Bare metal parts rarely leave the factory unfinished. Surface treatments fight corrosion, reduce wear, improve appearance, and change electrical or thermal behavior. Choosing the wrong one means parts that rust, seize, or peel within months.

Anodizing (aluminum) grows a controlled oxide layer electrochemically. The part becomes the anode in an acid bath; oxygen ions migrate into the aluminum surface, building Al₂O₃ that's harder than the base metal. Type II (sulfuric, 0.0001"–0.001" thick) is decorative and corrosion-resistant — what you see on MacBook cases. Type III (hardcoat, 0.002"+) is structural; used on pistons, gun parts, and aerospace housings. Critically, anodizing grows into the part by ~50% of layer thickness and builds out by ~50%, so tight-tolerance features need pre-machining undersize.

Electroplating deposits metal from solution onto a conductive substrate. Zinc plating (5–25 µm) is the cheap default for steel fasteners — sacrificial, corrodes preferentially to protect the base steel. Nickel plating gives a hard, bright finish but is cathodic to steel: a scratch through nickel accelerates rust underneath. Hard chrome (25–500 µm) is for wear surfaces like hydraulic cylinder rods. Electroless nickel deposits uniformly into blind holes and threads where line-of-sight plating fails.

Powder coating is electrostatically sprayed thermoset polymer, cured at 350–400°F. Thicker than paint (50–150 µm), tougher, and no solvents. Excellent for outdoor steel: patio furniture, electrical enclosures, fence posts. Limitations: doesn't tolerate sharp inside corners (powder pulls away from edges) and adds enough thickness to interfere with press fits and mating surfaces. Mask threads and bearing bores before coating.

Galvanizing is hot-dip zinc — parts dunked into molten zinc at 850°F. Coating is thick (50–100 µm) and metallurgically bonded, lasting 50+ years outdoors. The reason every highway guardrail and transmission tower uses it.

Rule of thumb — coating thickness and tolerances:

• Type II anodize: budget 0.0005" diameter growth on shafts, 0.0005" bore shrinkage
• Zinc plate: 0.0003"–0.001" per surface
• Hardcoat anodize: 0.002" per surface, double-sided features lose 0.004" clearance
• Powder coat: 0.005"–0.010" per surface — never apply to threads or bearing seats

Real-world example: A custom aluminum enclosure with 6-32 tapped holes gets Type III hardcoat. The threads bind because anodize grew 0.002" per flank, eating the 0.0035" thread clearance. Fix: mask threads with silicone plugs before anodizing, or tap holes oversize and pre-account for growth.