Impressed current system components

Impressed-Current Cathodic Protection System An impressed current system consists of the following components ... 

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. 

This is another vital part of an impressed current system. The T/R must be rugged and reliable with minimal maintenance requirements. It should be easy to maintain with good instruction manuals, circuit diagrams for maintenance and easy access to fuses and other consumable and replaceable components. Compared with pipeline or marine CP applications (steel piles, etc.) the power demand is modest. Most steel in concrete needs less than 10 mA-m to provide protection, usually at less than 10 V. The power for a 100 watt light bulb will protect 10,000 m. This means that a single phase, air cooled T/R will usually protect even the largest structure and power consumption is rarely an economic concern. 

Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

In contrast to the galvanic anode system, the flow of current from the anode to the cathode is forced from a DC source in the impressed current system. Thus, whereas the current is provided by the corrosion of the electrode in the anodic galvanic system, the electrode acts as a conductor and hardly corrodes in the impressed system and the AC input is transformed and rectified to a varying DC voltage. A transformer rectifier is the most important component of the system. 

Coral sands and mud lend themselves to standard cathodic protection materials, resulting in the use of low-cost components for either galvanic or impressed current systems. In such marine environments in semiarid areas, consideration must be given to atmospheric corrosion of components as well as proper monitoring procedures. 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

The sensitivity of a lead system that detects the electric dipole moment of a volume source consists of three orthogonal components (Figure 27.2a). Each of these is linear and homogeneous. In other words, one component of the electric dipole moment is detected when the corresponding component of all elements of the impressed current density P are detected with the same sensitivity throughout the source area. 

The components of an impressed current cathodic protection system... 

THE COMPONENTS OF AN IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM... 

It may appear that Table 1 contains an essentially complete summary of patterns that may form in electrochemical systems. This impression is misleading, since Table 1 only roughly summarizes results observed so far or predicted with models. These are investigations concentrating on phenomena that can be described with two essential variables (two-component systems). This survey is certainly not yet completed. Furthermore, numerous examples of current or potential oscillations involve complex time series. Only in a few cases does the complex time series result from the spatial patterns. In most cases, the additional degree of freedom will be from a third dependent variable, such as from a concentration that adds an additional feedback loop into the system, as discussed in Section 3.1.3. Spatial pattern formation in three-variable systems is an area that currently develops strongly in nonlinear dynamics. In the electrochemical context, the problem of pattern formation in three-variable systems has not yet been approached. 

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