Guides / Finishing

Titanium Anodizing: What Type II and Type III Actually Do

Titanium anodizing confuses people who know aluminium anodizing, because the two processes share a name and almost nothing else. Here is what actually happens and what to specify.

It is not aluminium anodizing

On aluminium, anodizing grows a relatively thick porous oxide that absorbs dye. The colour is in the dye. The layer is thick enough to matter dimensionally, and hard anodize is thick enough to need planning into the tolerance.

On titanium, the oxide is extremely thin and it is not porous, so it takes no dye. The colour you see is light interference: white light reflects off both the top of the oxide and the metal underneath, and depending on the oxide thickness some wavelengths cancel and others reinforce. Change the oxide thickness and you change the colour.

Oxide thickness is set by voltage. That is the whole colour control system.

The AMS 2488 types

The controlling specification for most work is AMS 2488, which groups titanium anodizing by intended use.

Type	What it is	Appearance	Used for
Type I	Higher temperature alkaline process, thicker oxide	Dark grey	Aerospace applications, often as a base for other treatments
Type II	Room temperature process, moderate oxide	Even matte grey	Wear resistance, anti galling, general engineering
Type III	Very thin oxide, voltage controlled	Interference colours	Colour coding, identification, appearance

For a buyer the practical split is simple. Type II is the one you ask for when threads gall or surfaces slide. Type III is the one you ask for when the part has to be a colour.

Why anti galling is the real argument for Type II

Titanium galls against titanium. Threads seize, sliding faces pick up and tear, and the failure is sudden rather than gradual. This is the single most common reason a titanium assembly disappoints in service.

Type II anodizing puts a harder, chemically stable layer between the two surfaces and takes most of that problem away. It is cheap relative to redesigning the joint, and it is a one line drawing callout.

If your drawing has a titanium fastener threading into a titanium boss with nothing specified, that is the first thing to fix. Options are Type II anodize, an anti-seize compound, or a stainless insert.

The colour sequence

Type III colours arrive in a repeatable order as voltage rises. Roughly:

Approximate voltage band	Colour
Low	Bronze and brown
	Deep blue
	Light blue
	Yellow and gold
	Purple and magenta
	Teal and green
High	Pink and rose

The exact voltages depend on the electrolyte, the alloy and the surface preparation, which is why colour matching between two finishers is genuinely difficult. If colour consistency matters, specify a single approved finisher and get a physical sample approved before production.

Black and true red are not available through interference. Black titanium parts are usually PVD coated or treated by a different process entirely.

Surface preparation decides the result

This is the part machining buyers most often miss. Anodizing is transparent. Whatever the surface looked like before, you will see through the colour.

• Fine milled surfaces come out smooth and saturated.
• Coarse tool paths scatter light and give a muted, uneven colour.
• A witness line from a tool change shows up clearly.
• Bead blasting before anodizing gives an even matte colour and is the usual route for a consistent appearance.
• Polishing before anodizing gives the deepest, most saturated colour.

So a colour callout is also, implicitly, a surface finish callout. Say both on the drawing, or ask the finisher what preparation the colour you want requires.

What to specify on the drawing

A usable callout names the specification, the type, the colour if any, and the preparation.

Anodize per AMS 2488 Type II, all over. No colour requirement.

Or for a cosmetic part:

Bead blast, then anodize per AMS 2488 Type III, colour to approved sample. Masked on the bore marked X.

Masking matters. Anodizing a bearing bore or a threaded hole that needs a precise fit is usually not what you want, and a finisher will mask only what you tell them to mask.

Lead time and cost

Anodizing is an outside process for most machine shops. Budget three to seven days on top of machining, plus transport each way. Cost per part is usually modest and drops quickly with batch size, because the setup is per batch rather than per piece.

Two practical consequences: do not add anodizing to a rush job without checking, and do not split one batch across two finishers if colour has to match.

The short version

Ask for Type II when the part slides or threads against other titanium. Ask for Type III when it has to be a colour, and approve a physical sample first. Ask for neither when the part is internal, industrial and nobody will see it, which is more often than most drawings suggest.

Frequently asked questions

Keep reading

• Surface Finish on Machined Titanium and What Each Option Is For
• Drilling Titanium: Speeds, Heat and Why Holes Fail First