Duplex coatings, consisting of thermally sprayed aluminium or zinc topcoated with an organic coating system, is expected to have a lifetime in the order of 50 years. A company has used a coating system consisting of 200 μm (8 mil) TSA, tiecoat, 200 μm (8 mil) epoxy mastic and 75 μm (3 mil) polyurethane topcoat on two offshore platforms and a gas treatment plant onshore. Contrary to expectations, the coating system was severely degraded after less than ten years operation for all three installations. The degradation is particularly severe around bare steel in galvanic contact with the coated structure. The TSA corrodes rapidly and the organic coating blisters due to the voluminous corrosion products. The coating degradation spreads to large areas in short time.

A mechanism for the degradation is suggested. When the duplex coating is in galvanic contact with bare steel, galvanic corrosion of the thermally sprayed aluminium is initiated. Cathodic oxygen reduction takes place at the bare steel, while anodic dissolution of the thermally sprayed metal takes place under the organic coating. In chloride containing environments, like in marine atmosphere, chloride ions migrate in under the organic coating to maintain the charge balance. Aluminium chloride is then formed. This is highly unstable in the presence of water, reacting to form hydrochloric acid. The electrolyte under the organic coating will therefore be acidic. Cathodic hydrogen evolution will start under the organic coating, which will increase the corrosion rate of the TSA. The TSA will not be passive in the low pH electrolyte. Hence, the thermally sprayed aluminium corrodes actively and rapidly. The organic coating holds the aggressive electrolyte at the surface. TSA with only a thin sealer will not suffer from this type of degradation since the aggressive electrolyte will migrate out of the sealer.

Based on these findings we conclude that for structures to be exposed in corrosive environments, TSA should not be coated by thick organic coatings.

Subject
Steel structures, Coatings, Aluminum spraying, Oxides, Aluminum, Coating degradation, Steel, Sealers, Organic coatings, Chlorides, Galvanic corrosion, Electrolytes, Coating systems
Keywords:
Thermally sprayed aluminium, organic coatings, duplex coatings, coating degradation
© 2004 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).
2004
Association for Materials Protection and Performance (AMPP)
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