Anodising is not a coating applied to aluminium. It is the metal’s own oxide layer, grown deliberately and thickly in an electrolytic bath, which is why it cannot peel, flake or be chipped off in the way paint can. The layer is part of the section rather than sitting on top of it.
That single property explains almost everything a client needs to know about specifying it, including the two things that surprise people most: the finish is extremely durable, and it cannot be repaired on site.
Thickness is the specification
Anodic coatings are specified by thickness class, written as the letters AA followed by the thickness in micrometres. The classes correspond to exposure, and picking one is the first and most consequential decision.
| Klasse | Typical use |
|---|---|
| AA15 | Sheltered exterior |
| AA20 | General outdoor exposure |
| AA25 | Marine and severe environments |
The method of specifying is set out in ISO 7599, and thickness is verified by measurement rather than by assertion, usually with an eddy current instrument. A specification that asks for anodising without a class has asked for whatever the coater’s standard process produces, which may be a fraction of what the environment requires.
For structures within reach of salt spray, the marine class is the honest starting point. Coastal exposure is not limited to the waterline, and a handrail two hundred metres inland on a windward shore sees more chloride than most people assume.
Sealing matters as much as thickness
The anodic layer is porous when it comes out of the bath, and it has to be sealed for the corrosion resistance to be realised. An unsealed or poorly sealed coating of the correct thickness will still stain, absorb dirt and lose its appearance quickly, and the defect is invisible on delivery.
This is the main argument for requiring work under a recognised quality label rather than relying on a thickness figure alone. Qualanod, which conforms to the ISO specifying method, covers the process as a whole rather than a single measurement, and licensed plants are subject to inspection of the parameters that a client cannot check on a finished component.
Colour is where expectations go wrong
Anodising offers a narrower palette than paint, and the reason is physical. Colour arrives either by absorbing metal salts into the porous layer, which gives the stable bronzes, greys and blacks used in architecture, or by dyeing, which gives a wider range with much poorer lightfastness outdoors.
Two properties of the process then create the practical problems. The first is that the alloy affects the result: extruded 6xxx sections anodise evenly and predictably, higher-magnesium alloys and castings do not, and a structure combining them will show the difference clearly. The second is batch variation. Colour is a chemical outcome rather than a mixed pigment, and two batches of nominally identical components can be visibly different when laid side by side.
The requirement that prevents this is one line: all components visible together shall come from the same anodising batch, and where that is impossible, adjacent components shall be matched against an approved sample. This costs nothing to write and is very expensive to resolve afterwards.
There is no touch-up
An anodic layer is grown in a bath, so it cannot be applied on site. A scratch, a saw cut, a drilled hole or a weld made after anodising exposes bare metal that will never match, and no proprietary product closes the gap convincingly in daylight.
The consequence is a sequencing requirement rather than a finish requirement. All cutting, drilling, welding and fitting has to be complete before anodising, which means the fabrication has to be fully resolved before the components leave for the coater. Site modification of an anodised structure is possible and always visible, and any client who anticipates later alterations should know that before choosing the finish.
This is also why anodised structures suit bolted assembly from finished components. The same logic that keeps welds in the factory under the aluminium design rules keeps the finish intact, since every operation moved to site is an operation that has to happen on treated metal.
What it costs and what it saves
Anodising is generally more expensive than powder coating at the outset and cheaper over a long service life, because it does not require recoating. The layer is harder than paint and resists abrasion well, which matters on handrails, stair treads and anything people touch or walk on.
What it does not offer is repairability, and that is the real trade-off rather than the price. A powder coated structure can be patched after damage; an anodised one cannot. Which of those matters more depends on whether the structure is in a place where it is likely to be hit.
Hva skal stå i spesifikasjonen
State the thickness class and the standard it is specified under. Require the work to be carried out by a plant holding a recognised quality label. State the colour by reference to an approved sample rather than by name alone. Require all visible components to come from one batch. Require all machining and welding to be complete before treatment. Ask for the thickness measurement records with the delivery documentation.
Six lines, and they close the gap between a finish that looks right on handover and one that still looks right in year fifteen.
Before the components leave the workshop
Almost every complaint about an anodised structure traces back to one of three things: a thickness class too low for the exposure, components from different batches placed side by side, or a hole drilled after treatment. All three are settled before anything reaches the bath.
Our engineers will set the class against the actual exposure of the site rather than the general case, and the aluminium outdoor staircases and handrail structures we supply are sequenced so that no cutting is required after treatment.