Anodising aluminium

At a glance

From the NAAMM Metal Finishes Manual (AMP 500-06, 2006, US), converted at 1 mil = 25.4 micrometres. BS EN ISO 7599:2018 and the Qualanod specification were not read.

What anodising is, and how colour is made

Anodising is a film made from the metal, not a coat on top of it. The aluminium part is the positive electrode in an acid bath, and its surface is converted to a hard oxide layer full of tiny pores. Because the film is made from the surface, it cannot flake off like paint. For the same reason it cannot be touched up on site: the NAAMM Metal Finishes Manual says an anodic coating can be restored only by removing the item and refinishing it in the shop.

Sealing matters as much as thickness. After anodising, the pores are closed, usually in boiling deionised water or with metal-salt sealants, so the film does not absorb stains and resists corrosion. The manual calls sealing a prime essential, and lists quality tests for it, including a stain test (ASTM B 136), acid dissolution (ASTM B 680), mass loss after immersion in phosphoric chromic acid (ISO 3210) and admittance or impedance (ISO 2931 and ASTM B 457). Ask which test is used and what the result was.

Four ways to make colour. The manual describes: clear (natural) film from the sulphuric acid process; integral colour, produced in the film by the alloy and process together, in the bronze to grey to black range and noted for good ultraviolet resistance; electrolytically deposited colour, where metal salts are deposited in the base of the pores, light bronze to black, also used outside; and impregnated colour, where organic or inorganic dyes fill the pores. A palette of bronze, champagne and black is therefore easy; a bright red or blue is a dyed finish, so ask the anodiser for lightfastness evidence if it will sit in sun.

Preparation sets the look. The manual says most architectural products today are simply etched and anodised: etching gives an attractive matt that conceals minor marring. Buffed, directional and patterned mechanical finishes can be done before anodising. A “brite” (bright) finish uses a chemical brightening or an electropolishing-type process before anodising and is used mainly for bathroom fittings.

Classes and thickness

Thickness is the number that matters, and it comes in different systems. The manual, which is a US document, defines two architectural classes. Class I coatings are 0.7 mil (about 18 micrometres) or more and are the hardest and most durable; Class II coatings are 0.4 to 0.7 mil (about 10 to 18 micrometres). A mil is a thousandth of an inch, 25.4 micrometres. It says coatings for exterior use in polluted air, or without regular maintenance, should have at least 0.7 mil. In Europe, anodising is commonly named by its minimum average thickness in micrometres, with class numbers such as 5 to 25 reported by secondary sources. BSI lists BS EN ISO 7599:2018 as a method for specifying decorative and protective anodic oxidation coatings on aluminium, and the Qualanod scheme certifies anodisers; neither was read here, so ask the anodiser to write the class and the minimum average thickness in micrometres into the quote and the certificate.

Thickness and class here are read from the 2006 US manual. Class names, tests and tolerances differ in the European standard and in certification schemes, so the quote must name the system it uses.

How thickness is checked. The manual names an eddy-current test (ASTM B 244) for coating thickness and a weighing test (ASTM B 137) for coating weight, taken on samples chosen at random from the rack loads. Ask for the measured thickness on each batch, not a statement of class alone.

Alloys, welds and why two parts differ

Each alloy anodises to its own shade. The manual says each alloy produces its own characteristic colour, that a specific colour can be had only from certain alloys, and that alloys must not be mixed in an assembly. Welded assemblies of different alloys, or even of two grades in the same series, will not colour the same. Metal that has been cold-worked heavily, such as a tapered tube, can show colour variation within one alloy that cannot be controlled. For the alloys themselves see aluminium alloys for architectural interiors.

Welds, filler and joints. Arc welding of an assembly to be anodised should be done only in concealed areas; the wrong filler rod can turn an exposed weld black, and even the common anodising fillers (5356 or 5856) leave a visibly different shade at the weld. Joints between large anodised panels should be broken with divider strips, shadow gaps or changes of plane so that small shade differences do not read as errors. See weld finish and visible welds.

