Resistance coating

In electrochemical protection the necessary range of protection current is achieved by an appropriate arrangement of the electrodes. It follows that measures which raise the polarization resistance are beneficial. Coated objects have a coating resistance (see Section 5.2), which can be utilized in much the same way as the polarization resistance in Eq. (2-45). Therefore, the range in the medium can be extended almost at will by coatings for extended objects, even at low conductivity. However, the range is then limited by current supply to the object to be protected (see Section 24.4). 

In analyzing the results on a cathodically protected pipeline, the protection current density and coating resistances should be calculated for individual sections of the pipeline in addition to the on and off potentials, the pipe current, and the resistances at insulating points and between the casing and the pipeline. The results should be shown by potential plots to give a good summary [15] (see Fig. 3-20). 

Fig. 3-13 Determination of the protection current density and coating resistance of a pipeline (explanation in the text).

The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

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) ... 

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]). 

Table 5-2 Corrosion rate and specific coating resistance r ° after a long exposure of steel pipe sections with damage-free coating.

The coating resistance decreases slightly with time. 

Protection current density and coating resistance are important for the current distribution and for the range of the electrochemical protection. The coating resistance determines, as does the polarization resistance, the polarization parameter (see Sections 2.2.5 and 24.5). For pipelines the protection current density determines the length of the protection range (see Section 24.4.3). 

The cell current is determined by the coating resistance r and the size of the coated surface S. Neglecting the anode resistance, the cell current from these two quantities is given by [see Eq. (2-43)] ... 

Tank/soil potential measurements cannot be made on objects to be protected with very high coating resistances which are found in rare cases of defect-free coating, and particularly with resin coatings. Off potentials change relatively quickly with time, similar to the discharge of a capacitor, and show erroneous values that are too positive [8]. This is the case with coating resistances of 10 Q m. If there are defects, the resistance is clearly much lower. The advice in Section 3.3.2.2 is then applicable for potential measurement. 

The minimum value of G corresponds to the highest value of of the coating resistance according to Eq. (23-39) ... 

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