"Type II or Type III?" sounds like a question about quality — as though Type III were the upgraded option you take when the budget allows. It is not. The two are different processes for different problems, and treating Type III as simply better is the most expensive mistake in aluminium finishing.
Type III will not give you the colour you wanted. It will eat a tolerance you thought was safe. And on a cyclically loaded part it can take fatigue life out of the component in exchange for wear resistance the part may not even have needed. None of that is a reason to avoid it — it is a reason to decide on purpose.
Where the names come from
Type I, Type II and Type III are not industry marketing tiers. They are a designation from the US military specification for anodic coatings on aluminium, and they distinguish the electrolyte and the operating regime, not a ladder of quality.
| Type | Electrolyte | What it is for |
|---|---|---|
| Type I | Chromic acid | Thin coatings where fatigue life and precision matter more than wear — aerospace work, and parts where a residue of electrolyte in crevices would be a problem |
| Type II | Sulfuric acid, run warm | The general-purpose coating: corrosion resistance, an even appearance, and colour. The one most people mean when they say "anodised" |
| Type III | Sulfuric acid, run cold | Hard anodising or hardcoat — a thick, dense coating for wear resistance on aluminium |
The process parameters behind these — electrolyte concentration, bath temperature, voltage, achievable thickness and hardness — are tabulated on our aluminium anodizing equipment page. What follows is the part that table cannot tell you: what each difference costs you downstream.
The one difference that explains the rest
Anodising is not a coating that is laid on top of the metal. It is the aluminium surface itself, converted to aluminium oxide — grown out of the substrate. And the growth is a competition: the oxide forms, and the acid bath simultaneously dissolves it back.
Bath temperature is what sets the balance between those two. Run the bath warm and dissolution keeps pace, leaving a more open, more porous structure. Run it cold — Type III runs near freezing — and dissolution is suppressed, so the coating that forms is denser and harder.
Almost everything people find surprising about hard anodising falls out of that one fact:
- It is harder because it is denser.
- It is darker. The dense structure scatters light differently and traps more of the alloying elements in place. Type III comes out grey to near-black, and often bronze on copper-bearing alloys, before anything is done to it.
- It is far harder to dye, because the open pores that absorb dye in Type II are exactly what the cold bath suppressed.
- It needs real cooling, which is an equipment problem, not a chemistry problem — and it is the single biggest thing that separates an anodising line from a plating line.
What it does to your dimensions
Anodic coating grows in both directions: into the substrate and outward from the original surface. The practical figure your drawing has to survive is the outward share, because that is the part that changes the size of the finished part.
That share is much larger for Type III than for Type II. On a coating measured in tens of microns, the outward growth is measured in tens of microns too — which is fine on a cosmetic panel and fatal on a bearing seat.
Where this bites, in practice:
- Bores get smaller and shafts get bigger. A coating on a cylindrical surface changes the diameter by twice the outward growth, because both sides of the diameter grow.
- Threads are a specific problem. A thick coating on a thread can stop the fastener fitting, or gall it on assembly. The usual answers are to mask the thread, to cut it after coating, or to start from an oversize tap — and which one is right depends on the part, so it is a conversation rather than a rule.
- Masking is not free. Masked areas are uncoated, so they lose the corrosion protection the rest of the part gets. On a part that will see weather, that is a real trade.
- Pre-machining for the coating is normal. Where a Type III coating is specified on a dimensioned surface, the drawing often has to be released at the pre-anodise size. That is a machining decision, and it has to be made before the parts are cut, not after they come back from the anodiser.
The exact split between inward and outward growth depends on the alloy and the process, and it is worth confirming with your anodiser for the specific part rather than assuming a figure. What is safe to assume is that a Type III coating is never dimensionally free, and that a Type II coating on a non-critical tolerance usually is.
Fatigue: the trade that is rarely mentioned
This is the one worth slowing down for.
