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6 CHAPTER TWO CATHODIC PROTECTION TECHNOLOGY There are two distinct types of chemical reactions that occur IMPRESSED CURRENT CATHODIC on the surface of metals embedded in concrete, anodic and PROTECTION SYSTEM cathodic. These reactions are electrochemical in nature and, as the name suggests, electrical energy is associated with Impressed current cathodic protection is achieved by driving a these chemical reactions. These reactions occur at the metal/ low voltage direct current from a relatively inert anode material concrete interface. The loss of metal (i.e., corrosion) occurs as through the concrete to the reinforcing steel. Direct current a result of the anodic reaction. The rate of these reactions is of sufficient magnitude and direction is applied to shut down controlled by the magnitude and the direction of the local the anodic reaction and support the cathodic reaction on electric field and other factors. A cathodic protection system the steel surface. The direct current is supplied by an external applies an electric field such that it favors the cathodic and power source, most often a rectifier that converts alternat- deters the anodic reactions. When the magnitude of the applied ing current to direct current. Recently, solar power and spe- electric field exceeds the threshold for the local environment, cially designed batteries have been successfully used as an the anodic reactions stop; that is, corrosion stops. external power source (12). The direct current is distributed to the reinforcing steel by the anode. Figure 1 shows the basic layout required for impressed current cathodic protec- The material that imposes the electric energy on the metal tion systems. to be protected is called an anode. It is the primary compo- nent and, generally, a cathodic protection system is described by the anode material it uses. The strength of the electric field There are various different materials and configurations and the resistance of the system control the magnitude of that can be used as anodes in impressed current cathodic pro- the electric current that flows in the system. The principal tection systems. In some systems only one anode material is requirement of the anode material is that it have the capacity used and in others more than one is used. When more than to transfer the electrical charge from its surface to the elec- one anode material is used, the material that receives power trolyte. The electrolyte is a conductive solution, such as pore from the external power source is called the primary anode. water in concrete, through which the cathodic current flows It is important that the primary anode have as low an electri- to the surface of the metal to be protected. In the process of cal resistivity as possible so that current can be distributed to transferring the current, the anode will corrode (i.e., will be longer distances with minimal loss. The secondary anode, consumed). Therefore, the slower the consumption rate of which has a much larger surface area, receives current from the anode per unit of cathodic current, the longer it will last. the primary anode and distributes it uniformly over the area In addition, the anode must be durable in the environment it to be protected. is to be used in and be able to withstand the loading it may be subjected to. For example, the anode material placed on the There has been an evolution in anode materials for use on deck of a bridge must be capable of withstanding the weather reinforced concrete structures. During the developmental and the traffic. The lower the electrical resistivity of the anode process, some of the anodes did not perform as expected and material, the larger the surface area of the concrete element it were eventually eliminated. Unfortunately, agencies initially can uniformly distribute the current to. experimenting with the cathodic protection systems used some of these underperforming anodes and were left with an There are two different ways of imposing an electrical field unfavorable impression of the cathodic protection technol- on the metal to be protected. The one termed impressed current ogy. Listed here and defined are anodes that have been used uses an external electrical power source to drive a current in impressed current cathodic protection systems. through the anode toward the metal to be protected. In the other method, galvanic cathodic protection, another metal (anode), Platinum niobium wire: This anode is comprised of a cop- which is more electronegative than the metal to be protected is per core, a niobium substrate, and a platinum cladding. Plat- placed in its vicinity and electrically connected to it. The dif- inum forms the surface of the wire and is an excellent anode ference in the natural electrical potential between the two mate- material with a very low corrosion rate and does not form rials in the given environment generates an electrical field to an insulating layer in most electrolytes. The niobium sub- drive the protective current that flows from the anode to the sur- strate is used to provide dimensional stability and the cop- face of the metal to be protected. per core is used for its high conductivity and lower price. As

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7 RECTIFIER Repair (NEG) (POS) Embedded Area & Impressed Current Overlay Anode System Negative (Structure Ground) Original Concrete Reinforcing bars Continuity (Cathode) Bond FIGURE 1 Basic layout of an impressed current cathodic protection system. the dimensions of this anode are very small (see Figure 2), it Zinc: It is one of the most widely used anode materials and requires either a very conductive electrolyte or a secondary can be used as either an impressed current or a galvanic anode. anode to distribute current over larger areas. Although zinc is available in many different configurations, in impressed current systems it has generally been applied Carbon fiber: A fiber comprised of graphite, which is very to the concrete surface as a thin metallic coating using the arc similar in application to the platinum niobium wire, except its spray technique. conductivity is not as good (see Figure 3). Aluminum­zinc­indium alloy: This alloy can be used as an impressed current anode. It is applied to the entire surface of the concrete element using arc spray technique. Mixed metal oxide: This uses titanium as a dimensionally stable base material, which is protected by thin, self-healing, tightly adherent oxide films. It is acid resistant and resists the passage of current in the anodic direction. The mixed metal oxide coating functions as the anode. The mixed metal oxides, formed on the surface of titanium through a process FIGURE 2 Platinum niobium wire anode. FIGURE 3 Carbon fiber anode.

