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CORROSION
A 1998 EPRI study, released in October of 2001, estimated the cost of corrosion in 1998 to steam generating facilities alone at $5.37 billion. The study, EPRI Report 1004662, also concluded that 22 percent of these costs are avoidable. Corrosion remains an ongoing problem for power generators and the industry has spent, and will continue to spend, millions of dollars on a wide range of corrosion protection measures.
One of the proven technologies for controlling corrosion is cathodic protection. Although used in numerous applications in power plants, there are still many untapped opportunities to apply this technology. Power plants using cathodic protection are able to reduce the economic impact of power plant corrosion considerably.
WHAT IS CORROSION?
Corrosion can be defined as the degradation and destruction of a metal by its chemical reaction with the environment. Corrosion reactions, with the exception of some forms of high temperature corrosion, are electrochemical oxidation/reduction reactions. Figure 1 shows a typical corrosion cell that is formed when small variations in potential occur over the metal's surface. The corrosion is caused by differences in the metal or its surrounding electrolyte. The difference in potential results in a current flow from the anode to the cathode.
CATHODIC PROTECTION
Cathodic protection is one option for controlling corrosion. In the typical corrosion cell, the metal structure has both anodic (area where metal is lost) and cathodic (area with no metal loss) regions resulting from electrical potential differences. Even small differences in potential can result in significant metal loss over time. Cathodic protection is accomplished by intentionally substituting the slightly anodic region of the structure to be protected with an even more anodic component called an anode.
The anode is intentionally coupled with the protected structure. In a galvanic system, current results from the inherent potential differences between the anode and the cathode. An impressed system, on the other hand, uses a DC power supply. As long as sufficient protective current is maintained, an impressed system can eliminate further corrosion.
GALVANIC SYSTEMS
The simplest system is the galvanic anode system, Figure 2. With a galvanic system, the potential difference results entirely from the electrical characteristics of the anode versus the structure to be protected. Although this type of system will protect the...