Design for the bath. Assemblies need drainage holes so acid does not stay trapped and later stain the face. No other metals should be in the bath except lead and titanium, which means brass screws, steel pins and other metal parts cannot be fixed to the aluminium before anodising; they are added afterwards. Heavy sheet that is not ground, polished or buffed should be a specialty sheet to avoid structural streaking, and the manual does not recommend specialty sheet where a mechanical finish will be applied.

Batches. Colour is set by the alloy and temper, the acid concentration, and the time, temperature and current of the bath, so two batches can differ even in the same alloy. The manual recommends that a specification require colour samples showing the range to be expected in production for any specific colour and shade. Approve against those, and read metal finish sample approval and batch variation for the process.

Cleaning. Ordinary wax cleaners, soap or mild detergent and water are generally satisfactory; do not use alkaline or acid cleaners (NAAMM). See cleaning and maintaining metal finishes.

Anodising compared, and what to write

Against powder coat. Anodising keeps the metal visible and the film is part of the part; a powder coat is a paint film that hides the metal, can be any RAL colour and can be touched up. If the part will be hit, scratched or worn, the repair route matters: an anodised part is refinished in the shop, a powder-coated part can be sanded and recoated. See powder coat or wet paint.

Against PVD and plating. PVD and decorative plating are more often used on stainless steel and brass than on aluminium; see PVD coatings explained and decorative plating. For micrometre thicknesses side by side see plating and coating thickness.

What to write in the brief. State the alloy and temper, the pre-treatment (etched, brushed, polished), the colour method and the sample it must match, the class and the minimum average thickness in micrometres, the seal and the seal test, the thickness test, and a certificate with each batch. Keep all visible parts of one run in one alloy and, where the partner can, one batch.

Aluminium [alloy and temper, e.g. 6063 T5] anodised to [BS EN ISO 7599:2018], class [ ] with a minimum average film thickness of [ ] micrometres, measured by [eddy current] on every batch. Pre-treatment [etched / brushed / polished]. Colour method [clear / integral / electrolytic / dyed], matching approved sample [ref] within the range shown on [colour range samples of the production alloy]. Sealed [hot water / mid-temperature], seal quality tested by [test method and limit]. Anodiser [certified to the scheme named in the quote]; certificate and test results with each batch.

All visible parts of [elevation or room] to be of the same alloy and, where possible, the same anodising batch. Welds [concealed / not permitted] on visible faces; filler [ ] agreed with the anodiser before welding. Joints between panels to be broken by [shadow gaps or divider strips]. Drainage and vent holes as [ ]. No non-aluminium parts fixed before anodising. Approve against [sample ref] before manufacture.

Standards and sources

• NAAMM/NOMMA Metal Finishes Manual AMP 500-06 (2006, US): Chapter 1 and Table 1-5 read 7 Nov 2026; US units.
• BS EN ISO 7599:2018: Listings: current; method for specifying decorative and protective anodic oxidation coatings. Not read.
• Qualanod specification: Certification scheme for sulphuric acid anodisers. Edition on request; not obtained.

Questions we are asked

What class of anodising do I need for an interior?

The NAAMM manual says Class II (about 10 to 18 micrometres) suits interior items not subject to excessive wear, and Class I (about 18 micrometres and over) suits interior items with normal wear. Ask the anodiser for the class and the minimum average thickness in micrometres in the quote, because European and US class names differ.

Can anodised aluminium be repaired on site?

No. The manual says an anodic coating can be restored only by removing the item and refinishing it in the shop. If the part will take hard wear, specify a heavier film or choose another finish.

Why do two anodised parts look different?

Each alloy has its own shade, welds and cold-worked metal anodise differently, and the bath time, temperature and current change the colour. Approve against colour range samples of the actual alloy and process.

Can any colour be anodised?

Clear, bronze to black integral colour and electrolytic bronze to black are straightforward. Brighter colours are dyed finishes, so ask for lightfastness evidence if they will sit in sun. Colour is also limited to certain alloys.

Is anodising better than powder coating?

They suit different jobs. Anodising keeps the metal visible and the film is part of the part; powder coat is a paint film in any colour that can be touched up. Compare them for the use, wear and repair route in the brief.