Protection with Impressed Current

Cathodic Protection with Impressed Current Anodes... 

Cathodic protection with impressed current anodes is used predominantly with cables or steel casing in which the cable is inserted, outside built-up areas where it is possible to build large anode installations without damaging interference with other lines. In densely populated areas, protection with impressed current anodes is often only possible with deep anodes, with surface anodes or locally at individual problem points (local cathodic protection, see Chapter 12). 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

Today loading piers are mostly cathodically protected with impressed current. At moorings for tankers, cathodic protection rectifiers are installed on extinguisher bridges as far as possible from the hazardous area. Otherwise, they must be of an explosion-proof type. 

In protection with impressed current installations, it depends on the conductivity of the harbor water whether olf potentials can be measured and also whether the IR component can be taken into account. Only very few inland harbors require such expenditure usually the conductivity is sufficiently high. 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

Anodic Protection with Impressed Current 21.4.2.1 Preparatory Investigations... 

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. 

Examples of cathodic protection with impressed current are, at the present time, protection of steel pipelines in maritime environments or in subsoil. An important example of anodic protection is in the storage of acids in steel tanks—the anodic current passivates the steel (see Fig. 16.1a). 

The high-alloy silicon irons are used in draining pipelines, pumps, valves, other process equipment, and anodes for cathodic protection with impressed current. 

Figure 10.16 Schematic potential-distance diagram for cathodic protection with impressed current. Definition of Ea, E and AE as in Figure 10.15. Pj = electrical potential in the anode material, Pstmcture = electrical potential in the structure (the cathode material), Pj = the terminal voltage of the external current source.

Figure 11.7 Principle of cathodic protection with impressed current (schematic).

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