When effectively protected from corrosion, steel is ideal for facade systems requiring high load-bearing capacity and large spans due to its significant strength-to-weight ratio, while its strong tension and compression characteristics allow for versatile designs such as curved or irregular shapes.

Steel-intensive façades are building envelopes that prominently feature steel elements as a primary structural and aesthetic component. Used in compact-cross sections, steel weighs about 60 per cent less than concrete, and is eight times stronger in terms of tension and shear strength.

A recent study noted that steel façade systems had attracted increasing interest from the construction sector, due to their strength, flexibility and recyclability characteristics.

This is seen across all steel façade applications, including built-up wall steel insulated panel cladding systems, sandwich panel or liner-tray wall systems, single-skin façade panels, and modular cassettes.

Using metal façades gives architects a broad range of design possibilities, such as modern, smooth metal panelling or intricate patterns using perforated metal, and have significant durability and longevity benefits.

Digital fabrication using computer-aided design (CAD) also allows for complex geometric patterns to be created with precision.

Despite being lightweight, metal façades can withstand harsh weather conditions; resist corrosion and fading; and maintain their appearance over time, reducing the need for frequent repairs or replacements.

Importantly, steel as a façade material is intrinsically linked to sustainable practice, due to its inherent recyclability, energy efficiency, and seamless integration with photovoltaic solar panels.

A research paper published last year noted that façade systems made of galvanised steel did not have the disadvantages inherent in aluminium and plaster façades, as the substructure of reinforced fixing brackets could be mounted vertically, horizontally or crosswise.

The authors added that while aluminium façades were once the most common material used for commercial premises, now aluminium – as well as titanium and zinc – was more often used for cladding a gable or as individual parts of a façade.

Cladding is the non-loadbearing layer of a complete wall system, controlling weather intruding from the outside and allowing water vapour to escape from the inside – it also contributes to acoustic and thermal insulation, as well as fire resistance.

They said: “The cladding of ventilated façade systems can be made with metal siding, linear panels, corrugated board, façade cassettes or porcelain stoneware, depending on the purpose, building type and architectural design.

“When using façade systems, endless possibilities open up for the external design of buildings and the implementation of almost any architectural ideas due to the variety and widest range of colours of these materials.”

PREVENTING STEEL CORROSION IN FAÇADES

There are multiple case-specific methods for managing corrosion in steel façades, but prevention with sound design principles can alleviate serious degradation.

While its many benefits have made it a proven and durable construction material, structural steel is not naturally resistant to corrosion due to the reactive nature of its main component, iron.

Steel is also impacted by environments with high concentrations of chlorides, which hinder its ability to form a protective oxide outer layer. However, steel can easily be alloyed with other metals to markedly improve its corrosion resistance.

A common alloy used is chromium, which at about 12 per cent of the alloy together with smaller amounts of other elements like manganese, silicon, nickel, and molybdenum, forms what is otherwise known as stainless steel.

The corrosion resistance of stainless steel in atmospheric environments is higher than zinc and copper, which is the main factor behind the use of stainless steel in façade systems.

Stainless steel also has mild self-healing properties which make it the first consideration where corrosion might cause issues with a building. Even though it may generally provide corrosion resistance, stainless steel can suffer from some types of corrosion in severe environments.

When steel corrodes, the resulting rust takes up a greater volume than the steel, creating tensile stress by expanding and can eventually cause cracking, delamination, and spalling.

Steel corrodes because it is not a naturally occurring material – iron ore is smelted and refined to make steel, a process which adds energy to the metal.

Under normal conditions, steel like most metals (except for gold and platinum) is thermodynamically unstable and will release energy and revert to its natural state of iron oxide, or rust.

The design of structural steel can play an important role in minimising corrosion – for example, building designs should avoid creating cavities or crevices where moisture and dirt can become trapped.

Contact with other materials such as timber or other metals should be insulated where necessary and the overall design should ensure that selected protective coatings can be applied efficiently.

Other design factors to be considered include providing adequate drainage around the steelwork and access for future maintenance, as well as the general notion of large flat surfaces being far easier to protect than more complicated shapes and structures.

Effective ways of avoiding or minimising steel corrosion in façade systems include material selection (choosing an appropriate steel alloy matching the site hazard exposure),

eliminating any one of the requirements for corrosion, reduction of the cathode and/or electrolyte activity (polarisation), anode protection (passivation), changing of the micro-environment, and protective coating.

A façade needs to manage water ingress and minimise water retention in the wall, requiring durability and protection from deterioration.

There are three main methods used to prevent corrosion in steel: passive barrier protection, active protection, and sacrificial protection (cathodic or galvanic protection).

Passive barrier protection works by covering the steel with a protective coating that forms a tight barrier and prevents further exposure to oxygen, water, and salt – the protective qualities are enhanced the lower the coating’s permeability to water.

Active protection is when reactive chemical compounds are applied directly to the steel to disrupt the normal process of anode formation on the surface of the steel.

Zinc is the most widely used metal in steel corrosion control and can provide sacrificial protection through its preferential oxidation when in direct contact with the steel substrate.

Along with preferential oxidation, zinc metal corrodes at a generally slower rate as well.

Different kinds of metallic coatings can be applied to steel and their composition will depend on the corrosive environment and its severity, including organic coatings, paint, and powder coatings such as acrylic, vinyl, epoxy, nylon, polyester, and urethane.