This work presents a numerical model of the coupled interactions between temperature profile, electrolytic potential drop, and steady-state oxygen concentration gradient in soils surrounding buried pipelines. Three different soil types are considered (sand, clay, and peat), with porosity ratios varying between 0.4 and 0.8. Two volumetric wetness ratios are simulated for each soil type, representing moisture changes during successive soil drying-wetting cycles. The motivation behind this study is to model the interdependencies of heat transfer, cathodic protection, and oxygen diffusion on pipeline steel corrosion in various soil environments. A key benefit of the developed model is its rapid scalability, allowing the simulation of these interrelated phenomena for different geometries, dimensions, and boundary/initial conditions. The results of a select number of cases are presented in this paper.
Based on the oxygen diffusion, cathodic protection, and iron oxidation behavior of an exposed 90˚ arc on the pipeline’s external surface facing a magnesium cathodic protection anode, it is found that drier sand and clay soil structures cause the most corrosion. The geometric location of the coating holiday relative to the ground surface and the cathodic protection anode has a particular influence on oxygen concentration and iron oxidation. Temperature fluctuations during seasonal weather cycles have observable effects on iron oxidation rates due to influences on heat transfer and oxygen diffusivity. An overall trend of decreased oxygen concentration and iron oxidation in wetter and warmer soils is detected and quantified.
Keywords: downloadable, pipeline steel, soil corrosion, oxygen diffusion, finite element modelling, cathodic protection