Complete Material Surface Treatment Process for CNC Parts

• 1
  Key takeaways
• 2
  What the complete material surface treatment process actually covers
• 3
  Matching the treatment to aluminium, steel, stainless and titanium
• 4
  Surface preparation before any coating or plating
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  Running the treatment: anodize, plating and coating parameters
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  Where the process goes wrong and how to catch it early
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  Laser marking, engraving and masking in the same run
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  Step by step: running the complete material surface treatment process
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  Treatment choice by failure mode and tolerance cost

Surface treatment is any controlled layer built on top of a base material so the part behaves differently at the skin than it does in the core. The base metal carries load. The surface carries corrosion, wear, friction, electrical contact and appearance. Treat them as two separate design problems, because they are solved with different parameters.

A complete material surface treatment process has four stages you cannot skip: substrate preparation, the treatment itself, post-treatment sealing or curing, and inspection. Plating shops that fail usually fail at stage one or stage four. A zinc layer over a smeared edge will blister in a salt spray cabinet. A hardcoat over trapped coolant residue will show white spots after sealing.

For machined parts the treatment choice is constrained by geometry long before it is constrained by chemistry. Deep pockets, blind holes, Ø1 mm cross-holes and 0.5 mm wall sections all behave badly in anodize baths and powder lines. If a part needs a Class A finish, design the drainage path and the racking point at the CAD stage.

The table below is the decision shortcut we use in quoting. It maps the failure mode you care about to the treatment that solves it, and flags the tolerance cost.

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  Corrosion onlyBlack oxide with oil, zinc plating, or clear anodize on aluminium.
• 2
  Wear and abrasionHardcoat anodize, electroless nickel, or a hardened stainless substrate instead.
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  Conductivity that must surviveConductive anodize or masked hardcoat, never standard anodize.
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  Cosmetic class ABead blast plus anodize, or polish plus clear coat. Both need a coupon.

Aluminium is the easiest substrate to treat and the easiest to ruin. Alloy selection drives the color. 6061 and 6082 anodize to a clean silver-grey. 7075 and 2024 contain copper and go dark or muddy in clear anodize, and they pit badly if the bath is out of balance. If the customer expects a bright uniform finish across a mixed alloy assembly, specify one alloy for all visible parts.

Cast aluminium such as ADC12 has porosity and silicon that show through anodize as blotches. For die-cast housings, powder coating or painting hides porosity far better than anodize. If anodize is mandatory, expect a matte or textured look and approve a sample from the actual casting process, not from bar stock.

Carbon steel takes zinc, black oxide, phosphate or paint. 1018 and 1045 respond predictably to black oxide with a light oil. 4140 and 4340 are usually plated or coated instead, because black oxide on them offers little corrosion protection. Hydrogen embrittlement is a real risk on high-strength steels above roughly 32 HRC, so a bake step after plating is not optional.

Stainless is normally passivated rather than coated. 303 passivates less uniformly because of its sulfur content, and free-machining grades may show a slight dulling. 304, 316 and 17-4PH passivate cleanly. Titanium is different again: anodize on Ti-6Al-4V gives color and some wear resistance but is thinner and more process-sensitive than aluminium anodize. For titanium wear, specify a nitriding or PVD route and check the geometry first.

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  One alloy per visible assemblyMixed 6061 and 7075 parts will not match after clear anodize.
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  Bake high-strength steelAsk for a post-plate bake on parts above 32 HRC.
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  Passivate, do not coat, stainlessCoating stainless usually hides a machining defect.

Preparation is where a finish is won or lost. Start at the machine. A 0.2 mm edge break on every external edge stops coating from pulling back at sharp corners. Burrs left in a Ø3 mm cross-hole will trap plating solution and bleed out later as a stain or a corrosion seed.

Degreasing is the next gate. Machined aluminium carries cutting fluid in every scratch and pore. Alkaline cleaning at 55–65 °C for 5–10 minutes, followed by two rinse stages, is a normal baseline. Hand-wiping with solvent is not a substitute. If a part is handled with bare hands after cleaning, skin oils will cause spotty anodize.

Mechanical prep sets the cosmetic baseline. Bead blasting with 120–180 grit glass beads produces a fine matte that hides light tool marks and gives a consistent anodize appearance. Tumbling rounds edges and polishes small parts in bulk. Brushing leaves directional lines that show under gloss coatings, so only use it when the design calls for a brushed look.

