Type II vs Type III hard anodizing: which should you specify?

The short answer

The process difference in one paragraph

Both types are sulfuric acid anodizing, and both convert the aluminium surface into aluminium oxide rather than adding a foreign layer. The difference is entirely in how the bath is run. Type II runs close to room temperature at a moderate current density, which grows a porous layer quickly and leaves it receptive to dye. Type III drops the bath temperature to near freezing and raises the current, which slows the reaction and produces a denser, less porous oxide that is thicker and far harder but almost impossible to dye. Everything else that separates the two, from colour to dimensional growth to cost, follows from that one change in conditions.

Side by side: thickness, hardness, colour, corrosion

Put the two on the same table and the decision usually makes itself. A Type II layer of 5 to 25 microns runs at 200 to 350 HV and takes any dye colour, which is why it is the default on enclosures, panels, trim and consumer hardware. A Type III layer starting at 25 microns and reaching 125 runs at 400 to 500 HV, roughly ten times more wear-resistant than Type II in abrasive testing, and that is what makes it the choice for pistons, valve bores, slide rails and any surface that mates with a moving part. The corrosion figures favour Type III as well, at 1,000 hours and up of neutral salt spray when sealed, against 336 to 500 hours for a sealed Type II.

What the choice does to your dimensions

Because anodizing is a conversion, both types grow the part by about half the coating thickness on each side, and the difference in thickness becomes a difference in fit. A Type II layer of 20 microns adds about 10 microns per surface, which is negligible on most parts. A Type III layer at the common 50 micron nominal adds about 25 microns per surface, which is 50 microns across a diameter and enough to lose a press fit or jam a thread. In practice hard-anodized parts are machined with a stock allowance, or the drawing says anodize and grind, and threads are either masked or cut oversized. Ignoring the growth is the single most common reason a hard-anodized batch fails inspection, and the fix costs nothing if it is designed in at the drawing stage.

Cost and lead time

Hard anodizing costs two to four times Type II, and the reasons are all process-related rather than arbitrary. The bath must be held near freezing while kilowatts of electrical energy are pumped through it, so the refrigeration load alone is substantial. Cycle times are longer, because building a 50 micron layer takes roughly three times the tank time of a decorative coat. Racking is more labour-intensive, since hard-anodized parts often need bolted titanium fixtures and careful masking of contact points to avoid burning. Thicker layers also raise the reject risk, because uncontrolled growth cracks. The practical consequence is that Type III should be specified only where the wear or insulation performance is genuinely required, and a part that is merely cosmetically important should stay on Type II.

Which type for which application

The decision tree is short. If the surface slides, rubs or abrades against another surface, it is a Type III application: hydraulic valve bores, pistons, cylinder walls, cams, gears, slide rails and tooling fixtures. If the surface is static and mostly seen, it is Type II: electronics enclosures, instrument panels, brackets, trim and any part where a consistent colour across a batch matters. If the requirement is electrical insulation on a heat sink or a housing, Type III carries the thicker dielectric barrier. If the requirement is tight tolerance with minimal growth, Type II wins because it removes the least from the allowance. And if fatigue life under cyclic load is critical, neither thick layer is ideal, and a thin chromic Type I is usually the correct specification.

What to write on the drawing

An ambiguous finish note generates rework, and anodizing is unusually prone to it because the default thickness is set by the finisher unless the drawing overrides it. A clear callout names the type, gives a thickness range rather than a single number, states the colour or the word natural, lists the surfaces that must stay uncoated, and references the acceptance standard. Threads and sealing faces belong in the masking list by name. The two errors to avoid are assuming that machined threads will still fit after hardcoat, and designing a close-tolerance fit without allowing for coating growth. Every figure here is a planning range for the two dominant sulfuric types; the layer a given line achieves depends on alloy, racking and bath control, so a thickness measurement on your own part is the only number that settles it.

See surface finishing for the anodizing types we run, CNC machining for the pre-finish stock allowance, and aerospace and aviation parts for the kind of fatigue-sensitive work where a thin chromic layer often beats hardcoat.