Cathodic protection by impressed current

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. 

Figure 20-13 shows current and potential time curves for a stainless steel 500-liter tank with cathodic protection by impressed current and interrupter potentiostat. 

Suppose metal M in problem 4 is to be cathodically protected by impressed current. 

Lastly, it may be useful to recall that the resistivity has an important role in the field of cathodic protection by impressed current because, especially in soils [18], it helps to determine the real polarization potential of the structure considered. In all of these cases the attainment of the optimum degree of protection depends on the a priori knowledge of the medium resistance between the reference electrodes and the structure to be protected. 

Carbon possesses high resistance to corrosion (except against halogens and oxidizing acids) and high electrical and thermal conductivity, but it is brittle. Graphite anodes are used in cathodic protection by impressed current. 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

Coatings must be considered for aU applications of steel. Cathodic protection should be considered for steel pipe where soil or groundwater resistivity is less than 10,000 t2-cm, and where steel win be in contact with process streams. Cathodic protection of steel is strongly recommended where resistivity is less than 5000 Q-cm. For aU exposures, steel should be electrically isolated from dissimilar metals to prevent the formation of unfavorable galvanic corrosion ceUs. In areas where abrasive materials are hkely to damage coatings, cathodic protection by impressed current or galvanic anodes may be desirable. 

Figure 8-5. Cathodic protection by impressed current 4pp in neutral aerated water [Jones (1996), reprinted by permission].

Figure 12.14 Cathodic protection by impressed current method...

A typical basic appreciation of cathodic protection by impressed current/cathodic control is as follows ... 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

Cathodic protection by means of impressed current is very adaptable and economic because of the long durability of anodes and the large number of anode materials and shapes. Some examples are described here. Internal cathodic protection of fuel oil tanks has already been dealt with in Section 11.7. The internal protection of water tanks is described in detail in Chapter 20. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

Environment Reduce kinetics of cathodic reaction Lower potential of metal Cathodic inhibition Reduce a , reduce O2 concentration or concentration of oxidising species lower temperature, velocity agitation Cathodically protect by sacrificial anodes or impressed current sacrificially protect by coatings, e.g. Zn, Al or Cd on steel Formation of calcareous scales in waters due to increase in pH additions of poisons (As, Bi, Sb) and organic inhibitors to acids... 

Cathodic protection is an electrochemical technique of providing protection from corrosion [38]. The object to be protected is made the cathode of an electrochemical cell and its potential driven negatively to a point where the metal is immune to corrosion. The metal is then completely protected. The reaction at the surface of the object will be oxygen reduction and/or hydrogen evolution. Cathodic protection may be divided into two types, that produced using sacrificial anodes and the second by impressed current from a d.c. generator [39]. 

Fig- 4.25 Components used to impose and monitor conditions providing cathodic protection by an impressed external current. Note Power supply may be either a galvanostat or a potentiostat. In the latter, the electrometer provides feedbackto the potentiostatto control to constant potential. Electrometer provides check to show that the metal is at the protection potential. 

Fig. 4.26 Schematic polarization curves used in the analysis of cathodic protection by an impressed external current. Cathodic reaction is under Tafel control.

The ohmic drop exerts a sensible influence on the evaluation of the electrochemical parameters as well as on the definition of the reaction scheme that is most suitable for describing the behaviour of a metal in a given environment. It also determines the success of many operations, such as cathodic protection by means of sacrificial anodes or impressed current and corrosion rate monitoring. 

Definition Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode using an impressed current or attachment to a sacrificial (galvanic) anode. It is a process that reduces the anodic corrosion reaction by creating an electric field at the surface of the metal so that the net flow of current is into the metal. 

Figure 10.13 Cathodic protection by a) sacrificial anodes and b) impressed current.

Figure 10.18 Cathodic protection of a buried pipeline by impressed current [10.19].

---