The mechanism of underfilm corrosion in atmospheric conditions of painted precoated automotive steels has been investigated using electron microscopy techniques. The materials that were compared were hot-dip and electrogalvanized steel, electrogalvanized zinc-iron-coated steel and galvannealed steel, which were phosphated and electrocoated. Most of the panels were also topcoated. In some of these systems, a region of interfacial delamination preceeds a region where the metal coating has been converted into corrosion products. In hot-dip and electrogalvanized steel this delamination is usually at the phosphate-zinc interface; in the zinc-iron-coated steels delamination is at the phosphate-electrocoat interface. In electrogalvanized ZnFe coatings which were electrocoated only, delamination and corrosion were found to proceed through longitudinal cracks in the metal coating. Models for the mechanism of corrosion propagation and suggestion for improved systems are presented.

Subject
Zinc rich coatings, Materials, Acidity, Corrosion products, Phosphates, Interfaces, Steel, Metallic coatings, Zinc, Corrosion mechanisms, Delamination, Chlorides, Paints
© 1988 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).
1988
Association for Materials Protection and Performance (AMPP)
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