Article - Issue 24, September 2005

Cracking Up

Paul Lambert

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Exposure in a 1920s building showing the corroded frame behind the crack © Mott MacDonald

Exposure in a 1920s building showing the corroded frame behind the crack © Mott MacDonald

Many of the architecturally important structures in our major cities are falling victim to an insidious form of attack from within. When the ferrous framework of these structures reverts to iron oxide it results in cracking and displacement of the façades. Paul Lambert explains how a technique used hundreds of years ago to protect ships from corrosion could now help to preserve our architectural heritage.

The rapid development of the commercial and municipal infrastructure in our cities in the late 19th and early 20th centuries embraced the new technology of steel framed construction. The steel frameworks allowed fast construction of higher, lighter and more elegant buildings with large windows and imposing entrances that could never have been built with load bearing masonry.

On such buildings, the masonry is relatively thin and is primarily there for aesthetic reasons. While the frame may have originally had a thin bituminous emulsion or cement wash coating, this was little more than a holding primer. The only protection against the environment was the surrounding masonry which was often of poor or variable quality, and in a relatively short time the steel would start to corrode.

Rust swells

The corrosion itself would be bad enough given that it could, in time, reduce sections of the steel member to a point where it was no longer structurally adequate. However, for these buildings, it is the corrosion product or ‘rust’ that is the main cause of the problems. When ferrous alloys such as steel corrode, the corrosion product swells to several times the volume of the metal from which it was generated. In time, this voluminous corrosion product has the force to bend steel, crack concrete and displace masonry.

Once corrosion is established and the rust starts to push on the masonry, stones and bricks crack and joints open up to allow in even more moisture and oxygen, so the process may accelerate until the façade is sufficiently damaged to require repair, reconstruction or demolition.

This degradation mechanism, first reported in the 1940s, was nicknamed ‘Regent Street Disease’. For a time the degradation put great pressure on both owners and conservation professionals. A well known example is BBC’s Broadcasting House, whose soft porous stone is supported by simple steel frames. Now we discover, perhaps surprisingly, that the answer to many of the associated problems has been around for longer than most of the buildings.

Humphry’s heritage

In 1824, Sir Humphry Davy, President of the Royal Society, described how the iron nails and copper cladding of ship’s hulls could be protected from corrosion by attaching zinc plates and taking control of the electrochemical processes through a method now known as ‘cathodic protection’. At the time it was considered only a partial success as it prevented the copper from dissolving and acting as anti-fouling. The critical press coverage that followed this ‘failure’ has been linked to Davy’s stroke and early death.

However, Davy would have been pleased to know that what initially proved helpful for ships also appears to be the most reliable and least disruptive way of preserving iron and steel structural elements in our historic buildings. But to achieve this, it is first necessary to develop radically new hardware and innovative approaches to the design of such systems.

While cathodic protection has been around for over 180 years and in common use for over 50 years it is only very recently that developments in anodes, power supplies and monitoring systems have made it feasible to consider such technology for applications other than heavy marine, chemical and civil engineering applications.

What is cathodic protection?

The electrochemistry of cathodic protection is simple and elegant. When a metal such as iron corrodes, it does so through the formation of what is essentially a battery, forming electrically positive and negative areas referred to as anodes and cathodes. During the process of corrosion metal is lost from the anodes – ultimately resulting in the formation of the characteristic rust – while the cathodes remain unaffected. What Davy recognised has since been turned into a workable technology, i.e. if one could make all the metal behave as a cathode, then corrosion would cease. It would be, as Davy himself stated, ‘cathodically protected’.

The first way of achieving this, which is still in use today because of its relative simplicity, is to attach pieces of a more reactive metal that corrodes preferentially to the component to be protected, by acting as a galvanic or ‘sacrificial’ anode. Zinc is the first choice for this role although aluminium and magnesium are commonly used in large marine applications. A more elegant, durable and controllable approach is to use ‘inert’ anodes powered by a low voltage DC power supply which reproduce the behaviour of the galvanic anodes but are not consumed in the same way.

