Anodizing is the most common surface treatment for aluminum parts in consumer products, industrial equipment, and machined enclosures, and it delivers wear resistance, corrosion protection, and a finished appearance in a single step. The cost is usually modest — for machined parts it typically adds a few percent up to around a tenth of the part cost in production quantities — but small lots are dominated by minimum lot charges, which is why a single prototype part can cost a surprising amount to anodize. Unlike paint, the coating is not applied on top of the metal; it is grown out of the metal electrochemically, so it cannot peel or chip off. That also means it has to be planned during design, not requested after the parts are already machined.

What Actually Happens in the Tank

The part is immersed in an acid bath and wired as the anode in an electrical circuit. Current causes the natural oxide layer on the aluminum surface to grow in a controlled way into a hard, porous ceramic film. Those pores are what make coloring possible: dye penetrates them, and a sealing step then closes them to lock the color in and complete the corrosion barrier.

Because the film grows partly inward and partly outward, the finished part grows on each surface by roughly half the coating thickness — a dimensional fact that has to appear on the drawing.

Everything is decided before the tank, though. Cleaning, degreasing, etching, and sometimes polishing or bead blasting set whether the result is uniform or blotchy. That preparation is also why a small handling scratch stays visible after anodizing: the coating is semi-transparent and does not hide defects, it highlights them. Parts must be protected in transit between machining and finishing.

Types of Anodizing and What Each Is For

  • Type II (conventional decorative/protective). A film in the range of a few microns up to roughly 25 microns. This is the standard choice for enclosures, front panels, and visible parts, and it accepts dye well.
  • Type III (hard anodize). A much thicker film — tens of microns — with very high hardness and excellent abrasion resistance. Used on moving parts, pivots, wear surfaces, and pistons. It is typically dark gray to black even undyed.
  • Type I (chromic) and thin films. Very thin coatings used mainly as a paint or adhesive base, or in aerospace applications where fatigue performance matters.

Colors: What Is Reliable and What Is Not

Black, gray, blue, red, and bronze come out consistently. Light, highly saturated, and pastel shades require careful bath control and vary from lot to lot; white is effectively impossible in a conventional dye process. The critical point designers miss is that anodizing is not opaque paint — the final shade is influenced by the alloy, the grain direction, and the mechanical finish applied beforehand.

Practical consequences:

  • Silicon-rich cast alloys anodize dark and mottled. Extrusion and machining alloys such as 6061 and 6082 give clean, even results.
  • Parts from different alloys or different suppliers will not match, even with identical process settings. If two parts must match visually, specify the same alloy and the same finisher.
  • Lock the shade against an approved, signed physical sample plus a written material and finish specification. A color name in an email is not a specification.
  • Anodized dye is not fully lightfast. Outdoor products in strong sun should use fade-resistant or inorganic coloring, and black is generally the most stable choice.

How It Changes the Design and the Dimensions

Because the coating adds material, precision holes and threads can fall out of tolerance. The practical rule is to either allow for the growth on critical dimensions or mask those features, and to state that explicitly on the drawing rather than leaving it to the finisher's judgment. Hard anodize needs even earlier attention, because it adds a meaningful fraction of a millimeter and rounds off sharp corners.

Points to settle with your engineer before release:

  • Threads and bearing seats. Mask them, or machine them after coating.
  • Rack marks. The part hangs on a fixture, and the contact point always leaves a small mark. Decide in advance where it will be — ideally a hidden surface.
  • Electrical contact and grounding. The anodic film is an insulator. Every chassis-ground point needs a masked, uncoated area, and that has to be drawn.
  • Tight tolerances. Note on the drawing which dimensions are measured before coating and which after. This single note prevents most anodizing disputes.
  • Bending after coating. Do not do it. The film is brittle and will craze on the outside of a bend, so sheet metal parts are formed first, coated second.

The general framework for calling out these features is covered in our guides to GD&T basics and design for manufacturing.

Cost, Quantities, and Lead Time

The plating itself is cheap relative to machining, and for CNC parts its effect on the final unit cost is small — our breakdown of CNC machining for prototypes puts the machining side in context. What surprises first-time buyers is the minimum lot charge: a shop that charges a minimum for a given color charges it whether you send three parts or three hundred, so a single prototype in a non-standard color can cost more than the part itself. Custom colors carry setup costs of their own.

Typical lead time is several business days for standard black or clear, and longer for a specific dyed color or hard anodize. Cost drivers worth knowing:

  • Surface area, not part count — pricing is generally by area or by rack.
  • Color — standard colors are cheapest; specific matched shades cost setup and yield.
  • Masking — every masked feature is manual labor and priced accordingly.
  • Pre-finish — bead blasting or polishing before anodizing is a separate operation with its own price.
  • Inspection level — a cosmetic Class A surface with a written acceptance standard costs more than a functional finish.

On early prototypes it is usually smart to skip anodizing entirely, or to accept a plain clear finish, and to spend the money once the geometry is settled.

When to Choose a Different Finish

Anodizing is not universal. It works only on aluminum and its alloys — never on steel or stainless — it is attacked by strong alkaline cleaners, and it will craze if a part is formed after coating. If the part sees harsh marine exposure, aggressive cleaning chemicals, or high temperatures, consider powder coating, e-coating, or a conversion coating instead, and in some cases change the material outright. That decision starts upstream, with the choice of metal itself, covered in our comparison of aluminum vs steel. Sealing performance also interacts with your enclosure's ingress requirements — see IP ratings explained.

Specify the Finish, Not Just the Part

Projects House specifies alloys, finishes, masking, and tolerances as an integrated part of mechanical design, so parts arrive looking and fitting the way the drawing intended. Have an aluminum part that has to look professional and hold up in service? Send it through our contact form and we will define the alloy, finish, and tolerance scheme for it. More in our manufacturing technologies knowledge center.