Protection by Impressed Current

Cathodic protection also can be accomplished by lowering the electrode potential to E M, the equilibrium potential for the metal to be protected, by an external power source. The circuit used to accomplish this is the same as shown in Fig. 2.12. With slight modification, it is again shown in Fig. 4.25 in which the metal to be protected is iron and the cathodic reaction supporting corrosion is either hydrogen-ion reduction, oxygen reduction, or both. 

Interpretation of cathodic protection of iron in an environment of PH = 1 may be made by reference to Fig. 4.26. Without an external current, steady-state corrosion occurs under the conditions, Ecorr and icorr. If electrons are supplied to the metal, the potential will decrease, and at any arbitrary reduction of potential (e.g., Ej), a current balance requires that Iex = Iox M - Ired x, or iexA = iox MA - ired xA for a given area A (assuming that Ac = Aa = A), or iex = iox m - bed x- This external current density is represented in Fig. 4.26 as the span between the respective polarization curves at Ej. It is evident that for corrosion to be stopped, E must be reduced to E Fe, and to maintain this protection, the external 

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. 

In aerated neutral environments, corrosion will be supported by the cathodic oxygen reaction and will normally occur under oxygen-diffu- 

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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. 

Table 10.1 gives a brief summary of the behaviour of some impressed current anodes and protection by impressed current is discussed in more detail in Section 10.3. 

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. 

On ship hulls protected by impressed current, flush-mounted anodes are used in order to avoid additional hydrodynamic resistance. Around the anode, the nearest steel surface is covered with a dielectric shield or coating with the aim of obtaining a better current distribution. 

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. 

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... 

Anodic protection, normally done potentiostatically, by application of a potential within the passive region. Given the form of Fig. 16.6, it is not so easy to control the potential by impressed current. The advantages are that there is no release of hydrogen and that often the current, and thus the energy consumed, is low. 

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. 15.3 Estimation of corrosion current, applied current for the Fe substrate protected by impressed...

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

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

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