Decorative vs functional. Thin vs thick. Color vs wear.

Type III anodize produces much harder surface than Type II — 300-500 HV vs 200-250 HV. Achieved by: colder electrolyte temperature (-5 to +5°C vs room temperature), higher voltage, longer process time. Resulting oxide layer is denser and harder. Wear resistance: 5-10× Type II. Used specifically for functional wear applications.

Type III coating is darker and more porous than Type II, absorbing dyes differently. Black dye penetrates well, producing uniform dark black. Other colors appear muted and less vivid than Type II. For bright colors: Type II. For black wear-resistant: Type III. Some Type III colors possible (bronze, red-brown, dark blue) but limited palette.

Type III is thick: 25-75 µm adds significant dimension. Grows 50% into surface, 50% out. For tight tolerance features (bearing bores, mating surfaces), account for growth or mask features. Common approach: machine slightly undersize on features requiring Type III, anodize, achieve target dimension with coating. Masking used for internal threads, bearing bores — costs extra but ensures dimensional accuracy.

Difficult — different processes, single anodize run produces single type. Workarounds: (1) Mask areas for second anodize pass (expensive, 2× lead time). (2) Use Type II for entire part, upgrade only wear areas by other means (PVD coating, insert). (3) Redesign to use consistent anodize type. For most parts, pick one type — mixed is specialty work.

Type III has higher dielectric strength (800+ V/µm vs 200-400 V/µm Type II) due to thicker denser coating. Used for electrical isolation in aerospace and industrial applications. Alternative: conductive anodize (rare) or specifically non-anodized surface with insulation sleeve. For high-voltage aluminum applications, Type III provides useful electrical isolation.

Both Type II and III significantly improve corrosion resistance vs bare aluminum. Type III is thicker and denser, so better long-term corrosion protection especially in severe environments. For coastal/marine outdoor 20+ year service: Type III preferred. For indoor consumer electronics: Type II adequate. Sealing (post-dye) further improves both.