Anodised surfaces on aluminium

Anyone ordering anodised aluminium parts will quickly come across designations such as E0, E6, C-0 or C-33. These indicate the pre-treatment and the desired colour tone of the surface.

These abbreviations are important, because anodising does not simply apply a colour to the aluminium. The nature of the base material, the mechanical or chemical pre-treatment and the subsequent anodising process together determine how the finished surface looks.

In this article you will learn what the common abbreviations mean and what to look out for when choosing an anodised surface.

What does anodising mean?

During anodising – technically also known as anodising – the surface of aluminium is electrochemically converted into an oxide layer. Unlike painting or powder coating, no separate coating is applied to the aluminium. The protective layer is formed from the aluminium itself.

As a result, the typical metallic character of the material is retained. At the same time, depending on the process, the surface can be made matt, glossy, textured or coloured.

Pre-treatment plays an important role here.

Pre-treatments from E0 to E8

The abbreviations E0 to E8 describe different types of surface pre-treatment. They influence in particular the structure, gloss and uniformity of the subsequently visible anodised surface.

Abbreviation	Pre-treatment
E0	No special pre-treatment
E1	Ground
E2	Brushed
E3	Polished
E4	Ground and brushed
E5	Ground and polished
E6	Chemically matted or pickled
E7	Chemically or electrochemically brightened
E8	Polished and then chemically or electrochemically brightened

The VOA lists these pre-treatments in accordance with DIN 17611. Internationally, the same abbreviations are also found in the current QUALANOD specifications.

E0 – without special mechanical pre-treatment

With E0, no additional mechanical processing such as grinding, brushing or polishing takes place before anodising.

However, this does not mean that the surface enters the anodising bath completely untreated. The cleaning and chemical preparation steps necessary for the process still take place.

With E0, visual characteristics of the starting material can remain comparatively clearly visible. These include, for example, rolling marks or other surface features caused by production.

E1 – ground

With E1, the aluminium surface is ground. This can reduce unevenness and existing surface marks.

The result is a more uniform, rather matt surface with a visible grinding structure.

E2 – brushed

With E2, the surface is mechanically brushed.

This produces a typical directional structure, as desired for many decorative aluminium surfaces. Depending on the brushing process and the starting material, the structure can vary in how pronounced it is.

E3 – polished

E3 denotes a polished surface.

The mechanical processing smooths the surface and gives it a significantly higher degree of gloss. However, how strong the final gloss actually is also depends on the alloy and the subsequent anodising process.

E4 – ground and brushed

With E4, two mechanical processing steps are combined.

First, the surface is ground in order to reduce existing surface defects or processing marks. It is then given its characteristic structure by brushing.

The process is suitable when a relatively uniform surface with a visibly directional structure is desired.

E5 – ground and polished

With E5, the surface is likewise ground first. It is then polished.

Grinding improves the uniformity of the substrate, while polishing provides a smoother and glossier appearance.

E6 – chemically matted

E6 is one of the most frequently encountered pre-treatments.

The surface is chemically pickled and thereby acquires a more uniform, matt to satin-matt appearance. The VOA currently describes E6 as "chemically matted". In older documents and in practical usage, terms such as pickled, E6-pickled or special pickling are also found for this.

This surface is particularly widespread for architectural profiles and other visible aluminium components.

It is important to note: pickling can make certain structural or material differences in the aluminium visible in the first place. The choice of a suitable aluminium alloy and clean processing before anodising are therefore crucial.

E7 – chemically or electrochemically brightened

With E7, the aluminium surface is chemically or electrochemically brightened.

The aim is a smoother, more strongly reflective surface. The result appears significantly glossier than a chemically matted E6 surface.

E8 – polished and brightened

E8 combines mechanical polishing with subsequent chemical or electrochemical brightening.

This allows particularly smooth and high-quality glossy surfaces to be achieved.

Why pre-treatment is so important for the appearance

The designation of the anodised colour alone is not sufficient to clearly describe the subsequent appearance of a component.

For example, a colourless anodised aluminium profile can look completely different with an E0, E2 or E6 pre-treatment.

The pre-treatment influences, among other things:

• the visible surface structure,
• the degree of gloss,
• the uniformity of the surface,
• the visibility of processing and rolling marks,
• the perception of the subsequent colour tone.

For visually demanding components, pre-treatment and colour tone should therefore be specified together.

Standard colour tones for anodised aluminium

Certain colour tones have become established for anodised aluminium surfaces.

The current VOA anodised colour chart contains six particularly frequently used standard colours: C-0 and C-31 to C-35.

Colour tone	Current VOA designation
Colourless	C-0
Light bronze	C-31
Pale bronze	C-32
Medium bronze	C-33
Dark bronze	C-34
Black	C-35

In addition, anodising companies can offer further colour tones. These include, for example:

Colour tone	Abbreviation
Light grey	C-36
Medium grey	C-37
Dark grey	C-38

The VOA expressly points out that C-36, C-37 and C-38 can also be offered by companies, but they do not belong to the six colour tones of the current standard colour chart.

What do EV1 to EV6 mean?

Particularly in older drawings, bills of quantities and tenders, you will frequently still come across designations such as EV1, EV2 or EV5.

These designations come from earlier systems and are still used colloquially today. For current specifications, however, today's designations should be preferred.

The formerly common assignment is:

Colour tone	Current designation	Former EURAS designation	Former EV designation
Colourless	C-0	C-0	EV1
Light bronze	C-31	C-31	EV2
Pale bronze	C-32	C-32	EV3
Medium bronze	C-33	C-33	EV4
Dark bronze	C-34	C-34	EV5
Black	C-35	C-35	EV6

The former EURAS colour chart is no longer available. The VOA therefore points out that the designation "EURAS colour chart" should no longer be used today. It has been replaced by the VOA anodised colour chart.

So if, for example, you read "EV1" in an older tender, this generally means a colourless anodised surface. For new orders, a clearer specification with pre-treatment and current colour tone is advisable.

Example of a clear surface specification

Instead of merely specifying

"aluminium anodised, natural"

a description such as

"aluminium, pre-treatment E6, anodised C-0 colourless"

is much clearer.

For a bronze-coloured variant, the specification could for example read:

"aluminium, pre-treatment E6, anodised C-33 medium bronze"

This describes both the desired surface structure and the colour direction.

Why a colour tone still does not always look exactly the same

Anodised surfaces are not painted RAL surfaces.

The final colour impression is influenced, among other things, by:

• the aluminium alloy,
• the surface condition of the starting material,
• the pre-treatment,
• the anodising and colouring process used,
• material and process tolerances.

Even components with the same colour designation can therefore differ slightly from one another.

The VOA also points out that anodised colour tones can vary depending on the colouring process and the composition of the base material. A colour chart therefore serves first and foremost as a guide. Where there are high visual requirements, binding colour and limit samples should be agreed with the company carrying out the work.

This is particularly important for façades, window profiles, visible parts and larger contiguous surfaces.

Pre-treatment and colour tone belong together

When choosing an anodised aluminium surface, it is worth taking a closer look. Abbreviations such as E0 to E8 describe the pre-treatment and thus largely the structure and gloss of the surface. C-0 to C-35 stand for the VOA standard colour tones commonly used today, from colourless through various bronze tones to black.

Older designations such as EV1 to EV6 will still be encountered in drawings and tenders, but for new specifications they should wherever possible be replaced by current, clear details.

If the visual result is particularly important, it is also advisable to agree it on the basis of real samples. After all, with anodised aluminium the appearance always arises from the interplay of material, pre-treatment, anodising process and colouring.