Cathodic protection current density

Typical Cathodic Protection Current Density Ranges.111... 

FIGURE 10.3.17. Primary current distribution profiles for various MDC cathode geometries. (Cathodic protection current density = 0.099 A m. )... 

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

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). 

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... 

If, however, it is assumed from Eq. (2-40) that the protection current density corresponds to the cathodic partial current density for the oxygen reduction reaction, where oxygen diffusion and polarization current have the same spatial distribution, it follows from Eq. (2-47) with = A0/7 ... 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

A check on the cathodic protection of the pipeline should be carried out annually according to Section 10.4, where, of course, only the on potential should be measured. This value should also be compared with the values of the measurements in Section 10.4. If there are no changes in the on potentials and the protection current densities for the individual sections of the pipeline, it can be concluded that the off potential has not changed. The values can easily be compared using computers and represented in plots. If the protection current and potential distribution have changed, or in any case every 3 years, the off potentials as well as the on potentials should be measured. 

For economical and complete cathodic protection against external corrosion without harmful effects on nearby installations, the storage tank to be protected must have good coating and therefore require a low protection current density. In addition, it must have no electrical contacts with other buried installations, such as... 

At the relatively low protection current density of 200 llA m and with the anode positioned on one side, it is to be expected that with this storage tank sufficient reduction in potential would be achieved on the other side of the tank from the anode. The off potential was measured using a measurement point at a depth of about 2 m as f/cu-cuso4 = -0.88 V at the tank. At the other side of the tank as well as above it, off potentials of-0.90 to -0.94 V were found. These potentials were measured with a protection current of 10 mA (anode 1 6 mA, anode 2 4 mA) with an additional resistance of 8 Q in the protection current circuit (see Fig. 11-2). With a direct connection between the tank and the group of magnesium anodes, the initial current was about 16 mA, which after 1 h of polarization decreased to about 14 mA. The reserve current, based on a long-term current of 10 mA, amounted to ca. 40% in the operation of the cathodic protection installation. 

Cathodic protection of different materials in installations of dissimilar metals is only possible if the protection potential ranges of the individual materials overlap. Section 2.4 gives information on the protection potential ranges of various systems. If there is no overlapping, then insulating couplings must be installed. This is also appropriate and even necessary if the protection current densities are very different. 

Structures or pits for water lines are mostly of steel-reinforced concrete. At the wall entrance, contact can easily arise between the pipeline and the reinforcement. In the immediate vicinity of the pit, insufficient lowering of the potential occurs despite the cathodic protection of the pipeline. Figure 12-7 shows that voltage cones caused by equalizing currents are present up to a few meters from the shaft. With protection current densities of 5 mA mr for the concrete surfaces, even for a small pit of 150 m surface area, 0.75 A is necessary. A larger distribution pit of 500 m requires 2.5 A. Such large protection currents can only be obtained with additional impressed current anodes which are installed in the immediate vicinity of the pipe entry into the concrete. The local cathodic protection is a necessary completion of the conventional protection of the pipeline, which would otherwise be lacking in the pit. 

After a few years the tank base can no longer be cathodically protected by the initial protection current density of 3 mA m Only after installing additional anodes can the tank base be again cathodically protected with protection current densities of 10 mA m" with a total protection current of 80 A. The reason for the high protection current requirement is unsatisfactory insulation of the tank foundation. 

Table 16-3 Protection current densities for the cathodic protection of uncoated steel [1-3]...

In planning cathodic protection, the specific resistivity of the water, the size of the surfaces to be protected and the required protection current densities have to be determined. The protection current density depends on the type and quality of the coating. Thermosetting resins (e.g., tar-epoxy resin coatings) are particularly effective and are mostly used today on coastal structures. They are chemically... 

After measuring the zero profile, AU measurements are carried out with the injection of a cathodic protection current. In contrast to the zero profile measurements, the distance between the individual measurements is 25 to 50 m. Shorter distances between the measuring points are used only at depths where there are unusual AU profiles. Current should be injected at at least three different levels. The protection current density of about 12 mA m obtained from experience should be the basis for determining the maximum required protection current. As shown by the results in Fig. 18-3, the AU profiles are greater with increasing protection current. The action of local cells is suppressed when the AU values no longer decrease in the direction of the well head. This is the case in Fig. 18-3 with a protection current I = 4A. 

Table 19-1 Protection current densities and potentials of the cathodically protected steel test pieces in Fig. 19-1...

The arrangement and distribution of anodes in gravel and activated charcoal filters is different. Cathodic protection of activated charcoal filters is basically feasible but requires a large number of electrodes and high protection current densities that are twice those for gravel bed filters, so that an electrically insulating layer can be deposited on the steel wall. 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

The application of new insulation materials and technologies has led to a decrease in the required protective currents. The unit insulation resistance is a significant parameter determining the usefulness of structural insulation in cathodic protection. It characterizes the CP current transfer resistance from the grormd to the pipehne. The interaction range of cathodic protection and the protective current density change depend on its value. 

If the detached coating does not shield the cathodic protection current, the current density can assume locally very high values, and lead to phenomena of embrittlement due to hydrogen evolution. On the other hand, if the detachment is such that the current is shielded, the anaerobic condition of... 

Let us assume that a safe cathodic protection current required to protect this area is 10% of the total current density, and calculate the distances on the top of the finger from the sharp comer. These results are tabulated in Table 10.3.1. If = 1.5-1.8cm, and the welds are located 0.6 cm from the sharp comer, a ratio a/P of 0.25 will protect... 

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