Context 139 - May 2015

20 C O N T E X T 1 3 9 : M A Y 2 0 1 5 DAVID FARRELL Cathodic protection in church towers Cathodic protection, the application of a small negative charge to the iron or steel, is being used increasingly to control corrosion of embedded iron within historic buildings. Corrosion of iron tie bars, ring beams and cramps is becoming an increasing problem in our heritage building stock. Although the rate of corrosion is relatively low, in the long term it results in the fracturing of stones and sometimes the loss of structural integrity. In the past architects have attempted to remove as much as possible of the original iron fittings and replace them with more corrosion-resistant materials. The policy now followed in the UK for heritage buildings is that of ‘minimum intervention’, with the intention of retaining as much of the original fabric as possible. In the 18th and 19th centuries, tie bars, ring beams and cramps were usually made from wrought iron, which is susceptible to corrosion when exposed to air and moisture. The situation may be exacerbated if sedimentary stones, such as Portland or Bath, are used in the construction. These sometimes contain chloride and/or sulphate salts, which result in the depassivation of the iron surface and an acceleration of corrosion. Corrosion rates are also significantly higher where iron is in direct contact with damp stone as compared to exposure to air. For major construction work during this period the wrought iron was sometimes surrounded by lead. Lead corrodes at a very slow rate in this environment. If it completely surrounds the fitting, the iron should be protected from corrosion. However, this is rarely the case for heritage buildings, and especially for church towers, where the melting of lead on wooden scaffolding during construction would have been minimised. For unprotected iron the corrosion results in expansion forces, which eventually exert such pressure on the stone that it cracks or spalls. The volume ratio between iron and rust can be as high as 1:7. Ring beams were originally incorporated into towers to resist the masonry buckling outwards and to strengthen the walls to resist wind loading. Normally they are embedded within the external masonry to maximise support to the tower, but sometimes they are embedded within the internal masonry if the thickness of the wall changes due to the presence of windows or louvres. The conventional remedy has usually involved surgery to remove the iron fittings and cramps. They may sometimes be replaced with non-corroding phosphor bronze or stainless steel, or even left without replacement, prior to repair to the stonework. Conventional treatments can sometimes be highly invasive, involving large-scale opening up to expose and treat affected components, or even demolition to replace the iron. Cathodic protection offers a non-intrusive, alternative approach to the treat- ment of rusting iron embedded in masonry and stone. In 1824 Sir Humphrey Davy presented a series of papers to the Royal Society describing how cathodic Expansion and repointing of the joints associated with continuing corrosion of three ring beams at the tower of St Aldhelm’s Church, Doulting External damage to the tower due to corrosion of iron cramps at St Michael and All Angels Church, GreatWitley protection could be used to prevent the corrosion of copper sheathing in the wooden hulls of British naval ves- sels. Since then it has been applied to many other areas, including marine applications, and for the preservation of buried underground structures such as pipelines and tanks. Over the past 30 years the technology has been applied to concrete to protect steel reinforcement from corrosion. Over the past 20 years it has also been applied to iron and steel embedded in brick, masonry and stone in heritage buildings. Corrosion is an electrochemical reaction involving both anodic and cathodic sites on an iron or steel surface. At the anodic site corrosion occurs as iron gives up electrons