What does a European anodising specification actually include?

Direct Answer: Six items, and only one of them is about appearance. A European specification names the film thickness class, the sealing route, the alloy and temper, the third-party certification, an approved appearance sample, and the protection applied for transit.

The document a European specification is built on is EN ISO 7599, the international standard for specifying decorative and protective anodic oxidation coatings on aluminium. It is worth knowing what it actually does, because it is more specific than the sales language around anodising: it defines the thickness class system, it lists the test method for each characteristic, and it states in clause 4.2 exactly what the customer has to tell the anodiser before the work starts.

That clause is the specification. The customer has to give the reference to the standard, the intended service use of the part, the specification of the aluminium, which surfaces are significant, the surface preparation, the required coating thickness class, and whether the product is to be sealed and by which method. A purchase order that omits any of those is incomplete by the standard’s own definition, not by ours.

The thickness class is the item that changes most in practice. EN ISO 7599 grades coatings by the minimum average thickness and designates the class with the letters AA followed by the thickness grade: AA 5, AA 10, AA 15, AA 20 and AA 25. Class AA 15 means a minimum average of 15 µm, and the standard also fixes a floor for the lowest single reading, at 12 µm. The rule behind it is that no local thickness may fall below 80 per cent of the minimum average. A class is therefore a minimum with a stated tolerance rather than a target, and a buyer who writes AA 15 has to accept a local reading of 12 µm.

Certification is the piece European buyers value most and mention least. In anodising the recognised third-party scheme is Qualanod, the quality label for sulphuric-acid anodising founded in 1974 by the European aluminium and anodising associations, with around 219 licensed plants in 44 countries. It is a product certification scheme rather than a quality management system: it defines the coating requirements and has the plants audited against them. A buyer does not need a certificate number in the article text; they need to ask for the scheme name and the scope the licence covers, and to check that the scope matches the process being quoted.

Does anodising change profile dimensions?

Direct Answer: Yes. The oxide forms partly within the original surface and partly outside it, and roughly half of the coating thickness is added to the outside. A 20 µm class therefore raises each coated surface by about 10 µm, which is 20 µm across a slot mouth or a hole.

It is often claimed that anodising preserves dimensions, and the useful version of that claim is narrower than it sounds. An anodic layer does not distort the section the way heat can, and it does not build up unevenly the way some coatings do. But it does grow from the surface, and the growth is split: the oxide occupies part of the metal it consumed and part of the space above it. The rule used across the industry is that a coated surface rises by about half the coating thickness, which means the dimensional consequence can be calculated from the class alone.

The direction of the change is what matters. The coating closes gaps rather than opening them, so a slot mouth narrows, a hole narrows, and a thread changes its pitch diameter. On a general framing profile that is negligible. On a slot mouth that has to accept a fastener, on a tapped hole, or on any feature with a stated fit, it is a real change and it has to be designed for.

There are two honest ways to handle it. Anodise first and verify the critical feature afterwards, accepting that the coating is present and the dimension includes it. Or mask the critical features and coat everything else, accepting the cost of masking. What does not work is to specify a tight dimensional tolerance and a coating class and to assume that the two are independent, because they are not.

Is sealing chemistry the real compliance issue?

Direct Answer: Yes. For a European buyer the regulatory question is the sealing route, not the solvent claim. Two of the common sealing chemistries, hot nickel-acetate sealing and cold nickel-fluoride sealing, both contain nickel, and REACH Annex XVII entry 27 limits nickel release from articles in prolonged skin contact to 0.5 µg/cm² per week.

Anodising is usually described as environmentally favourable because it is a water-based inorganic process with no solvents. That is true, and it is not where the regulatory questions sit.

After anodising, the porous layer is sealed, and the purpose of sealing is to reduce the porosity and absorption capacity of the coating. The standard treats this as a real property rather than a finishing detail: EN ISO 7599 gives sealing its own clause and its own test methods, and those tests are surface-specific, measuring the resistance of the sealed surface to acid attack rather than the quality of the whole coating thickness. The referee method is the loss of mass after immersion in acid solution, with a maximum accepted loss variously set at 30 or 40 mg/dm², and there are also the dye-spot test and the admittance measurement.

