Council for Aluminium in Building

Finishing

Durability is an important quality for construction elements. Aluminium is extremely durable because it naturally forms a layer of aluminium oxide on exposure to air, in a process known as “passivation”, which describes a material becoming “passive” and less affected by its environment. This makes aluminium an excellent choice for your building, including windows, roof lights, doors, curtain walling, cladding systems, roofing and roofline products.

“Mill finish” refers to aluminium as it comes off the rolling or extrusion mill, with no additional surface finishing applied. Here, the aluminium will quickly form a surface layer of aluminium oxide on exposure to air, “passivating” and providing a protective grey oxide coating.

There are two principal methods to further enhance the corrosion resistance of aluminium in construction, and to create decorative and durable finishes.

Anodising is an electrolytic process that forms a stable oxide layer on the aluminium surface. The resulting oxide layer is thicker and stronger than aluminium’s inherent oxide coating. It is possible to dye the coated surface a variety of colours, electrolytically or with organic dyes, before it is sealed, with bronze shades often used for architectural purposes. Where the colour results from optical effects in the oxide layer, due to absorption and reflection of specific light frequencies, the colour is termed fade-free.

Powder coatings are solvent-free paints applied to metals and other conductive surfaces. After suitable pre-treatment of the material to be coated (see our pre-treatment Position Paper), the powder coating is applied electrostatically and is cured under heat to allow flow and formation of a hard finish, tougher than conventional paint. Polyester is the most widely specified resin system for powder coatings and a very wide range of colours and gloss levels are available.

Case studies

The dome of the San Gioacchino in Prati Church, completed in 1897, is the earliest known example of external aluminium cladding still in service. It is in excellent condition after over 120 years, undergoing regular inspection and testing. The architect chose aluminium to clad the dome because it was lightweight, durable and economical in comparison with other materials.

The New Bodleian Library, completed in 1938, boasts anodised aluminium windows. As part of a refurbishment project in 2014, it was only necessary to clean and re-glaze the windows. Based on the durability of anodised aluminium, it is reckoned that the finish is satisfactory for another 60 years or more.

Studies* have shown that polyester powder coated aluminium windows and curtain walling, when appropriately detailed, installed and maintained, can exhibit excellent colour and gloss retention compared with the original coating after more than 40 years.

* Aluminium and Durability. Towards Sustainable Cities. Michael Stacey Architects 2014.