Corrosion rate of buried steel after cathodic protection interruption
Proceedings Publication Date
Dr. Erwan Diler
Erwan Diler, Marie Roustan
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Steel buried structures such as pipelines are usually protected from corrosion by a combination of organic coatings and cathodic protection (CP). In the field, according to the ISO 15589-1, the potential E to protect such structure is bounded by the protection potential Ep and the critical limit potential El E El.

Under service conditions CP interruption or depolarisation might occur due to maintenance cycles, rectifier breakdown and seasonal fluctuations. The knowledge of the consecutive corrosion rate after CP interruption is crucial to assess the structure integrity over time. This is particularly true as corrosion events are cumulative.

Under CP, at the cathode, the reduction reactions induce local pH increase and oxygen consumption. Such alkalinity interacts with the soil, following complex phenomena expressed as the soil pH Buffer Capacity (pHBC)., involving possible protonation/deprotonation of acidic groups on organic matter, oxides, and hydroxides dissolution/precipitation. Thus, after CP interruption, the local environment at the vicinity of the defects, as well as its corrosiveness might strongly differs in contrast to undisturbed soil. Under some conditions, after CP, an effective corrosion protection can be achieved by the local alkalinity and passivation, and/or oxygen depletion. However, such phenomena are expected to be limited with time.

In this study an experimental setup is proposed to assess the key parameters driving the time to corrosion after CP interruption in soils at different moisture levels, and pHBC. For this purpose, a set-up with pH and dissolved oxygen probes, as well as real time corrosion sensors are proposed. Beyond, the key role of the pH and oxygen, under some conditions the local modification of the environment seems to strongly increase the corrosion rate. A strong polarisation, prior to the CP interruption appears thus not to be fully beneficial in terms of global corrosion mitigation.

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