Anodic coatings are brittle. Under cyclic loading the coating cracks, and those cracks act as stress raisers at the surface of the substrate — which is precisely where fatigue cracks start. The result is that anodising, and hard anodising in particular, reduces the fatigue strength of the part. Chromic acid anodising (Type I) is the least damaging of the three, which is a large part of why aerospace work uses it despite its thin, soft coating.
If your part is statically loaded or cosmetic, this does not matter. If it is a rotating shaft, a stressed structural member, or anything that sees millions of cycles, it can matter more than the wear resistance you were buying — and it is not something a thickness table will warn you about.
The usual mitigations are design and preparation rather than process: generous radii instead of sharp edges (the coating cracks first at edges and corners), and surface treatments such as shot peening applied before anodising to put the surface into compression. Where the fatigue requirement is genuinely critical, the honest answer is that it needs to be designed for and probably tested, not chosen from a table. Raise it with your anodiser early.
Sealing, and the trade inside it
Anodised aluminium is porous. Sealing closes those pores, and it is what gives the coating its corrosion resistance and stops dye from bleeding out. Hot water sealing and nickel acetate sealing are the common routes; the sealing tanks are part of the line.
But sealing and wear pull in opposite directions. The same pores that let the coating absorb dye and hold a seal are what make it able to absorb oil and tolerate rubbing. Seal the coating fully and you improve corrosion resistance while giving up some of the surface's wear behaviour.
So there is a genuine choice here, and it is application-specific:
- Wear is the priority — a hydraulic component, a mould, a sliding surface — and the part often stays unsealed or minimally sealed, accepting less corrosion protection in exchange for the surface staying hard.
- Corrosion and colour are the priority — architectural work, consumer products — and it is sealed properly, because an unsealed part will stain and the colour will drift.
- Both matter, and the part sits in the middle: this is a case for saying what the part actually does and letting the anodiser propose, rather than ticking a box on a form.
What an anodising line needs that a plating line does not
Anodising looks like plating from a distance — tanks, racks, a rectifier — and the equipment differences are real. For Type III they are not optional.
| Item | What changes | Why |
|---|---|---|
| Cooling | Forced chilling with tight control and real circulation, not just a coil in the tank | The cold bath is the process. The current passing through the load generates heat in the bath (Joule heating) and pushes the temperature up; if it drifts, the coating dissolves as fast as it forms and the result is a soft, burnt, powdery surface. Type II needs cooling too, but Type III lives or dies on it. See plating chillers. |
| Rectifier | Much higher voltage capability, with constant-current and constant-voltage control | Type II runs at a low voltage. Type III needs several times that, and the voltage climbs as the coating thickens and its electrical resistance rises. A plating rectifier will not do this job. See rectifiers. |
| Racking and contacts | Titanium, with titanium-clad copper where current capacity demands it | Aluminium racks and contacts would anodise along with the work, building an insulating oxide at the contact point — the part loses its electrical connection and the coating stops. Titanium resists the acid and keeps conducting. |
| Agitation | Solution movement across the surface, and matched cathode area | Both the cold and the current have to reach the surface uniformly. Where they do not, thickness and colour vary across the part. |
| Fume extraction | Acid mist control | The bath is a strong acid run at temperature with gas evolving at the electrodes. See exhaust treatment. |
| Wastewater | Rinse treatment for acid plus dissolved aluminium — and for dye and nickel where colouring and sealing are on the line | Anodising rinse water is not the same stream as a plating rinse, and a line that adds colouring adds another. See wastewater treatment. |
| Colouring and sealing | Dedicated tanks downstream of the anodise tank | Two-step colouring and sealing are separate processes with their own chemistry and their own rinses, and they set the line length. |
If the line is being built rather than bought off a shelf, the same configuration questions apply as for any finishing line — see how to choose an automatic plating line for the rack-versus-hoist and manual-versus-automatic trade-offs, which carry over.
