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.
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.