|IAIN MCCAIG, KEVIN DAVIES AND DAVID FARRELL
Cathodic protection of iron and steel
Major works to historic buildings can sometimes be avoided by using electric current to suppress the corrosion of metal structures and components.
|Elizabeth Longford Almshouses, Whitchurch, Shropshire|
|Metal dowels and cramps were often incorporated in traditional masonry structures to secure stones, such as copings, parapets and cornices, that might otherwise be prone to movement or displacement. They were also widely used in ordinary ashlar walls to tie relatively thin stone facings back to the core. Dowels and cramps were also embedded in the facing itself to help maintain its structural integrity.
In 18th and 19th century buildings, dowels and cramps were usually made from wrought iron, which is susceptible to corrosion if exposed to air and moisture. In ashlar masonry of this period it is not unusual to find rainwater penetration occurring through perpendicular joints which have not been filled to their full depth. The narrowness of these joints makes effective repointing very difficult, so water penetration continues, causing the embedded cramps to rust. The expanding rust eventually exerts such pressure on the stone that it cracks or spalls. The conventional remedy involves major surgery to remove the cramps, replace them with noncorroding phosphor bronze or stainless steel and then repair the wounded stonework.
|Inserting and attaching electrical contact to buried cramp. .|
Some 19th and 20th century masonry-clad buildings incorporate steel frames which are also liable to corrosion. Again, conventional treatments can be highly invasive involving large-scale opening up to expose and treat the effected components. Cathodic protection offers an alternative approach to the treatment of rusting iron and steelwork buried in masonry and stone.
Cathodic protection (CP) encompasses a range of techniques used to suppress corrosion of metal structures and components. CP is not a new process: in 1824 Sir Humphrey Davy presented a series of papers to the Royal Society describing how CP could be used to prevent the corrosion of copper sheathing in the wooden hulls of British naval vessels. S ince then it has been applied in many areas, including marine applications and for the preservation of buried underground structures such as oil pipelines and tanks. Over the past 20 years, CP technology has been applied to reinforced concrete to protect steel reinforcements from corrosion. More recently it has also been applied to iron and steel embedded in brick, masonry and stone in historic buildings.
|Schematic diagrams of ICCP and SACP systems for protecting corroding iron.
Mason using 2mm thick diamondtipped saw to clear joints in which the connecting wiring is run.
|CP systems work on the principle that corrosion is an electrochemical reaction in which one part of a piece of iron or steel acts as an anode while adjacent metal acts as a cathode. At the anode, corrosion occurs as iron gives up electrons and forms soluble iron ions. At the cathode, free electrons combine with water and oxygen to form hydroxide ions. In CP systems, the metal to be protected is forced to act as the cathode, as on this side of the reaction the surface of the metal is unaffected by the reaction, preventing further corrosion. When used to protect structural iron and steel, this is achieved by applying small DC electric currents, via the building material. There are two methods of achieving this, either sacrificial anode cathodic protection (SACP) or impressed current cathodic protection (ICCP).
SACP systems use sacrificial anodes (zinc, aluminium or magnesium) placed in close proximity to the corroding metalwork and electrically connected to it. As the sacrificial anode corrodes, it generates a current that passes through the building material to provide protection to the embedded metalwork. The current is ionically conducted by means of pore water contained within the building material. These systems are capable of protecting small metal components such as embedded iron cramps or restraints set into walls, floors or roofs of a building.
ICCP systems use transformer rectifiers, normally mains powered, to provide the DC current to the iron or steel being protected. These systems use corrosion resistant anodes, fixed close to the metalwork, to provide part of the current pathway. ICCP systems are more complex than the SACP systems, but are suitable for providing CP to much larger areas of embedded steel such as 'I' beams, supports and columns, and where the stone or masonry has inherently higher electrical resistance.
A SACP system was installed to protect rusting iron cramps in the stone facade of four Grade II listed almshouses in Whitchurch, Shropshire, in 1999. This was the first application of its kind in the UK. The stones formed an interlocked frontage with iron cramps fitted between adjacent blocks. Water ingress through joints in the stonework had allowed the iron cramps to corrode, especially in the vertical joints around windows and doors. The expanding corrosion product had introduced internal stresses into the stonework that had resulted in some of the stones cracking and spalling.
