Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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36 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 CATHODIC PROTECTION ESSENTIALS Assessing CP Effectiveness on Pipelines at Corrosion Defects Generally, corrosion can result in several types of defects on pipe surface, such as mac- roscopic holes due to general corrosion oc- curring over a large area, corrosion pits due to localized corrosion, and cracks generated by stress corrosion cracking (SCC). Although cathodic protection (CP) used in conjunc- tion with coatings is recognized as the most efective method for corrosion mitigation, the efectiveness of CP may be reduced at surface defects such as corrosion pits and cracks. Field experiences have found a signif- cant number of corrosion defects on the ex- ternal surface of cathodically protected pipe- lines after several years of service. While a numerical method has been used in CP de- sign, a model that quantifes CP performance at various geometrical corrosion defects on pipelines has not been available. Tis is par- tially due to the experimental difculty in ac- cessing the environment inside pits and cracks with a measuring probe or electrode. Using a homemade potential microprobe, researchers with the University of Calgary (Calgary, Alberta, Canada) investigated the efectiveness of CP at corrosion defects on a steel pipeline by measuring the local potential and its distribution at artifcial defects with various geometries on a pipeline steel speci- men (UNS T30102). Results showed there was a nonuniform potential distribution at corro- sion defects. Te CP was at least partially shielded at the defect, especially at its bottom. Tis efect was enhanced with increasing depth and decreasing width of the defect. A fnite element model was developed to simulate the potential distribution inside the defects, and empirical equations were derived to enable calculation of CP efectiveness at corrosion defects, which will provide industry with recommendations for accurate assess- ment of further corrosion allowance and re- maining life of pipelines in the feld. Te experimental procedure and results are discussed in CORROSION 2015 paper no. 3850, "A Direct Assessment of Efectiveness of Cathodic Protection at Corrosion Defect on Pipelines," by L. Xu and F. Cheng. Evaluating Coating Degradation and CP Design on Offshore Installations Anode consumption on a subsea frame during 21 years of exposure.

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