Cathodic Protection

Cathodic protection (CP) is a widely used electrochemical method for protecting a structure or important components of a system from corrosion. A CP system is essentially an electrochemical cell and must have a cathode, an anode, an electrical connection between them and an electrolyte. The principle behind CP is that dissolution of a metal (cathode) can be suppressed by supplying it with electrons, and in effect, controlling the corrosion. Corrosion is then targeted on the anode instead of the metal. Since an electrolyte is required for this method of protection, CP is not effective for systems in air or other environments that resist current flow between the anode and cathode.

There are two main classes of CP: active and passive. Active cathodic protection, also called impressed-current, requires the use of an external power supply. In this type of protection, the negative terminal of the power supply is connected to the metal to be protected, and the positive terminal is connected to an inert anode. The anode, however, is often not more anodic to the metal, and can be even more cathodic than the metal. The impressed-current ensures that current flows such that the metal acts as the cathode and is therefore protected from corrosion. Moreover, the anodes are not typically consumed by corrosion in impressed-current CP systems, since they do not undergo the typical corrosion reactions. It is possible to overprotect a system using impressed-current CP. If the voltage is too high, the metal can experience hydrogen embrittlement (e.g. steel) or possibly accelerated corrosion (e.g. aluminum). Therefore, proper conditions for the system should be determined in order to optimize the protection.

Passive CP systems are simpler than impressed-current systems and involve the galvanic coupling of the metal to be protected to a sacrificial anode, which corrodes preferentially. The anode in this type of system must be more anodic than the metal and must also readily corrode without passivation in order for the system to be successful. In some instances the sacrificial anode must be replaced after it has been consumed to ensure protection of the structure. A comparison of the characteristics of the active and passive CP systems is provided in Table 56.

There are several anodes that are available for use in cathodic protection applications. For passive CP systems magnesium, aluminum and zinc are commonly used. Characteristics of some sacrificial anodes are given in Table 57. Furthermore, there is a variety of anodes available for active CP systems. These include high-silicon cast iron, graphite, polymers, precious metals, lead alloys, and ceramics. Table 58 gives a comparison of the consumption rate between various sacrificial and impressed-current anodes.

Impressed current cathodic protection is sometimes not practical, such as when the metal is in an extremely corrosive environment, which would require a prohibitively high current. Therefore, CP is sometimes used in conjunction with other protection methods in order to enhance the level of protection and avoiding an impractical system. It is common for pipelines, for example, to be coated with an organic coating, and CP is used to protect the structure from corrosion where there are weaknesses or defects in the coating, known as holidays.

A notable disadvantage of CP, specifically active CP, is the resulting stray-current effects it may impose on nearby systems or structures. Stray currents can be picked up by metallic components or structures that are in close proximity to the CP system, potentially resulting in accelerated corrosion of that metal component or system, as depicted in Figure 43.

Cathodic protection systems are usually designed and implemented by a company that specializes in this field. Choosing the right system and then designing it is not a straightforward process, and usually requires expert knowledge to determine what is best for a specific system in a specific environment. Therefore, it is generally recommended that an expert company be contracted or at least consulted to do such work.