Anodic protection currents

Number Tank Volume (m ) Surface area (m ) No. of anodes Protection current (A) Voltage (V) Soil resistivity (Q m) [after Eq. (3-44)]... 

Test potential Cu-C S04(V) Medium at the anode Protection current density (mA m ) Anode 1 Anode 2 ... 

The four types of mixed potential models presented in Figs. 12.6 to 12.9 are simplistic and do not necessarily reflect the complete behavior of carbon steel in Kraft liquors because the models all assume some sort of steady states. Figure 12.10 depicts typical curves from an in situ test in a white liquor clarifier at different scan rates. The passive state does not exist until after the active-passive transition is traversed. Therefore, unless sufficient anodic current density is discharged from carbon steel by a naturally occurring cathodic reaction or an applied anodic protection current, the carbon steel liquor interface remains monostable (active) because the passive film and its low current density properties do not exist. 

Figure P.l Passivation curve (Ia = anodic protection current)...

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

Cases I and II are examples of protection by cathodic polarization and III and IV of protection by anodic polarization. In cases I and III, the protection current can be uncontrolled, while in cases II and IV the potential has to be controlled. The development of operationally safe and widely available rectifiers (see Chapter 21)... 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

Here 7 X is the protection current, 5 is the surface area of the anode, and and are the grounding resistances of the anode and the object. Information on the... 

In the application of magnesium anodes for enamelled boilers, the consumption rate of the anodes is determined less by current supply than by self-corrosion. The calculation of life from data on protection current requirement, /, and anode mass, m, is difficult because the a value is so low. 

Ohmic voltage drops resulting in losses cannot be ignored in the connecting cables with long anode cables and high protection currents [28]. Cable costs and losses must be optimized for economic reasons. The most economic calculated cable dimension depends primarily on the lowest cross-section from the thermal point of view. For various reasons the permitted voltage drop usually lies between 1 and 2 V, from which the cross-section of the cable to be installed can be calculated from Eq. (3-36). 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

In choosing a site for protection installations for steel-water structures, decisive factors are the location in the harbor area and the need to keep the lengths of cable to the protected object and anode as short as possible where very high protection currents are involved. 

Rectifiers working according to the control diagram in Fig. 8-6 are used for anodic corrosion protection in passivatable systems that go spontaneously from the passive to the active state when the protection current is switched off [12]. The predetermined nominal voltage between reference electrode and protected object is compared with the actual voltage f/j in a differential display unit D. The difference AU = is amplified in a voltage amplifier SV to VqAU. This... 

Too low protection current or none Cable or contact break Measure pipe/anode resistance, locate cable failure, test connecting terminals... 

Protection current too high Anode resistance reduced by ground water or soil moisture Do not alter rectifier setting as anode resistance will increase in summer... 

Voltage cones also occur where the protection current enters through defects in the pipe coating (see Sections 3.6.2 and 24.3.4). Figure 9-1 shows schematically the variation of the voltage cone of an anode bed and a cathodically protected pipeline that results from the raising and lowering of potential. 

The current output of galvanic anodes depends on the specific soil resistivity in the installation area and can only be used in low-resistivity soils for pipelines with a low protection current requirement because of the low driving voltage. Impressed current anode installations can be used in soils with higher specific soil resistivities and where large protection currents are needed because of their variable output voltage. 

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... 

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