Impressed current system

The grounding resistance of different types of anodes can be calculated from the equations in Section 24.1 (see Table 24-1). The use of magnesium anodes is convenient and economical for relatively small protection currents. In the case of an increase in the protection current demand, because the voltage is fixed at about 0.6 V, the current can only be raised by lowering the grounding resistance of the anodes, i.e., by installing more anodes. Alternatively, the voltage can also be increased by an impressed current system. 

In contrast, impressed current installations have the advantage of a choice of voltage, so that the protection current can be imposed stepwise or without steps. Formerly, impressed current systems were only used for protection currents above 0.1 A. Today impressed current installations are employed almost exclusively. 

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. 

A particular advantage of impressed current systems is the ability to control the output voltage of the rectifier. Also, there are the comparatively low installation costs and relatively uniform current distribution. The costs of impressed current protection compared with aluminum anodes are 0.8 1. With ships this ratio depends on the length of the ship with larger ships it is 1 2.5 since the calculation is made in comparison with zinc and aluminum anodes. The order of magnitude of the annual costs depends on the structure and the investment costs. 

The various types of materials used as anodes in impressed-current systems may be classified as follows ... 

Failures of impressed-current systems may occur not because of anode failure in a specific environment but because of poor integrity of the anode/cable connection or the use of an inferior cable insulation. Particular... 

The use of an impressed-current system or sacrificial anodes will both provide satisfactory cathodic protection, but each has advantages and disadvantages with respect to the other (Table 10.24). 

Economics After evaluating these variables, it must then be decided which type of system, i.e. sacrificial anode or impressed current, would be the most economical under the prevailing conditions. For instance, it would obviously be very expensive to install an impressed-current system on only 100 m of fire main. Similarly, it would be equally uneconomic to install a sacrificial-anode... 

Impressed-current systems for power stations are somewhat more sophisticated than those required for pipelines or marine structures inasmuch that a large number of items of plant, with a wide range of current requirements, are protected by one transformer-rectifier. Each section of every water box in order to provide even current distribution requires one or more anodes. In the case of a large circulating water pump as many as 30 anodes may be required to provide the current distribution necessary. Three types of system should be considered as follows ... 

Figures 10.29a and b give an indication of the relative numbers of anodes that may be involved for sacrificial anode and impressed-current systems.

Impressed current systems are normally based upon anodes of silicon iron, platinised titanium or platinised niobium. The method of anode installation is usually by suspension. The anode configuration and number must be such as to ensure uniform current distribution. Considerable use is made of wire-type platinised-titanium, and niobium anodes which offer minimal weight and relative ease of mounting/suspension. 

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. 

Steel with cathodic protection, using either dielectric coating, field installation by a corrosion expert, or impressed current system. 

The two types of cathodic protection are (i) sacrificial anode and (ii) impressed current systems. 

Figure 7.26 Impressed current system line diagram...

The principle of an impressed current system involves the supply of a protective current from some direct current power source, e.g. storage batteries, rectifiers or d.c. generators, through an auxiliary anode. Fig. 15.4 illustrates the arrangement. 

Impressed current systems require low-voltage high-current DC power. The voltages are typically 12, 25 and 50 volts. The currents are typically 100 to 800 amperes from one unit. The power is supplied by transformer rectifier units in which the transformer coils and the power rectifier are usually immersed in insulating oil to improve heat removal. The AC supply is usually three phase at LV voltage, e.g. 380 to 440 volts, and the supply power factor is about 0.75 lagging. 

Up to now several million square metres of concrete have already been treated worldwide with impressed current systems that utilise a variety of anodes. 

The most common source of electricity for impressed current systems is a local power utility. Power normally involves the DC rectifier arrangement. Remote locations can use solar cells, thermoelectric current sources, special fuel-driven electric generators, or even windmills. Impressed current systems are preferred when current requirements and electrolyte resistivity are high. These systems require an inexpensive source of electrical power, are well suited to long-time operation and large... 

In this technique, the electrical current is delivered to the structure to be protected from a direct current (dc) power source through an auxiliary electrode. The structure acts as a cathode and the auxiliary electrode becomes the anode in the cell [26, 27]. Figure 8 shows an impressed current system used to protect a pipeline. Both the buried anode(s) and the pipeline are coimected to an electrical rectifier, which supphes direct current to the buried... 

It should be noted that in contrast to the impressed current system in which a driving voltage of up to 100 V may be made available, in the sacrificial anode system the maximum driving voltage is controlled by the open circuit potential difference between the anode and the structure with the result that it carmot exceed about 1.0 V. 

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

Impressed-Current Cathodic Protection The impressed-current system consists of the following ... 

Both systems performed well for 14 years prior to removal because of failure of the asphalt overlay and the necessity of widening the structure. Although the sacrificial anode CP systems perform well, the majority of the CP systems on bridge decks are impressed-current systems. 

Impressed-current CP systems are extensively used in transmission pipelines. Impressed-current systems can be readily adjusted to compensate for changes in the amount of current required to adequately protect the structures however, they may also contribute to the interference of other structures in the vicinity. 

CP or other protection systems should be incorporated, if necessary, in the design phase of the ship. CP system is a secondary defense against corrosion when holidays or cracks form in the coating. CP systems use either sacrificial zinc anodes or impressed-current systems to mitigate corrosion. Other corrosion prevention equipment and materials are inert gases to drive out corrosive gases. Corrosion inhibitors are also used. 

The two types of cathodic protection are (i) sacrificial and (ii) impressed current systems. The sacrificial anode system typically uses magnesium, zinc, or aluminum and their alloys (Fig. 5.25). These metals or alloys act as anodes when coupled with steel and its alloys. These metals or alloys act as anodes when coupled with steel and preferentially corrode. Magnesium is often used in fresh water media while zinc and aluminum are used in seawater and brackish water media. 

Based on the polarization used to protect the structure, the CP systems are divided into sacrificial anode and impressed current systems (ICS). 

As shown in Fig. 15.6, external DC current is supplied from a power source such as a rectifier. The external DC current is used to cathodicaUy polarize the pipeHne. Impressed Current System (ICS) can be used to protect bare and poorly coated pipeHnes because of high current capacity. The anodes are made of durable materials that resist wear or dissolution. Iron with 14% silicon, carbon, and graphite are some commonly used anodes for pipeline protection [17,18]. All impressed current CPs require routine maintenance because they involve a power supply and more electrical connections than sacrificial systems. 

J.A. Jakobs, A comparison of anodes for impressed current systems. Mater. Performance 20 (1981) 17-23. 

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