Resistance and Protection Current Demand

The following terms apply to the specific coating resistance which is related to the surface, S r is the value calculated from the specific resistance of the coating material using Eq. (5-1)  

Its determination follows from current and potential measurements  

With large resistances and at high voltages the replacements U = AU and 1 = AI can be made (see Refs. 2-5). At lower voltages and for buried objects (f/on - = AU can be replaced with / = A/ [8]. Such a measurement is described 

The r values from Eq. (5-1) for the most important materials for pipe coatings are given in Table 5-1. Table 5-2 contains results of long-term field experiments. For comparison, the values of r°are included in Table 5-1. It can be seen that values are always smaller than r values, which is apparently due to the absorption of water when the coating is immersed in the medium. A marked reduction in the coating resistance has been observed with increasing temperature for resins [9,13,14] (see Fig. 5-2 [14]). 

The coating resistance decreases slightly with time. 

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