Reference

Electroplating Spec Guide

Zinc, zinc-nickel, electroless nickel, hard chrome, tin, and precious-metal finishes compared by purpose, typical thickness, and governing standard, so you can call out the right plating.

Electroplating deposits a thin metal layer onto a part by passing current through a plating bath. The metal you choose depends on the job: zinc and zinc-nickel are sacrificial corrosion protection for steel; electroless nickel and hard chrome add hardness and wear resistance; tin aids solderability and corrosion resistance; gold and silver deliver conductivity and cosmetic value. Each finish has a governing ASTM or AMS standard that fixes thickness classes and test requirements.

Common electroplated finishes compared

Thickness is usually called out as a class tied to a service condition. The values below are typical ranges; the standard for each finish defines the exact classes and tests.

Finish	Purpose	Typical thickness	Typical base metal	Standard
Zinc	Sacrificial corrosion protection	5-25 um (SC1-SC4), plus chromate	Steel, iron	ASTM B633
Zinc-nickel (alloy)	High corrosion protection, better than zinc	5-20 um typical	Steel, iron	ASTM B841
Electroless nickel (Ni-P)	Uniform corrosion and wear resistance	5-75 um (SC1-SC4)	Steel, aluminum, copper alloys	ASTM B733 / AMS 2404
Hard chrome	Wear, abrasion, low friction, build-up	Commonly 10-250 um; thin classes to 2.5 um	Steel, cast iron	AMS 2406 / ASTM B650
Tin	Solderability, corrosion, low contact resistance	Commonly 2.5-15 um	Steel, copper alloys	ASTM B545
Gold	Conductivity, contact reliability, corrosion	Commonly 0.05-5 um	Copper alloys, nickel underplate	ASTM B488
Silver	High conductivity, solderability, cosmetic	Commonly 2.5-25 um	Copper alloys, brass	ASTM B700

Typical ranges only. Precious metals are often plated over a nickel or copper underplate. Confirm the exact class, underplate, and post-treatment your part needs.

Zinc thickness classes (ASTM B633)

ASTM B633 ties zinc thickness to a service condition, from mild indoor exposure to very severe corrosive environments. A chromate or passivate conversion coating is applied on top for extra corrosion resistance and color.

Class	Min thickness	Service condition
Fe/Zn 5	5 um	SC1 - mild (indoor)
Fe/Zn 8	8 um	SC2 - moderate
Fe/Zn 12	12 um	SC3 - severe
Fe/Zn 25	25 um	SC4 - very severe

Electroless nickel vs hard chrome

Both add wear resistance, but they work differently. Electroless nickel (nickel-phosphorus, per ASTM B733 and AMS 2404) plates without electric current, so it deposits at a uniform thickness even inside bores and on complex geometry, and it gives excellent corrosion resistance. Hard chrome (per AMS 2406 or ASTM B650) is electrodeposited, is harder and better for high-wear sliding surfaces, but plates unevenly on complex shapes and needs racking. Choose electroless nickel for uniform coverage and corrosion; choose hard chrome for maximum hardness on wear surfaces.

Hydrogen embrittlement relief

High-strength steels (roughly 40 HRC and above) can absorb hydrogen during acid cleaning and plating, which causes brittle failure. Standards require a post-plate bake, typically several hours at about 190 to 220 C, to drive the hydrogen out. Always call out embrittlement relief when plating hardened or high-strength steel fasteners and springs.

Values reflect ASTM B633, B841, B733, B650, B545, B488, B700 and AMS 2404 / 2406, plus common practice. Verify the exact class, underplate, and post-treatment a shop can hold on your base metal before release.

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Frequently Asked Questions

What is the difference between zinc and zinc-nickel plating?

Both protect steel sacrificially, corroding in place of the base metal. Zinc-nickel alloy plating (per ASTM B841) offers substantially better corrosion resistance than plain zinc (ASTM B633) and holds up better at high temperature, which is why it is common on automotive and demanding parts. Plain zinc is cheaper and fine for milder service.

Which standard governs electroless nickel plating?

Electroless nickel-phosphorus coatings are covered by ASTM B733, and often also AMS 2404 for aerospace. These define service-condition thickness classes from about 5 microns (SC1) up to 75 microns (SC4), plus phosphorus content and test requirements.

How thick is hard chrome plating?

Hard chrome (AMS 2406 or ASTM B650) is applied over a wide range depending on the job. Thin functional layers can be a few microns for tight-tolerance parts, while wear surfaces and build-up of worn parts commonly run tens to hundreds of microns and are ground to size afterward.

Do I need hydrogen embrittlement relief after plating?

Yes, for high-strength and hardened steels, roughly 40 HRC and above. Acid cleaning and plating can introduce hydrogen that causes brittle failure, so standards require a post-plate bake, typically several hours around 190 to 220 C. Call it out for hardened fasteners, springs, and high-strength parts.

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