Type II vs Type III anodizing for aluminium EDC parts

Both are sulphuric acid anodizing. The difference is bath temperature, current density and time, and the result is two finishes with genuinely different jobs.

Type II — decorative and corrosion resistant

Run warm, roughly 5 to 25 microns thick. The oxide layer is porous before sealing, which is what lets it take dye. That is why Type II is the one that gives you a colour range: red, blue, green, purple, gold, and a clear that just protects.

Use it for: scales and clips where colour is part of the design, internal parts, anything where the wear is light.

Cost: low. It is the default aluminium finish in most of the world, and the bath charge at EDC batch sizes is usually smaller than the setup cost of the machining.

Limits: dyed Type II fades under UV and wears through at contact points. A bright red clip will be pink at the corners after a year in a pocket.

Type III — hard anodize, also called hardcoat

Run near freezing at much higher current density, roughly 25 to 50 microns and considerably denser. It is genuinely hard — the reason flashlight bodies have used it for decades.

Use it for: flashlight bodies, heads and tail caps, threads, any surface that slides or gets dropped.

Colour: limited, and it is not really dye. The natural colour of a thick hardcoat on 6061 is a deep grey-black; olive drab and dark bronze are achievable. Anything bright is not Type III.

Cost: higher than Type II, and more likely to have a minimum batch charge worth planning around.

The alloy matters

6061-T6 hardcoats to a more even, deeper black than 7075-T6, which tends toward a greenish or brownish cast because of its zinc content. If you need a true black and the part is not structural, specify 6061. See 6061 vs 7075 for EDC for the rest of that trade-off.

Cast alloys and high-silicon alloys anodize badly in general. Stick to wrought 6xxx and 7xxx for EDC parts that will be anodized.

Dimensional growth — the thing that catches people

Anodizing converts the surface rather than coating it. Roughly half the layer grows outward and half consumes base metal, so an external dimension grows by about half the total thickness per surface.

On Type II that is small enough to ignore on most features. On Type III it is not: 25 to 50 microns per surface on a threaded pair or a sliding fit is enough to make parts that do not assemble.

Two rules:

1. Say in the title block whether dimensions are before or after finish.
2. Tell the shop the anodize type before the part is machined, so tight features get relieved.

Flashlight threads covers what this does to a head-and-body pair specifically.

Masking

Anything that must stay conductive or dimensionally exact needs masking: threaded inserts, electrical contacts, bearing bores, the spring contact on a tail cap. Masking is manual labour and it is priced per part, so call out exactly what to mask on the drawing rather than describing it in an email.

Batch consistency

Colour drift across a batch is the most common anodize complaint in EDC, and it is usually racking rather than chemistry — parts at different points in the tank see different current. Ask for a sample set before a production run, and ask whether the whole batch goes through in one load. On a 200-piece drop, two loads on different days is a real risk of two slightly different colours.

Quick answer

Colour and cost: Type II. Wear and threads: Type III. Both on one part is possible with masking, and it is how some two-tone designs are done, but it doubles the finishing handling.

Send the part with the anodize type stated and the quote will come back with the right allowance already in it.

Sealing, and why colour fades

After the oxide layer forms it is porous. Sealing — usually hot deionised water, sometimes nickel acetate — closes those pores and locks the dye in.

An unsealed or poorly sealed Type II part will fade, bleed colour onto a cloth when wiped, and corrode faster. It is invisible at delivery and obvious three months later, which makes it one of the harder defects to catch.

Ask what the sealing process is. Hot water sealing takes real tank time, and it is one of the corners a shop under schedule pressure can cut.

Type III is usually left unsealed or only lightly sealed, because sealing slightly reduces hardness. For a flashlight body that is the right trade.

Thickness and where to specify it

Anodize thickness is specified in microns and it is worth putting a number on the drawing rather than just a type.

• Type II decorative: 5 to 12 microns is typical
• Type II for corrosion: 15 to 25 microns
• Type III standard: 25 to 50 microns
• Type III heavy: up to 75 microns, rarely needed in EDC

More is not better. A very thick hardcoat is more brittle and can craze at sharp edges, and it consumes more base metal. For an EDC light, 25 to 35 microns is a sensible target.

Relevant standards if you want one on the drawing: MIL-A-8625 Type II and Type III, which is the specification most shops recognise.

Edges, corners and racking marks

Two cosmetic issues that surprise first-time brands:

Sharp edges anodize badly. Current concentrates at a sharp corner, producing a thicker, more brittle layer that can chip. Break every edge with at least a small radius or chamfer. This is worth doing for function as well as appearance.

Every part has a contact point. The rack has to touch the part somewhere to carry current, and that contact point does not anodize. On a flashlight body it is usually hidden inside a thread or a bore; on a flat scale it has to go somewhere.

Tell the shop which surfaces are cosmetic so it can plan where the rack mark lands. If you do not, it will land wherever is convenient.

Two-tone and masked designs

A part that is Type III on the threads and Type II colour on the body is possible and it is how some premium designs are done. It means two separate anodizing operations with masking between them, so it roughly doubles the finishing cost and adds a week.

It is a real design option, not a free one. Decide before the first article, because retrofitting it changes the machining tolerances.

Cost at EDC volumes

Anodizing is a batch process with a minimum charge per load. That means 30 parts and 150 parts can cost nearly the same to run.

Two practical consequences:

1. Small anodize batches have a high per-part cost that has nothing to do with your part
2. If you are planning two colourways, running them as one machining batch and two anodize loads is much cheaper than two separate projects three months apart

Ask what the minimum bath charge is. It frequently changes the quantity decision.

FAQ

Can Type III be coloured black reliably? Yes on 6061 — a thick hardcoat is naturally near-black. On 7075 expect a cast. If you need true black on 7075, you are looking at dyed Type II over a thinner layer, with the wear trade-off that implies.

Does anodizing change the weight? Negligibly. It changes dimensions in a way that matters for fit, but not mass in any way a customer notices.

Can anodizing be stripped and redone? Yes, chemically, and it is the normal remedy for a colour that came out wrong. Stripping removes some base metal, so a part can only go through it a limited number of times before it is undersize. Two is usually safe; beyond that, remake.

Is anodized aluminium safe against skin? Sealed anodize is inert and widely used in products worn or carried against the body. Unsealed dyed anodize can transfer colour, which is another reason to ask about sealing.

Why does my part have a slightly different colour than last batch? Almost always racking or a second tank load — see titanium anodizing colours, where the same mechanism applies more visibly. Ask for one load and an approved sample.

Can I anodize a part with a stainless insert? Not without masking or removing the insert. Dissimilar metals in the bath cause problems for both. Design for the insert to be pressed in after finishing.