Surface finishing: specified, sequenced and measured
A finish is a process, not a coat of paint. Thickness changes the dimension, pretreatment changes the corrosion result, and masking decides whether the bore still fits.
Finish options and what they change
| Finish | Substrate | Thickness | Dimensional effect |
|---|---|---|---|
| Anodizing AA5–AA30 | Aluminium | 5–30 µm | Grows about half the thickness per surface |
| Hard anodizing | Aluminium | 25–150 µm | Up to 75 µm per surface, mask precision bores |
| Chromate conversion | Aluminium | 0.3–1 µm | Negligible, keeps conductivity |
| Passivation | Stainless | Nil | None, removes free iron only |
| Electroless nickel | Steel, aluminium | 10–50 µm | Uniform build, tight bore tolerance |
| Zinc plating | Steel | 5–15 µm | Threads need allowance |
| Powder coating | Any metal | 60–120 µm | Large build, mask all fits |
| PVD | Steel, aluminium, titanium | 1–5 µm | Decorative and wear resistant, tight control |
Sequencing rules that prevent rejects
If a bore must finish at Ø20.00 H7 after 15 µm of electroless nickel, it is machined oversize by twice the coating thickness and checked after plating. Machining to nominal and plating on top produces a part that will not accept the bearing.
Masking is quoted with the finish. A masked thread and a plated thread are different parts, and an anodized thread will not take a mating screw without chasing it, which removes the coating and the corrosion protection with it.
Blasting after anodizing breaks the sealed oxide layer and leaves a mottled finish. The blast belongs in the sequence before the anodic layer is grown.
Free iron left by tooling starts corrosion in storage. Passivation removes it, and doing it before the final wash and pack is what makes the difference on a part that travels by sea.
How finish is inspected
Thickness is measured on coupons or on the part with a non-destructive gauge and recorded per batch. Adhesion is checked by tape or bend test on a coupon from the same load. Colour and gloss are compared against an approved sample, because two anodize loads of the same alloy can differ visibly.
For cosmetic parts the drawing should say where the finish matters. A visible face and a hidden mounting face are not the same requirement, and treating both as cosmetic is how finish cost doubles without improving the part.
| Spec | AA5–AA30 per GB/T 8013.1, hard anodize per GB/T 19822, salt spray hours |
|---|---|
| Colour | Pantone or RAL reference, plus "dye lot variation acceptable" or not |
| Masking | Bores, threads, grounding pads, mating faces |
| Critical surfaces | Mark cosmetic versus functional areas |
Questions we get on this process
Yes, to a Pantone or RAL reference. Anodized colour varies with alloy and dye lot, so if two batches must match exactly, plan them as one run rather than two orders months apart.
Yes, by roughly half the coating thickness per surface, up to about 75 µm on a 150 µm coating. Precision bores should be masked, or machined undersize with the growth accounted for on the drawing.
Yes, for the finishes supplied and the material purchased. Declarations are issued against the batch, not as a generic company statement, because plating chemistry can change between loads.
Sometimes, quoted separately and inspected on arrival. If incoming parts have burrs or handling marks the finish will show them, and fixing that is not part of a plating price.
Send the drawing with the finish callout on it.
We will confirm whether the coating thickness affects any tolerance, and quote masking as a line item so nothing is discovered after plating.
From the shop notes
Longer answers to the questions these pages raise.