Black Anodizing Effect After CNC Machining Small Parts

Anodizing is an electrochemical conversion of the aluminum surface. The part sits in an acid bath and current is passed through it. The surface oxidizes into a porous aluminum oxide layer, and dye is pulled into those pores before the part is sealed. On 6061-T6 a Type II sulfuric acid layer typically builds to 5–25 μm; hardcoat, which is the same chemistry run colder and at higher current density, reaches 25–50 μm.

Black is not a pigment mixed into the metal. It is dye absorbed into the pore structure, so the shade depends on layer thickness, alloy, and how the surface was cut. Two batches of the same part can land a shade apart when the bath ages or the dye concentration drifts.

• 1
  Type IIDecorative black, 5–25 μm, keeps tight dimensions
• 2
  Type III hardcoat25–50 μm, wear resistant, darker and less uniform
• 3
  SealingHot DI seal closes pores; cold seal leaves them slightly open

Roughly half the oxide grows inward and half outward from the original surface. A 20 μm Type II layer adds about 10 μm to the outside, or 0.01 mm per side. That matters on a Ø6 mm shaft with a ±0.005 mm tolerance: the coating alone eats twice the tolerance band.

Threads are the usual casualty. A 1/4-20 or M3 thread anodized without masking will not accept a nut cleanly, because both flanks grow. The standard fix is to mask threads, or to cut them oversize before anodizing and chase them after. We normally mask and re-tap after coating so the fit stays class 2B or 6H.

Holes behave the same way. A Ø3 mm dowel hole closed by 0.02 mm on diameter turns into an interference fit. If the hole is a locating feature, call it out as masked on the drawing, or plan a reaming pass after anodizing. Blind holes trap dye and rinse water, so we add a note to drain and rinse them; trapped electrolyte keeps working on the wall long after the part leaves the tank.

• 1
  Mask before coatingThreads, dowel holes, electrical contact pads
• 2
  Cut oversize, then re-cutSealing faces and bearing bores that must stay in tolerance
• 3
  Leave bareNon-critical cosmetic surfaces where growth does not matter

Anodizing does not hide tool marks, it amplifies them. Dye sits in the oxide and any scratch, burr, or chatter line stays visible in the finished part. A 1200 grit brushed or bead blasted surface gives a uniform satin black; a mirror polished surface gives a deep glossy black that shows fingerprints immediately.

We usually bead blast aluminum before black anodizing. It scatters light, hides minor handling marks, and gives the dye a consistent pore structure to grab. Parts that must stay reflective skip the blast and go straight from a fine machined finish at Ra 0.2–0.8 μm.

Deburring comes first, always. A burr on an edge traps dye and reads as a dark line after sealing. If the burr is heavy enough, it also breaks off in the tank and leaves a bright pit where bare aluminum shows through. That defect cannot be reworked without stripping the whole part.

The coating is hard and electrically insulating. If a part needs grounding continuity, anodizing the whole surface will break it. The usual answer is a masked contact pad or a conductive anodize variant, and the pad has to be called out on the drawing with its own finish note.

Corrosion resistance improves but has limits. The sealed oxide resists salt spray and humidity far better than bare 6061, yet it is brittle. A sharp impact chips the layer and exposes bright metal underneath, which is why edge breaks and radii help on parts that see handling.

Color matching across batches is the other limit. Black dye is not a color number the way paint is. If a customer needs two runs to match, we run them on the same bath day and note the batch on the certificate. That is the practical control, not a guarantee.