Aluminium alloys for architectural interiors

At a glance

From UK and European stockholder datasheets quoting the 2008 and 2009 editions of EN 485-2, EN 755-2 and EN 573-3, an extruder's alloy page and a US trade-body manual: confirm against the current edition and the mill certificate.

Alloy numbers and tempers decoded

An aluminium alloy is named by a four-digit number and a temper. EN AW-6063 [AlMg0.7Si] is the alloy; T5, T6 or H34 is how it was worked and aged. The stockholder datasheets we read define the tempers: T4 is solution heat treated and naturally aged, T5 is cooled from the shaping process and artificially aged, T6 is solution heat treated and artificially aged, T651 is the same with a stress-relieving stretch, H22, H24 and H26 are work hardened and annealed to quarter, half and three-quarter hard, and H34 is stabilised.

The drawing places six alloys by their two main additions. The 6000 series, which has both magnesium and silicon, includes the usual extrusion alloys. The 5000 series carries magnesium with little silicon and is the usual sheet family. 6082 sits highest because of its silicon and manganese, which is why it is the strong one.

The standards behind the numbers. The datasheets quote the 2009 edition of BS EN 573-3 for composition and the 2008 editions of BS EN 755-2 (extrusions) and BS EN 485-2 (sheet) for mechanical values. BSI Knowledge now lists BS EN 573-3:2026 (current, under review), BS EN 755-2:2025 (current) and BS EN 485-2:2016+A1:2018. Stockholder sheets lag the standards, so confirm the current edition and the mill certificate. For architectural extrusions in 6060 and 6063 the dimensional standard is EN 12020-2, whose 2008 edition (BSI) was written for profiles “mainly for architectural use” with stricter surface requirements for visible faces; that edition is withdrawn, and a 2022 edition appears at a standards seller.

Which alloy for extrusion, which for sheet

Extrusions. 6063 and 6060 are the appearance alloys. An extruder says 6063 offers more strength than 6060 at a modest cost to surface finish, anodises well and consistently across production runs, and recommends 6060 where decorative or anodising criteria are very demanding. For significant structural stress it points to 6082, and to 6061 for heavy welding and machining. The stockholder sheets list 6063 for window frames, doors and shopfittings, with T6 for rails and posts and T4 for formed bends, and 6060 for interior fittings, lighting and furniture.

Sheet. 5005 is the sheet alloy a sheet supplier recommends for anodising, with 6063 for extrusion. 5251 and 5754 are stronger sheet alloys. The 5251 sheet describes high corrosion resistance, particularly in marine conditions, and the 5754 sheet lists treadplate and shipbuilding. Neither gives an anodising statement, so do not assume they anodise to a decorative standard.

The chart compares minimum proof stress for the tempers named on the datasheets. It is a strength comparison only: a stronger alloy is not a better-looking one, and extrusion and sheet figures come from different standards.

Anodising: alloy, quality and the 6082 caution

Caution on 6082. The research behind this sheet flagged 6082 as an alloy that often gives a poor decorative anodised surface, and the sources disagree about how far. A European plate stockholder rates 6082 plate 1 (very good) for technical anodising and 1 for hard anodising but 3 for decorative on a scale from 1 (very good) to 6 (inapplicable), and adds “technical anodising only, no warranty towards optical demands”. The UK stockholder’s extrusion sheet makes no anodising claim but says the extruded surface is not as smooth as other alloys of similar strength in the 6000 series. A third stockholder was reported to call its anodising properties good. If a visible 6082 face matters, anodise a sample of it.

Anodising quality. The alloy number does not guarantee an even surface. A sheet supplier, quoting the Aluminum Anodizers Council’s chart, separates alloys into anodising quality (A.Q.) and other types, lists 6063 as A.Q. and says 5005 should be held to tight silicon and magnesium limits to avoid oxide streaks. A UK stockholder tells buyers of 5005 sheet to specify “special anodising quality” if the finish is critical. The NAAMM manual says the same of thick sheet: ask for a specialty sheet product to minimise “structural streaking”.

Rules from the NAAMM manual (a US document, 2006).

• Each alloy produces its own characteristic shade, and alloys, even 6061 and 6063, will shade differently when anodised together. Do not mix them indiscriminately.
• Anodising should not be specified on composite or extensively cold-worked structures where appearance differences are unacceptable.
• Arc welding is best kept to hidden areas: if the filler is wrong a weld can turn black, and even with 5356 or 5856 filler a weld area shows a different shade.
• Include colour samples showing the range to be expected in production.
• A coating can be restored only by removing the item and refinishing it, so a heavily worn item needs a heavier coating or a different finish. The manual’s Class II coatings are 0.4 to 0.7 mil thick (about 10 to 18 micrometres) for interior items not subject to excessive wear, and Class I are 0.7 mil or more for interior items subject to normal wear.

How to specify the coating. BS EN ISO 7599 is a method of specifying anodic coatings. BSI Knowledge shows the 2010 edition as withdrawn on 23 February 2018, and a standards seller lists the 2018 edition as current. The Qualanod label has its own specification, edition 01.07.2026, available on written request, which we did not read. Our coating thickness sheet and sample approval sheet cover thickness and batch variation, and powder coating metalwork covers the alternative.

Movement, welding and contact

Movement. The stockholder sheets give 23.4 to 24 millionths per kelvin for 6060, 6063, 5005, 6082 and 5754 and 25 for 5251, roughly twice steel. See thermal expansion of metals for the movement in millimetres and our steel and glass screens sheet for why aluminium and steel frames are detailed differently.

