This chapter derives important equations for corrosion protection that are relevant to the stationary electric fields present in electrolytically conducting media such as soil or aqueous solutions The equations are also applicable to low frequencies in limited areas, provided no noticeable current displacement is caused by the electromagnetic field.. For the simplest case, the grounding resistance of a spherical anode in surrounding space is derived. The major part of the resistance lies in the soil in the immediate vicinity of the anode. The total grounding resistance of the anode—that is, the resistance between its lead and the infinitely large and distant remote ground, is composed of three terms: (1) the resistance in the lead and the anode itself, which usually is so small that it can be neglected, (2) The transition resistance between the surface of the metal and the electrolyte, and (3) The grounding resistance that is given by the current and potential distribution in the electrolyte. If anode installations for cathodic protection consist of several individual anodes of length, spacing, and grounding resistance, then these are usually spaced far enough from each other so that their mutual interference can be treated by the potential distribution of a spherical anode.

Corrosion protection measures are divided into active and passive processes. Electrochemical corrosion protection plays an active part in the corrosion process by changing the potential. Coatings on the object to be protected keep the aggressive medium at a distance. Organic coatings applied primarily to protect pipelines and storage tanks include paints, plastics, and bituminous materials. All organic coatings show varying degrees of solubility and permeability for components of the corrosive medium, which can be described as permeation and ionic conductivity. An absolute separation of the protected object and the medium is not possible because of these properties. Cement coatings are usually applied as linings for water pipes and water tanks, but occasionally also for external protection of pipelines. Cement is not impervious to water, so electrochemical reactions can take place on the surface of the object to be protected. In addition, enamel coatings are applied for internal protection of storage tanks. Enamel is impervious to water—that is, it separates the protected object and the corrosive medium. Metal coatings are applied in special cases where the protective action has to be ensured by the coating metal or its corrosion products.

Cathodic protection of ships extends from the external protection of the underwater area including all attachments and openings, to the internal protection of various tanks, pipework, and bilges. Ships are exposed to water of very different compositions. The salt content and conductivity are particularly important because they have a profound influence on the action of corrosion cells. In addition, on ships problems of dissimilar metals must be considered. A distinction is made between complete and partial protection of the underwater area, depending on the extent of the protected region. In partial protection, only the stern is protected; it is particularly endangered because of the high flow rate and aeration as well as the formation of cells on attachments such as the propeller and rudder. Partial protection can also be extended to the bow, which also experiences high rates of flow. The complete protection of the ship with galvanic anodes or impressed current is becoming increasingly important because defects in the coating due to mechanical damage are more frequent at the bow and amid ships. The extent to which attachments such as propellers and rudders can be covered by the protection or be given individual cathodic protection depends on the design of the ship and the method of protection. The design of cathodic protection must involve the calculation of the Protection Current Requirement, Protection by Galvanic Anodes, and Protection with Impressed Current.

The chapter discusses the deficiencies in the thickness and the density of concrete structures that require additional corrosion protection measures in a construction process. The corrosion protection of reinforcing steel in steel-reinforced concrete and prestressed concrete structures acts through the alkalinity of the porous concrete water content because the steel passivates under many conditions. The application of cathodic protection to reinforced concrete structures requires proper technical and economic considerations such as design and installation; determination of the state of corrosion of the reinforcing steel; assuming an extended electrical continuity through the reinforcing steel; making use of different concrete replacement systems for cathodic protection and proper maintenance; and commissioning and control of the cathodic protection. Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is a positive experience in the case of a direct connection without coated pipes; this is the protection of buried prestressed concrete pipelines by zinc anodes.

Electrochemical corrosion protection of the internal surfaces of reaction vessels, tanks, pipes, and conveyor equipment in the chemical, power, and petroleum industries is usually carried out in the presence of strongly corrosive media. The range stretches from drinking water through more or less contaminated river and seawater frequently used for cooling, to reactive solutions such as caustic soda, acids, and salt solutions. Galvanic anodes are used for smaller objects to be protected such as water heaters and feedwater tanks. Large objects, such as power station condensers and inlet structures, are mainly protected with impressed current on economic grounds. Galvanic anodes are, however, also installed if an account has to be taken of protection against explosion and safety regulations regarding products of electrolysis. Areas of application include tanks for ballast, loading, petrol, water, and exchange tanks of ships as well as crude oil tanks. The choice of protection method and protection criteria should always be properly tailored. Coatings are usually applied to reduce the protection current requirement as well as to improve the protection current distribution in cathodic internal protection systems because they raise the polarization parameter. If the polarization parameter lies in the same range as the protected object, the current distribution is sufficiently good.

