A surface finish protects the part, changes how it slides or looks, or adds electrical function. Pick it for the job — plating is cheap insurance against corrosion and wear, but each process has trade-offs.
Corrosion protection
Zinc plating (with a chromate/passivate) is the workhorse for steel — sacrificial, cheap, good for indoor/mild use. Zinc-nickel and hot-dip galvanizing last far longer outdoors. Passivation of stainless removes free iron and restores the chromium-oxide layer (it adds no thickness). Electroless nickel gives an even, hard, corrosion-resistant layer that plates into bores and threads uniformly.
Aluminium: anodizing
Type II anodize is decorative/corrosion (and takes dye). Type III (hardcoat) is a thick, very hard, wear-resistant layer — but it grows and penetrates the surface (~50/50), so account for the dimensional change on tight fits and threads. Anodize is an insulator; mask where you need electrical contact or grounding.
Wear and friction
Hard chrome for hardness and wear on shafts and rams. DLC (diamond-like carbon) and nitriding give low-friction, very hard surfaces for tooling and moving parts. PTFE-loaded coatings lower friction on fasteners and sliding faces.
You specify 12 µm of zinc plating on a Ø20 h7 shaft. What happens to the fit?
Plating goes on all round, so a diameter grows by twice the thickness: 24 µm on a band that is only 21 µm wide at this size. The part leaves the plating shop out of tolerance even though the plating was perfectly to specification. The fix is to dimension the part before plating with the growth allowed for, and say so on the drawing — "dimensions apply after plating" is a note that changes what the machinist has to cut. It is the most common way a correct drawing produces incorrect parts.
Try it — what the coating does to the fit
Cosmetic and process notes
Powder coat and paint for colour and outdoor durability over a primer. Beware hydrogen embrittlement on high-strength steel from acid pickling and electroplating — it needs a post-plate bake. And always call out which surfaces are masked, and the thickness, on the drawing.