Location of coating defects

For quantitative evaluation of the extent of the damage, it is convenient to use dc because the coating has a lower resistance with ac because of capacity leakage. 

The following considerations are based on the assumption that the part of coating that is free from holidays acts as an insulator. Two limiting cases are considered a single circular defect and an almost porous coating with numerous small and equally distributed defects. 

The impressed current enters the pipe surface at a defect of radius r. The defect can be considered as a point source at a sufficiently large distance compared to the diameter of the pipe. The potential on the surface of the soil is from Table 24-1, line 5  

Here x is the electrical conductivity of the soil in the vicinity of the defect, which need not necessarily be the same as the average conductivity of the soil, x, in Eq. (3-48). In the area of the defect from Eqs. (3-15) and (3-16) 

Information on defects can be obtained with good approximation from Eq. (3-5 la). The value of AU is all that is necessary for an overview. should be as high as possible to increase the sensitivity. In addition, to eliminate foreign voltages in the soil, it is necessary to switch the polarization current on and off with the help of a current interrupter periods of about 2 s off and 18 s on are convenient. Potential differences independent of the polarization current that are the result of foreign currents or electrode faults (see Section 3.2) are totally excluded by this method. On the other hand, the IR component of a compensation current can also be 

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