Current density and impedance mapping measurements have been used to locate and monitor corrosion and defects on painted surfaces in solution. Measurements are reported for painted zinc and aluminum-zinc alloy coated steel surfaces. When scratched, current density mapping showed corrosion of the metal coating started at localized sites in both dilute chloride and sulfate solutions. Different scribing techniques were tested exposing only the metal coating and both the coating and the underlying steel. Effect of roll forming was investigated. Current density mapping located corrosion susceptible defects on painted roll formed materials that were not readily discernible optically. Scanning ac mapping showed that artificially formed defects were readily observed. Local impedance variations with frequency were measured for simulated defects and defect free areas of painted surfaces. Variations in paint thickness and the presence of defects were detected using the ac techniques.

Subject
Zinc rich coatings, Steel surfaces, Coatings, Voltage, Impedance, Current densities, Cathodic sites, Anodic current, Steel, Coating defects, Zinc, Electrodes, Paints
Keywords:
zinc, aluminum-zinc alloy, paint, corrosion testing, surface defects, ac impedance, localized corrosion
© 1995 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1995
Association for Materials Protection and Performance (AMPP)
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