A pinhole defect and the resulting cathodic disbonding in pipeline coatings was examined using an ac impedance method. An equivalent circuit model was assumed and the ac impedance characteristics was calculated using a computer digital simulation. The results showed that the disbonded area of the coated steel could be estimated from the frequency where the phase shift was a maximum. This method was applied to experiments. Fusion bonded epoxy films were examined on a mild steel substrate polarized cathodically in 3% sodium chloride. The ac impedance was measured before and after the polarization. After the polarization, the impedance decreased with increasing frequency and the phase shift showed a distinct peak at a certain frequency, f1. The experimental results indicated that the disbonded area on the coated steel in the presence of a pinhole could be estimated from the characteristic frequency, f2, which was calculated from f1. The experimental results agreed well with the theoretical analysis.

Subject
Circuits, Films, Coated steel, Coatings, Impedance, Cathodic disbondment, Capacitance, Impedance measurements, Experimental data, Metals, Corrosion resistance, Polarization, Pinholes
© 1989 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).
1989
Association for Materials Protection and Performance (AMPP)
You do not currently have access to this content.