Anodising, plating, passivation and blasting: picking a finish that quotes cleanly
August 28, 2026
Finishing is where quotes quietly stop being comparable. Two shops read “black anodised” three different ways, and the parts that arrive prove it. The fix is not a longer email. It is naming a process, a class and a thickness.
As machined is a finish too
Every part comes off the machine with a finish. Typical milled surfaces land around Ra 1.6 to 3.2 micrometres, turned surfaces a little better. That is fine for most brackets, housings and internal components.
If that is all you need, write it down: “as machined, deburred, no coating”. Shops routinely add a bead blast for cosmetic uniformity because they assume you want it, and then you are paying for an operation you did not ask for.
Mechanical finishes
Deburring. Not optional. Assume every shop does it, but call out edge condition if it matters, for example “break all sharp edges 0.2 to 0.4 mm”. On sealing faces and O-ring grooves this is worth stating explicitly.
Bead blasting. Glass bead or aluminium oxide media gives a uniform matte texture and hides tool marks. Cheap, fast and usually in house. Specify the media if the appearance matters, because glass bead and aluminium oxide look different.
Tumbling and vibratory finishing. Best for high quantities of small parts. It rounds edges, which is good for handling and bad for sharp sealing features.
Polishing. Labour intensive and priced by the hour. Only specify it where a surface is optical, sealing or in contact with a moving part.
Aluminium: anodising is the default
Type II sulphuric anodising gives corrosion resistance and colour. Common thicknesses run 5 to 25 micrometres. Colour consistency between batches is a real issue, so if the parts are cosmetic and visible, say that they must match within a batch and approve a first sample.
Type III hard anodising is thicker and much harder, used for wear surfaces. It builds dimension significantly and changes the colour of the part toward dark grey or black regardless of dye.
Chromate conversion coating, often called by its trade names, is a thin film that keeps aluminium conductive while providing corrosion protection. If your part is an electrical ground path, this is usually what you want instead of anodising, which is an insulator.
Two details that cause rejects: threads that were anodised without being chased afterwards, and no masking on a bore that needed to stay bare. Both belong on the drawing.
Steel and stainless
Zinc plating with a clear or yellow passivate is the cheap corrosion answer for carbon steel. Ask for the thickness and whether hydrogen embrittlement relief baking is required, which matters on hardened parts.
Black oxide is a conversion coating, dimensionally negligible, mildly corrosion resistant when oiled. Good for tooling and fasteners, not for outdoor exposure.
Nickel plating, electroless in particular, gives an even build on complex geometry and good corrosion resistance. It is the usual answer when a steel part needs to look and behave like stainless.
Passivation for stainless steel is not a coating. It is a treatment that removes free iron from the surface so the natural oxide layer can form properly. If your stainless part will see moisture or a medical or food environment, specify passivation to a standard such as ASTM A967 and say which nitric or citric method is acceptable.
Powder coating is thick, tough and hides surface defects. It is not a precision finish. Mask anything that has to fit.
What a finish costs you
| Finish | Typical added lead time | Cost driver |
|---|---|---|
| Deburr and bead blast | 0 to 1 day | Usually in house |
| Type II anodise | 3 to 5 days | Batch size, colour, masking |
| Type III hard anodise | 4 to 7 days | Thickness, masking |
| Zinc or nickel plating | 3 to 7 days | Batch size, spec, baking |
| Passivation | 2 to 4 days | Batch size, method |
| Powder coat | 3 to 5 days | Colour changeover |
Small orders suffer most, because the finisher has a minimum batch charge and your 30 pieces are paying for a tank that could hold 3,000.
How to write the finish line
A finish line a shop can quote without asking anything looks like this:
All over: Type II anodise per MIL-A-8625, Class 2, black, 10 to 15 micrometres. Mask bore A and the four tapped holes. Chase threads after anodise. Colour to match approved first article.
It names the process, the standard, the class, the colour, the thickness, the masking and the acceptance basis. Every quote against that line means the same thing.
Masking, threads and the details that cause rejects
Most finishing failures are not about the finish itself. They are about the places the finish was not supposed to go.
- Threads. Anodising and plating build inside a thread and can make a fastener bind. Either mask the threads or chase them after finishing, and say on the drawing which you want.
- Press fit bores and bearing seats. Mask them or machine after finishing. A 25 micrometre build on a diameter is 50 micrometres on the fit.
- Electrical ground paths. Anodising is an insulator. If a face must conduct, mask it or use a conversion coating instead.
- Racking marks. Parts hang in the tank on racks and the contact point leaves a small mark. Specify where a rack mark is acceptable, or expect it somewhere visible.
- Sealing faces. A blasted surface will not seal against an O-ring the way a machined one does.
Getting colour right the first time
For anything visible, three steps prevent an expensive disagreement:
- Approve a physical first sample and keep it. Screens lie about colour.
- State that the batch must match the approved sample within a stated tolerance, judged in daylight.
- Say whether parts from different alloys or different batches will sit next to each other in the assembly, because that is where mismatch shows.
If you are still deciding, ask the shops to quote the part as machined and the finish as a separate line. You can then compare the machining honestly and treat the finish as the variable it is. That approach is covered further in the guide on comparing three quotes, and when the drawing is ready you can send it here.
Questions we get on this
Why does the finish add a week to my lead time?
Most machine shops do not anodise or plate in house. The parts leave the shop, sit in a queue at a finisher, come back and get inspected. Three to seven working days is normal, and it runs in series with machining, not in parallel.
Do I need to specify a standard or is a description enough?
A description gets you something that looks right. A standard gets you something you can reject. For anodising, MIL-A-8625 Type II or Type III with a class and a thickness is the common language. For plating and passivation the ASTM specifications do the same job.
Will anodising change my dimensions?
Yes. Type II builds roughly half its thickness outward, so a few microns on each surface. Type III hard anodising builds more and is often 25 to 50 microns total. On press fits and threads that matters, so either mask those features or machine them undersize by agreement.
Have a part like this?
Send the drawing. We shortlist the shop we place it with that run the process and material, and answer within 24 business hours.
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