A steel part with no coating starts rusting the week it leaves the shop. Plating is how you stop that, and it is also how you control appearance, hardness, solderability, wear, and electrical contact resistance. The problem for most product teams is that plating gets treated as a finishing detail — a line on the drawing that says "zinc plate" — when it is actually a specification with a dozen variables that decide whether the part survives three years on a truck or three months in a bathroom.
The four options you will actually choose between
Zinc
Electroplated zinc is the workhorse for steel: cheap, widely available, and sacrificial, meaning the zinc corrodes preferentially and protects the steel even where the coating is scratched. Standard commercial thickness runs 5–8 µm; heavier service classes go to 12–25 µm. The coating is then given a chromate conversion passivate, which is what determines both color and most of the corrosion life. Clear, blue, yellow, and black passivates are all common. Zinc is specified to ASTM B633, which defines service condition classes (SC1 through SC4) and passivate types, so "zinc plate per ASTM B633 SC3 Type III" tells a plater far more than "zinc plate."
Expect roughly 96 to 200 hours of neutral salt spray to white rust on a typical trivalent clear passivate, and considerably more with a topcoat sealer. Zinc is the default for fasteners, brackets, stamped hardware, and anything hidden inside an enclosure.
Zinc-nickel
An alloy deposit, typically 12–15% nickel, that outperforms plain zinc dramatically — 500 to 1,000 hours of salt spray to red rust is normal, and it holds up far better at elevated temperature. It costs meaningfully more per part and fewer shops run it. This is the choice for automotive underbody parts, marine hardware, and anything where a warranty claim costs more than the plating line item. It is also the standard replacement where cadmium was once used.
Nickel — electrolytic and electroless
Bright electrolytic nickel gives a decorative, reflective, hard finish and is usually the underlayer beneath decorative chrome. Electroless nickel is the more interesting engineering option: deposited chemically rather than with current, so it plates at uniform thickness into blind holes, threads, and internal bores where an electroplated coating would starve. Phosphorus content sets the properties — low phosphorus is harder, high phosphorus (10–13%) is more corrosion resistant and effectively non-magnetic. Electroless nickel per ASTM B733 is the go-to for hydraulic bodies, valve internals, molds, and precision parts that must keep their dimensions, since thickness holds within about ±10% across the whole geometry.
Chrome
Two completely different processes share the name. Decorative chrome is a very thin bright layer, often well under 1 µm, applied over nickel; the nickel does the corrosion work and the chrome supplies the color and tarnish resistance. Hard chrome is 20–250 µm of chromium applied for wear resistance and low friction on shafts, rods, and rollers, typically at 800–1,000 HV, usually ground to final size after plating.
Hexavalent chromium processes carry serious environmental and worker-safety obligations in the US under OSHA and EPA rules, which has pushed lead times up and shop counts down. Trivalent decorative chrome is now widespread and looks slightly warmer or darker than hexavalent. If your part does not truly need chrome, alternatives are often faster to source.
Comparing at a glance
| Finish | Main reason to pick it | Typical thickness | Relative cost |
|---|---|---|---|
| Zinc + trivalent passivate | Cheap sacrificial corrosion protection on steel | 5–12 µm | Lowest |
| Zinc-nickel | Long corrosion life, heat exposure | 8–15 µm | 2–3× zinc |
| Electroless nickel | Uniform thickness, hardness, complex geometry | 12–50 µm | High |
| Decorative chrome over nickel | Appearance, tarnish resistance | Under 1 µm chrome | High |
| Hard chrome | Wear surfaces, low friction | 20–250 µm | Highest |
The details that ruin plated parts
Hydrogen embrittlement. Acid cleaning and plating drive hydrogen into high-strength steel. Any part above roughly 40 HRC — hardened fasteners, springs, clips — must be baked, usually 4 to 24 hours at about 375–400 °F (190–205 °C), within a few hours of plating. Skip the bake and the part can crack days later with no warning. Call the bake out on the drawing; do not assume the plater knows the alloy and hardness.
Thickness is not uniform in electroplating. Current density concentrates on edges and corners and starves in recesses. A part that measures 10 µm on a corner may have 2 µm at the bottom of a pocket, and that is where it will rust. Design with generous radii, avoid deep blind features that need coverage, and specify where thickness is measured. Electroless nickel is the way out when uniformity is non-negotiable.
Racking and masking. Barrel plating is cheap and fine for small hardware, but parts tumble and can nest or dent. Rack plating costs more and leaves contact marks somewhere — decide with the plater where those marks are allowed. Masked areas for grounding, sealing surfaces, or press fits add real cost per part and should be minimized.
Base metal decides everything. Aluminum needs a zincate pretreatment before it will accept nickel, and for most aluminum parts anodizing is the better answer anyway — compare the economics in our guide to anodizing types, colors, and cost. If the material is still open, our comparison of aluminum versus steel is the right place to start, because the metal constrains the finish list.
The surface underneath shows through. Plating is thin and follows the substrate. A decorative chrome part needs a polished, low-Ra surface before it goes in the tank, and that polishing is often more expensive than the plating — see how to specify surface finish before promising a mirror.
When not to plate at all
Plating is not the only route to a durable finish. For sheet-metal enclosures and outdoor housings, powder coating often gives better corrosion life per dollar and any color you want, though it adds thickness that fasteners and fits must accommodate. Pre-galvanized stock is a good answer for many sheet-metal designs, and for high-volume stamped parts plating a coil before forming can be cheaper than plating finished pieces. Stainless steel simply avoids the question at higher material cost. The wider decision framework is laid out in corrosion protection for metal parts that last years.
Projects House specifies finishes as part of mechanical design — matching the coating to the environment, writing the drawing callout a plater can quote from, and qualifying it with real salt-spray and adhesion testing. If you have a part that keeps corroding or a finish quote that looks wrong, send us the details through our contact form.