NACE SP21424

Abstract

General AC (alternating current) corrosion is defined as corrosion initiated and propagating under the influence of alternating current. AC corrosion on cathodically protected underground pipelines is commonly the result of a combined action of the AC voltage, the cathodic protection conditions, a coating defect—usually small—and the chemical and physical conditions of the soil. If the AC component is either entirely removed or limited to a certain level, the corrosion will be mitigated. AC corrosion is also influenced by direct current (DC). As such, in addition to mitigation by limiting the AC component, AC corrosion can be reduced by adjusting the DC-component – through the cathodic protection (CP) system.  An AC corrosion evaluation process (Figure 1) should include an analysis which results in development and implementation of a mitigation strategy, development of a monitoring strategy, and implementation of that monitoring strategy. If subsequent monitoring indicates risk of AC corrosion, the analysis as such should be reviewed, the mitigation strategy should be improved, or—in case values of the monitoring parameter are violated but it is documented that this does not lead to corrosion—the monitoring strategy can be modified.  The provisions of this standard should be applied under the direction of competent persons, who, by reason of knowledge of the physical sciences as well as the principles of engineering and mathematics, acquired by education and related practical experience, are qualified to engage in the practice of corrosion control of buried ferrous piping systems. Such persons may be registered professional engineers or persons recognized as corrosion specialists or CP specialists by NACE if their professional activities include suitable experience in external corrosion control on buried ferrous piping systems and AC interference and mitigation. This standard should be used in conjunction with the references contained herein.