The three-coat system consisting of a zinc-rich primer, epoxy midcoat, and polyurethane topcoat has been widely adopted for corrosion protection of structural steel bridges. Corrosion mitigation is afforded by the barrier characteristics of the various layers as well as beneficial galvanic coupling of the zinc pigments in the primer to the steel substrate when subjected to corrosive environments such as with moisture ingress and coating defects. The dispersed zinc pigments in the binder require adequate electronic coupling in order to provide the beneficial electrochemical coupling of the zinc to the steel. The application of the electrochemical noise (EN) technique to identify coating corrosion performance was reviewed in controlled laboratory tests. Zinc pigment activation and transition to passive-like conditions in defect and defect-free coating specimens could be identified by EN. The characteristic charge of noise events resolved by shot noise analysis was demonstrated to be related to anodic polarization of extended active zinc pigments adjacent to coating defect sites galvanically coupled to the exposed steel and passivated zinc pigments. Electrochemical noise resistance was shown to convey zinc/steel interfacial behavior in cases with sufficient activity but largely reflects coating barrier quality for intact coatings.

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