Coupled Thermal-Electrochemical Modelling of Localised Corrosion Under Disbonded Coatings on Buried Pipelines in Tropical Nigerian Soils

This study develops a coupled thermal-electrochemical finite element model for simulating localised corrosion processes occurring under disbonded pipeline coatings in tropical Nigerian soil environments, addressing a gap in mechanistic understanding that directly affects the reliability of cathodic protection effectiveness predictions for buried pipelines. Localised corrosion under coating disbondments is a major contributor to pipeline integrity failures in Nigeria, and the effectiveness of impressed current cathodic protection is highly sensitive to the shielding geometry and soil resistivity beneath the disbond. The model couples heat transfer in the soil domain, governed by the transient diffusion equation with seasonal surface temperature boundary conditions representative of Edo and Delta State soil profiles, with a Butler-Volmer electrochemical reaction kinetic model for the disbonded steel surface within the disbond cavity. Ion transport within the disbond electrolyte is modelled by the Nernst-Planck equation with coupled electrostatic potential. The full model is implemented in COMSOL Multiphysics and validated against laboratory-scale disbond cell experiments using synthetic Nigerian lateritic soil electrolytes. Parametric studies investigate the influence of disbond geometry, soil resistivity, cathodic protection level, and seasonal temperature fluctuation on shielding distance and maximum corrosion rate within the disbond. The model produces original predictions of the critical pipe-to-soil potential window that ensures adequate protection under Nigerian tropical soil conditions. Keywords: localised corrosion, disbonded coating, pipeline, cathodic protection, finite element modelling.