Cathodic Protection

Overview

Cathodic Protection is a corrosion mitigation technique first introduced in the British navy in the 1820’s to protect ships in seawater. This corrosion mitigation technique supplies electrons to the corroding metal to suppress metal corrosion and dissolution. Current is supplied by applying a potential usually relative to a reference electrode to the structure that is more negative than the corrosion potential. One of two methods is used to supply the current (control the potential). The first is to use an external power supply. The second is to galvanically couple the structure to an alloy that is more active electrochemically than the alloy of the structure. Cathodic protection has been used to protect steel in concrete, structures such as tankage and piping that are below ground, and ship hulls and other structures in seawater. Two references which provide introductory information and examples are "Corrosion Engineering", M. G. Fontana, Mc-Graw Hill Book Company, New York, 1986 and "Cathodic Protection", R. Heidersbach, Volume 13A, Metals Handbook, ASM International, 2003. The latter reference provides information on pertinent design calculations. Also, numerous web sites exist with company specific information.

Theory

When a metal corrodes freely as, for example, a piece of steel in the presence of water, the metal undergoes several electrochemical processes. The metal is oxidized to metal ion, solid metal oxide, or metal hydroxide, i.e. the anodic reaction. Balancing that electrochemical reaction is the simultaneous reduction of some species on the metal surface, for example oxygen reduced to water or hydrogen ion reduced to hydrogen gas, i.e. the cathodic reaction. Multiple anodic and cathodic reactions can proceed simultaneously. These reactions proceed to produce the normally measured corrosion rate when the system is allowed to corrode freely, i.e. without external artificial influences. The potential at which these reactions proceed in this manner is the corrosion potential.

When a potential is applied cathodic or active with respect to the corrosion potential, the anodic or oxidation reactions are depressed and the cathodic or reduction reactions are accelerated. The applied current required to cause this effect is the basis for the cathodic protection current. This relationship is illustrated in the following idealized partial polarization curve which shows the overall anodic and cathodic reactions. Note that only the solid portion is observed when a polarization curve is measured. Further information on generating polarization scans can be found in http://www.argentumsolutions.com/tutorials/polexpert_tutorialpg3.html

As shown in the above figure, at the corrosion potential the oxidation and reduction reactions proceed at the same rate. When the potential is artificially changed to the point labeled as the Control potential the reduction reaction rate (current) is much greater than the oxidation reaction rate (current). In order to maintain the potential at that point, the current difference between the two reactions is supplied artificially. This cathodic current is the theoretical ‘’cathodic protection current’’ at that controlled potential. Note that the anodic or oxidation reaction still proceeds albeit at a much lower rate than under conditions in which there is no applied potential.

Practice

The cathodic protection current can be generated in two ways, using an external power source supplying current (impressed current) or coupling the structure to a more active alloy whose corrosion supplies the current (galvanic current).

Impressed Current

Cathodic protection systems that use impressed current might be depicted as in this figure:

The negative terminal of the power supply is connected to the structure to be protected. The positive terminal is connected to the anode which is often made from an inert material e.g. graphite. The power supply provides the current. The circuit is completed through the medium such as soil or water which surrounds the structure and anode. In applications such as protection in soils or seawater the structure is often polarized to a potential relative to a reference electrode. The potential chosen is known to induce protection for that alloy and structure. One of the most common reference electrodes used for this purpose is the copper/copper sulfate reference electrode (http://www.argentumsolutions.com/tutorials/refelectrode_tutorialpg10.html). As an example, a voltage sometimes reported in the literature for protecting steel is approximately -0.85V relative to the copper sulfate reference electrode.

Galvanic Current

Cathodic protection systems that use galvanic coupling to a more active material might be depicted as in this figure:

The structure is directly connected to an anode such as magnesium, both of which are in the same medium. Corrosion of the anode provides the current to protect the structure. The name sacrificial anode is often applied to this type of anode. The circuit is completed through the medium such as soil or water that surrounds the structure and anode. Sometimes measurement of the potential relative to a reference electrode such as the copper/copper sulfate reference electrode (http://www.argentumsolutions.com/tutorials/refelectrode_tutorialpg10.html) is used to guide the number and size of the sacrificial anodes.

Additional Considerations

  • Stray current is current in the earth generated by a source other than the cathodic protection system that affects the structure being protected. For example, in areas where many buried pipelines might exist, the current protecting one pipeline may adversely affect another underground structure nearby.
  • The operating conditions are, for the most part, determined by testing. Appropriate voltages and currents can usually not be determined a priori.
  • The -0.85V vs copper/copper sulfate voltage mentioned to protect steel structures does not compensate for the voltage drop between the measuring point of the reference electrode and the structure. This voltage drop can be large in environments with large electrical resistivity.
  • Reference electrode contamination can become an issue in some environments. A different reference electrode such as a silver/silver chloride reference electrode (http://test.argentumsolutions.com/tutorials/refelectrode_tutorialpg5.html) can sometimes be used as an alternative.
  • Some examples of sacrificial anode materials are magnesium, zinc, aluminum, and aluminum-zinc-tin alloy.
  • Some examples of inert anode materials are silicon-cast iron, graphite, lead, and mixed metal oxides.

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