Titanium Anodizing: Color (Type II) and Hardcoat (Type III) Explained

Titanium color anodizing (commonly called Type II, per AMS 2488) grows a transparent oxide whose thickness is set precisely by the applied voltage. That oxide produces color by light interference, not pigment, so there's no dye involved and the color is the same physics that makes a soap bubble iridescent. Roughly, low voltages give bronze and gold, mid voltages give blue and purple, and higher voltages give yellow, magenta, and green, with a repeatable voltage-to-color map specific to each bath chemistry. This is heavily used in medical devices: surgical implants and instruments are color-anodized for size and type identification (spinal screws, bone plates, dental components) because the oxide is biocompatible, adds no meaningful thickness, and won't flake into the body. In aerospace it codes fasteners. The catch is that the color is purely a thin interference film, so a deep scratch goes through it and the underlying titanium shows; it offers no wear protection. Surface prep matters enormously, because the same voltage on a bead-blasted versus a polished surface reads as a different shade.

Titanium's worst tribological habit is galling, it cold-welds to itself and to other metals under load and sliding, seizing threads and fretting mating surfaces. Type III titanium anodize (per AMS 2487) grows a thicker, harder, matte-gray oxide specifically to combat this, dramatically improving anti-galling and fretting performance on threaded fasteners, lugs, and sliding joints. It's not as thick or hard as aluminum hardcoat, but it's the standard surface treatment for titanium fastener systems and many airframe titanium fittings. Grade 5 (Ti-6Al-4V) and Grade 23 (the low-interstitial ELI version of Grade 5) are the typical candidates because they're the structural and implant grades that see the highest loads and galling risk. Commercially pure Grade 2 anodizes for color readily but is softer and less often hardcoated. Type III titanium anodize is matte gray and not chosen for color, though limited color control exists. For severe wear it's often paired with or replaced by other treatments, see the alternatives section.

Titanium anodizing is excellent for color ID and meaningful for galling, but it is not a true hard wear coating like aluminum Type III, and buyers expecting bearing-grade wear resistance will be disappointed. For real abrasion and erosion resistance, titanium parts get other treatments: thermal oxidation/hardening, plasma nitriding, or DLC and TiN PVD coatings that deliver genuine surface hardness. These are the right call for valve trim, pump components, and sliding wear surfaces in oil-gas and energy service. Passivation per ASTM A967 is also common before or instead of anodize on medical titanium to clean the surface and ensure the protective oxide is uniform. And for the cosmetic crowd, remember the anodize color is bath- and surface-dependent: to match a sample across lots you must control the alloy, the surface prep, and the rectifier voltage tightly. Mixing Grade 2 and Grade 5 parts in one color batch will give visibly different shades at the same voltage because the alloy chemistry shifts the oxide refractive behavior.