Numerical simulations of soil physicochemistry and aeration influences on the external corrosion and cathodic protection design of buried pipeline steels

• External corrosion of buried transmission pipeline simulated using finite element
• Heat transfer, cathodic protection, and O2 diffusion aspects considered
• Integrating findings from X100 pipeline steel corrosion testing improved results
• Combination of dry and cold soil environment found to be the most aggressive
• Model successfully used to identify ideal cathodic protection design parameters

Corrosion of oil and gas transmission pipelines is a serious industrial problem with potentially catastrophic environmental and financial consequences. A finite element model of the external corrosion of buried steel pipelines at coating failures is developed here to better predict degradation in different soil and cathodic protection (CP) environments. Synergistic interactions between steady-state temperature, potential, and oxygen concentration profiles in the soil surrounding the pipeline structure are quantified and discussed. Conductivity and oxygen diffusivity of soil conditions are represented as functions of soil matter, air porosity, and volumetric wetness. Theoretical formulations are uniquely merged with corrosion experiments conducted on actual pipeline steel samples, greatly improving simulation results. Overall, drier sand and clay soil structures cause the most corrosion, whereas wetter conditions impede oxygen diffusion and significantly augment hydrogen evolution. Geometric location of the coating breakdown site relative to the ground surface and the CP anode has a particular influence on oxygen concentration profiles and pipeline corrosion. Model convergence is tested with a mesh sensitivity study, and the model’s ability in evaluating practical design changes in the CP system is demonstrated.

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