One major difference between applying impressed current cathodic protection (ICCP) to an atmospherically exposed reinforced concrete structure and a buried or submerged structure is the proximity of the anode to the steel being protected. While anodes can be placed at intervals hundreds of feet from each other or from buried or submerged pipelines, a rule of thumb for anodes in concrete is a spacing of not more than 18 in (0.46 m). The result is that ICCP anodes for atmospherically exposed steel in concrete are very different from anodes for underground and underwater applications. The predominant anode material is mixed metal oxide (MMO)-coated Ti, but it comes in the form of expanded mesh, expanded ribbons, rods, and tubes. The big advantage of MMO Ti is the service life. The current densities for steel in concrete are low compared to other MMO Ti applications, so the anodes can easily be designed for a life of a century or more of active use.
Mesh anodes are generally fixed to the concrete surface and a cementitious overlay applied by casting or spraying. The overlay is generally 1 in (25 mm) thick so this anode increases the load and the cross section of the structure, and changes the surface finish. The interface between the overlay and the parent concrete can be the weak point of this system. Cables can be embedded in the overlay and are, therefore, protected from damage.
Ribbon anodes can also be applied with an overlay but they frequently are embedded in chases cut into the concrete surface and backfilled with mortar. This avoids changes in the loading and dimensions but does change the finish. Ribbons are generally spaced 12 to 18 in (0.3 to 0.45 m) apart. Care must be taken to avoid short circuits if the cover over the steel is low.
Rod or tube anodes are embedded in holes in the concrete. These are also on a 12- to 18-in spacing. A titanium wire, usually in a backfilled chase, connects the anodes into groups/zones. The potential for short circuits to the steel must also be given particular attention when drilling the holes. One proprietary anode that does not use catalyzed Ti is a tube of conductive titanium oxide (TiO) ceramic.
In addition to Ti-based anodes, there are also conductive coatings. These mainly use graphite or carbon fibers as a conductive “pigment,” although some have conductive binders as well. These have been very widely used in the past but are now used by a limited number of applicators, mainly in Europe. Coatings are easily applied with no significant changes to load or profile but can be susceptible to the effects of water. Even in the absence of excessive moisture, they are not expected to last for more than about 25 years, although some installed on motorway bridge beams in the United Kingdom in the late 1970s are still working.
Carbon fibers are also used in a proprietary conductive mortar. This has the disadvantages of being an overlay, but is easier to apply than MMO Ti mesh with an overlay and has excellent adhesion. Thermal-sprayed zinc is also used as either an ICCP anode or as a galvanic anode system.
Anode technology is not static. High silicon iron “pancakes” in conductive asphalt and conductive resins in slots were once state-of-the-art but are now obsolete. New designs such as adhesive strips and hybrid ICCP/galvanic anodes are being brought to market. Anodes should be independently tested under NACE International test methods1-2 prior to being used on site. Any CP system should be designed according to suitable standards3-4 by a qualified and experienced engineer.
References
1 NACE TM0105-2016, “Test Procedures for Organic Based Conductive Coating Anodes for Use on Concrete Structures” (Houston, TX: NACE International, 2016).
2 NACE TM0294-2016, “Testing of Embeddable Impressed Current Anodes for Use in Cathodic Protection of Atmospherically Exposed Steel-Reinforced Concrete” (Houston, TX: NACE, 2016).
3 NACE SP0290-2007, “Impressed Current Cathodic Protection of Reinforcing Steel in Atmospherically Exposed Concrete Structures” (Houston, TX: NACE, 2007).
4 BS EN ISO 12696, “Cathodic Protection of Steel in Concrete” (Geneva, Switzerland: ISO).
This article was adapted by John Broomfield, FNACE, from Corrosion Basics—An Introduction, Second Edition, Pierre Roberge ed. (Houston, TX: NACE International, 2006), pp. 201-203 and Corrosion of Steel in Concrete—Understanding, Investigation, and Repair, Second Edition, John Broomfield (London, U.K.: E & F N Spon, 2007), pp. 153-171.