Surface qualityReading time 12 min
Corrosion, wear or appearance? How to choose a surface treatment
Surface treatment is the last operation and often the first thing forgotten, forgotten that it adds thickness, and that it moves the tolerances.
01Comparing the common 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 |
02Dimensions change, and not always in the same direction
- 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.
So the drawing has to say whether the dimensions are before or after treatment. Without it, both sides argue at acceptance. The usual convention is that drawing dimensions are finished dimensions including treatment, but a convention is not a specification.
03Hydrogen embrittlement: the hidden killer in high-strength steel
Hydrogen generated during plating diffuses into the steel and, under stress, causes delayed fracture, a part can snap hours or days after assembly, with a fracture surface showing no plastic deformation.
- 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.
04How to call it out on the drawing
| 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 |