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8 FIGURE 5 Ceramic tubular anodes. appropriate additives and coke breeze as the filler material. It has excellent freeze­thaw durability, bonds to concrete, and has electrical resistivity in the 10 ohm-cm range. Conductive coatings: These coatings are essentially paints with graphite added to improve conductivity. The available anode materials can be used in various com- binations and configurations to meet the requirements of the FIGURE 4 Mixed metal oxide mesh anode. structure. Combinations of anodes and configurations that have been used to date are discussed here. of thermal decomposition, have good electrical conductiv- ity and anodic properties. The wear rate of these anodes is On bridge decks, the configurations used to date can be extremely low, uniform, and constant over all current densities. categorized as follows: These anodes are supplied as expanded mesh (see Figure 4) and ribbon. A solid ribbon is also available. 1. Conductive overlay systems. 2. Non-overlay slotted. Cast iron anodes: High silicon cast iron in various geometric 3. Non-conductive overlay. shapes has been used as an anode material. These anodes were only used in the asphalt coke breeze overlay systems and are The only type of conductive overlay that has been pro- not used anymore. moted to date is the coke breeze overlay. Conductive coke breeze overlay systems use silicon­cast iron plate anodes Conductive rubber: The conductive rubber anode is manu- placed on the deck surface or in recesses on the concrete deck factured from ethylene­propylene­diene monomer containing surface. A conductive asphalt overlay is then placed. This is 25% by volume acetylene black conductive carbon and is followed by placement of a conventional bituminous mixture, produced as sheets with corrugation on one face. which serves as the wearing course. Ceramics: These are supplied as tubular anodes and are The slotted non-overlay system requires sawing slots into manufactured from ceramic/titanium composite (see Figure 5). the concrete, which form a uniform grid over the entire sur- These anodes are designed to provide protection in a local face (see Figure 6). Anodes are then placed in the slots, which area and do have good characteristics as an anode material. are backfilled with a conductive material. Several different anodes can be used in the slots and they include platinum nio- The following have been used as secondary anodes on bium wire, carbon fiber, and mixed metal oxide ribbon. The reinforced concrete structures: slots are backfilled with an FHWA conductive polymer ma- terial when the primary anode is the platinum niobium wire or Conductive polymer grout: This material was developed the carbon fiber. A cementitious backfill material can be used by the FHWA and is manufactured with vinyl ester resin with with the mixed metal oxide ribbon anodes. The older systems

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9 material over the entire surface of the concrete. The most common surface-applied anodes are conductive paint and thermally sprayed zinc. The conductive paint anode is applied by spray or roller and a decorative overcoat is then applied, if desired, for aesthetic purposes (see Figure 8). A thin layer of zinc is applied to the concrete surface, often using the arc spray technique. On hammerheads and caps, mixed metal oxide anode encapsulated in shotcrete has been attempted. In one application, the mixed metal oxide anode was placed on the underside of a roadway of a tunnel and encapsulated with shotcrete. In another, mixed metal oxide mesh was installed on the top and bottom surfaces of historic arches and encapsulated with shotcrete. The aluminum­ zinc­indium alloy has also been installed using the arc spray method. FIGURE 6 Slotted non-overlay system (Platinum niobium wire anode and FHWA conductive polymer). On bridge substructure elements the configurations used can be categorized into: in Missouri were overlaid with a wearing surface of asphalt 1. Surface applied, and the newer ones are overlaid with concrete. 2. Encapsulated, and 3. Non-encapsulated. Several different combinations of anodes and configura- tions have been used with a non-conductive overlay. One of the earlier designs used a grid of anodes on the concrete sur- face, usually platinum niobium wire or carbon fiber encapsu- lated with a mound of conductive polymer to increase the surface area of discharge. A cementitious overlay was then placed to restore the wearing surface. This configuration is not used any more. The other alternative involves placing a mixed metal oxide mesh anode on the surface of the deck as shown in Figure 7 and overlaying it with either a portland cement or latex modified concrete. On the underside of the deck and other superstructure elements such as beams, girders, diaphragms, hammer heads, and caps the surface-applied systems are generally used. The surface-applied systems involve application of an anode FIGURE 7 Placement of mixed metal oxide mesh on a deck surface. A cementitious overlay is normally placed on the mesh FIGURE 8 Conductive paint systems with decorative overcoat anode. on hammerheads and columns.