A Two-Dimensional Model for Steel Corrosion Under a Disbonded Coating Due to Oxygen With or Without Cathodic Protection—Part 2: Model Simplification for Practical Application Available to Purchase

The complex two-dimensional (2D) model developed in Part 1 to predict pipeline steel corrosion rate under a disbonded coating with or without cathodic protection (CP) has been simplified in this work for easy application of this model. It has been found that steel corrosion under a disbonded coating due to dissolved oxygen (O₂) can be reproduced in otherwise deaerated conditions as long as the potential at the holiday is kept the same. With the model simplified to 1D, steel potential (ψ) anywhere in the disbonded region at a distance, z, from the holiday can be estimated with:$ψ−EOCP ψ0−EOCP =e−C1za$⁠, where ψ₀ is the potential at holiday, “a” is the disbondment gap size or separation distance between the inner surface of the disbonded coating and steel surface, and $C1=aρRp$⁠. ρ is solution resistivity in disbondment and E_OCP and R_p are, respectively, open-circuit potential (OCP) and linear polarization resistance of pipeline steel in deaerated solution with the same ionic chemistry as in the disbondment. The distance that CP can penetrate into a disbonded region from holiday is roughly $3aC1$⁠.