Shielding Effects of Coatings in Cathodic Protection Systems: Experimental Techniques, Observations, and Case Histories

Abstract The effectiveness of cathodic protection (CP) in mitigating external corrosion is strongly influenced by the behavior of protective coatings. When coatings disbond yet remain intact, they may act as a barrier to CP current, creating shielded regions that foster aggressive localized corrosion and stress corrosion cracking (SCC). Conversely, non-shielding coatings allow CP current to penetrate beneath disbondments, maintaining protection of the steel substrate. This paper reviews the mechanisms of CP shielding, describes laboratory and field techniques used to evaluate shielding tendencies, and presents case histories from pipeline, utility, and water/wastewater systems. A high-confidence inspection approach is emphasized, integrating visual examination, thickness measurement, and electrochemical potential testing to identify risks that are often underestimated when relying solely on visual assessments. The role of shielding in hidden corrosion failures is examined through both experimental data and field case histories, with recommendations for coating selection and integrity management.