Surface Finishing for CNC Parts: Anodising, Plating, and Powder Coating Guide

Surface finishing transforms a precision-machined part into a component that can survive its service environment. The right finish provides corrosion protection, wear resistance, electrical conductivity control, or a combination. Choosing incorrectly — or omitting the finish specification entirely — leads to premature failure, costly field returns, or aesthetic defects that damage brand perception.

Surface Finish Quick Reference

The table below summarises eleven common CNC part surface finishes by material compatibility, thickness, and best application.

Why Surface Finishing Is an Engineering Decision, Not an Afterthought

Surface finishing is one of the most impactful post-machining decisions in part design, yet it is frequently specified late in the design process — or omitted from drawings entirely — and left to the machining supplier to interpret. This approach creates two types of problems. First, dimensional: most surface finishes add material to the part surface (or remove it, in the case of electropolishing), which directly affects tolerances on bores, shafts, mating faces, and thread fits. A part designed to a nominal dimension without considering anodise build-up will fail the post-finish CMM inspection. Second, functional: specifying the wrong finish for the service environment leads to premature corrosion, wear failure, or coating delamination in service — often with no visible warning signs until the product fails.

The correct sequence for a precision CNC program is: (1) select the surface finish based on service environment and functional requirements; (2) determine the build-up or removal thickness for the selected finish; (3) calculate the pre-finish machined dimensions that will achieve the required post-finish dimensions; (4) machine to the pre-finish dimensions; (5) apply the finish; (6) verify post-finish dimensions where critical. Lewei Precision’s engineering team performs this pre-finish dimension calculation as part of the DFM review on every surface-finished order, flagging any dimensions where finish build-up will affect toleranced fits.

Surface finishing also affects lead time significantly. Most finishing operations are subcontracted by CNC shops to specialist processors, adding 3–10 business days to total delivery time depending on the process (anodising is typically 2–4 days; powder coating 3–5 days; hard chrome or electroless nickel 5–10 days). Planning the finish into the program timeline from the beginning prevents late-stage schedule surprises.

• Dimension planning: always calculate pre-finish dimensions before machining to ensure post-finish tolerances are met
• Lead time impact: finishing adds 2–10 business days; include this in the program schedule from day one
• Material compatibility: most finishes are specific to one base material group — anodising only works on aluminium; passivation only on stainless steel
• Specification clarity: always include finish specification, thickness, and colour (where applicable) on the engineering drawing — never leave it to supplier interpretation

Anodising: The Primary Finish for CNC Aluminium Parts

Anodising is an electrochemical process that converts the surface of aluminium into aluminium oxide (Al₂O₃) by immersing the part in a sulphuric acid bath and passing an electrical current through it. Unlike plating, which adds a foreign metal layer, anodising converts the aluminium itself — the oxide layer grows both into the surface and out from it, approximately 50% inward and 50% outward. This means anodise is part of the aluminium, not a coating sitting on top of it, giving it exceptional adhesion and eliminating the delamination risk that exists with plated or painted finishes.

Lewei’s CNC-Bearbeitungsdienst coordinates standard Type II and hard Type III anodising with qualified anodising partners, including pre-finish and post-finish CMM inspection on all tolerance-critical dimensions.

Type II — Standard Sulphuric Acid Anodising

Type II anodising produces an 8–25 µm oxide layer with good corrosion resistance suitable for indoor and mild outdoor environments. The porous oxide layer can be dyed in any colour before sealing. Common colours: clear (natural silver-grey), black, red, blue, gold, and custom Pantone-matched colours. After dyeing, the pores are sealed by boiling in deionised water or with a nickel acetate sealer, locking the dye and closing the pores. Sealed Type II anodise passes 336–500 hours of salt spray testing (ASTM B117), making it appropriate for electronics enclosures, precision instruments, and non-marine outdoor applications.

Type II anodise hardness (60–80 HV, equivalent to approximately 5–10 HRC) is not suitable for wear-critical surfaces. A Type II anodised sliding guide will wear through the oxide layer within thousands of cycles — less than a year of service in most industrial applications. For wear-critical aluminium surfaces, Type III is required.

