Anodised Aluminium Profile 40 x 40 mm AA15 Clear Finish for Visible Frames

01What does AA15 actually mean on a drawing?

It means a nominal coating mass or thickness of 15 micron, in the designation system of EN ISO 7599. The letters are not a quality grade and they are not a brand — they are a thickness class, and the four classes you will see on aluminium extrusion drawings are AA10, AA15, AA20 and AA25. A 40 x 40 mm anodised aluminium profile with an AA15 callout is the middle of that range and is what most visible machine frames in Europe are specified to.

The important word is *nominal*. A 15 micron class does not mean every point on the profile measures 15.0 micron. It means the coating meets the minimum and mean values the class defines when measured the way the standard says to measure it. Buyers who write “15 micron +/- 1 micron” on a drawing create a specification that cannot be inspected, because the natural variation across a single extrusion is wider than that.

02Which faces do you measure the coating on?

Not the corners, not the slot mouths, and not the cut ends. Anodising grows from the surface outward and into the metal at the same time, and the current density is highest at edges and corners, so corners always read thicker and the flat face is the only honest place to measure. On a 40 x 40 section that means the middle of a flat face, clear of the slot mouth by at least a few millimetres.

• Eddy current, ISO 2360 — the method used for goods-in check. Fast, non-destructive, and reliable on a flat face. It reads high near an edge, so keep the probe back.
• Microscopical cross-section, ISO 1463 — the referee method. Slow and destructive, used when a dispute has to be settled or when the coating is very thin.
• Coating mass — the alternative expression of the same thing, in grams per square metre. Some drawings use it, and it converts to thickness through the density of aluminium oxide. Ask which one the drawing means before arguing about a result.
• Cut ends are bare aluminium. Anodising does not cover a face that was cut after coating. Every cut end, every drilled hole and every tapped thread is exposed metal, and that is where corrosion starts on a coated frame.

This is the single most common misunderstanding on an incoming inspection report: the inspector measures near a corner, gets 21 micron on an AA15 part, and rejects a batch that is entirely correct. Agree the measurement position in writing before the first shipment, and put it on the drawing.

03Why is there always a colour difference between batches?

Because clear anodising is not colourless. The coating is transparent, and what you see is the aluminium underneath it, slightly darkened and shifted by the oxide layer. That means the visible colour is a function of the alloy, the billet chemistry within the alloy, the extrusion temperature and the anodising bath — four variables, none of which is fully fixed by “6063-T5”.

The practical instruction is blunt: if two parts will be seen side by side, they must come from the same lot. That is a purchasing decision, not a technical one, and it costs almost nothing if it is made before the order rather than after the complaint. Where a project runs for months, we hold a retained sample and match each subsequent lot against it — but only if the customer asks for it at the quotation stage.

04Rack marks: why they are unavoidable, and where to put them

Anodising is an electrolytic process. Every profile has to be held by a contact that carries current, the contact has to be aluminium, and the point where the jig touches the work does not get anodised. That is not a defect. It is the process.

The useful conversation is therefore not “can you avoid rack marks” but “which face may carry them“. On a 40 x 40 frame post there is usually one face that ends up against a panel, facing a wall, or inside a closed assembly. Tell us which one, and the jig is set to contact there. If no face is acceptable, the alternatives are a masked contact area that is later covered, or a different finish entirely.

05Anodise before machining, or machine before anodising?

Almost always machine first, anodise last — with two exceptions that are worth knowing because they catch people out.

1. The coating moves the surface. Sulphuric anodising grows roughly half into the metal and half outward, so a 15 micron coating raises the original surface by about 7 to 8 micron. On a 40 x 40 frame that is irrelevant. On a machined datum face, or on a bore that has to take a bearing, it is not — mask those faces.
2. The coating tightens threads. A tapped M6 hole anodised without masking will not accept a standard M6 bolt cleanly. Either mask the thread, or anodise first and tap afterwards, or specify a thread that is cut oversize to allow for the coating. The third option is common in Europe and needs to be stated, because a standard tap will produce a thread that is too tight.
3. Cut ends stay bare. Anodising after cutting means the cut end is coated only on the faces that were already there. If the cut end is visible, that bare edge is a different colour and will oxidise to white powder outdoors. A coated end cap solves it more cheaply than a second anodising operation.
4. Masking is a manual operation and it is priced per part. If a drawing has twelve masked features, that is twelve operations, and the price will say so.

06Is black anodising UV stable?

It depends entirely on how the black was made, and this is where a lot of outdoor projects go wrong. There are two routes and they behave very differently in sunlight.

Neither route makes anodising into a paint system. An anodised layer is thin, hard and integral to the metal, which makes it excellent against abrasion and poor at hiding anything underneath. If the requirement is a specific colour held to a number over many years outdoors, powder coating is the honest recommendation and anodising is not.

07What does the coating do to electrical continuity?

It breaks it. An anodised layer is an electrical insulator — that is the same property that makes anodised aluminium useful as a substrate in electronics. On a machine frame that has to be earthed, or on an ESD workbench, a fully anodised frame is electrically the same as a plastic frame until you deliberately create bare-metal paths.

This matters more than it looks. A guard frame that is anodised everywhere and earthed through one bolt into a painted machine base has no earth at all in practice. The test is a low-resistance bond from the far end of the frame back to the earth point, and it fails on anodised assemblies far more often than people expect.

08What does seal quality mean, and how is it checked?

After anodising, the coating is porous. Sealing closes those pores, and an unsealed or badly sealed coating looks identical to a good one until it starts to stain, chalk or corrode. There are two standard checks and both are laboratory tests rather than production line tests.

On salt spray, one caution that saves arguments: a coated part with bare cut ends will fail in salt spray at the cut ends long before the coating fails. If the test coupon includes a saw cut, the result describes the cut, not the coating. Specify whether coupons are cut or as-anodised, and compare like with like.

09Questions buyers ask before the first order

Related sections and categories

Browse Anodised Aluminium Profiles, 40 Series Aluminium Profiles, the 40 x 40 mm four slot section for the full section data, or the 40 x 40 mm single slot section if the frame also needs a machined datum face.