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.
Base material and priority narrow the reference options. Results are not ranked recommendations; confirm substrate preparation, thickness, service environment and the governing process specification with the finisher.
Anodize bars show the outward build-up (~50% of total coating thickness); the list shows total coating thickness. 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
No finishes match — try widening the filters.
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 and electroplating can charge high-strength steel with hydrogen. ASTM B850 calls for a post-plating relief bake on steel at tensile strength ≥1000 MPa (≈31 HRC and up), typically around 375 °F, with the hold time set by the B850 class for the part's strength (longer as strength rises). 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.
Keep exploring
Related tools & references
Decorative vs hard anodize, in depth — MIL-PRF-8625.
Choosing the stainless passivation method — ASTM A967 / AMS 2700.
Avoid dissimilar-metal corrosion when picking a coating.
The hardening step that often precedes plating and the embrittlement bake.
How to call out a finish so it's unambiguous on the drawing.
Picking the right coating?
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