Impressed current systems advantages

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. 

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 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). 

Sacrificial anodes are relatively inexpensive, easy to install, and in contrast to impressed current systems, can be used where there is no power supply. The method has the added advantage that there is no expensive electrical equipment to buy and current cannot be supplied in the wrong direction. Sacrificial anodes are very suitable in small-scale applications (Fig. 13.4), though they are also used extensively and with equal effect on large-scale structures (Fig. 13.5). 

These are very commonly used for the impressed current system. A typical vertical anode installation is shown in Fig. 5.27. This type of ground-bed has a low resistivity which is an outstanding advantage. Almost all of the current flow is from the anode to the backfill by direct contact so the consumption of the material is mostly at the edge of the backfill. Vertical anodes are installed with a carbonaceous backfill. 

This sacrificial anode system does not require the use of an external power source as in the impressed current system. A schematic description is shown in Fig. 12.16. Use is made of zinc and magnesium anodes which corrode and supply electrons to steel bars embedded in concrete. The current flow circuitry is same as in the impressed current system. The anode life is, however, shorter than the life of inert anode. As the current generated by the corroding anode is a function of environment, such as temperature and moisture, it is difficult to adjust and control the current. However, a major advantage is that the risk of over-protection which is inherent in impressed ourrent system is minimized. i.e. operator dependence is removed, advantageously. The galvanic... 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

The advantages of galvanic system over its impressed-current counterpart can be enumerated as follows ... 

Impressed-current-type cathodic protection systems have a number of advantages ... 

Galvanic cathodic protection has its own advantages and disadvantages relative to impressed current cathodic protection and the other electrochemical and conventional rehabilitation techniques. The different anode systems also have their own merits and limitations. 

The advantage of this technique over the impressed current is that it does not require a power supply since the structure and the anode are coupled through a wiring system or by mounting the anode on the structure forming a galvanic cell. Table 8.2 lists relevant data for common sacrificial anode materials used... 

The following are the advantages and disadvantages of impressed current anode systems ... 

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