Analysis of Cathodic Protection Criteria

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Abstract
The criterion of cathodic protection is that level of cathodic potential change (polarization) which is sufficient to give adequate protection for a given application. Ideally, it is preferred to polarize to the reversible potential of the anodic reaction, but this potential is often impossible to determine by experiment or to calculate from theory for complex corrosive electrolytes. Polarization to a specific potential (e.g., −0.85 volts) or to various potential changes (e.g., 100 to 300 mv) have also been proposed, but none have been universally accepted. Electrochemical kinetic theory has been very successful in predicting and correlating the corrosion behavior of metals. However, the theory has not been applied to cathodic protection until recently. It has been shown that the anodic Tafel constant determines the cathodic polarization necessary to effect a given reduction in corrosion rate. Thus, polarization required to obtain a given degree of protection can be predicted if the anodic Tafel constant is known. Determining the anodic Tafel constant may be difficult for complex structures in service. However, even an estimate from a small sample in the required corrosive would be very helpful in making the criterion of cathodic protection more quantitative.

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