Type III

Type III hardcoat anodizing builds a thick, dense aluminum oxide layer for high wear resistance and electrical insulation, with measurable dimensional buildup.

Overview

Type III anodizing (hardcoat anodizing) grows a thick, dense aluminum oxide layer on aluminum using a low-temperature sulfuric acid anodize process. It delivers high abrasion resistance, improved corrosion performance, and strong electrical insulation, with a typical finish ranging from dark gray to black (dye options are limited and color can vary by alloy).

Choose Type III when surfaces see sliding wear, grit abrasion, or repeated handling and you need a durable oxide—not paint. It’s common on machined parts where hardness and long service life matter more than cosmetic appearance.

Tradeoffs: the coating adds thickness (dimensional buildup) and can tighten fits; some growth is inward and outward. Sharp edges and thin features can burn or grow unevenly, and holes/threads may require masking or post-process sizing. Alloy, temper, and surface finish influence final color, thickness uniformity, and corrosion performance.

Common Materials

  • Aluminum 6061
  • Aluminum 7075
  • Aluminum 2024
  • Aluminum 5052
  • Aluminum 6082

Tolerances

±0.002–0.005 in after anodize (unless masking/post-sizing is specified)

Applications

  • Hydraulic cylinder bores and pistons
  • Sliding wear plates and rails
  • Firearm receivers and handguards
  • Robotic gripper jaws and end effectors
  • Bicycle suspension components
  • Aerospace actuator components

When to Choose Type III

Choose Type III for aluminum parts that must resist wear, galling, and abrasion while maintaining a stable oxide surface. It fits low-to-medium production where durability is driven by function, not cosmetics. Plan for coating buildup on precision fits, threads, and sealing surfaces.

vs Type I anodizing

Choose Type III when wear resistance and surface hardness are primary requirements and you can accept thicker buildup. Type I is thinner and often used where minimal dimensional change is critical, while Type III is preferred for sliding contact and abrasive environments.

vs Type II anodizing

Choose Type III when the part sees mechanical wear and needs a harder, thicker oxide layer. Type II is better for decorative color and moderate corrosion protection, but it won’t hold up as well in high-wear contact areas.

vs Electroless nickel plating

Choose Type III when you want an oxide layer integral to aluminum with strong abrasion resistance and electrical insulation. Electroless nickel can offer very uniform thickness and better coverage in recesses, but it changes conductivity and can be higher cost for larger parts.

vs Hard chrome plating

Choose Type III when you need a durable wear surface on aluminum without introducing a thick metallic plating system. Hard chrome can provide excellent wear and low friction, but it’s typically used on steels and has regulatory and process constraints that can drive lead time and cost.

Design Considerations

  • Call out coating thickness range and whether thickness is per-side; specify any critical-to-fit surfaces that must be masked or post-sized
  • Add anodize allowance on tight bores, sliding fits, and thread classes; don’t assume the shop will “hold size” through coating without instruction
  • Break sharp edges and add small radii/chamfers to reduce burning, improve thickness uniformity, and protect the coating in handling
  • Specify which holes/threads must remain conductive or gaugeable; plan for masking plugs and caps in the model/print
  • Define cosmetic requirements realistically (color, gloss, visible streaking) and reference a standard like MIL-A-8625 Type III if certification matters
  • Avoid ultra-thin fins and knife edges; hardcoat grows unevenly and can crack or spall on fragile features