Abstract
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. In aggressive environments and adverse conditions, the coatings may require early maintenance before its service life. Corrosion mitigation is afforded by the barrier characteristics of the various layers as well as beneficial galvanic coupling of the zinc pigment in the primer to the steel substrate when subjected to corrosive environments such as with moisture ingress and coating damage such as holidays and scratches. 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 noise resistance correlated well with the polarization resistance to identify zinc activity when coating defects exposed the metal surface and could differentiate coating resistance upon wetting with time of exposure.
