Glossary · Single word
Corrosion
In short
the deterioration of a metal surface due to chemical reaction or environment
Corrosion describes the chemical deterioration of metal components within a cladding system, often caused by environmental exposure. Understanding this process is vital for maintaining the structural integrity and long-term durability of a building’s external envelope.
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Corrosion
Definition and Chemical Principles
In the context of building envelopes, corrosion refers to the deterioration of a metal surface due to chemical or electrochemical reactions with its environment. Within the UK cladding industry, this most frequently involves the oxidation of metallic components when exposed to moisture, oxygen, and atmospheric pollutants. The process represents a natural tendency for refined metals to return to a more chemically stable form, such as oxides, hydroxides, or sulphides.
The rate and severity of corrosion depend on the specific alloy used, the presence of protective coatings, and the micro-climate surrounding the building. In external wall systems (EWS), corrosion is rarely a uniform process; it often manifests as localised pitting or galvanic corrosion, the latter occurring when two dissimilar metals—such as an aluminium bracket and a stainless steel fastener—come into electrical contact in the presence of an electrolyte like rainwater.
Impact on Building Fabric and External Wall Systems
Corrosion affects both the aesthetic integrity and the structural performance of cladding systems. While surface discolouration or "tea staining" may be the first visible sign, the primary technical concern relates to the integrity of the subframe and fixings. In many rainscreen applications, the primary cladding panels are supported by an aluminium or galvanised steel grid system. If the protective zinc coating on steel components is breached, or if the protective oxide layer on aluminium is compromised by acidic environments, the structural capacity of the support system may diminish over time.
Furthermore, corrosion can lead to "oxide jacking" or "rust bursting." As iron-based metals corrode, the resulting iron oxide occupies a significantly larger volume than the original metal. This expansion can exert substantial pressure on surrounding materials, potentially cracking masonry, displacing sealants, or loosening mechanical fixings. Within a cavity, where airflow is intended to manage moisture, persistent dampness caused by blocked drainage paths or poor ventilation can accelerate these chemical reactions, often hidden from view behind the external rainscreen.
Regulatory Context in England and Wales
The management of corrosion falls under the broader remit of ensuring functional stability and weather resistance as required by the Building Regulations. Specifically, Requirement A1 (Loading) and Requirement B4 (External Fire Spread) are relevant, as the structural failure of a cladding component due to corrosion could lead to the detachment of panels or the compromise of fire-stopping integrity.
In the current regulatory landscape, particularly following the introduction of the Building Safety Act 2022, there is an increased emphasis on the "golden thread" of information. This includes documentation regarding the specification of materials and their suitability for the local environment (e.g., proximity to coastal salt spray or industrial pollutants). Competent professionals conducting PAS 9980 assessments or EWS1 surveys typically scrutinise evidence of corrosion as part of their evaluation of the building’s overall condition. The durability of materials is also governed by British Standards, such as BS EN ISO 12944, which classifies environments by corrosivity (from C1 for heated buildings with clean atmospheres to CX for extreme offshore environments).
Considerations for Building Owners and Managers
For those responsible for the tripartite management of residential blocks—such as Residents' Management Companies (RMC) or Right to Manage (RTM) directors—understanding the lifecycle of metallic components is essential for long-term maintenance planning. Corrosion is not always a sign of systemic failure, but it does necessitate professional investigation to determine if the degradation is superficial or structural.
Regular inspections are a standard industry recommendation to identify early signs of coating failure or electrolytic action. If corrosion is detected during a routine facade survey, the remediation strategy will depend on the depth of the penetration. While surface corrosion on certain aluminium alloys might be self-limiting due to the formation of a protective oxide layer, corrosion on carbon steel components usually requires intervention, such as the application of sacrificial coatings or the replacement of affected sections to prevent further loss of section thickness.
Corrosion
Common Misunderstandings
A frequent misconception is that all "rust" is identical. In technical terms, rust refers specifically to the corrosion of iron and its alloys. Other metals corrode but do not "rust" in the strictest sense. For instance, the white powdery substance found on aluminium is aluminium oxide; while it indicates corrosion, it behaves differently than the red iron oxide found on steel.
Another misunderstanding involves the assumption that stainless steel is entirely immune to corrosion. While highly resistant, stainless steel can suffer from pitting or crevice corrosion if the wrong grade (e.g., Grade 304 vs Grade 316) is specified for a high-chloride maritime environment. Furthermore, the belief that "out of sight is out of mind" is particularly hazardous regarding cladding; the most critical corrosion often occurs within the cavity, affecting the wall ties, brackets, and rails that ensure the system remains secured to the primary structure.
Related Terminology and Environmental Classifications
To accurately describe and manage corrosion, the industry employs several specific terms that define the mechanism and the environment.
| Term | Technical Context |
|---|---|
| Galvanic Corrosion | Occurs when two different metals are in contact in a wet environment. |
| Pitting | Localised corrosion that creates small holes or cavities in the metal. |
| Patina | A thin layer that forms on the surface of metals like copper or bronze due to oxidation, often protective. |
| Sacrificial Anode | A metal used to protect another by corroding in its place (e.g., galvanising). |
| C-Category | The ISO classification system (C1 to C5) for atmospheric corrosivity. |
Process and Remediation Context
When corrosion is identified as a significant risk during a structural or fire safety assessment, the process typically involves a detailed condition survey. This may include non-destructive testing (NDT) to measure the remaining thickness of structural members or invasive inspections where sections of the cladding are removed to inspect the subframe.
Remediation varies significantly. In some instances, localised cleaning and the reapplication of protective paints or cold-galvanising sprays may be sufficient. In cases where the cross-sectional area of a load-bearing component has been reduced beyond permissible margins, full replacement of the subframe may be necessary. Any such work must be specified by a structural engineer or a specialist facade consultant to ensure that the replacement materials are compatible with the existing system and suitable for the building’s specific geographic and environmental exposure.
Corrosion