Cathodic protection current sources

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

For another method of cathodic protection, the source of electrons is an impressed current from an external dc power source, as represented in Figure 17.23h for an underground tank. The negative terminal of the power source is connected to the structure to be protected. The other terminal is joined to an inert anode (often graphite), which, in this case, is buried in the soil high-conductivity backfill material provides good electrical contact between the anode and the surrounding soil. A current path exists between... 

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

It is little known that Thomas Alva Edison tried to achieve cathodic protection of ships with impressed current in 1890 however, the sources of current and anodic materials available to him were inadequate. In 1902, K. Cohen achieved practical cathodic protection using impressed direct current. The manager of urban works at... 

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. 

Electrical sources static electricity, electrical current, lightning, stray currents (radiofrequency electromagnetic radiation, overhead high voltage transmission lines, galvanic and cathodic protection stray currents)... 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

The effect of stray currents arising from a d.c. source or from cathodic protection of an adjacent structure are considered in Sections 11.5 and 11.6. 

Fig. 10.4 Polarisaiion diagram representing corrosion and cathodic protection. A corroding metal takes up the potential corr spontaneously and corrodes at a rate given by /con. If the potential is to be lowered to a current equal to( /c - li) must be supplied from an external source the metal will then dissolve at a rate equal to li...

Figure 10.7 illustrates the use of an external power supply to provide the cathodic polarisation of the structure. The circuit comprises the power source, an auxiliary or impressed current electrode, the corrosive solution, and the structure to be protected. The power source drives positive current from the impressed current electrode through the corrosive solution and onto the structure. The structure is thereby cathodically polarised (its potential is lowered) and the positive current returns through the circuit to the power supply. Thus to achieve cathodic protection the impressed current electrode and the structure must be in both electrolytic and electronic contact. 

In an impressed-current cathodic protection system the power source has a substantial capacity to deliver current and it is possible to change the state of polarisation of the structure by altering that current. Thus effective control of the system depends on credible potential measurements. Since the current output from any given anode is substantial, the possibility of an IR error which may reach many hundreds of millivolts in any potential measurements made is high. Thus the instant-off technique (or some other means of avoiding IR error) is essential to effective system management. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

An impressed-current cathodic protection system circuit comprises an anode, the power source, the structure and the environment in which it... 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

A comparison of typical properties of cathodic protection materials is given in Table 10.23, but is by no means comprehensive. It is obvious that the modification of an alloy, environment or other important factors will be reflected in the life and output characteristics. In some cases the maximum voltages and current densities recommended can be vastly exceeded. In others, particularly where abnormal levels of environmental dissolved solids are met, factors of safety should be applied to modify the proposed figures. Acceptance of a much reduced or uncertain life, weighed against a possible economy, may also influence the chosen working limits. For example, the life of ferrous alloy anodes may, in practice, be only two-thirds of that expected because of preferential attack eventually leading to disconnection of all or part of the anode from the source of e.m.f. 

Advantages No external source of power is required installation is relatively simple the danger of cathodic protection interaction is minimised more economic for small schemes the danger of over protection is alleviated even current distribution can be easily achieved maintenance is not required apart from routine potential checks and replacement of anodes at the end of their useful life no running costs. 

Establishing electrolyte resistivity To enable a satisfactory cathodic-protection scheme to be designed, it is necessary to determine the resistivity of the electrolyte (soil or water). This information is necessary to enable the current output of anodes to be determined together with their position and power source voltage, and it also provides an indication of the aggressiveness of the environment in general the lower the resistivity the more aggressive the environment. 

Impressed Current cathodic current supplied by a d.c. source to a structure in order to lower the potential to the protective potential for cathodic protection. 

Cathodic Protection. Steel can be protected by cathodic current, supplied either from sacrificial anodes or an external direct current source. The method is effective for completely immersed steel—i.e., for surfaces on structures below the low-tide level. Current consumption can be greatly reduced by applying a suitable paint system to the steel before it is immersed in sea water. Such a paint system should be alkali resistant. 

Cathodic protection. Another way of keeping metal in a cathodic state is to connect it to an external direct current source. For smaller structures such as boats and domestic water water heaters the current is supplied by a sacrificial anode made of aluminum or zinc. For larger extended structures such as piers and buried pipelines, an external line-operated or photovoltaic power supply is commonly used. (All interstate oil piplines in the U.S. are required by law to employ cathodic protections.)... 

Fig. 12.38. Cathodic protection can also be accomplished by using an inert auxiliary electrode and sending current into the circuit from an external current source.

At points where the current enters the structure, the site will become cathodic in nature because of changes in potential, while the area where the current leaves the metal will become anodic. Electric railways, cathodic protection, electrical welding machines, and grounded DC electrical sources are subject to stray current corrosion. (Craig)5... 

Impressed-current cathodic protection entails the use of an external power source in combination with a stable anode. The potential of the specimen being protected is forced to negative values with respect to its open-circuit corrosion potential, and its rate of anodic dissolution is consequently reduced. The result of impressing a cathodic current on the structure is shown in Fig. 21M for the parameters used to draw this figure we obtain i = 48.8 pA/cm and E = - 0.554 V, NHE. Applying a cathodic current density of 72... 

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