Anodized Aluminum Enclosures: Type II & III

An enclosure that comes back from anodizing the wrong colour, with cracked corners and holes that no longer accept the fastener, was not ruined at the anodizing tank. It was ruined on the drawing — usually by a single line that says “anodize, black” and nothing else.

That line leaves nine decisions open. Whoever makes them will be the applicator, working from habit rather than from your spec: which alloy shade they happen to have, where the rack grips the part, whether the bore keeps its size, whether the earth path survives. Some of those habits are fine. Some of them produce a part you cannot assemble.

This page is about the nine decisions, in the order they bite. It is written for buyers specifying anodized aluminum enclosures, not as a chemistry lesson.

What anodizing is, in the two sentences that matter to a designer

Anodizing is not a coating applied on top of the metal. It is an electrolytic process that converts the surface layer of the aluminum itself into aluminum oxide — the film grows out of the part. Two consequences follow from that, and both are design constraints rather than finishing details:

• It cannot flake or peel. There is no interface to fail. That is the whole reason to choose anodize over paint on a part that gets handled.
• It changes the size of the part. The film grows roughly half outward from the original surface and half into it. A bore therefore loses close to the full coating thickness from its diameter. Small numbers, but they land inside real tolerances.

If you want the process itself rather than the specification — the tank sequence, the dyeing and sealing steps, and the standards behind them — that is written up separately on anodizing aluminum sheet metal. This page assumes you know what the process does and are now deciding what to write on the drawing.

The third consequence is the one that catches enclosure designers: aluminum oxide is an insulator. A hard anodized surface does not conduct. If your enclosure needs an earth path or an EMC contact, that has to be designed in before the part goes to the tank, not argued about afterwards.

Type II or Type III: what each one is actually for

The names are from the US military specification buyers most often cite, and the two classes are not “good” and “better” — they suit different jobs and they do not look the same.

The thickness numbers above are the industry-typical ranges for each class. What a specific bath can hold is narrower than that. Where thickness is load-bearing — a wear surface, a bore that must stay in tolerance — we confirm the achievable range with the applicator before we quote, and we tell you which line the work will run on.

The specifications buyers most often name are MIL-A-8625 (Type II and III), ISO 7599 (sulfuric acid anodizing) and ISO 10074 (hard anodic coatings). Tell us which one governs your part; if none does, tell us the service environment instead and we will propose a class.

Seven things that go wrong, and where they are actually decided

1. The alloys on one assembly do not anodize the same colour

This is the most common cosmetic rejection, and it is decided when the material is specified — weeks before finishing. 5052 and 5754 (aluminum-magnesium) anodize bright and close to colourless. 6061 carries more silicon, magnesium and copper, and clear anodize over it reads slightly grey or warm. Put a 5052 door on a 6061 chassis and the two panels will not match, however good the applicator is.

Decide on the drawing: one alloy per visible assembly. If two alloys are unavoidable, expect a shade difference and agree it on a limit panel before the batch runs. Castings and extrusions in the same assembly are the worst case — cast material anodizes mottled.

2. The weld bead goes grey

Filler wire choice decides this, not the anodizer. ER4043 (aluminum-silicon) is the general-purpose filler and it turns noticeably dark grey to black after anodizing. ER5356 (aluminum-magnesium) stays much closer to the parent metal. On a cosmetic welded enclosure this is the difference between a seam you can see from three metres and one you cannot.

Decide on the drawing: name the filler wire on cosmetic welds. It is one line, and it costs nothing at the welding stage. Our welding is done in-house on 10 stations, so the wire is our decision to follow — not something the applicator discovers later. More on how we plan welded aluminum is on the sheet metal welding service page.

3. The part is formed after it is anodized

Aluminum oxide is hard and brittle. Bend it and it crazes — a fine network of cracks along the bend line that looks like a defect and, at the edge of the bend, exposes bare metal. The fix is sequence, not process: anodize last. Cut, bend, weld, dress, then finish.

Decide on the drawing: state the sequence if there is any chance a rework loop will form the part again. A second bend after finishing is a refinish.

4. A hole that was in tolerance is not any more

Because the film grows outward from every surface, a bore shrinks by roughly the coating thickness on its diameter. A 25 µm hardcoat takes about 0.025 mm out of a hole. On a loose clearance hole that is nothing. On an M3 clearance hole already cut at the bottom of its tolerance, or on a bore that locates a shaft, it is a parts-bin problem at your assembly line.

