19 Apr Cathodic Protection
Metal has long been used in the construction of buildings, typically embedded into masonry as restraining cramps and, more recently, beams and steel frames. The long-term issue is that ferrous metals will corrode when exposed to moisture and oxygen. As the ferrous oxides form on the surface of the metals, their volume increases. This expansion applies tensile stresses to the stonework causing fracturing and displacement. As corrosion continues, the fractures will widen, leading to an increased rate of corrosion because more moisture is able to reach the surface of the metal. In order to address the issue of failing metalwork, the conservator/restorer has to undertake invasive means to expose the corroding ferrous material, and either treat it or replace it (ideally with a non-ferrous alternative such as stainless steel).
An alternative approach in recent decades has been the use of Cathodic Protection. Cathodic Protection basically reduces the corrosion rate of a metal by reducing its corrosion potential, bringing the metal closer to an immune state. It is employed to control the corrosion on a metal’s surface by making the metal the cathode of an electrolytic cell. This procedure is relatively simple and involves either connecting the original metal component to a more reactive metal, which consequently acts as the anode (eg. Galvanic Protection, sometimes referred to as Sacrificial Anode Protection), or applying an electrical current through the component to ensure that it acts as the cathode (eg Impressed Current Cathodic Protection).
Galvanic Protection (or Sacrificial Anode Cathodic Protection)
This is the process of preventing corrosion by attaching a galvanic anode (a piece of a more reactive metal) to the vulnerable metal component. An electrolytic cell is created where both metals are exposed to an electrolyte. In our context, the electrolyte is usually either airborne moisture or saturating rainwater. Water percolating through clinker concrete becomes sulphuric acid, which is an extremely effective electrolyte. Sea water is also an effective electrolyte. The galvanic anode will continue to corrode until eventually it must be replaced. To ensure that the anodes function properly, the galvanic protection system needs to be tested periodically, typically every two to four years.
Impressed Current Cathodic Protection (ICCP)
Impressed current systems are used when galvanic protection is considered inappropriate. ICCP employs an external electrical current to disrupt the flow of electrons which would otherwise occur between the anode and the cathode to the detriment of the anode. This power supply is typically regulated by means of a site based network control unit. It gives greater longevity to the protection as the system does not rely on the corrosion of anodes to protect the vulnerable metal components.
Anodes are installed into the masonry taking care to ensure that they are not in direct contact with the metal components. There are two types of anodes: discrete or ribbon. Discrete anodes are installed into de-dusted holes filled with suitable conductive material. Ribbon anodes are best used within the bed and perpend joints of reconstructed masonry to ensure that full embedment into the jointing mortar is achieved. PAYE have found from experience surveying facades on behalf of potential purchasers that there is a high incidence of failure in systems using ribbon anodes inserted into open masonry joints and pointed over. This is due to installers leaving the ribbons in voids behind the pointing mortar. In such circumstances, acids form on the surface of the ribbons and eventually break down any mortar that might be in contact with them.
The power supply is provided by means of cables inserted into masonry joints and pointed over.
To properly design an ICCP system, it is essential to ensure that all metal components within the anode reactive zone (typically a 1500mm radius from each anode) are identified and earth bonded. If this is not achieved, a possibility exists whereby stray current into the metal component within the anode reach zone will cause the component to corrode.
The anodes are arranged into zones, often on a floor by floor basis. Reference electrodes are installed into each zone to monitor local conditions and provide data back to the network control unit. Any potential changes can then be measured and logged, and data can be accessed remotely via the internet for ongoing monitoring.
ICCP does have a limited life due to the nature of the components used. If properly designed and installed, the manufacturers advise a likely performance life of 25 years, though some component replacement will be required during this period.
Using CP on historic buildings
PAYE repair many millions of pounds of fractured masonry each year, much of it damaged by the expansion of embedded corroding metal components. Cathodic Protection clearly has the potential to control this corrosion, and this should be welcomed by the conservation community.
However, we have found that the quality of design and installation varies considerably between CP contractors. Furthermore, the cost of designing and installing a CP system can be very high compared to the cost of undertaking basic conservation or masonry repair works.
Maintaining or replacing CP components during the limited life span of the system can also become expensive taking into account the cost of access scaffolding.
Essentially, decisions on how best to maintain historic masonry need to be based on an accurate understanding of the failures, and the causes of failure, together with a clear knowledge of the owner’s requirements in terms of planned maintenance cycles.
PAYE are very experienced in this field and will always provide a clear concise analysis of the issues. Ultimately, it comes down to a simple question: what does the building need?