Plating for CNC Machined Parts: Electroless Nickel vs Zinc

Your Plated Part Came Back Too Tight — Here's the Real Reason

You called out "plate per ASTM B633" and the parts returned a few thousandths too tight. That isn't the plater's mistake — it's a dimension you didn't plan for. Plating is a deposit, not a coating that soaks in, so every surface grows by the full thickness: a bore, a thread, a press fit each loses that much clearance. The fix isn't a bigger ream after plating; it's machining undersize before it. Tell us the finished dimension you need and we'll set the pre-plate size so it lands on target the first time.

What it is

Electroless Nickel vs Zinc — Which You Should Ask For

Both stop corrosion, but they protect differently and they treat your tolerances differently. Pick from the function, not the silver color.

Property	Electroless Nickel (ASTM B733)	Zinc plating (ASTM B633)
Protection mechanism	Dense barrier coating	Sacrificial (anodic) — protects steel even when scratched
Substrates	Steel, stainless, aluminum, copper, brass (zincate first)	Carbon & low-alloy steel only
Typical thickness	5–50 μm (5–25 μm common)	5–25 μm per side (SC1 5 / SC2 8 / SC3 12 / SC4 25 μm)
Hardness	550–650 HV as-plated; 850–1000+ HV heat-treated	Soft coating; low wear resistance
Thickness uniformity	Even everywhere, incl. bores & threads (±10%)	Uneven — thick on edges, thin in recesses
Corrosion (ASTM B117)	High-P EN 1000+ h at 25 μm	Trivalent clear ~24–72 h; yellow 200–500 h; SC4 500+ h
Best for	Wear surfaces, complex geometry, precision fits	Cost-sensitive steel, fasteners, brackets
Cost level	Medium–high	Low–medium

These are the ranges in ASTM B733 and ASTM B633 and the ASTM B117 salt-spray results reported by finishers; the class on your drawing is binding. Electroless nickel is further graded by phosphorus content — Type II/III (low, 1–4%) is hardest as-plated, Type IV (mid, 5–9%) is the general-purpose default, and Type V (high, 10%+) gives the best corrosion and is non-magnetic. See the surface finish comparison chart for how plating sits beside anodizing, passivation and black oxide.

The Dimension Trap: Plan for the Coating Before You Machine

Plating builds outward on every wetted surface, so the coating thickness is a real dimension you machine around — not an afterthought. A 25 μm electroless-nickel layer deposits 25 μm on each wall of a bore, shrinking a 10 mm hole to 9.95 mm (a 50 μm loss) and growing a 20 mm outside diameter to 20.05 mm. Zinc is similar in magnitude, 5–25 μm per side, but it piles up on edges and thins inside recesses, so a 12 μm zinc coat (SC3) on an M6 external thread grows the major diameter about 24 μm — often enough to turn a free fit into a tight one.

Feature	Before plating (machine to)	After 25 μm EN / 12 μm zinc
10 mm bore (finished)	10.050 mm / 10.024 mm	10.000 mm / 10.000 mm
20 mm OD (finished)	19.950 mm / 19.976 mm	20.000 mm / 20.000 mm
M6 external thread	undersize to leave ~50 μm / ~24 μm	full major diameter

The practical rule: machine critical bores about 2× the coating thickness oversize, or mask the feature so it stays bare; for threads, cut undersize or mask and tell us which. We flag this during quoting so the allowance is built in and the finished part measures where you drew it. The full tolerance chart shows the bands we hold.

When to specify

Pick the Finish From the Job, Not the Look

The right call comes from the base material, the environment, and how tight the fit has to be.

Choose this when…	Specify
Carbon-steel part, rust is the only enemy, tolerances are open	Zinc plating ASTM B633 SC1–SC2, clear or yellow chromate
Steel outdoors / automotive / marine, longer life needed	Zinc ASTM B633 SC3–SC4 yellow chromate, or zinc-nickel for 720 h+ red-rust
Wear surface, sliding contact, or hardTooling	Electroless nickel ASTM B733 Type IV, heat-treated for hardness
Complex geometry, bores, blind holes, tight precision fit	Electroless nickel ASTM B733 — uniform ±10% everywhere
Aluminum part needs corrosion + conductivity or color	Anodizing (MIL-A-8625), not plating
Stainless part needs corrosion, no buildup	Passivation (ASTM A967 / AMS 2700)

Two hard rules we repeat so the drawing is right: zinc plating is for steel only — it does not belong on aluminum or stainless; and high-strength steel (about 31–32 HRC and up) needs a hydrogen-embrittlement relief bake per ASTM B850 after plating. We include that bake automatically when the material calls for it.

How We Handle Plating on Your Parts

We machine your parts in-house and arrange plating through our finishing partner per ASTM B733 or ASTM B633, so the quoted lead time already includes the handoff — the same route as our other surface finishes. On the drawing, call it out explicitly: "Plate per ASTM B733 Type IV SC3" or "ASTM B633 SC3 yellow chromate." Tell us the function — wear, corrosion, electrical — and we'll pick the phosphorus class or chromate and machine the allowance before plating so the finished dimension lands where you need it. For high-strength steel we add the hydrogen-embrittlement relief bake per ASTM B850, and our partner returns plating certs and thickness reports with the shipment. Material choice drives the pre-treatment, so we confirm the substrate before the bath goes in.