Powered anodes have been in common use in impressed current cathodic protection (CP) systems since the 1950s but the large size of both the anodes and the power supplies (transformer-rectifier units or ‘TRs’ in CP parlance) made them unsuitable for anything other than large buried or submerged structures.

Implementing CP

The ideal impressed anode should be able to pass a large current over many years without losing significant mass. The very best material would be platinum, but there are obvious commercial and less obvious metallurgical reasons why this would be a bad idea in practice. Fortunately by coating titanium with a thin layer of platinum or, better still, a blend of platinum-group metal oxides it is possible to produce very effective anodes at lower cost and better mechanical properties than would be the case for platinum. A similar level of performance can be achieved by ceramic anodes manufactured from titanium oxide.

Such developments in anode technology allowed the commercial application of impressed current cathodic protection to bridges and tunnels from around 1990. While there were now anode systems that could, in theory, be applied to steel framed building structures, there were still problems with reducing the size and complexity of the control equipment and wiring requirements so that they could be discretely integrated into a building environment.

As with most things in modern life, digital technology came to the rescue by allowing multiple small power and monitoring systems to be linked by a minimum of wiring and report back to a central monitoring and control system that can be interrogated via a modem, local area network or the internet.

Steel framed buildings today

The challenge we have in the early part of the 21st century is to provide an affordable and durable solution to the century of cumulative damage and well meaning but inappropriate repairs to the insurance offices, town halls, libraries and banks that form the core of our cities. Cathodic protection promises to be one such solution.

To assist in the eventual development of such standards, research is currently underway to better understand and define the criteria for the cathodic protection of steel framed structures. Part of this work has involved the modelling of the protection currents so that the effect of variables such as masonry type, mortar composition and joint width can be established. Such research has already allowed the type and location of anodes to be optimised so as to ensure full protection with minimum intrusion into the structure.

So, from little more than a Georgian scientific novelty, cathodic protection has already developed into the backbone of corrosion protection in the water, oil, gas and maritime industries. Within only the last few years, developments in anode design and computer-controlled power and monitoring systems have allowed the technology to be applied to sensitive heritage structures and the most demanding of commercial environments with equal levels of success and customer satisfaction.

While cathodic protection has been around for over 180 years and in common use for over 50 years it is only very recently that developments in anodes, power supplies and monitoring systems have made it feasible to consider such technology for applications other than heavy marine, chemical and civil engineering applications.

Typical CP system for steel framed buildings

Cathodic protection systems for steel framed structures typically employ strings of discrete anodes. The anodes are grouted into small diameter holes drilled from either the exterior or interior of the building and interconnected with thin but durable titanium wires buried in masonry joints or chases.

The strings of titania or coated titanium anodes, installed along the line of the steel frame but not touching it, are connected to a low voltage power supply with a connection to the steel completing the circuit.

A small DC current shifts the potential of the steel to a more negative value making it the cathode in the circuit and thereby preventing it from corroding. The applied current and resulting potential can be monitored and adjusted remotely to optimise the performance of the system.

The use of networkable power supplies and monitoring equipment, interrogated via the internet, has greatly simplified both the installation and operation of such systems and made them a viable option for such applications.

Once installed, the system is completely hidden and can operate unobtrusively for many years with the minimum of external interaction.

Biography – Paul Lambert

Paul Lambert is Technical Director of Materials & Corrosion Engineering at Mott MacDonald. He is Visiting Professor and Royal Society Industry Fellow at the Centre for Infrastructure Management, Sheffield Hallam University, where he researches electrochemical methods for the preservation of steel framed heritage structures. He is a Fellow of the Institute of Corrosion and the Institute of Materials, Mining and Minerals, and is a Chartered Materials Engineer.

Further reference

Corrosion Prevention Association Technical Notes 1 to 10, available for free download from the CPA website: www.corrosionprevention.org.uk

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