Type III — Hard Anodising

Type III (hard anodising) uses a lower bath temperature (0–5°C vs. 18–22°C for Type II) and higher current density to grow a denser, harder oxide layer up to 75 µm thick. The resulting oxide has a Vickers hardness of 350–600 HV (60–70 HRC equivalent), making hard anodised aluminium surfaces comparable to through-hardened steel for wear resistance. Hard anodise colour is always dark grey to black — it cannot be dyed to other colours because the dense oxide layer does not allow dye penetration.

Hard anodise is specified for: hydraulic valve bores (where the hard oxide surface resists the lapping action of particulate contamination in hydraulic fluid), firearm receiver rails and guide surfaces, precision tooling and fixtures, and aerospace actuator components where weight is critical but wear resistance is needed. The critical dimension consideration for hard anodise is that a 50 µm hard anodise layer on a bore reduces the bore diameter by approximately 100 µm (50 µm each side) — machined bore diameter must be 100 µm larger than the required post-anodise diameter.

Powder Coating: The Workhorse Industrial Finish

Powder coating applies a dry polymer powder electrostatically to the part surface and cures it in an oven at 160–200°C. The cured powder film (typically 60–120 µm thick) provides excellent corrosion resistance, UV stability, impact resistance, and a wide range of colours and textures. Powder coating is significantly more environmentally friendly than liquid paint (no solvent VOC emissions) and more durable than most liquid paint systems.

Powder coating is ideal for: steel sheet metal enclosures and housings, aluminium structural frames, industrial equipment panels, and any large part where anodising is impractical. It is less suitable for small precision CNC parts with tight tolerances (60–120 µm build-up makes it incompatible with close fits) and parts with internal threads (the powder coats over threads and must be masked or post-processed). For threaded holes in powder-coated parts, always mask threads before coating or specify thread re-tapping after coating.

Electroplating and Electroless Plating

Electrolytic zinc plating (galvanising) is the most common and cost-effective corrosion protection for steel fasteners and simple steel CNC parts. A 5–15 µm zinc layer provides mild corrosion protection (96–240 hours salt spray per ISO 9227) at very low cost. Trivalent chromate passivation of the zinc layer (clear or yellow) extends protection to 500+ hours. Zinc-plated steel is not suitable for marine or severe industrial environments.

Hard chrome plating (decorative chrome is different) deposits a dense chromium layer (60–70 HRC) directly on steel or cast iron surfaces for hydraulic cylinder rod ODs, bearing journals, and any high-wear, high-contact-stress surface that exceeds the load capacity of a zinc or nickel coating. Hard chrome can be applied in variable thicknesses from 5 to 500+ µm, making it the choice for worn component restoration (rebuilding an undersized shaft OD by chrome plating and then grinding to final dimension). Chrome plating processes use hexavalent chromium (Cr VI), which is highly regulated under EU REACH and US EPA rules — trivalent chrome alternatives are available but achieve lower hardness.

Passivation for Stainless Steel

Passivation is a chemical treatment — not a coating — that removes free iron contamination from the surface of stainless steel CNC parts and restores the passive chromium oxide layer. The process (per ASTM A967 or AMS 2700) immerses the part in nitric acid or citric acid solution for a defined time and temperature, dissolving the free iron without significantly attacking the stainless steel itself. Passivation is mandatory for stainless steel parts in: medical device applications (ISO 13485), food and pharmaceutical processing (FDA 21 CFR), marine and offshore (chloride environments), and any application where the stainless steel must perform at its rated corrosion resistance. Lewei’s Präzisionsbearbeitung service includes passivation coordination for all stainless steel orders where specified.

How to Specify Surface Finishing on CNC Drawings

A complete surface finish specification on a CNC drawing includes: finish type (e.g., “Hard Anodise Type III per MIL-A-8625 Type III”); minimum and maximum thickness (e.g., “25–50 µm”); colour (e.g., “Black, no dye specification required for Type III”); masking instructions (e.g., “Mask bore ø20.000–20.020 mm from thread – exclude from anodise”); and the applicable standard reference. For electroplating, also specify the base plating, any chromate conversion coating, and the testing requirement (e.g., “Salt spray 96 hours per ASTM B117”). Including these callouts eliminates ambiguity and ensures the finishing house produces parts that meet the engineering intent on the first attempt.

