Soils impressed current cathodic

Soil resistivity surveys are often impractical in built-up areas, but in such areas impressed-current cathodic protection is usually avoided on account of the danger of interaction. Under such conditions adequate protection can be achieved by installing magnesium anodes in the pipe trench should the soil resistivity measurements made when the trench is opened indicate that this is necessary. 

A. Deb, D. Sarma, M.K. Banegee, Significance of logarithmic averaging of soil resistivity data for underground pipelines using impressed current cathodic protection system, J. Met. Mater. Sci. 47 (2005) 169-176. 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

The principles of galvanic anode cathodic protection were discovered by Sir Humphrey Davy in 1824. His results were used over the next century or so to protect the submerged metallic parts of ships from corrosion. In the early decades of the 20th century the technology was applied to underground pipelines. Impressed current cathodic protection was developed when it was found that the electrolytes like soils had too high electrical resistance for galvanic systems to be effective. 

In soils, stray current corrosion can be caused by close proximity to other buried metal systems that are being protected by an impressed current cathodic protection system. These stray currents can leak onto a buried aluminum structure at one point, then off at another (where corrosion occurs), taking a low-resistance path between the driven buried aiwde and the nearby structure being protected. (Totmnon bonding of all buried metal systems in close proximity is the usual way to avoid such attack (Ref 39 and 40). 

On the other hand, the costs for an average cathodic protection station for 6 A come to = 40,000 DM according to Table 22-2. For very small installations as, for example, the external cathodic protection of a tank, the costs of an impressed current system where a current supply is already available without cost, with lower current output, can be reduced to about 4000 DM. With larger tanks and greater soil resistivity, the following considerations point to the increased suitability of an impressed current system. 

The decision on whether cathodic protection with impressed current or with magnesium anodes is more economical depends on the protection current requirement and the soil resistivity. This estimate only indicates the basic influence of the different variables. In the individual case, installation costs can vary widely so that a specific cost calculation is necessary for every project. 

A typical soil resistivity survey is shown in Fig. 10.22. Soil resistivities will normally indicate whether a cathodic-protection system is advisable in principle and whether impressed current or sacrificial anode schemes in particular are preferable. It may, as a result of the survey, be considered desirable to apply protection to the whole line or to limit protection to certain areas of low soil resistivity or hot spots . 

Backfill the soil replaced over the pipe in the trench (general connotation). In cathodic protection, special backfills are packed around the anodes. These backfills are selected to lower circuit resistance of the anode for sacrificial anodes a gypsum/bentonite mixture is used, and for impressed-current anodes, coke breeze. 

Drainage (electric drainage) (a) flow of positive current through the soil or the electrolyte solution from the cathodically protected structure to the groundbed of the impressed current system, or (b) protection of an immersed structure from electrochemical corrosion by making an electrical connection between the structure and the negative return circuit (rail, feeder, busbar) of a d.c. electric traction system. 

Lastly, it may be useful to recall that the resistivity has an important role in the field of cathodic protection by impressed current because, especially in soils [18], it helps to determine the real polarization potential of the structure considered. In all of these cases the attainment of the optimum degree of protection depends on the a priori knowledge of the medium resistance between the reference electrodes and the structure to be protected. 

FIGURE 19.12 Schematic view showing impressed-current-type cathodic protection installation for an underground steel pipeline (a) in soil and (b) in seawater [5]. 

In soils with high resistivity, it is advisable to locate the impressed current anodes immediately next to the pipeline [12]. The pipelines then lie within the voltage cone of the anodes. Figure 12-6 shows the arrangement of the anodes for local cathodic protection of a pumping station. The distance of the anodes from the protected objects should be chosen according to Rgs. 9-5 and 9-6 so that the pipe/soil potential is reduced by the protection current to t/ = -1.2 V. The voltage cones of the individual anodes will thus overlap. 

Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

Coatings must be considered for aU applications of steel. Cathodic protection should be considered for steel pipe where soil or groundwater resistivity is less than 10,000 t2-cm, and where steel win be in contact with process streams. Cathodic protection of steel is strongly recommended where resistivity is less than 5000 Q-cm. For aU exposures, steel should be electrically isolated from dissimilar metals to prevent the formation of unfavorable galvanic corrosion ceUs. In areas where abrasive materials are hkely to damage coatings, cathodic protection by impressed current or galvanic anodes may be desirable. 

Impressed current cells in which the soil is the electrolyte are of two kinds accidental and deliberate. An example of a deliberate cell is an impressed current system which supplies cathodic protection to increase the life of a structure by forcing an anodic current to anodes in a remote location. Accidental systems, however, may exist under a variety of circumstances. Any direct current flowing in the soil from any source whatsoever can, if it finds a pipeline or other metal object in its path, collect on the pipe in one area and discharge from it in another. The area where it collects becomes a cathode while the area where it discharges is an anode, and thus corrosion occurs. 

Another approach for reducing corrosion is to employ mechanisms that can modify the electrochemical processes that consume materials. Cathodic protection, either through the use of sacrificial anodes or an impressed current system, can convert a material that normally will corrode quite readily into a material that resists corrosion. This approach, which is the topic of Chap. 13, works very well for protecting fixed assets in contact with potentially corrosive environments such as soils, seawater, or any other electrolytically conducting medium. 

The two systems, impressed current and galvanic anode, are used in protection of steel in soil and water and also used for the protection of steel embedded in concrete. However, the set up of a cathodic protection system for steel in concrete differs from the set up for protection of steel in soil, in the following respect ... 

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