For this structure, a novel repair technique using SACP was adopted. Damaged stones located on the outer edges of the facade were replaced with new stones fitted with stainless steel cramps and an SACP system was installed to control further corrosion of the remaining iron cramps.
To provide the cathodic protection, six magnesium anodes were buried in the pavement in front of the cottages. These were connected directly through to each of the iron cramps in a 'daisy chain' circuit. Electrical connections were made to the cramps us: minimise daming
The project was carried out with grant aid from the North Shropshire Conservation Area Partnership Scheme.
A number of historic structures in the London area have been fitted with ICCP over the past few years. It protects the embedded steel of an external staircase at Kenwood House, Hampstead, and iron cramps at the Inigo Jones Gateway at Chiswick House.
|Wellington Arch, Hyde Park Corner, London.|
|An ICCP system has recently been installed within the Grade I listed Wellington Arch, Hyde Park Corner. The arch is built of Portland stone with a concrete roof slab supported by steel 'I' beams. It arch has recently undergone extensive renovation, including the repair of the steel and concrete structure which supports the bronze sculpture of the quadriga. During the inspection it was discovered that some of the key steel 'I' beams within the roof had suffered significant corrosion. This was partly due to rainwater penetrating the roof structure. The worst corrosion was where the beams had been in direct contact with the Portland stone.
Portland stone, having a neutral pH, offers no corrosion protection for steel. On the other hand, concrete is alkaline and, when in direct contact with steel, helps to passivate the surface, inhibiting further corrosion. In fact, as a sedimentary rock formed in a marine environment, Portland stone may contain significant concentrations of chlorides or sulphates (salts) which, in the presence of moisture, can accelerate the corrosion process.
|Detail of the repaired deck; the lines running across the surface of the concrete indicate the location of the mixed metal oxide coated titanium ribbon anode.
|Iain McCaig has specialised in building conservationon for over 25 years for much of this time with the GLCs historic buildings division and English Heritage. He is now buildings at risk officer for North Shropshire District Council.
Kevin Davies is a specialist corrosion engineer who has been responsible for the design, supervision, installation, commissioning and monitoring of more than over 50 cathodic protection systems for civil engineering structures and buildings.
David Farrell is a corrosion engineer and a director of Rowan Technologies, a consultancy and r&d company specialising in the conservation of metals, stones and concrete in historic buildings and structures.
Corrosion of the arch's 'I' beams had several consequences.
English Heritage did not wish to replace the steel beams as this would have been both expensive and disruptive, possibly requiring lifting of the quadriga. Ultrasonic thickness readings of the steel beams showed that their present thickness was sufficient to support the roof slab and quadriga, provided that corrosion could be controlled.
To provide efficient long-term CP performance, titanium expanded mesh ribbon anodes, with a mixed metal oxide (MM0) coating, were cast into the top surface of the concrete slab above the steel frame structure. This ribbon anode system provides maximum anode surface area for efficient transfer of the electrical current through the structure's concrete sections.
Small, discreet MMO coated titanium rod anodes were used to protect the steel beams where they were laid over the Portland stone. These anodes were embedded into 12mm diameter, 300mm deep holes at centres of 300mm in the mortar pointing of a brick course under the stone, using a conductive backfill to provide electrical continuity. These discreet anodes provide current into the depth of a wall to protect embedded steelwork.
System wiring from the beams and anodes was installed within the internal roof space back to an instrumentation cabinet. The cabinet houses the system electronics and computer for system control and monitoring.
One of CP's principal advantages in the protection of embedded metalwork is that it provides corrosion protection without changing the immediate physical environment. There may still be now, or in the future, damp concrete or stone adjacent to the metal which would previously have allowed corrosion to continue. Cathodic protection provides the electrochemical conditions to control this corrosion process.
The implication of this is that there is no need to gain full access to the structure by removing the surrounding material and the structure can remain largely intact. All that is required is to install the necessary cables and anodes that form part of the CP system. These can usually be installed in such a way as to have little or no impact on the structure's visual appearance. In the case of the Whitchurch almshouses, this avoided the need to dismantle the stone facade and remove the expanded corroded cramps, which would have had uncertain consequences. On the Wellington Arch, major structural upheaval was avoided.
CONTEXT 71 : SEPTEMBER 2001