The quick answer
Type II for corrosion protection, color, and appearance — which covers 80% of anodized parts in industry. Thinner (5–25 µm), cheaper, takes dye well, preserves tight tolerances. Type III (hardcoat) for wear resistance, electrical insulation, and severe-duty corrosion — thicker (25–75 µm), harder, limited color options, requires dimensional compensation.
The most common over-specification: Type III hardcoat on parts that never see wear or abrasion. A customer sees "hard" in the spec name and thinks harder is always better. For non-wearing surfaces, Type III doubles the finishing cost while providing no functional benefit over Type II. If the only specification driver is "looks like anodized aluminum," Type II is correct.
The process difference
Both processes pass current through aluminum parts in a sulfuric acid electrolyte, growing aluminum oxide on the surface. The surface oxide is hard, corrosion-resistant, and electrically insulating by nature. The difference between Type II and Type III is entirely in the process conditions — same chemistry, different parameters.
Type II runs at 18–22°C electrolyte temperature, 15–20 V DC, for 15–30 minutes. The warm electrolyte dissolves the oxide at the pore walls as fast as it forms, giving an open, porous structure that accepts dye. Thickness: typically 5–25 µm.
Type III runs at 0–5°C electrolyte temperature (requires chilling), 20–40 V DC, for 40–90 minutes. The cold electrolyte minimizes oxide dissolution, producing a dense, closed-pore structure. The resulting coating is 3–10× harder than Type II and 2–5× thicker. Thickness: typically 25–75 µm, with 50 µm being the most common spec.
Head-to-head comparison
| Property | Type II | Type III (Hardcoat) |
|---|---|---|
| Coating thickness | 5–25 µm | 25–75 µm |
| Hardness (Vickers) | 200–400 HV | 500–700 HV |
| Rockwell equivalent | ~45 HRC | ~65 HRC |
| Electrolyte temp | 18–22°C | 0–5°C (chilled) |
| Processing time | 15–30 min | 40–90 min |
| Dimensional build (per side) | 2.5–12.5 µm | 12.5–37 µm |
| Dielectric strength | ~500 V | 1500–2500 V |
| Salt spray corrosion | 336 hrs | 1000+ hrs |
| Color options | Full spectrum | Black, dark grey, natural |
| Surface finish change | ±0.1 µm Ra | ±0.3 µm Ra |
| Typical cost | 1.0× | 2.0–3.0× |
When Type II is the right answer
Specify Type II for: enclosures and housings where corrosion protection and color are the main drivers, consumer product faceplates and trim, architectural components (anodized aluminum railings, panels), brackets and non-wearing structural parts, electronic heatsinks, sanitary food-contact parts (with appropriate sealing), decorative parts requiring specific dye colors.
Type II with Class 2 (dyed) finish in common colors — matte black, clear, gold, red, blue — is the standard consumer-facing anodize. It's what you see on laptop chassis, camera bodies, bicycle components, and pen barrels. For 95% of those applications, Type II with proper sealing provides adequate corrosion resistance and a uniformly colored, cleanable surface.
When Type III is required
Specify Type III (hardcoat) for: sliding or rotating surfaces that would wear Type II through (pistons, bearing surfaces, valve spools), surfaces with repeated contact abrasion (handles, slides, levers on heavy equipment), electrical isolation applications (dielectric strength 3–5× Type II), military and aerospace components under MIL-A-8625 Type III requirements, high-wear consumer parts (firearms components, premium tool housings), salt-spray-exposed equipment (marine hardware, coastal outdoor components).
A real example: a pneumatic cylinder piston running inside an anodized aluminum bore. Type II oxide would wear through in thousands of cycles. Type III survives millions. The same piston housing with a non-wearing flange could be selectively masked so only the bore gets hardcoat and the flange gets Type II — cutting finishing cost while keeping wear performance where it matters.
Dimensional compensation
Anodize grows roughly 50% into the base metal and 50% out from the original surface. For a part machined to 20.00mm OD with Type III targeted at 50 µm coating, the post-anodize diameter will be approximately 20.00 + (2 × 25 µm) = 20.05mm. Bore diameters shrink by the same amount.
Tight-tolerance features usually need pre-anodize compensation: machine shafts 25 µm under nominal OD before Type III, or mask the features and leave them bare aluminum. Threaded holes M5 and smaller typically need masking — the coating reduces effective thread engagement enough to cause fastener issues. Our workflow applies these offsets automatically during CAM programming, but it's good practice to discuss critical dimensions during the DFM review to make sure tolerance stack-up is correct.
Color options and visual effects
Type II accepts a wide range of colors: the porous oxide absorbs dye, which is then sealed into the pores. Standard colors include black (absorbs most), red, blue, gold, green, bronze, clear (no dye, shows natural aluminum with anodize gloss). For color-matched production, we pull dye samples from each tank and compare against a reference under controlled lighting. Lot-to-lot consistency is good but not perfect — expect minor shade variations between production runs.
Type III has limited color flexibility. The natural color of undyed Type III ranges from light bronze (on 6061) to dark grey or near-black (on 7075) depending on alloy. Dyeable options are basically limited to black — and even black Type III has a slightly different appearance than black Type II due to the denser oxide. If your product line requires matched anodize finish across parts with different wear requirements, Type II on everything is simpler than trying to match Type II to Type III.
Masking and selective anodize
Sometimes you want anodize on some surfaces but not others — threaded holes, electrical grounding surfaces, press-fit bearing seats. The shop solution is masking: applying a chemically-resistant tape, plug, or wax that protects the surface from the electrolyte. After anodize, the mask is removed and the masked area remains bare aluminum.
Masking is a per-part cost adder: figure $0.50–2.00 per mask point for small parts, more for complex masking patterns or high-volume production. For high-volume parts, custom-fabricated masking fixtures amortize this cost. Specify masking requirements with a marked view on the drawing — either a "DO NOT ANODIZE" callout with hatching, or an explicit "MASK PRIOR TO ANODIZE" instruction with a detailed dimension. Ambiguous masking specs are the most common cause of finish-related quality escapes.