The annulus of an unbonded flexible pipe contains armouring layers, which are typically made of carbon steel wires. The annulus is characterized by having a low ratio of free volume (V) to steel surface area (S). Typically the V/S ratio is below 0.1 ml/cm2. The free volume may be wetted with water and influenced from influx of corrosive gasses like CO2 and H2S permeating from the bore stream through the innerliner into the annulus. The resulting environment is complex and unique in relation to corrosion.

It has previously been reported that the corrosion rates in such environments are very low with uniform corrosion at rates far below 10 μm/year. In this work this corrosion process in the annulus environment of flexible pipes have been studied under varying environmental conditions and in different experimental setups with the aim to further document the expected corrosion rates under different environmental conditions and to understand the governing mechanisms in the process. The experimental setups are described and the results reported.

All experiments are carried out in experimental setups simulating the annulus environment. The setups are based on dense packing of wires in glass vessels. The wires are exposed to water solutions in saturation with CO2. Corrosion rates are determined based on weight loss measurements. In addition, electrochemical potential measurements and measurements of pH have been performed.

The corrosion rates after relative short exposures (30 days) were found to be relatively high, but with increased exposure time results confirm that long-term corrosion rates are very low. Corrosion rates below 1 μm/year have been measured in the later part of the exposure for tests running for more than 300 days. Results also show that corrosion is uniform with low corrosion rates. Finally, experimental data, where the level of V/S ratio varied within the test cell, suggest that high super saturation levels of iron ions in solution influence the corrosion rate far more than the thickness of the electrolyte film at the steel surface.

Subject
Steel surfaces, Piping, Electrochemical cells, Corrosion rate, Glass, Annulus, Iron, Wiring, Reservoirs, Specimen preparation, Steel, Electrolytes, Electrode potential
Keywords:
CO2 corrosion, unbounded flexible pipe, annulus environment, confined environment, inhibitor
© 2012 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).
2012
Association for Materials Protection and Performance (AMPP)
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