Guides / Finishes

Zinc, Zinc-Nickel, Electroless Nickel, Anodize or Powder Coat: Matching the Finish to the Part

September 22, 2026

Most finishing decisions come down to two questions: what is the part made of, and what must the finish do? Answer those two and the list of sensible finishes drops from twenty to two or three. This guide compares the five finishes buyers ask about most, then shows which one fits which job.

All thickness figures here are typical ranges. The spec and class on your drawing decide the actual number.

The short version

Finish	Base material	Main job	Typical thickness	Common spec
Zinc plating	Steel	Low cost corrosion protection	5 to 25 µm	ASTM B633
Zinc-nickel	Steel	Higher corrosion protection	5 to 15 µm	ASTM B841, AMS 2417
Electroless nickel	Steel, aluminum, copper, stainless	Even coating, wear, corrosion	5 to 75 µm	ASTM B733, AMS 2404
Anodize Type II and III	Aluminum	Corrosion, color, wear	5 to 25 µm, 25 to 75 µm	MIL-PRF-8625
Powder coat	Steel, aluminum	Color and barrier protection	50 to 100 µm	Customer or AAMA spec

Zinc plating

Zinc is the default corrosion finish for carbon steel. It works as a sacrificial layer: the zinc corrodes first and protects the steel underneath, even at small scratches. A passivate on top, clear, yellow or black, slows down white rust and sets the color. Most shops now use trivalent passivates rather than hexavalent chromates.

Use it for brackets, fasteners, stampings and general hardware that sees indoor or mild outdoor conditions.

Write on the drawing the spec, service condition and passivate type, for example ASTM B633 with the Fe/Zn class and type. Say whether thread gauges apply before or after plating.

Watch for hydrogen embrittlement on high strength steel. Above about 39 HRC, plan a relief bake.

Zinc-nickel

Zinc-nickel alloys, usually around 12 to 15 percent nickel, give much longer salt spray life than plain zinc at a similar or lower thickness. That is why automotive and aerospace programs use it on fittings, brake parts and fasteners, and why it has replaced cadmium on many drawings.

Use it for steel parts exposed to road salt, marine air or outdoor service where plain zinc would not last, and for parts that need thin coatings on threads.

Write on the drawing ASTM B841 or AMS 2417 with class and minimum thickness, the passivate, and any sealer or topcoat.

Watch for a smaller supplier pool and higher price than zinc. It still needs a bake on hardened steel.

Electroless nickel

Electroless nickel deposits without electric current, so it builds the same thickness in bores, threads and recesses as on outside faces. That uniformity is its main advantage over every electroplated finish. Phosphorus content changes the properties: low and mid phosphorus deposits are harder, high phosphorus deposits resist corrosion better and are close to non-magnetic.

Use it for valve bodies, hydraulic parts, molds, aluminum housings that need wear and solderability, and any part with internal features that must be coated evenly.

Write on the drawing ASTM B733 or AMS 2404, the service condition or minimum thickness, the phosphorus range if it matters and any heat treatment for hardness.

Watch for pretreatment on aluminum and stainless. Adhesion lives or dies on the activation step. Ask the shop how they handle your alloy.

Anodize: Type II and Type III

Anodizing is not a coating laid on top of aluminum. It grows an oxide out of the aluminum itself. Type II, sulfuric acid anodize, is thinner and takes dye well, so it is used for color and corrosion. Type III, hard anodize, is thicker and much harder, used for wear surfaces, pistons and sliding parts.

Part of the oxide grows outward and part penetrates into the metal, roughly half each on hard anodize. A bore specified at 20.00 mm before hard anodize will come out smaller. Size critical features for the finished condition and tell the shop which dimensions apply after anodize.

Use it for aluminum housings, heat sinks, consumer products, aerospace parts and wear surfaces.

Write on the drawing MIL-PRF-8625 with type, class (dyed or undyed), thickness and seal. Mark grounding points to mask, because anodize is an electrical insulator.

Watch for alloy. 6061 and 6063 anodize cleanly. Cast alloys and high copper alloys such as 2024 give darker, less even results. For conductive surfaces, chem film to MIL-DTL-5541 is the usual partner finish.

Powder coat

Powder coat is a dry paint sprayed electrostatically and cured in an oven. It builds thick, typically 50 to 100 µm, and gives strong color, gloss control and a good barrier. On steel it goes over a phosphate or zinc layer for better corrosion resistance. On aluminum, over a conversion coating.

Use it for enclosures, frames, brackets, architectural parts and anything where color and appearance matter.

Write on the drawing the color by RAL or sample, gloss level, film thickness range, pretreatment and any performance spec, such as AAMA 2603, 2604 or 2605 for architectural aluminum.

Watch for thickness on threads and bores. Mask them or plan to chase threads after coating. Cure temperature is typically near 200 °C, which can soften some low tempered steels.

Which finish for which job

Job	Steel	Hardened steel	Stainless	Aluminum
Corrosion	Zinc-nickel, zinc	Zinc-nickel with bake, zinc flake	Passivation	Type II anodize, chem film
Wear	Hard chrome, EN	PVD, hard chrome with bake	PVD, EN	Type III anodize, EN
Color	Powder, e-coat	Black oxide, PVD	Electropolish, PVD	Dyed anodize, powder
Conductivity	EN, tin	EN with bake	Plate the contact area	Chem film, EN
Tight fit	Black oxide, phosphate	Black oxide, PVD	Passivation	Chem film

EN = electroless nickel.

Three drawing notes that prevent most problems

1. Name the spec and class, not just the finish name. “Black anodize” leaves thickness, seal and dye open.
2. State dimensions before or after finish. Especially on threads, bores and press fits.
3. Mark masking and contact points. Where it must stay bare and where a rack mark is acceptable.

See also: Metal finish selection chart by base metal and need.

Still not sure?

Describe what the part must survive and send the drawing. We are a matching service, not a plating shop: we pass your RFQ to up to three finishing shops that run the process and spec, and they can recommend the finish when you are unsure. See how to choose a metal coating supplier or go straight to the RFQ form.

Frequently asked questions

Keep reading

• Zinc Flake Coatings for Fasteners: When to Specify Them and What to Check
• Wear Surfaces: Choosing Between Hard Anodize, Nitriding and Hard Chrome