Cathodic protection continued controlled 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. 

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. 

The cell potential for such a process is shown in Fig. 15.28 (Dandapani, 1987). In fact, at about 80 °C and pH 14, polysulfides are first produced, but pure sulfur precipitates on cooling. The process has been developed to an engineering stage (Petrov and Srinivasan, 1996). It is necessary to protect the cathode against catalytic inactivation by polysulfide (i.e., a membrane must be used). For continuous use, the pH must be controlled. The cooling and production of S may be best carried out outside the cell. 

The potential control in anodic protection installations has two functions. First, the potential must be measured and compared to a desired preset value. Second, a control signal must then be sent to the power supply to force the dc current between the cathode and vessel wall. In early systems, this control function was done in an ON-OFF method because of the high costs of electronic drcuitiy. The recent progress in power electronics has resulted in all systems having a continuous proportional-fype control. Packaging these electronic components occasionally involves special requirements because most of the installations are made in chemical plants. Explosion-proof enclosures are sometimes required, and chemically resistant enclosures are necessary in other installations. 

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