Choosing, in practice
Strip away the process detail and the decision usually comes down to what the part is actually for:
| If this is what the part needs | The usual answer | Because |
|---|---|---|
| A colour, or a decorative finish | Type II | Type III's dense structure will not take dye the same way and arrives dark |
| Corrosion resistance on an architectural or visible part | Type II, sealed | The porous structure takes a proper seal and holds it |
| Wear resistance on an aluminium part | Type III | This is the problem hard anodising exists to solve |
| Tight dimensional tolerance | Type II, or Type III with the drawing released at pre-anodise size and the critical surfaces masked | Type III's outward growth will consume a tolerance it was not budgeted for |
| Cyclic loading, fatigue-critical | Type I, or reconsider whether to anodise at all | Type III reduces fatigue strength; Type I is the least damaging of the three |
| Thin coating, precision part, aerospace | Type I | That is what chromic acid anodising is for |
| Wear resistance on a steel part | Not anodising at all | Anodising is an aluminium process. For steel, the equivalent conversation is a hard chromium deposit — see hard chrome plating |
What to tell an anodising supplier
More of the outcome is decided by what you send with the enquiry than by which supplier you pick:
- The alloy, specifically. Not all aluminium anodises the same. Copper-bearing alloys and castings behave differently from wrought 6000-series material — the coating can come out darker, less uniform, or harder to build to thickness. If the alloy is unusual, or the parts are castings, say so up front, because it changes what is achievable.
- Which type, and to what thickness, ideally against the standard or the customer's drawing rather than a verbal description.
- Which surfaces are critical, and which may be masked. This is the single most useful thing on the list.
- Colour — and whether it has to match an existing part or a sample, which is a different and harder requirement than just naming a colour.
- Whether the part is fatigue-critical. Say it explicitly; do not assume it will be inferred.
- The sealing and colouring requirement, or a statement of what the part has to survive, if you would rather the anodiser proposed.
- Part size, quantity and the finish the part starts from — as-machined, cast, or previously anodised, since stripping and re-anodising is its own operation.
If you are specifying a line rather than buying finishing services, that list becomes the brief the line is designed around — contact us to work through it.
Frequently asked questions
- Is Type III anodizing better than Type II?
- No — they solve different problems. Type III gives a thicker, denser, harder coating for wear resistance on aluminium. Type II gives a coating that takes colour, seals well and suits corrosion resistance and appearance. Choosing Type III for a part that needed colour, a tolerance or fatigue life is a downgrade, not an upgrade.
- How thick is Type III hard anodizing?
- Substantially thicker than Type II, and the useful figure is the one your drawing or the referenced standard calls out. The parameter table on our anodizing equipment page lists the working ranges for all three types. What matters more than the number is the outward growth, because that is what changes your dimensions.
- Why does hard anodizing come out dark?
- Because the cold bath produces a denser coating that takes up more of the alloying elements in place and scatters light differently. It arrives grey to near-black, sometimes bronze, before any colouring step. That is also why hard anodising is difficult to dye to a bright or light colour.
- Does anodizing change the dimensions of a part?
- Yes. The coating grows both into the substrate and outward, and the outward share is what changes the finished size. It is small for Type II and considerably larger for Type III, where it can consume a tolerance or stop a thread from fitting. Where a Type III coating is specified on a dimensioned surface, plan for it before machining.
- Does hard anodizing affect fatigue strength?
- It can reduce it. The coating is brittle, cracks under cyclic load, and those cracks concentrate stress at the substrate surface where fatigue cracks begin. Chromic acid anodising is the least damaging of the three types, which is why fatigue-critical aerospace work uses it. If the part sees cycling loading, raise it rather than assuming it is covered by the coating specification.
- Should anodized parts be sealed?
- It depends which property matters more. Sealing closes the pores and gives corrosion resistance and colourfastness, but it gives up some of the surface's wear behaviour. Wear-critical parts often stay unsealed or minimally sealed; visible and corrosion-critical parts are sealed properly. Where both matter, describe what the part does and let the anodiser decide.
- Can you anodize castings or high-copper aluminium alloys?
- They can be anodised, but they behave differently from wrought 6000-series material and the result is less predictable — colour and uniformity especially. The right move is to tell the anodiser the exact alloy and the form before committing a batch, rather than discovering it on the finished parts.