Aluminium shapes modern architecture: on façades, windows, doors, railings and solar shading. Many of these surfaces are natural in colour and show the typical metallic aluminium tone – others are bronze, black or coloured, matt or glossy. What they have in common is the surface treatment: it protects the metal and gives it its appearance at the same time. But how are surfaces created – whether natural or coloured – that still look even after years outdoors? A look at surface treatment.
Why aluminium is treated
In air, aluminium spontaneously forms a very thin oxide layer that protects the metal against further corrosion. However, this natural protective layer is not enough for a defined appearance and durably reproducible surface properties. Surface treatment therefore serves two purposes at once: it improves the protection of the aluminium and defines its appearance – from a metallic natural finish to defined colours. In architecture, two routes have become established above all: anodising and coating.
Route 1: Anodising
In anodising, a much thicker, controlled aluminium oxide layer is built up electrochemically. For weather-exposed architectural components, the required layer thickness is defined according to the environmental load and the relevant specification. For demanding exterior applications, a layer thickness of around 20 µm is often specified.
The surface is often left uncoloured, resulting in a natural anodised finish. The anodic oxide layer is largely transparent, so the metallic character of the pretreated aluminium surface is retained. The layer increases surface hardness and, with a suitable layer thickness and sufficient sealing, can achieve high weather resistance. Whether the surface looks matt, satin or glossy is determined largely by the pretreatment – for example etching for a uniformly matt finish, or grinding and brushing for a directional look.
Colour is an optional additional step. It is not created by paint, but within the porous oxide layer itself. A well-established method is electrolytic colouring with tin salts. Alternating current is used to deposit tin at the base of the pores, producing shades from light bronze to black depending on the amount deposited. For coloured surfaces, the subsequent sealing additionally keeps the shades light- and weather-stable. Relevant references for anodised architectural components include ISO 7599, DIN 17611 and the current QUALANOD specifications.
Route 2: Coating (powder and liquid paint)
In coating, powder or liquid paint systems are applied to the pretreated aluminium and then cured. Unlike natural anodising, the focus here is usually a defined colour – with a very wide range of colours and surface effects; high-quality liquid paints, for example, use PVDF binder systems. Long-term colour and weather resistance depends, among other things, on the coating system, its performance class, the pretreatment and proper curing. As pretreatment, approved chromium-free systems are common today, including titanium- or zirconium-based processes. For coating materials, the international QUALICOAT quality label defines different performance classes.
Why the aluminium surfaces last
Whether a coloured or natural surface retains a uniform appearance over time depends on several factors: layer thickness, process control, sealing or curing, pretreatment and the load at the installation site. The required surface quality therefore also depends on where a component is installed. Humidity, industrial atmospheres, intense UV radiation and, above all, salt-laden air can stress the surface to different degrees.
Under high salt loads, for example near the coast, higher requirements apply to pretreatment and the coating process. QUALICOAT Seaside or GSB Sea Proof define additional requirements for this. Depending on the system, pre-anodising can also be used to reduce the risk of filiform corrosion – a thread-like undermining of the coating.
What it comes down to
Whether architectural aluminium retains a uniform appearance over time is not decided at a single point. Four factors work together: a suitable material with defined pretreatment, the choice between an anodic oxide layer and/or an organic coating, proper process control with quality assurance, and matching the surface to the installation site, weathering as well as a later façade cleaning. It is precisely this interplay that creates surfaces that last – whether in a metallic natural tone or in a strong colour.
Surfaces with system
For durable anodised surfaces – natural or electrolytically coloured – Alufinish offers the process chemistry along the entire anodising line, from cleaning and etching to sealing; for reproducible bronze and black tones, for example, the Tin-ColorAL process. For the pretreatment of aluminium before coating, various QUALICOAT-approved, chromium-free systems are available, including the Alfipas 745 + 746 system, Alfipas 7816 and Alficoat 748 and 748/3. For material that has already been pre-anodised, ENVIROX PREANO is available as an approved post-treatment. And if you want to maintain your anodised surfaces – use the ALUPOLISH cleaner.