The compliance point is the chemistry. A hot-water seal is the traditional route and contains no regulated metal. The two faster routes that dominate industrial production, nickel-acetate hot sealing and nickel-fluoride cold sealing, both put nickel into the process. Where the end use involves prolonged contact with skin, or where a customer’s own restricted-substances list covers nickel, a nickel-free or hot-water route has to be requested. Both routes are available, and the choice is not visible in the finished appearance, so it has to be stated.

Two smaller items belong in the same conversation. Restricted-substances rules limit lead, cadmium and hexavalent chromium, and in an aluminium context the relevant sources are impurities in the alloy and any chromate-based conversion treatment, which is a different process from an anodic layer. And the fasteners and accessories that ship with the profile carry their own finish: a frame can hold a compliant coating on the profile and a non-compliant one on the fixings, which is a common and avoidable audit finding.

On our own supply the standard alloy and temper is EN AW-6063-T5, with 6061-T6 available on request, and the profiles are anodised in-house. The anodised aluminium profile range is quoted with the class and the sealing route as separate lines, so a nickel-free requirement does not have to be inferred from a colour name.

Why do anodised batches show tone variation?

Direct Answer: Because anodic tone depends on the alloy composition and on the anodising line and bath, not only on the process name. Two heats of the same alloy designation can produce slightly different tones, and two lines can differ even with the same alloy.

“Consistent across production runs” is a common claim, and it is worth understanding where the risk actually lies, because it is not where most buyers expect.

Anodic tone depends on the alloy composition and on the anodising process, including the bath chemistry and the thickness achieved. For a single machine frame this is invisible. For a facade, or for a multi-unit order where components will be seen side by side, it is a visible defect.

The controls are the ones a specification can actually impose. Fix the alloy and the temper and do not allow substitution, because a substitution that looks equivalent on a mill certificate is not equivalent in tone. Place multi-unit orders as one batch where the geometry permits, and require the same anodising line. And agree a retained appearance sample that both sides keep. EN ISO 7599 supports exactly this: where colour variation has to be limited, it states that the acceptable limits should be identified by agreed limit samples. The sample is the part that settles disputes, because tone is judged by comparison and a description cannot do the job.

Where tone is genuinely critical, the honest position is that a dyed or powder-coated finish gives more control over colour than a natural anodic finish, at the cost of the other properties that made anodising attractive. That trade belongs in the conversation rather than in a brochure.

What fails outdoors: alkali, chloride, abrasion?

Direct Answer: Three things fail, and a fourth is not a failure at all. Alkali such as concrete, mortar and alkaline cleaners etch the layer. Chloride from coastal and de-icing exposure pits it. Abrasion wears through it locally, because it is hard but thin. Salt spray ranks coatings against each other and does not predict years of service.

The layer is hard, integral and does not peel or chip the way a paint film can. It is also thin, and thinness is the whole story of how it fails. The protective window is a pH range of roughly 4 to 9; outside that range the oxide is attacked rather than protective.

Alkalis. Concrete, mortar and strongly alkaline cleaning products attack the layer, and the resulting etching and staining is often blamed on the coating rather than on the chemistry. On a building site, splash and run-off are a design case, not an accident.

Chlorides. Coastal and de-icing exposure pits the surface. A thicker class helps; it does not make the material immune, and a coastal installation needs an inspection interval rather than a no-maintenance assumption. Cut ends and drilled holes are the first places to look, because they expose uncoated metal.

Abrasion. The layer is much harder than the base metal and it is a fraction of a millimetre thick. On a surface that is repeatedly handled, dragged against other parts, or cleaned with an abrasive, it can be worn through locally, and once it is worn through the protection is gone.

Salt spray. ISO 9227 is a comparative test. It ranks one coating or supplier against another on a consistent basis, and it does not predict how many years a coating will last outdoors. Percentage claims about extended service life should be treated with the same caution.

What should I write into a European purchase order?

Direct Answer: Eight lines, and they are the same eight whether the order is one machine frame or a facade package: thickness class, sealing route, alloy and temper, certification, critical features after coating, appearance sample, restricted substances, and packaging.