MEASUREMENTS OF CORROSION AT DEFECTS IN PAINTED ZINC AND ZINC ALLOY COATED STEELS USING CURRENT-DENSITY MAPPING

Current density mapping was used to locate and monitor the early stage s of corrosion at defects on painted surfaces in solution. Principles of the technique and methods of analysis were reviewed, and the effect s of different scribing techniques were investigated. Results for pain ted zinc and aluminum-zinc alloy coated steel surfaces showed corrosio n occurred initially at localized sites on the exposed zinc in both di lute chloride and sulfate solutions and when zinc was galvanically cou pled to or isolated from steel. Current density mapping was shown to l ocate corrosion-susceptible defects on painted roll-formed materials t hat were not readily discernible optically.