In the context of this chapter, “ac interference” refers to the action of a high-voltage power plant on a pipeline by capacitive, ohmic, and inductive coupling. The results in frequent crossings and contiguity, and in stretches of lines running parallel for several kilometers, can lead to ac interference. This results not only in danger to the pipeline maintenance personnel, but also in damage to the cathodic protection. There are various kinds of interference: (1) capacitive interference (the production of electrical potentials in conductors due to the influence of alternating electrical fields), (2) ohmic interference (the production of electrical potentials in conductors by electrical contact), and (3) inductive interference (the production of electrical potentials in conductors due to the induction from alternating magnetic fields). Depending on the length of time the interference processes are active, a distinction is made among short-term and long-term interference. Short-term interference is a rare occurrence resulting from a failure in a high-voltage installation and very high interference currents. Long-term interference is a continuing action that involves lower voltages than short-term interference. To avoid disruption or danger to installations and people, recommendations for avoiding interference should be taken into account in planning pipelines and high-voltage installations. These should include regulations for routing energy corridors, measures to be taken in the construction and operation of pipelines, and the methods used to determine pipeline potentials under long- and short-term interference and safety requirements.

Cathodic protection of long pipelines, distribution networks, pipelines in industrial plants, and other buried installations with a high protection current requirement is achieved mostly by means of impressed current anodes. With a high protection current requirement, the grounding resistance of the anodes, as the highest resistance in the protection current circuit, determines the necessary rectifier voltage and also the power of the protection station. The lowest grounding resistance should be designed to keep down the electric power and therefore, the operating costs. The protection current is transmitted to the soil via the impressed current anode bed. Therefore, the current density and the field strength, the voltage drop per meter, are greatest in the vicinity of the anode bed; they decrease with the increase of distance from it. Where there is available ground and the specific resistivity of soil in the upper layers is low, the anodes are laid horizontally. Deep anodes are installed where the resistivity is high in the upper layers of soil and decreases with increasing depth. This type of installation is recommended for densely populated areas and for local cathodic protection on account of the small space needed and the smaller voltage cone, which avoids interference with foreign structures.

The greatest possible safety against corrosion damage is achieved by passive protection with coatings in combination with cathodic protection. Therefore, coating and cathodic protection of pipelines that have strong safety requirements are compulsory to protect both people and the environment. The cost and economics of cathodic protection depend on a variety of parameters. Because of this, it's hard to come up with general statements on costs.. In particular, the protection current requirement and the specific electrical resistance of the electrolyte in the surroundings of the object to be protected and the anodes can vary considerably, thereby affecting the costs. Usually, electrochemical protection is particularly economical if the structure can be ensured a long service life, maintained in continuous operation, and if repair costs are very high. Electrochemical protection often allows older structures to be kept functional that otherwise would have to be replaced due to local corrosion damage. In certain cases the operation has been technically possible only by using economical materials protected electrochemically.

The chapter discusses the internal cathodic protection of water tanks and boilers, which is most economical if it is taken care of at the design stage. It also details the description and function of objects to be protected, such as boilers with enable linings and electrolytically treated water and water storage and filter tanks. In uncoated cathodically protected tanks, inserts must have an electrical connection to the tank to avoid damage due to anodic interference in the cathodic voltage cone of the surfaces of the object to be protected. Materials for metal tanks and installations include plain carbon steel, hot-dipped galvanized steel, stainless steel, and copper and its alloys. The corrosion resistance of these materials in water is very variable. Plain carbon steel that is uncoated is not corrosion resistant. Stainless steel can suffer chloride-induced localized corrosion. The rest of the materials are corrosion resistant due to the formation of protective films but can be attacked if the films have defects locally. Different linings and coatings are used to provide corrosion protection for the tank, which often makes economical cathodic protection possible. Organic coatings include phenol formaldehyde, epoxy, polyacryl and polyacryl acid resins, polyamide, polyolefin, bitumen and rubber. Cathodic blistering is not expected with high-resistance polyolefins and rubberized coatings; however, where there are defects, local reduction in adhesion (cathodic disbonding) is apparent. Enamel linings have the best resistance against cathodic influence.

The basis of conventional cathodic protection is the electrical separation of the protected object from all installations that have a low grounding resistance. This separation, however, creates technical difficulties in industrial installations because of the large number of pipes with very large nominal diameters. Technical difficulties arise in installations where there is a danger of explosion and with pipelines that transport electrolytic solutions. Local cathodic protection has the objective, not only of compensating for the cell current of the foreign cathodic structures, but of sufficiently cathodically polarizing the protected object so that the protection criterion is fulfilled. A disproportionately large part of the protection current flows to the foreign cathodes due to the low contact resistance between the protected object and the foreign cathodic structures, and the very low grounding resistance of the latter. In setting up the impressed current anode beds, the objective is to increase the part of the current for the protected object. In contrast to conventional cathodic protection, the protected object lies mainly within the voltage cone of the impressed current anodes.