Turned steel and alloy components ready for plating — **Turned steel parts**
Shafts, fittings, connectors.

CNC milled flange — precision body for plated or passivated finish — **Milled bodies**
Flanges and housings.

Precision CNC parts — electroless nickel plated for wear or corrosion — **Precision parts**
Plated for wear or corrosion.

FAQ

Plating — Common Questions

Should I specify electroless nickel or zinc plating on my CNC part?

Default to zinc plating for carbon-steel parts when the goal is cheap corrosion protection and tolerances are forgiving; choose electroless nickel when you need wear resistance, uniform thickness on bores and threads, or tight tolerances on complex geometry. Zinc is a sacrificial coating applied only to steel; electroless nickel plates steel, stainless, aluminum and copper uniformly with no electric current, so it lands evenly in deep holes and threads where electrolytic plating thins out, and it heat-treats to about 850–1000 HV. When it does not apply: do not specify zinc on aluminum or stainless — those substrates need anodizing or passivation instead, so calling for zinc there wastes the job. Basis: ASTM B633 (zinc), ASTM B733 / AMS 2404 (electroless nickel), ISO 4527.

How much does plating change my finished dimensions?

Plan for 5–50 μm added per surface — a 25 μm electroless-nickel coating shrinks a 10 mm bore by 50 μm and grows an outside diameter by the same, because the deposit builds outward on every face. Electrolytic zinc is similar in magnitude, 5–25 μm per side, but uneven: thicker on edges and thin in recesses, so internal threads and bores need a machined allowance or masking; electroless nickel is far more predictable at about ±10%. When it does not apply: black oxide and chem film add almost nothing, so if you cannot absorb any thickness change, compare those finishes instead. Basis: ASTM B733 service conditions SC0–SC4 and ASTM B633 SC1–SC4; thickness values are standard minima, not shop limits.

Does plating work on aluminum parts?

Yes, but rarely as zinc — aluminum is normally anodized, and electroless nickel is the plating you would actually use, after a zincate pretreatment so the coating adheres. Electroless nickel is common on aluminum connectors, heat sinks and precision parts for solderability, wear and corrosion, and its uniform deposit offsets aluminum's softness; zinc plating on aluminum is not recommended because the benefit is low and adhesion is poor. When it does not apply: if the goal is corrosion plus conductivity or a colored finish, anodizing or chem film usually beats plating — pick the finish from the function, not the look. Basis: ASTM B733 (aluminum substrate noted); anodizing per MIL-A-8625 / ASTM B580.

Will plating make my threads not fit?

It can — plating grows external threads oversize and shrinks internal threads, so state on the drawing whether threads are plated, masked, or cut oversize to leave room for the coating. The standard fix is to machine threads undersize before plating, or mask the threaded section so it stays bare; for internal threads you usually plate after the thread is cut, and for external threads you plan the allowance up front. When it does not apply: this only matters for functional, close-tolerance threads — a decorative or clearance thread rarely needs the allowance, so do not over-specify and add cost. Basis: industry-typical plating practice; ASTM B633 / ASTM B733 thread handling.

How do you verify plating thickness and quality?

We verify coating thickness (by cross-section or X-ray gauge) and salt-spray performance per ASTM B117, and our finishing partner supplies plating certs and thickness reports with the parts. For electroless nickel we confirm phosphorus class and hardness; for zinc we confirm the ASTM B633 service condition and the chromate type, and we check critical dimensions after plating rather than only before. When it does not apply: salt-spray hours measure coating durability, not your part's fitness for service — the drawing's thickness and specification are what actually govern performance. Basis: ASTM B117 (salt spray), ASTM B733 / ASTM B633 (specifications), material certs to EN 10204 3.1 on request.

Standards referenced

Sources & Further Reading

ASTM B733 / B733M — autocatalytic (electroless) nickel-phosphorus coatings: types, service conditions (SC0–SC4), heat-treat classes.
ASTM B633 / B633M — electrodeposited zinc coatings on iron and steel by service condition (SC1–SC4) and chromate type.
ASTM B117 — operating salt spray (fog) apparatus for corrosion testing of coatings.
ASTM B850 — post-coating hydrogen embrittlement relief bake for plated high-strength steel.
ISO 4527 — electro-deposited nickel, autocatalytic nickel, and chromium coatings (international equivalent of B733 class structure).
AMS 2404 / AMS-C-26074 (formerly MIL-C-26074) — aerospace electroless nickel requirements.

Thickness and hardness figures above are the ranges in the cited ASTM/ISO standards and salt-spray results reported by finishers, not proprietary shop limits. Where this page says "industry-typical," it describes common plating practice rather than a published standard.

Need a part that's plated and still fits?

Send the drawing with the finish and the finished dimension called out. We'll machine it in-house, arrange plating through our partner per ASTM B733 or ASTM B633, build the allowance in up front, and return DFM feedback and a per-drawing quote within 24 hours — so the coating lands where your tolerance needs it.

Quote a Plated Part

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