Glossary · Two-word term
Galvanic Corrosion
In short
electrochemical deterioration when dissimilar metals are in contact with an electrolyte
Galvanic corrosion is the electrochemical process where dissimilar metals deteriorate when in contact through an electrolyte. This technical reference explains its impact on cladding systems, UK regulatory considerations, and the importance of material compatibility in building maintenance.
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Galvanic Corrosion
Understanding Galvanic Corrosion in Cladding Systems
Galvanic corrosion is a specific form of electrochemical deterioration that occurs when two dissimilar metals are in electrical contact while exposed to an electrolyte, such as moisture or airborne pollutants. In the context of building envelopes, this process leads to the accelerated corrosion of the more chemically active (anodic) metal, while the more noble (cathodic) metal remains relatively protected.
For a galvanic cell to form within a cladding assembly, three conditions must be met: there must be two metals with different electrode potentials, a physical or conductive path between them, and an electrolyte—typically rainwater, condensation, or sea spray—bridging the gap. The rate of degradation is influenced by the difference in the metals' positions on the galvanic series, the ratio of the surface areas of the two metals, and the conductivity of the electrolyte.
Technical Applications in Cladding Testing and Repair
During the investigation of external wall systems (EWS), surveyors and façade engineers scrutinise interfaces where different metallic components meet. This is particularly relevant in the United Kingdom, where the prevalence of aluminium rainscreen systems, steel subframes, and stainless steel fixings creates numerous potential galvanic couples.
Testing and inspection protocols typically involve visual assessments for characteristic signs of galvanic action, such as localised pitting, staining, or the accumulation of powdery corrosion products at junctions. In more detailed forensic investigations, technicians may use ultrasonic thickness gauging to determine if the structural integrity of a component has been compromised. When repairs are specified, the objective is usually to break the galvanic circuit, either by selecting compatible materials or by introducing insulating spacers, gaskets, or non-conductive coatings to isolate the dissimilar metals.
Building Fabric and External Wall Systems
The complexity of modern external wall systems often necessitates the use of various alloys to meet structural, aesthetic, and thermal requirements. Galvanic corrosion frequently occurs at the point where aluminium cladding panels are secured to galvanised steel or stainless steel support railings. While aluminium and stainless steel are often used together, the specific grade of the alloys and the environmental conditions play a significant role in the long-term stability of the interface.
Environmental factors in the UK, such as proximity to coastal salt spray or high levels of industrial atmospheric pollutants, increase the conductivity of the electrolyte (moisture), thereby accelerating the electrochemical reaction. The "area effect" is also a critical consideration in building fabric design: a large area of a noble metal (cathodic) in contact with a small area of an active metal (anodic), such as a stainless steel screw in a large aluminium panel, is generally more problematic than the inverse, as the corrosive current is concentrated on a small surface area.
Regulatory and Process Context in England and Wales
The management of cladding systems in England and Wales is governed by a framework that emphasises the functional requirements of the Building Regulations, particularly Requirement B4 (External Fire Spread) and Requirement A1 (Loading). While much recent attention has focused on fire performance, the structural permanence of the cladding system—which can be undermined by galvanic corrosion—falls under the remit of ensuring the building remains safe for its intended lifespan.
Under the Building Safety Act 2022 and the evolving regime overseen by the Building Safety Regulator (BSR), the "Accountable Person" is responsible for assessing and managing building safety risks. While galvanic corrosion is primarily a durability issue, if left unaddressed, it can lead to the failure of fixings or support structures. Consequently, periodic inspections and the maintenance of a "Golden Thread" of information regarding material specifications are becoming standard practice for higher-risk buildings (HRBs) to ensure that the original design intent regarding material compatibility is preserved.
Galvanic Corrosion
Guidance for Building Owners and Managers
Building owners, managing agents, and Right to Manage (RTM) directors should be aware that galvanic corrosion is a predictable and manageable phenomenon rather than a latent defect in all cases. It is a matter of material science that requires professional oversight during both the specification phase and the ongoing maintenance lifecycle of a building.
When reviewing survey reports or planned preventative maintenance (PPM) schedules, it is important to distinguish between superficial oxidation and systemic galvanic deterioration. A competent professional should assess whether the observed corrosion is likely to affect the structural capacity of the cladding fixings over time. In the event of cladding replacement or remediation, ensure that the interface between new components and the retained substructure is evaluated for electrochemical compatibility to avoid introducing new corrosion risks.
Common Misunderstandings
A frequent misconception is that all contact between different metals will result in immediate and catastrophic failure. In reality, many metal combinations are relatively stable in benign environments. The severity of the reaction depends heavily on the specific environment and the relative surface areas involved.
Another misunderstanding involves the use of "galvanised" steel. Galvanisation is itself a deliberate application of galvanic principles, where a zinc coating acts as a sacrificial anode to protect the underlying steel. However, if this zinc layer is depleted or if the galvanised steel is placed in contact with a more noble metal like copper or certain grades of stainless steel without adequate separation, the protective mechanism can be overwhelmed, leading to the rapid decay of the steel core.
Related Terminology and Material Properties
| Term | Description |
|---|---|
| Anode | The less noble metal in a galvanic couple that undergoes corrosion. |
| Cathode | The more noble metal that remains relatively unaffected by the electrochemical process. |
| Electrolyte | A liquid (often water with dissolved salts) that conducts electricity and facilitates ion movement. |
| Bimetallic Strip | A junction of two metals, often used in sensors but avoided in structural cladding without insulation. |
| Sacrificial Protection | The intentional use of a more active metal to protect a primary structural metal. |
| Pitting | A form of extremely localised corrosion that leads to the creation of small holes in the metal. |
In summary, galvanic corrosion is a fundamental consideration in the longevity of external wall systems. While the UK regulatory environment focuses heavily on life safety, the mechanical stability of the building fabric relies upon the correct management of these electrochemical interactions. Professional assessment remains the only reliable method for determining the status of metallic interfaces within a specific building's cladding assembly.
Galvanic Corrosion