Plating is chosen for what it does to the surface: protect steel from corrosion, harden a wear face, keep a surface conductive, or produce a bright decorative appearance.
Matching the process to the requirement
Zinc — the workhorse for steel parts and fasteners. Cheap, effective and available with clear, yellow or black passivation.
Electroless nickel — uniform thickness on complex geometry, good wear resistance and excellent corrosion protection, used on hydraulic parts, valve components and precision machine elements.
Decorative nickel and chrome — for parts that are seen, often over a polished or bright base.
Hard chrome — for wear surfaces such as shafts and guides where a thick, hard deposit is required.
Fit and masking
Plating adds a measurable layer, so any thread, bore or press fit affected by it is either masked or machined to a pre-plate dimension. Send the finished tolerance — not just the as-machined drawing — for any plating job.