Schlussfolgerung

Surface finishing selection is as engineering-critical as tolerance specification — the wrong finish will fail in service regardless of how precisely the part was machined. Use the guide above to select the correct finish for your material and service environment, plan pre-finish dimensions accordingly, and include a complete finish specification on your engineering drawing. Lewei Precision coordinates all common CNC surface finishes alongside precision machining under a single ISO 9001:2015 quality system, with pre- and post-finish dimensional inspection included.

Häufig gestellte Fragen

What is the difference between Type II and Type III anodising?

Type II (sulphuric acid anodising) creates a porous aluminium oxide layer 8–25 µm thick that can be dyed in virtually any colour before sealing. It provides good corrosion resistance for indoor and mild outdoor environments. Type III (hard anodising) uses a lower temperature bath and higher current density to grow a denser, harder oxide layer 25–75 µm thick, with a Vickers hardness of 350–600 HV (equivalent to 60–70 HRC). Type III is used for wear-critical aluminium parts: hydraulic valve bores, cam surfaces, sliding guides, and tooling.

Does anodising affect CNC machined tolerances?

Yes. Type II anodising adds 8–25 µm to each surface, which means the diameter of an anodised bore decreases by 16–50 µm (anodise grows equally inward and outward on external surfaces, but only inward on internal bores). For tight-tolerance fits, the machined dimension must be adjusted to account for anodise build-up. Type III adds up to 75 µm per surface, making it critical to machine bores and shafts to adjusted pre-anodise dimensions when tolerances are tighter than ±0.05 mm.

When should I use passivation on stainless steel CNC parts?

Passivation should be specified on all stainless steel CNC parts used in corrosive environments: marine, food processing, pharmaceutical, chemical, and medical applications. The CNC machining process exposes fresh metal surface and can embed tool particles into the stainless surface, disrupting the passive chromium oxide layer that gives stainless its corrosion resistance. Passivation per ASTM A967 or AMS 2700 removes free iron contamination and restores the passive layer. Without passivation, machined stainless will rust in chloride environments despite being a nominally stainless alloy.

What is electroless nickel plating and when is it preferred over electrolytic plating?

Electroless nickel (EN) plating deposits nickel from a chemical bath without electrical current, producing an extremely uniform thickness that covers internal bores, blind holes, and complex geometries equally — something electrolytic plating cannot achieve uniformly. The phosphorus content of EN (typically 5–12% P by weight) determines hardness and corrosion resistance: high-phosphorus EN (10–12%) is amorphous, non-magnetic, and excellent for corrosion; low-phosphorus EN (2–5%) can be heat-treated to achieve 60+ HRC. EN is preferred for parts with complex internal geometry, precise dimensional tolerance, and moderate wear and corrosion requirements.

Does Lewei Precision provide surface finishing services on CNC parts?

Yes. Lewei Precision offers anodising (Type II and Type III), powder coating, electroless nickel plating, zinc plating, chromate conversion coating, passivation, and black oxide as standard post-machining services. All finishing is coordinated under Lewei’s ISO 9001:2015 quality system, with pre-finish and post-finish dimensional inspection included where tolerance callouts require verification. Surface finish specifications should be included in the drawing or communicated at quoting to allow correct pre-finish dimension adjustment.

What is the most cost-effective surface finish for aluminium CNC parts in general engineering?

For general engineering aluminium parts requiring corrosion protection and a professional appearance, standard Type II anodising is the most cost-effective option. At typical production volumes (50–500 pieces), Type II anodising costs $2–$8 per part depending on size and colour. Clear anodise with no dyeing is the least expensive; black anodise is only slightly more. Powder coating is an alternative for larger, simpler parts where colour consistency and UV resistance are more important than dimensional precision, but for small precision CNC parts, anodising is usually the superior choice.