Decide on the drawing: either mask the bore, or size it for the post-anodize condition and say so. Threads are worse than plain holes — specify masking for threaded holes, or accept that the anodizer will plug them and that the plugs leave a visible boundary.

5. The rack leaves a bare mark where you did not want one

Anodizing is electrolytic: the part has to be gripped electrically. The contact point stays bare aluminum. Every part has one, and if you do not say where it may be, it will be wherever the rack fits most conveniently — often a visible face.

Decide on the drawing: mark the permitted contact area, usually an inside face, a flange edge that gets covered by a gasket, or a mounting foot. Related: hollow sections need a drain hole so solution does not get carried out and drip later. If your design has no drain path, add one, or the parts will arrive with staining around a seam.

6. The dye fades, or the colour drifts between batches

Two separate problems that get blamed on each other. Fading is a dye chemistry issue: organic dyes, particularly reds and blues, break down under UV. An outdoor enclosure in an organic blue will not be the same colour in two summers. Inorganic or electrolytic colouring (the bronze and black ranges produced by metal salts in the bath) is far more lightfast. Drift is a process issue: shade varies with alloy lot, bath temperature, current density and time in the tank. Two batches run a month apart can be visibly different even when both are “in spec”.

Decide on the drawing: name the colouring method, not just the colour. If the shade matters, agree a limit panel and keep it — a retained reference sample is the only thing that settles a shade dispute.

7. The enclosure no longer conducts

An anodic film is an insulator. On an enclosure that has to bond to protective earth, or that relies on metal-to-metal contact for EMC shielding, a fully anodized surface breaks the path. This is discovered at the test house, and by then the parts are finished.

Decide on the drawing: call out the contact lands and how they stay conductive — masked areas, machined-after-finishing faces, or a separate bonding stud fitted after anodizing. Where the whole enclosure must stay conductive and corrosion resistance is still needed, a conversion coating (chrome-free, RoHS and REACH compatible) is the usual alternative: micrometres rather than tens of micrometres, conductive, and it takes paint or powder later if the design changes.

The eight lines to add before you send the drawing

Anodize specifications fail by omission far more often than by being wrong. This is the checklist we work from; every line is something the applicator will otherwise decide for you.

1. Class and governing specification — Type II or Type III, and MIL-A-8625 / ISO 7599 / ISO 10074 if one applies.
2. Thickness as a range in µm — “8–12 µm”, not “standard”.
3. Colour and how it is produced — natural clear, or dyed, or electrolytic; with a retained limit panel where the shade matters.
4. Sealing method — hot water, nickel acetate, or a nickel-PTFE seal. Unsealed anodize absorbs whatever touches it and stains.
5. Masked areas, dimensioned — bores, threads, earth lands, sealing faces. Not “mask as required”.
6. Permitted rack contact area — an inside face, a covered flange, a mounting foot.
7. Filler wire on cosmetic welds — ER5356 where the seam has to disappear, ER4043 where it does not.
8. Whether dimensions are pre- or post-treatment — say it once, and the bore argument never happens.

Anodize, brushed, or conversion coating: which one for this enclosure

If you are still choosing between finishes rather than specifying one, the wider comparison by failure cause is on our stainless and aluminum enclosure finishes page. For the alloy and thickness side of aluminum work, start at aluminum sheet metal fabrication.

What we do, and what the applicator does

Being precise about this saves a wasted site visit. We do not own an anodizing line. In our own workshop we brush and mirror-polish aluminum and stainless; anodizing, along with passivation, electropolishing and plating, runs on audited partner lines that we specify and inspect.

What that means in practice:

• We write the specification, and we confirm the achievable thickness range, the colour and the sealing options with the applicator before we quote — not after you have accepted a price.
• We tell you which line the work will run on.
• We approve a sample panel before the batch runs, where the finish is cosmetic.
• We measure on receipt. Coating thickness is checked with coating thickness gauges; dimensional checks with height gauges.