Welding. The datasheets rate 6063, 5005 and 5251 good to very good for welding, and give 4043 or 5183 filler for 6063 and 5356 for 5251 to itself. 6082 loses strength in the weld zone. If a welded part is to be anodised, the filler and the visible weld need agreeing with the anodiser first (see above).

Contact with other metals. Our galvanic corrosion sheet reports aluminium against brass, bronze or copper as a pairing to avoid or isolate, and stainless fixings in aluminium as usually safe. See also bronze, copper and nickel silver.

Cleaning. Anodised aluminium is cleaned differently from powder coat: see cleaning and maintaining metal finishes.

What to put in a specification

• Alloy and temper by EN name, for example EN AW-6063 T6, never “aluminium” alone.
• The product standard for the form: BS EN 755 for extrusions (EN 12020-2 for precision architectural profiles in 6060 and 6063), BS EN 485 for sheet, composition to BS EN 573-3. Editions to be confirmed.
• Anodising quality, stated for sheet and for any visible extrusion, and a note that 6082 is not to be used on a visible anodised face without an approved sample.
• One batch per visible run, and sheet and extrusion shown together on one anodised sample, because different alloys shade differently.
• The anodic coating by standard, class or thickness from the anodiser’s own specification, sealing, colour and the range sample.
• Welds and fixings: where welds are allowed, the filler and whether they are visible; fixings of a compatible metal.
• A material certificate for each batch, kept with the project file.

Extruded sections in EN AW-[6063] temper [T6] to BS EN 755 [part and edition to be confirmed] [and BS EN 12020-2 for precision profiles], composition to BS EN 573-3. Anodising quality [required]. Visible runs from one batch where possible; material certificate [EN 10204 type 3.1] per batch. No substitution of alloy or temper without written approval. EN AW-6082 not to be used on a visible anodised face without an approved sample.

Sheet EN AW-[5005] temper [H34] to BS EN 485 [part and edition to be confirmed], [anodising quality / special anodising quality], [ ] mm thick, one batch per visible run, grain direction [ ]. Extrusion and sheet to be shown together on one anodised sample; a match between different alloys is not to be assumed.

Anodic oxidation coating to [BS EN ISO 7599, edition to be confirmed] by an anodiser working to [Qualanod, current edition], class or minimum thickness [from the anodiser's specification], colour [ ], sealed. Approved sample [ref] showing the range to be expected in production. All visible parts anodised in [one lot]. Welds [hidden / filler 5356 or 5856, shade difference accepted].

Standards and sources

• BS EN 573-3:2026; BS EN 755-2:2025; BS EN 485-2:2016+A1:2018: BSI Knowledge: current. Composition, extrusion and sheet properties. Texts not read.
• BS EN ISO 7599: BSI Knowledge: 2010 edition withdrawn 23 Feb 2018; 2018 edition current at a standards seller. Not read.
• BS EN 12020-2: BSI Knowledge: 2008 edition withdrawn 31 Dec 2016; 2022 edition at a retailer. Not read.
• Qualanod specification (edition 01.07.2026): on written request; not read.
• NAAMM/NOMMA Metal Finishes Manual, AMP 500-06 (2006), Chapter 1: US trade body; anodic coatings and precautions.

Questions we are asked

What is the difference between 6063 and 6082?

6063 is the architectural extrusion alloy, with a smooth surface and good anodising; its minimum proof stress on one stockholder's sheet is 170 MPa in T6. 6082 is stronger, 250 MPa on the same stockholder's T6 profile figure, and is used for highly stressed work, but sources say its extruded surface is not as smooth and one rates its decorative anodising lower. Use 6082 for strength and show a sample if it is visible.

Which aluminium alloy should be anodised?

A sheet supplier recommends 5005 for sheet and 6063 for extrusion, and says to order anodising quality if the finish is critical. A stockholder rates 6060 anodising very good. Neither 5251 nor 5754 has an anodising statement on the sheets we read.

Why does anodised aluminium vary in colour or show streaks?

Alloy, batch and metal structure all show through. The NAAMM manual says each alloy produces its own shade, that different alloys anodised together will not match, and that thick sheet can show structural streaking unless a specialty sheet is used. A stockholder warns that 5005 sheet can streak and should be ordered in special anodising quality.

Will an extrusion and a sheet match after anodising?

Not by default. They are usually different alloys, 6063 or 6060 against 5005, and each shades in its own way. Show both on one approved sample from the same anodiser, or finish one of them differently.

Can aluminium be welded and then anodised?

It can, but the weld shows. The NAAMM manual says a weld area has a different shade even with 5356 or 5856 filler, that the wrong filler can turn a weld black, and that welds are best hidden. Agree the filler and the visible weld with the anodiser.

What do T5, T6 and H34 mean?

They are tempers. T5 is cooled from the extrusion process and artificially aged. T6 is solution heat treated and artificially aged. H34 is a strain-hardened, stabilised sheet temper. They change strength and forming; they do not change the colour or the anodising.

Is anodising or powder coating better for an interior?

They are different finishes: anodising keeps the metal visible and cannot be restored locally, while powder coat gives a colour range and a different repair route. Our powder coating sheet covers the Qualicoat route for aluminium; the NAAMM manual says anodic coatings can only be restored by removing and refinishing the item.