Function first, appearance second
Surface treatment serves two purposes: protection and appearance. Our recommended order of decisions is: establish the service environment (humidity, corrosive media, indoor or outdoor), then the appearance requirement (colour, gloss, texture), and finally whether the dimensional effect is acceptable.
Common processes
Electroplating
- Zinc (blue-white, yellow, black): the most economical anti-rust option for carbon steel fasteners and structural parts
- Nickel: better appearance, wear and corrosion resistance than zinc; suited to visible and conductive parts
- Chrome (decorative or hard): high hardness and wear resistance; hard chrome is common on wear surfaces of shafts
- Copper: usually a strike layer, or for conductivity
Chemical conversion coatings
- Blackening (oxidation): thin, low cost, almost no dimensional change; suitable for tools, gauges and general structural parts. Limited corrosion protection, normally used with rust-preventive oil
- Passivation: for stainless steel; removes free iron from the surface and improves corrosion resistance without changing dimensions or appearance
- Phosphating: mainly a pre-treatment for painting
Anodising (aluminium only)
- Natural, black or coloured; coating thickness typically 5–25 μm, growing roughly half inward, so dimensional allowance is required
- Hard anodising is thicker and harder, with better wear resistance but a larger dimensional effect
Mechanical finishes
- Polishing, brushing and blasting improve appearance and feel but add no protection
- Deburring and degreasing are prerequisites for almost every part
Heat treatment
- Used to change hardness and strength, generally between or after machining operations; distortion must be allowed for
Calculate the dimensional effect in advance
This is the most frequently overlooked point, and the most common cause of assembly problems. Coatings are added to the outside surface, so:
- Outside diameters grow, bores shrink, and thread fits become tighter
- Typical thicknesses: zinc about 5–10 μm, nickel about 5–15 μm, hard chrome 20–50 μm and above (varies by process and supplier — confirm with the finisher)
- On mating dimensions toleranced at ±0.01 mm, a few tens of microns of coating is already enough to decide whether the part assembles
There are two ways to handle it: specify pre-plate dimensions and let the machinist back-calculate the turning size, or state clearly on the drawing whether dimensions are "after plating" or "before plating". Either works, as long as both sides read it the same way.
How to write it on the drawing
Material + process + colour + coating thickness + salt-spray duration if applicable. For example: "carbon steel, nickel plated, bright finish, 8–12 μm, 48 h salt spray no rust". Then neither the machine shop nor the finisher has to guess.