Investigation of the Mechanisms and Kinetic of Cathodic Disbondment in Seawater and Soils

To ensure the integrity of offshore and onshore buried structures against corrosion, a complementary approach combining cathodic protection (CP) and organic coatings is used. Following the ISO 15589-1, the efficiency of the CP is ensured by maintaining the CP potential within a protective range with a critical limit potential value of –1.200 V/CuCuSO₄. This latter, beyond the hydrogen embrittlement risk, was mainly defined to limit the cathodic disbondment (CD). For onshore buried structures, lower potentials were observed in the field. The influence of over cathodic protection on the coating is rather complex since it depends on many factors such as the nature of the coating, the media, the temperature, the electrochemical processes and the kinetic involved. In the literature, many CD testing procedures are discussed in terms of testing parameters. For instance, existing procedures are usually limited to chloride containing electrolytes that are not representative of underground environment. In this study, CD tests between 28 days and 12 months were realized in soils and seawater at different potentials on Fusion Bonded Epoxy (FBE) and three layers Polyethylene (3LPE) coatings. Different CD rate were observed in these medias, with in soils a strong influence of the moisture level.

Cathodic protection is used in addition to organic coatings to ensure the integrity of offshore and onshore buried structures against corrosion. The cathodic protection efficiency is usually ensured by keeping the potential of the structure to be protected in a narrow range following standard recommendations such as ISO 15589-1 and/or NF EN 12954.

For onshore buried structures, this potential range is limited by the protection potential E_p and the limit potential E_l. The protection potential is defined according to the material, the soil resistivity, and the temperature. The limit potential E_l is a function of the material, its mechanical properties, and the presence of organic coating. For carbon steels and low alloy steels with a yield strength lower than 550 MPa (N.mm⁻²), the hydrogen embrittlement risk can be neglected. In presence of organic coatings, E_l has also to be considered due to the risk of CD ¹ and should not be more negative than –1.200 V/CuCuSO₄. More negative than this potential, the structure under CP is considered as over polarized.