion protection and improves the adhesion of paint applied over it. It also largely preserves electrical conductivity — the deciding property on earthing faces and enclosures that need EMC shielding.
Side by side
| Anodising | Powder coating | Chromating | |
|---|---|---|---|
| What it does | Grows the oxide layer | Films over the surface | Thin conversion layer |
| Appearance | Metallic, clear or dyed | Opaque, plain or textured | Almost invisible |
| Wear resistance | High | Medium | Low |
| Hides defects | No — it accentuates them | Hides well | No |
| Electrical conductivity | Insulates | Insulates | Largely preserved |
| As a paint base | Not suitable | It is the top coat | Its main use |
The alloy shapes the appearance
This is where die cast parts produce the most surprises. Anodising is sensitive to alloy composition: on high silicon casting alloys the surface does not come out as clear and even as it does on wrought aluminium — it is darker, greyer and varies with the structure of the alloy. That is not a defect; it is a natural consequence of the alloy.
The practical conclusion: if decorative anodising is expected, the alloy decision has to be made for it from the start. An "let's anodise it" decision taken after the alloy is fixed, the die is built and series production is running usually means either lowering the expectation or starting over. Our alloy selection article covers this link in detail.
Powder coating is far more forgiving here: being an opaque film, it covers both the alloy's colour and small surface waviness.
Which one? A short decision guide
- The part works outdoors, exposed to weather → Powder coating. A thick film gives the widest safety margin against UV and moisture.
- It will be rubbed and scratched → Anodising. The hard oxide layer wins on wear.
- Electrical contact or EMC shielding is required → Chromating. This is the one case where you do not want an insulating layer.
- Colour range and branding matter → Powder coating. Colour flexibility decides it.
- A metallic look is wanted → Anodising, but with the alloy constraint understood. Settling the expectation against a sample is essential.
- The part will be painted later and the coating is an interlayer → Chromating.
Preparation matters as much as the treatment
Whichever method is chosen, the result depends on the state of the surface underneath. Burr residue, parting line marks, release agent residue or oil after machining — all of it stays under the coating and causes trouble there. That is why we treat deburring and surface preparation as a precondition of coating; blasting both cleans the surface and gives the texture the coating grips.
How is the result approved?
Surface finish is the hardest thing to specify in words: "make it grey" or "make it matt" produce two different pictures on the two sides of the table. That is why approval runs against a sample, not against a sentence.
- Colour reference. On powder coating, a standard colour code settles the matter. On anodising a code is not enough; you need a reference sample produced in the same bath, on the same alloy, with the same surface preparation.
- Gloss and texture. The same colour looks completely different in matt and gloss. Blasting grade shifts that picture too; a reference sample fixes both at once.
- Adhesion check. Whether the coating holds onto the surface is the most practical indicator that preparation was done right, and it is checked periodically in series production.
- Acceptable variation. Small tone differences between batches are possible. Unless the limit of that variation is agreed up front, every shipment turns into a separate argument.
Once approved, the reference sample is kept; later batches are judged against it. It is the most reliable way to put a visual expectation in writing.
A common mistake
Leaving surface treatment to the end of the project. The decision affects the alloy, the draft angle (a thick coating builds up on sharp corners), the tolerance calculation (coating thickness adds to the dimension) and which faces need masking. Discussed before the die is built, all of it is free; discussed afterwards, each item is a separate revision.
The second mistake is leaving coating thickness out of the tolerance calculation: on fitting faces and tapped holes the dimension changes after coating. Which faces are to be masked therefore belongs on the drawing.
Tell us the service environment of your part and we will set the surface route together — we run surface treatment and coating through an approved supplier network.