Cathodic protection from rectified current sources

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. 

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). 

In this technique, the electrical current is delivered to the structure to be protected from a direct current (dc) power source through an auxiliary electrode. The structure acts as a cathode and the auxiliary electrode becomes the anode in the cell [26, 27]. Figure 8 shows an impressed current system used to protect a pipeline. Both the buried anode(s) and the pipeline are coimected to an electrical rectifier, which supphes direct current to the buried... 

Rectifiers are used more than any other source of impressed-current power. Areas discussed include rectifier types, rectifier selection, specification requirements, and typical installation details. Various types of impressed-current anodes and components that make up an impressed-current system are also presented. Impressed-current-type cathodic protection systems provide cathodic current from an external power source. A direct current (DC) power source forces current to discharge from expendable anodes through the electrolyte and onto the structure to be protected. Although the current is not generated by the corrosion of a sacrificial metal/alloy, the energized materials used for the auxiliary anodes do corrode. 

When choosing rectifiers it is necessary to take into account the type of anodic material, which could be prone to corrosion attack caused by dc current ripples. The effect of dc ripples from cathodic protection rectifiers is relatively less known by installation users. At high thyristor ignition angles, when the current waveform is in the form of pins, corrosion of Ti/Pt anodes occurs. Efird (1982) has shown a disadvantageous effect of the shape of the current waveform on the premature failure of Nb/Pt anodes in a CP system of two North Sea offshore oil production platforms. Juchniewiczetal. (1981) have shown that the current waveform from a thyristor power source causes accelerated corrosion of platinum and platinized titanium anodes exposed in 0.1 M NaCl solution (see Fig. 8-15). 

In all the cells described so far, the source of the energy which makes a cell active has been within the cell. However, in stray current cells the energy comes from an electrical current external to the corrosion site per se. The source of energy for such cells may be a distant generator, a direct-current transmission line, a cathodic protection rectifier on some other line, a street car system, or an electric railway (Fig. 7.29). 

---