Skip to main content
True Precision Machining
Engineering hub

Engineering · Coatings & Finishes

Coatings & finishes reference.

Post-machining surface treatments — anodize, plating, passivation, powder coat, PVD, thermal spray. What each does, when to use it, and the standard specs to call out.

Why coat at all

Five reasons. Pick the right one.

Surface treatments are usually called out for one of five reasons: corrosion resistance, wear / hardness, electrical or thermal modification, cosmetic finish, or dimensional buildup (filling worn surfaces). The right coating depends on which one matters most — and on the base material.

A common mistake: spec’ing a coating that looks like the right answer but doesn’t bond to the base material, or that produces dimensional change that wasn’t accounted for. The tables below organize by base material so you start in the right column.

Coating finder

Find the right finish.

Pick your base material and what matters most — the chart and list filter to the finishes that fit, with the first-choice recommendation surfaced. Hover a bar or click any finish for full detail.

Base material
Priority
Added thickness · one-sided, log scale
Chem film (chromate) — 0.00001–0.0001″. Corrosion resistance + electrical conductivity (grounding). Yellow to gold. Chem film (chromate) 0.00001–0.0001″ Trivalent chromate (RoHS) — 0.00001–0.0001″. Same as chem film, RoHS-compliant alternative to hex chrome. Trivalent chromate (RoHS) 0.00001–0.0001″ Black oxide — 0.00005–0.0001″. Cosmetic black + mild corrosion. Negligible thickness — fits assemblies as-is. Black oxide 0.00005–0.0001″ Bluing (heat or chemical) — ~0.0001″ visible. Cosmetic only on firearms, tooling. Not for corrosion service. Bluing (heat or chemical) ~0.0001″ visible DLC (diamond-like carbon) — 0.5–4 µm. Ultra-hard (3000–5000 HV), low-friction (μ ~0.1) film. Bearings, optical mounts, tooling, high-end firearms. DLC (diamond-like carbon) 0.5–4 µm TiN / TiAlN / AlCrN (PVD) — 1–4 µm. Cutting-tool coatings — TiN (gold), TiAlN (purple-bronze), AlCrN (gray). 2000–3500 HV. Also decorative. TiN / TiAlN / AlCrN (PVD) 1–4 µm PVD coating (TiN, etc.) — 0.00004–0.0002″ (1–5 µm). Wear + cosmetic for cutting tools, jewelry, instruments. Many color options. PVD coating (TiN, etc.) 0.00004–0.0002″ (1–5 µm) Silver / gold plating — 0.00005–0.0003″. Electrical conductivity, RF shielding, oxidation resistance on contacts. Gold over nickel underplate. Silver / gold plating 0.00005–0.0003″ Black oxide on stainless — 0.0001–0.0003″. Cosmetic black for optical/military gear. Less durable than on carbon steel. Black oxide on stainless 0.0001–0.0003″ Phosphate (manganese) — 0.0002–0.0005″. Lubricant retention, wear-in. Common on fasteners + sliding parts. Phosphate (manganese) 0.0002–0.0005″ Zinc plating — 0.0002–0.0005″. General corrosion. Clear or chromate-finished. Zinc plating 0.0002–0.0005″ Cadmium plating — 0.0002–0.0005″. Aerospace fastener standard. Restricted in EU/RoHS — use Zn-Ni where allowed. Cadmium plating 0.0002–0.0005″ Zinc-nickel (RoHS) — 0.0003–0.0008″. Aerospace fasteners, automotive — better than zinc, RoHS-compliant. Zinc-nickel (RoHS) 0.0003–0.0008″ Type II anodize (sulfuric) — 0.0001–0.001″. General corrosion + cosmetic. Black, clear, and dyed colors. Type II anodize (sulfuric) 0.0001–0.001″ Electroless nickel — 0.0003–0.002″. Wear + corrosion + dimensional build-up. Hard (50+ HRC after bake). Electroless nickel 0.0003–0.002″ Electroless nickel — 0.0003–0.002″. Wear + uniform corrosion resistance. Bake to harden to 50+ HRC. Electroless nickel 0.0003–0.002″ Teflon (PTFE) impregnation — 0.0005–0.002″ added. Anodized aluminum infused with PTFE for low friction. Slide rails, dynamic seals, food contact. Teflon (PTFE) impregnation 0.0005–0.002″ added Type III hardcoat — 0.0005–0.004″. Wear surfaces, fluid contact, long-term outdoor exposure. Thick, dark gray to black. Type III hardcoat 0.0005–0.004″ Hard chrome plating — 0.0002–0.005″. Wear surfaces, hydraulic rods. Hardness 60–70 HRC. Hard chrome plating 0.0002–0.005″ Hard chrome — 0.0002–0.005″. Wear surfaces, hydraulic rods, dimensional rebuild. Hard chrome 0.0002–0.005″ Powder coat — 0.002–0.006″. Cosmetic + impact + corrosion. Wide color range. Powder coat 0.002–0.006″ Powder coat over phosphate — 0.002–0.006″. Industrial cosmetic + corrosion. Wide color range. Powder coat over phosphate 0.002–0.006″ Plasma-sprayed ceramic — 0.005–0.020″. Alumina, zirconia, chromium oxide. Thermal barriers, electrical insulators, wear resistance. Plasma-sprayed ceramic 0.005–0.020″ HVOF tungsten carbide — 0.005–0.025″. High-velocity oxy-fuel WC-Co. Wear + erosion. Replaces hard chrome on hydraulic rods. HVOF tungsten carbide 0.005–0.025″ 10 µin 100 µin 1 mil 10 mil

