51312-01495-Corrosion rates of carbon steel in confined environments

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Product Number: 51312-01495-SG

ISBN: 01495 2012 CP

Author: Adam Rubin

Publication Date: 2012

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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. 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 or autoclaves. The wires are exposed to water solutions in saturation with CO2 or CO2/H2S mixtures. Corrosion rates are determined based on weight loss measurements. In addition electrochemical potential measurements and measurements of iron in solution 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 even with partial pressures of CO2 up to 10 bar. Finally experimental data where the level of V/S ratio varied within the test cell suggest that both the low V/S ratio as well as high super saturation levels of iron in solution influence the corrosion rate.