What’s the actual difference between Type II and Type III?
Process conditions. Type III runs at lower bath temperature and higher current density, which produces a much thicker, denser oxide. Type II typically builds 0.0002″–0.0008″ of coating; Type III defaults to a nominal 0.002″ unless the drawing says otherwise — roughly three to ten times thicker. That thickness and density difference drives everything else: hardness, color, growth, and cost.
How much harder is hardcoat, really?
Type III hardcoat typically measures 400–530 HV (Gleco’s process runs 500–530 VPN on common alloys) and is on the order of ten times more wear-resistant than conventional Type II anodize in Taber abrasion testing. Type II resists scuffing and handling wear; Type III resists sliding, abrasive, and impact wear — pistons, cylinder bores, gears, cams, and any aluminum surface that mates with a moving part. If the surface sees repeated mechanical contact in service, that is a Type III application.
Which one for color and cosmetics?
Type II. Its thinner, more transparent oxide accepts dye uniformly — Class 2 covers black, blue, red, gold, and more, and Class 1 gives a clean natural finish. Type III’s thick oxide is inherently dark gray to bronze (darker on high-alloy material), colors inconsistently, and is realistically only dyed black (Class 2). If matched cosmetic color across parts and alloys matters, specify Type II; if you specify Type III, expect alloy-dependent gray/bronze and put “color not controlled” language in the notes unless black dye is called out.
How much will my part grow?
Anodizing is a conversion coating: roughly half the coating thickness grows outward and half penetrates the original surface. So dimensional growth per surface is about 50% of coating thickness — negligible for Type II (0.0001″–0.0004″ per surface), but real for Type III: a default 0.002″ hardcoat adds about 0.001″ per surface, which is 0.002″ on a diameter and enough to scrap a tight-tolerance bore. Machinists either pre-machine undersize/oversize to allow for growth, or the drawing specifies “anodize and grind.” Threads deserve special attention; hardcoating a threaded hole without allowance will change fit.
Does anodizing hurt fatigue life?
Type III can, significantly — published fatigue debits run 30–50% on some high-strength alloys, because the thick brittle oxide initiates cracks under cyclic load. Type II’s effect is much smaller but not zero. For fatigue-critical hardcoated parts, standard mitigations are shot peening before anodize, thinner coating on critical sections, and design attention to radii. This is an engineering decision that belongs on the drawing, not a default the plater can pick.
What do the MIL-PRF-8625 classes mean?
For both types, Class 1 is non-dyed and Class 2 is dyed. Sealing is a separate decision: Type II is normally sealed (hot DI water, nickel acetate, or dichromate) and must pass 336 hours of salt spray per spec; Type III sealing is optional — sealing improves corrosion resistance (sealed hardcoat can pass 1,000 hours salt spray) but measurably reduces abrasion resistance, so wear-critical hardcoat is usually left unsealed or PTFE-impregnated. State type, class, thickness, seal, and any masked areas in the callout, e.g., “Anodize per MIL-PRF-8625, Type III, Class 1, .002 thick, unsealed; mask datum A.”