Passivation, electropolish & bead-blast remove or only alter the surface (no added thickness) — they appear in the list, not the chart.

28 of 28 finishes

Anodize design notes

  • ·Build-up: Type II/III grow ~50% of total thickness into the base — account for it on tight tolerances.
  • ·Sealing: Hot DI water seal extends corrosion life; some specs require it.
  • ·Color match: 6061 anodizes lighter than 7075 — match alloy across an assembly.
  • ·Mask threads: Anodize won't clear small threaded holes — mask inserts and bushed holes.

Design considerations

Five things to remember when speccing.

Account for build-up

Type III anodize (0.0005–0.004″ total coating) grows the part ~0.00025–0.002″ per side — roughly 50% of the coating thickness builds outward from the original surface. Hard chrome 0.0002–0.005″. For tight tolerances, machine to a slightly undersize dimension and let the coating bring it to spec — or note 'after coating' on critical dims.

Mask carefully

Bores, threaded holes, sealing surfaces, and electrical contact points all may need masking from coating. Specify what gets masked on the drawing — don’t leave it to the coater to guess.

Hydrogen embrittlement bake

Acid-cleaning before plating absorbs hydrogen into high-strength steel. Required bake-out (375 °F, 8–24 hr) on parts ≥35 HRC after plating. Specify 'plate per AMS XXXX with hydrogen embrittlement relief per ASTM B850.'

Galvanic compatibility

If a coated part contacts dissimilar metal in moisture, choose coating to match the assembly. Cadmium and zinc are sacrificial protectors for steel against aluminum; nickel is not.

Fastener clearance

Screw threads especially: plating builds up the pitch diameter. Class 2A's external allowance usually absorbs standard plating thickness; on the female side, tap oversize with a higher H-limit tap sized for the coating thickness (Class 3B is the tightest internal class, not an oversized one), or specify 'mating threads to fit Class 2A external after coating.'

RoHS / REACH compliance

Hexavalent chromium (chromate) and cadmium are restricted under EU RoHS and REACH. For aerospace customers requiring compliance, specify trivalent chromate (TCP) or zinc-nickel as substitutes.

Spec syntax

Call it out clearly.

Hardcoat anodize per MIL-A-8625 Type III, Class 1, 0.002″ ± 0.0005″
Type III, undyed (Class 1), specified thickness window. Standard for aluminum wear surfaces.
Black oxide per MIL-DTL-13924 Class 1, applied after final machining
Specifies post-machining timing — important for tolerance-critical surfaces.
Citric passivation per AMS 2700 Method 2
Modern stainless treatment — call out citric (Method 2) over nitric (Method 1) when corrosion margin matters.
Electroless nickel per AMS 2404, high phosphorus (10–13% P per ASTM B733 Type V), 0.001″ thick, Class 1 — hydrogen embrittlement relief bake 375°F for 8 hr
Phosphorus content affects hardness and corrosion. Baking is critical on heat-treated steel.

Keep exploring

Related tools & references

Anodize Type II vs III→

Decorative vs hard anodize, in depth — MIL-A-8625.

Citric vs nitric passivation→

Choosing the stainless passivation method — ASTM A967 / AMS 2700.

Galvanic series→

Avoid dissimilar-metal corrosion when picking a coating.

Heat treatment reference→

The hardening step that often precedes plating and the embrittlement bake.

Drawing template & notes→

How to call out a finish so it's unambiguous on the drawing.

Technically reviewed by the True Precision Machining engineering team · Last reviewed June 2026

Standards referenced: MIL-A-8625, MIL-DTL-5541, AMS 2404, AMS 2406, AMS 2700, ASTM A967, ASTM B912

Picking the right coating?

Tell us the base material, the environment, and the function — we'll suggest the right combination and the AMS/ASTM spec to call out on the drawing.

Request a Quote Contact Us

Quotes in 24 hours