What we do not have is a coordinate measuring machine. We say this up front rather than at the first article. If your incoming inspection needs a 3D measurement report, say so in the enquiry and we will establish before quoting whether we can supply it or whether an external metrology service has to be scheduled in. Full detail of what we check and what travels with the shipment is on the quality inspection service page.

What anodizing does to price and schedule

Four things drive the cost, and three of them are decided on your drawing:

• Thickness. Tank time scales with it. Going from 10 µm to 40 µm is not a small increment.
• Masking. Every masked bore, thread or land is hand work. A drawing with twenty masked holes costs more to finish than one with two, regardless of part size.
• Racking. A part that cannot be gripped on a non-visible face, or that traps solution, needs a dedicated rack — sometimes a made fixture.
• Colour and seal. Dyed and sealed costs more than natural and sealed; some colours are simply harder to hold.

On schedule: we quote the finishing time into the route once the specification and the line are confirmed. We do not give a finishing lead time before that conversation, because a number we cannot hold is worse than no number. Our own production times are unchanged by the finish — simple samples 3–7 days, complex first article 15–20 days, production batch 25–35 days — and the finishing step is added to that route, not hidden inside it.

Questions buyers ask before they send the drawing

What thickness should I specify for an outdoor enclosure?

For corrosion resistance outdoors, Type II at the upper end of its range is normally enough; Type III is for wear, not for weather, and it costs more without adding weather resistance in proportion. The bigger outdoor issue is the colour, not the thickness — choose electrolytic or inorganic colour, or accept that an organic dye will fade. Tell us the environment and we will propose a class rather than a number pulled from a table.

Can you anodize a welded assembly?

Yes, and most enclosures we finish are welded. Two things to plan: the filler wire, because ER4043 goes dark grey after anodizing while ER5356 stays close to the parent metal; and the heat-affected zone, which can read a slightly different shade, particularly on 6061. Welds are dressed before finishing — a weld left proud shows as a ridge under a thin anodic film.

Will anodizing change the dimensions of my holes?

Yes. The film grows roughly half outward and half into the metal, so a bore loses close to the full coating thickness from its diameter — about 0.025 mm at a 25 µm hardcoat. Either mask the bore or size it for the finished condition and say which you have done. Threads need masking or an allowance.

Can you match a colour I already have?

We can aim at it, and we will tell you honestly how close is realistic. Anodize shade varies with alloy, bath condition and tank time, so a retained limit panel is the only workable reference — a Pantone or RAL number does not map onto an anodic film. Where an exact and repeatable match matters more than the anodic surface itself, say so early; a different process may be the honest answer.

Is an anodized enclosure still conductive?

No. Aluminum oxide is an insulator, and a hard anodized surface will break an earth path or an EMC contact. If your enclosure needs either, specify masked or machined contact lands on the drawing. Where the whole part must stay conductive, a chrome-free conversion coating gives corrosion preparation without the insulation.

Do you anodize in your own workshop?

No. Brushing and mirror polishing run in our own building in Ningbo; anodizing runs on audited partner lines that we specify and inspect. We write the specification, confirm the achievable range and the colour and seal options with the applicator before quoting, tell you which line the work will run on, approve a sample before the batch, and measure coating thickness on receipt.

Does black anodize fade in sunlight?

Organic black dye does, and so do most organic colours — reds and blues fade fastest. The bronze and black ranges produced electrolytically with metal salts are far more lightfast. If the enclosure lives outdoors, name the colouring method on the drawing, not just the word “black”.

Can you hard anodize 6061?

Hard anodizing is commonly applied to 6061, and to 5052 and 5754, but how thick a given alloy will build on a given line is not a number to assume — harder alloys and copper-bearing grades build differently, and thick coatings can craze on sharp radii. We confirm the achievable range with the applicator for your specific alloy before we quote.

Send the drawing, or send the problem

If you already have a finish specification, send it with the STEP and the PDF and we will confirm what holds. If you have an enclosure that keeps coming back wrong — faded, mismatched, cracked at a bend, or failing an earth bond test — send the photo and the drawing revision and we will tell you which of the nine decisions was left open. That review costs you one email and usually saves a batch.

Everything we do in aluminum, from cutting to welding to finishing, is summarised on custom sheet metal fabrication.

Where the anodized panel goes into a rail or transit cabinet, the earth-bond point above is usually what decides the drawing — see enclosures for the transportation industry.