Which Surface Finish to Specify for a Machined Part: Base Metal, Function and Drawing Requirements

Rule out finishes the base metal cannot take

Compatibility is a filter, not a trade-off. The sources list finishes by substrate family, so the first question is which processes the alloy can actually accept before corrosion, wear or cost are compared¹.

Aluminium and titanium are commonly paired with anodising, Type III where wear resistance is needed^1,4,15. Stainless steel is paired with passivation or electropolishing for corrosion resistance and cleanliness^1,18,25.

Carbon and alloy steel is paired with black oxide, zinc plating, nickel plating, electroless nickel or phosphate conversion^1,18. Copper and brass take electroless nickel, nickel, tin, silver or gold plating, chosen for conductivity, solderability and tarnish resistance¹.

Material family alone does not confirm feasibility. Alloy composition, hardness, heat treatment, welded areas, geometry and trapped chemistry in blind features can all affect whether a process works on a specific part, and two alloys from the same family may respond differently to the same chemical treatment¹.

Confirm the exact alloy before the finish is written on the drawing¹.

Corrosion, wear, or both: choosing the process family

Once the substrate allows the process, the functional requirement decides between the anodising types and the barrier coatings. Type II anodising is positioned for corrosion resistance and cosmetic colour on aluminium, and Type III hard anodising for wear resistance and surface hardness where the thicker oxide can be accommodated in the tolerance plan^3,4,15.

The two are not interchangeable: Type III gives greater wear resistance and a thicker oxide layer than Type II, with greater dimensional consequence and colour limits^3,15,35.

For aluminium parts that need corrosion and wear together, Type III hard anodising is the option the sources consistently point to, provided the thicker oxide fits the fit and tolerance plan^4,15,35.

No source in this set establishes a required coating thickness, hardness value or salt-spray duration that would make Type III mandatory, so those remain questions for the finisher ^3,15,35.

Powder coating is the alternative where anodising is not suitable: it is a durable, colour-varied barrier finish applicable to most metals, and it is often combined with bead blasting to produce a uniform appearance^4,11,36.

The operating environment sets how robust the finish must be — outdoor exposure, salt spray, high temperature and UV light can require more than indoor use^2,10.

Conductivity, grounding and impact resistance

Where the part must carry current or ground, the finish choice narrows. Powder coating builds a polymer barrier that impairs electrical conductivity, so it should be avoided on conductive paths or masked off them^8,35.

Anodising also insulates, and the sources list electrical insulation as one of its design consequences³⁵. Impact resistance and dimensional control pull in opposite directions.

Powder coating offers higher impact resistance than anodising but less dimensional control¹¹.

That trade is the reason a part that will be struck, chipped or abraded in service may still be powder coated even though the coating builds thickness, while a part with tight fits and no impact exposure is better served by anodising^11,35.

Powder coating is difficult to apply to internal surfaces, which matters for bores and recessed features¹¹. The sources do not establish a conductivity threshold or a required masking width, so the buyer should confirm with the finisher which surfaces must stay bare^8,35.

Tolerance allowance and masking on the drawing

Coatings change dimensions, and the size of the change depends on the process family. Conversion finishes are near-negligible: passivation on stainless removes free iron and supports corrosion resistance with negligible dimensional effect, and black oxide gives a controlled black appearance with minimal dimensional change²⁵.

Electropolishing removes material in a controlled way²⁵. Plating builds measurably — electroless nickel produces a measurable build-up — and plating stock should be subtracted from the machined dimension on tight-tolerance features such as bores, threads and press-fit seats^25,38.

Powder coating builds the most and requires masking. Two publishers give different typical thickness ranges, so the buyer should not let either number into a drawing without a governing standard^11,36.

Threads and precision fits must be masked, and multiple layers increase thickness further^11,36. Anodising grows both outward and inward from the original surface, so the allowance is not simply the coating thickness¹¹.

The sources do not publish a half-penetration figure, so the buyer should ask the finisher how much of the oxide sits above the original surface before setting the pre-machined dimension¹¹.

Post-plating bake for high-strength steel

Plating on high-strength steel carries a hydrogen embrittlement risk, and the sources indicate a post-plating bake as the mitigation. Hydrogen embrittlement is caused by hydrogen in the metal lattice, often introduced during plating or welding, and it differs from stress corrosion cracking³⁹.

The requirement is easy to miss in an uncontrolled supply chain, so it belongs in the purchase order rather than in a verbal instruction³⁸.

• ✓Confirm the base metal hardness before plating; the cited threshold is 39 HRC or above for zinc or cadmium plated steel.
• ✓Write the baking relief cycle into the PO, not just the drawing.
• ✓State that the bake must occur within four hours of plating, as cited.
• ✓Reference the governing standard the finisher will work to; the evidence cites ASTM F1941 for this requirement.
• ✓Ask the finisher to confirm the bake was performed and to provide the record with the shipment.

Does the lot size make the finish economic?

Finishing cost is driven more by volume than by treatment type. In a typical medium-volume run, post-processing — anodising, heat treatment, plating — is a minor share of total cost, while at very low volume setup charges dominate the per-part price^5,30.

The same anodising setup spread over a handful of prototypes produces a much higher per-part overhead than the same setup spread over a larger order³⁰. Masking is the other hidden cost.

Finishing time increases substantially for parts that need thread protection, precise masking lines or complex fixturing, and intricate geometries with internal features can multiply finishing expense beyond the base treatment cost³⁰. Powder coating follows a similar setup pattern but requires specialised masking for threaded or precision surfaces³⁰.

The sources do not publish a verified cost index, minimum lot charge or setup fee schedule, so the buyer should ask for the setup charge and the masking labour to be itemised separately from the per-part treatment rate^5,30.

What to obtain from the finisher

Several figures a buyer would normally expect on a finish specification are not present in this source set. They have to be requested as certificates or test reports rather than assumed from the process name.

• ✓Ask for a restricted-substance declaration covering RoHS, REACH and ELV; no source in this set addresses them.
• ✓Ask for the salt-spray test conditions and hours the finisher works to; no ASTM B117 conditions are provided here.
• ✓Ask for hardness values in a named scale; no standard or authoritative hardness figure is given for anodised surfaces.
• ✓Ask how much of the anodised oxide sits above the original surface; no half-penetration figure is published in this set.
• ✓Ask for the governing standard the finisher will certify against, since the sources cite different standards for different processes.

Where the sources disagree on a figure you may be quoted

Two publishers give different typical powder coating thickness ranges.

What the sources do not establish

• No source addresses RoHS, REACH, ELV or hexavalent chromium restrictions, so compliance must be requested as a declaration.
• No ASTM B117 test conditions or required salt-spray hours per environment are provided.
• No half-penetration figure for anodising is given.
• No HV or HRC hardness values from a standard or authoritative source are provided.
• No verified cost index, minimum lot charge or setup fee schedule is provided.
• No ASTM B850 content is present; only ASTM F1941 is cited for baking relief.
• The ASTM and NIST pages retrieved contain no relevant surface-finish content.