IMPRESSED CURRENT TECHNIQUE

The design and installation method for a buried structure using magnesium or any other suitable anode material as the source of current requires that the wiring system be well insulated with a good electrical resistance material otherwise, the insulation may be destroyed by the anodic reaction products once the anode is energized by a power supply. Also, moisture must exists at the backfiU-soil interface to provide a path for uniform current flow toward the active anode bed. 

Potential measurements of stmctures buried in soil are carried out along the soil surface directly above the pipeline at discrete intervals. Similarly, for immersed stmctures, a submersible RE is placed near and moved along the pipeline. The outcome of theses measurements is to determine if a stmctured is maintained polarized at the design potential. 

Furthermore, the length of the anode-backfiU column, known as active anode bed, must not be chosen arbitrarily because it is related to the current density entering the anode bed. Hence, the backfill current density is simply defined by the ground current divide by the column surface area 

Example 8.1 Suppose that a buried steel structure is to be cathodically protected using a rectifier capable of delivering 1 volt and 5 amperes through the wiring system. Assume that the soil resistance is 80% of the external resistance. Calculate a) the soil resistance when the anode bed is 4-meter long, and 0.10-meter in diameter and b) the current density. 

In summary, the impressed current technique requires the following 

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There are two principal methods of applying cathodic protection, viz. the impressed current technique and the use of sacrificial anodes. The former includes the structure as part of a driven electrochemical cell and the latter includes the structure as part of a spontaneous galvanic cell. 

Using the impressed-current technique the driving voltage for the protec-ive current comes from a d.c. power source. The sacrificial anode technique... 

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. 

A comparison of the sacrificial and impressed current techniques is given in Table 15.2 and is based on Ashworth [1986]. 

A comparison of sacrificial and impressed current techniques for cathodic protection... 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Comparison of Sacrificial Anodes and Impressed Current Techniques It is... 

The application of cathodic protection requires the delivery of electrons to the structure to be protected. This may be achieved by two separate means (1) using impressed-current techniques and (2) by a spontaneous galvanic effect using sacrificial anodes. The two techniques are discussed briefly below. 

Fig. 10.7 Schematic diagram of cathodic protection using the impressed-current technique...

Platinised titanium anodes (titanium carrying a thin surface film of platinum, of the order of 0-0025 mm thick) have proved successful in cathodic-protection systems employing impressed-current techniques, as electrodes for electrodialysis of brackish water, and in many applications where established anode materials suffer significant corrosion. Platinum-coated titanium anodes can operate without breakdown at very high current densities, of the order of 5 0(X)A/m, in sea water, as although the very thin platinum coating may be porous the underlying titanium exposed at the pores will become anodically passivated... 

The use of sacrificial anodes to protect ship hulls has become less favored than impressed current techniques, but it is still foimd on smaller vessels, where the impressed current method is uneconomical. Zinc is the most common anode material for seawater applications while aluminum and magnesium provide a higher voltage for less... 

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