Surface qualityReading time 12 min
Choose a Surface Finish Without Losing the Fit
Plating, coating and conversion treatment solve different problems. They also change dimensions and can affect tolerances. Specify the function, thickness and inspection point before release.
01Comparing treatments
| What to do | Materials | Typical thickness | Salt spray | Dimensional change |
|---|---|---|---|---|
| Anodising Type II | Aluminium | 5 ～ 25 μm | 300 ～ 1000 hr | About half the thickness grows outward |
| Hard anodising Type III | Aluminium | 25 ～ 100 μm | Above 1000 hr | As above, and significant |
| Zinc plating (trivalent chromium) | Steel | 5 ～ 15 μm | 96 ～ 240 hr | Grows outward by the coating thickness |
| Hot-dip galvanising | Steel | 45 ～ 85 μm | Thousands of hours | Very thick; threads must be opened up |
| Electroless nickel | Steel, aluminium, copper | 5 ～ 50 μm | 200 ～ 500 hr | Uniform; even reaches into deep holes |
| Black oxide | Steel | 1 ～ 2 μm | Low (needs oiling) | Almost none |
| Phosphate and zinc-flake coatings | Steel | 5 ～ 20 μm | Above 500 hr | Grows outward |
| Passivation (stainless) | Stainless | — | Improves existing corrosion resistance | None |
02How dimensions change
- Electroplating and electroless plating add material outward: holes get smaller, shafts get larger, by about the coating thickness per side.
- Anodising is different. Half the oxide grows outward and half consumes the base material, so a side grows by roughly half the coating thickness.
- Threads are affected most: the pitch diameter of an external thread grows by about four times the coating thickness because of the thread angle, which is why external threads are often specified 6g or even 6e before plating to leave room.
The drawing should make clear whether dimensions apply before or after surface treatment. A coating can change a fit even when the underlying part is within tolerance. State the inspection condition and any surfaces that must be masked, rather than relying on shop convention.
03Hydrogen embrittlement risk
Plating can introduce hydrogen into high-strength steel. Under sustained stress, that can contribute to delayed cracking after assembly. Where the risk applies, specify suitable material and process controls, including any required baking and verification.
- At risk: steels above HRC 32 or 1000 MPa tensile strength. High-strength bolts, springs and retaining rings are the classic cases.
- What to do: bake for de-embrittlement within four hours of plating, typically 190–220 °C for 4 to 24 hours, longer the harder the part (per ISO 9588).
- The more fundamental answer: use a process that generates no hydrogen, zinc-flake coatings, mechanical plating or physical vapour deposition.
04Specifying the treatment
| Element | Example | Why it is needed |
|---|---|---|
| Method and specification | Anodise per MIL-A-8625 Type II Class 2 | Specifies the process and class |
| Thickness | 10 ～ 15 μm | Left out and the treatment shop decides |
| Colour | Black / natural | Whether it is dyed changes the process |
| Extent | All over / except tapped holes | Masked areas must be clear |
| Dimensional basis | Drawing dimensions are after treatment | Avoids arguments at acceptance |
| Additional requirements | De-embrittlement bake, sealing, 500 hr salt spray | Mandatory on safety parts |