These early systems were also limited to horizontal application and were too thick and heavy for many situations, such as car-park decks.

Most of these problems were overcome with the development of activated titanium mesh anodes in the late 1980s and during this same period, MBT, now part of the Degussa Construction Chemicals Group, developed a unique patented spray-applied polymer-modified conductive cementitious anode called Emaco CP 60.

Both of these systems were evaluated in the two-year field trial on the Oseberg A platform.

Mesh anode

The mesh anode allows the applied current to be more evenly distributed over the surface of the structure to be protected, thereby greatly reducing the need for high anodic current densities and, consequently reducing the risk of localised acidification and premature anode consumption.

However it is important to note that the activated Ti mesh is only one component of the installed anode system. The mesh has to be mechanically fixed in intimate contact with the concrete surface, after which it is covered by a 20 - 30 mm layer of sprayed gunite.

This cementitious gunite overlay provides a continuous electrolyte through which the protective current passes into the host concrete and through to the reinforcement, in addition to mechanically protecting the mesh.

The gunite must be specially formulated to be electrically compatible with the host concrete and provide the highest possible pH buffer against the anodic acidification reactions. For this reason, mortars including fly ash or micro silica should be avoided, as these additives reduce the buffer capacity of the hardened concrete overlay.

Also, the gunite must exhibit the same longevity as the design life of the installed CP system and so must provide excellent mechanical and physical properties, such as good adhesion, good freeze/thaw resistance and resistance to cracking.

Too often, insufficient care is taken in specifying the overlays to Ti mesh anodes. This, together with high labour and installation costs, high material costs and difficulties fixing the mesh in overhead or onto complex structures, are the main drawbacks of what is now a proven method for the long-term protection of reinforced concrete. This is especially so in areas of high corrosion risk caused by high levels of chloride ions.

Emaco CP 60

This system developed in the early 1990s has also overcome the original problems of early CP anodes. Based upon a specially formulated polymer-modified sprayed cementitious mortar, the impressed anodic current is efficiently and evenly distributed over the whole surface of the structure to be protected.

Primary anode current feeders are fixed at intervals of 2 m, the same for the Ti mesh systems. The structure to be protected is then sprayed with the CP 60 anode. The natural conductivity of the hardened cement paste within this formulated product is enhanced further by a unique interlocking mesh of conductive carbon fibres distributed through the whole mass of the sprayed mortar overlay.

It has long been known that carbon has high conductivity. However, the acidic anodic reactions discussed previously cause rapid consumption of pure graphite anodes, a phenomenon causing some of the early dramatic failures of CP systems. This drawback has been completely overcome in CP 60 anode by individually coating each 6-m diameter fibre with an inert, highly-conductive nickel coating. The electron micrograph of a section of hardened CP 60 anode shows the distribution of the fibres and the nickel coating (see picture).

At typical operation current densities of 5 - 20 mA/sq m of concrete, the life expectancy of these fibres will exceed 30 years. Because the fibres are distributed through the whole mass of the mortar, a three-dimensional anode has been created which totally eliminates localised acidification caused by 'hot spots' of high current density. Therefore, interfacial acidification and adhesion problems are also eliminated. The use of polymers and presence of the carbon fibres also significantly improve the mechanical properties of the spray-applied mortar even at its recommended thickness of 8 mm. Low thickness, low weight, ease of installation and relatively low material costs are all significant advantages of the CP 60 anode.

Oseberg trial

In order to determine the feasibility of CP to extend the working life of the Oseberg oil platform, two anode systems were evaluated: a proprietary Ti mesh with sprayed concrete overlay and the Emaco CP 60 anode. The area of the trial was chosen with a high steel density 3 - 5 sq m of steel to 1 sq m of concrete surface. Each trial area consisted of 25 sq m divided into four anodic zones.

A Ferroeye transformer rectifier and control unit from Cyberdan, operating in constant current mode, was chosen for power supply and performance evaluation measurements. MnO2/NaOH/cement reference electrodes, supplied by Force Institut, Denmark, were used for recording reinforcement potentials, polarisation and depolarisation measurements.

Trial results

Both systems were energised in March 1999 and were evaluated for almost two years. The systems were energized in 6 mA increasing steps, corresponding to the minimum current output of the Ti. The aim was to achieve full cathodic protection using the criteria of 100 mV depolarisation within 24 hours of switching off the current. Table 1 provides data from the job site at the current setting of 18 mA.

The 100 mV depolarisation has not yet been achieved by either system although it is clear that this would be the case at 24 mV. Table 2 shows typical operating current densities for both anode types.

It should be noted that the actual operating current densities, even in this structure, which has a high steel density, are all below 5 mA/sq m of concrete surface.

In fact, this figure is typical of CP installations but is much lower than figures used by some design engineers. It must be remembered that the lower the applied current the better, as this will prolong the operating life of the system substantially.

Conclusion

From the results, engineers concluded that both anode systems were functioning equally electrochemically and that both would provide a similar and satisfactory level of protection to the structure as required.

However additional tests measuring the adhesion of the two systems showed a different picture. The results obtained are indicated in Table 3.

The inevitable conclusion from Table 3 is that localised acidification around the Ti mesh has caused some weakening effect that has resulted in tensile failures at the mesh/overlay interface. In the harsh environment of the North Sea it was considered important that the physical durability of the installed system was also sufficient.

The engineers also reported that the Emaco CP 60 anode was easier to install and had a lower overall cost.