For stainless and titanium, remove the heat tint and the disturbed layer from machining before passivation. A light acid pickle or electropolish removes free iron and embedded tool material. Skipping this step leaves a surface that looks clean but rusts at the weld or the machined corner weeks later.

• 1
  Edge break0.2 mm nominal chamfer or radius on all external edges.
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  Bead blast120–180 grit glass bead for a uniform matte at Ra 1.6–3.2 μm.
• 3
  No bare-hand handlingGloves after cleaning, or expect fingerprints in the finish.

Type II sulfuric anodize on aluminium typically runs at 15–20 °C with a current density around 1.5 A/dm² for 20–40 minutes, producing 5–25 μm. Type III hardcoat runs colder, near 0–5 °C, at higher current density, and takes longer. Cold bath temperature is what makes the layer dense and hard. A warm bath gives a soft, chalky coating that fails abrasion tests.

Electroless nickel is a chemical reduction, not an electrolytic one, so the layer thickness is nearly uniform on complex geometry. Mid-phosphorus baths deposit 10–25 μm in 60–90 minutes at 85–90 °C. This uniformity is why electroless nickel is the usual choice for valve bodies, manifolds and parts with blind features where throwing power matters.

Zinc plating on steel is usually 8–12 μm with a trivalent passivate. Trivalent systems are the current default because hexavalent chromium is restricted. A top seal adds corrosion resistance without adding much thickness. For threaded steel fasteners, watch the pitch diameter: 10 μm of zinc per side is enough to bind a tight Class 3 thread.

Powder coating is applied electrostatically and cured at 180–200 °C for 10–20 minutes. That cure temperature will anneal some aluminium tempers and distort thin sheet, so check the alloy and temper before specifying powder on a structural part. Liquid paint cures lower and is a better fit for heat-sensitive assemblies.

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  Cold anodize bath0–5 °C for Type III. Warm baths give soft coatings.
• 2
  Electroless nickel throwing powerBest option for blind holes and internal channels.
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  Thread allowanceSpecify 6g or larger on plated external threads.

Color drift is the most common complaint and it is rarely the plater's fault alone. Bath age, alloy lot and blast media all shift the shade. The fix is procedural: keep a signed coupon from the first article, store it in a dark drawer, and compare every run against it under the same light. If you compare parts from two different alloys, you will chase a problem that does not exist.

Pitting and white spots on anodized aluminium almost always trace back to trapped solution or contamination. Blind holes without a drain path hold acid. A part racked in a position that traps air gets an unanodized patch. Add a 0.5 mm drain hole, or specify a racking orientation, and the defect disappears.

Coating thickness versus tolerance is the third recurring issue. A ±0.005 mm bore cannot accept 25 μm of hardcoat per side. The standard fix is to coat the part oversize and then ream or grind the bore back to size. Tell the shop which features must be cut after coating, and mark them clearly.

Adhesion failure on powder and paint shows up as flaking at edges and around holes. It usually means insufficient cleaning, a phosphate step that was skipped, or too thick a film. Keep powder at 60–100 μm, keep the pretreatment line in spec, and test with a cross-hatch or impact method on the first article.

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  Keep a couponSigned first article, same light, every run.
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  Add drain holes0.5 mm minimum in blind pockets that enter a bath.
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  Coat oversize, then cutThe standard answer for tight bores under hardcoat.

Laser marking is normally done after the coating, not before. On anodized aluminium, a fiber laser burns through the oxide layer and leaves a light or dark mark depending on the parameters. Character height below 1.5 mm gets inconsistent on a textured surface, so keep part numbers and traceability codes at 1.5 mm or larger.

If the mark must survive abrasion, mark after coating and verify it on the coupon. Marking before anodize buries the mark under the oxide and it fades. Marking before powder coating hides it completely. Sequence matters more than laser power.

Masking is a design decision, not a shop-floor afterthought. Threads, connector pins, grounding pads and bearing seats should be called out on the drawing with a mask note. Masking costs time, and unmasked threads will need chasing after plating. Both are cheaper than a rejected assembly.

For parts that combine a cosmetic surface with a functional one, split the drawing into zones. Zone A is decorative and gets the full finish. Zone B is functional and gets masked or machined after coating. This removes ambiguity at the shop and prevents arguments at incoming inspection.

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  Mark after coating1.5 mm minimum character height for reliable contrast.
• 2
  Call out mask zonesThreads, pads, bearing seats, seal faces.
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  Zone the drawingOne finish callout per zone, not one per part.