Cathodic protection continued anode potential

In general terms, the systems for protection of steel in concrete are generally full wave rectifiers with smoothing to minimize interference and any possible adverse effects on the anode. A continuously variable output is usually specified. Most cathodic protection systems are run under constant current control, although constant voltage (or an option for both methods) is sometimes specified. Control by constant half cell potential against an embedded reference electrode is rarely specified for steel in atmospherically exposed concrete but may be applied to buried or submerged parts of structures. 

Observing the polarization diagram for the copper-zinc cell in Fig. 5.2, it is clear that, if polarization of the cathode is continued, using external current, beyond the corrosion potential to the thermodynamic potential of the anode, both electrodes attain the same potential and no corrosion of zinc can occur. This is the basis for cathodic protection of metals. Cathodic protection, discussed further in Chapter 13, is one of the most effective engineering means for reducing the corrosion rate to zero. Cathodic protection is accomplished by supplying an external current to the corroding metal that is to be protected, as shown in Fig. 5.14. Current leaves the auxiliary anode (composed of any metallic or nonmetaUic... 

Electrochemical protection is divided into cathodic and anodic protection. Cathodic protection based on the change of potential of a metal in the negative direction is realized in electrolytic environments, in most cases neutral, mainly of steel and reinforced concrete structures. A well-designed and correctly realized CP reduces the corrosion rate to almost zero. In practice it is realized with the use of an impressed current or protectors (galvanic anodes). The scope of application is enormous and continuously increases. With the use of this technology it is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete... 

The third widely used protection method is that of "cathodic protection", where a small negative potential is continuously applied to the metal surface to render it passive. Its counterpart, "anodic protection" can also be used to keep a metal in a permanently oxidized state, rendering it passive to corrosion. Quite evidently, this method is more cumbersome and expensive than most methods, although it does find niche uses where it is more practical, e.g. metal pipelines which have periodic control stations on the pipeline. 

Theoretically, it would be possible to hold the potential at a value where the metal is thermodynamically immune from corrosion. In practice, however, this Is likely to be too expensive. The continuous passage of current requires the consumption of power (j cell 0 tid, particulary if the medium between the protected surface and the anode is not highly conducting, the use of anything but a very low current will entail an unacceptably high energy consumption, The voltage required of the power supply also depends upon the active area of Che structure, the nature of the electrode reactions and resistance of the anode. The cathode potential is kept within the desired limits by ... 

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