Anodizing Types for CNC Aluminum Parts: How the Layer Grows and Which Type to Pick

• 1
  How the anodic layer grows on a machined surface
• 2
  Type II sulfuric anodizing: the default for machined aluminum
• 3
  Type III hardcoat anodizing: thicker, harder, more brittle
• 4
  Type I chromic anodizing: thin coat for tight tolerances
• 5
  Tolerance, masking and thread strategy before anodizing
• 6
  Alloy and surface finish effects on the anodized result
• 7
  Anodizing types for CNC aluminum parts compared

Anodizing passes current through the part in an acid electrolyte. The aluminum surface oxidizes and becomes a porous aluminum oxide layer. Nothing is deposited on top. The oxide consumes base metal as it forms, and it grows in both directions: roughly half outward, half inward. A 20 µm coating therefore raises the surface about 10 µm and eats about 10 µm into the part.

That split matters on CNC parts. A bearing bore machined to nominal will shrink by the inward growth on both sides. A thread cut to size will bind after coating. A press-fit shaft will grow and lose its clearance. The growth figure quoted on a finish spec is total coating thickness, not growth per side, so divide it before you adjust dimensions.

The porous structure is what makes dyeing possible. Pores run from the outer surface down toward the barrier layer. Dye molecules enter these pores before sealing closes them. Pore size and density depend on the electrolyte and temperature, which is why Type II, Type III and Type I behave differently even when the thickness looks similar on paper.

Growth is not perfectly uniform. Edges and corners draw more current and build slightly thicker oxide. Deep recesses and blind holes see less electrolyte exchange and can come out thin. On a part with both, the coating range across the surface may span several micrometers. That is normal, and it is one reason tight tolerance features are usually masked or finished after machining.

Type II is the workhorse. It runs in a sulfuric acid electrolyte at about 18–22 °C with a current density near 1.5 A/dm². Coating thickness typically lands between 5 and 25 µm, with 10–15 µm common for cosmetic and light-corrosion parts. Clear or dyed, it gives moderate abrasion resistance at a reasonable cost.

For enclosures, brackets, heatsink housings, front panels and general machine parts, Type II is usually the right first choice. It takes dye well, holds color across a batch reasonably well, and does not add much to the part cost. On a 6061-T6 machined housing, a 12 µm Type II clear coat is a routine call.

The limits show up under wear. Type II will not survive a sliding contact or a part that rubs against a mating surface for long. It also does not like sharp edges, where the oxide is thin and brittle. If the application involves abrasion, move up to Type III rather than trying to make Type II thicker.

Thickness above about 25 µm in Type II starts to cost more without adding much wear resistance. The coating gets more brittle and more prone to crazing. If a drawing calls for 30 µm of sulfuric anodize on a wear surface, that is usually a sign the process should be hardcoat instead.

Type III builds a denser, thicker oxide, typically 25–50 µm and up to 75 µm for severe wear. It runs at lower electrolyte temperature, often 0–5 °C, with higher current density. That combination costs more and takes longer than Type II, and the bath control is tighter.

Hardcoat is the right call for wear surfaces, sliding rails, pneumatic components, cam tracks, valve bodies and parts that must resist abrasion. On 6061-T6 and 7075-T6 it holds up well in sliding contact, especially when the mating part is softer or lubricated. It also gives better corrosion protection than Type II at the same thickness.

The trade-off is brittleness. Thick hardcoat cracks under impact or bending. On thin walls, sharp internal corners and threaded features, a 50 µm hardcoat can chip or flake at the edge. Designers sometimes ask for hardcoat on a part with a thin flange; that usually ends with a cracked coating and a rework conversation.

Hardcoat also darkens the surface. Clear hardcoat on 6061 reads as a gray-bronze tone, not bright silver. If the part is cosmetic and must stay light, hardcoat is the wrong finish even if the wear numbers look attractive. Color matching on hardcoat is approximate at best.

Type I uses a chromic acid electrolyte and produces thin coatings, usually 2–5 µm. The dimensional change is small enough that many precision parts can be anodized after final machining without adjusting dimensions. That is its main advantage on CNC work.

Fatigue performance is better than Type II or Type III because the coating is thin and less brittle. Chromic anodize is also favored for assemblies with faying surfaces and for parts where electrolyte entrapment in crevices is a concern. Aerospace and precision instrument work often specifies it for those reasons.

The drawbacks are real. Chromic acid is heavily regulated, so availability varies by region and shop. Corrosion protection is lower than Type II at similar thickness, and wear resistance is modest. Color options are limited, mostly clear to gray, and dyeing is not practical on such a thin layer.

If the part needs a conductive surface, none of these three types will do it. Anodizing is an insulator by nature. Conductive anodize uses a different chemistry and is specified separately, usually for grounding points or EMI paths. Do not assume a standard Type II coat will pass a resistance check.

Tolerance planning is where most anodizing problems start. Decide which features are critical and whether they are machined before or after coating. Bores and threads that must stay functional are best masked, plugged, or reamed after anodize. Each masked feature is a manual operation, so masking adds labor cost and lead time.

A practical approach is to call out anodize as a final finish, dimension critical features to the post-coat size, and note the growth allowance on the drawing. For a 12 µm Type II coat, that is about 6 µm per side. For a 50 µm hardcoat, it is about 25 µm per side, which is enough to close a light press fit.

Threads need their own decision. A standard internal thread anodized without masking will tighten, sometimes enough to fail a go gauge. Options are masking the thread, cutting it oversized before coating, or running a tap after anodize. Masking is cleanest but slowest. Re-tapping after coating risks chipping the oxide at the thread entry.

Sharp external corners and burrs should be removed before anodizing. The oxide is thin at a sharp edge and tends to crack or wear through first. A small edge break, 0.2–0.5 mm, improves coating coverage and appearance more than most designers expect. It also reduces handling damage during racking.

Alloy choice changes the result. The 6000 series anodizes cleanly with uniform color and is the easiest to specify. 6061-T6 is the safe default for cosmetic anodized parts. 6082 and 6063 behave similarly, though 6063 extrusions may show flow lines after dyeing.

The 2000 and 7000 series produce darker, less uniform coatings and are harder to dye consistently. 7075-T6 anodizes to a darker tone even when clear, and black dye on 7075 can look patchy next to 6061. High-silicon castings and 5000 series with heavy magnesium can anodize smutty or dark. If the design requires 7075 for strength, accept the color shift or specify a matte blast before coating.

Surface finish before anodizing carries through. Machining marks, tool chatter and burrs are amplified by the oxide layer, particularly on dyed parts. A finish of Ra 0.8 µm or better gives a clean cosmetic result. Ra 1.6–3.2 µm reads as machined texture and is fine for functional parts that will not be dyed dark.

Bead blasting before anodizing produces a matte, uniform appearance and hides tool marks. It also rounds edges slightly and changes the surface texture, so it is not a fix for a part with a sharp-edge requirement. On black dyed parts, blasting usually gives a more even color than a bare machined surface.