Corrosion of underground pipelines in clay soil with varied soil layer thicknesses and aerations

Abstract

Underground pipelines suffer from corrosion in the soil. In this work, weight-loss testing, electrochemical measurements and surface analysis techniques were used to investigate the pipeline steel corrosion in a Regina clay soil with varied soil layer thicknesses and gassing conditions (i.e., aerobic, CO2-containing and anaerobic). Results demonstrate that the steel corrosion is dependent on the soil layer thickness, where a maximum corrosion rate is recorded under the 5 mm thick soil due to the competitive impact of the blocking effect of the soil layer and its moisture-retaining ability on the corrosion. In thin soil layers, the corrosion is accelerated with the increasing soil thickness due to more water contained in the soil. With the further increase of the soil layer thickness, the blocking effect of the soil on diffusion of corrosive species is dominant. The corrosion of the steel also depends heavily on the gassing condition in the soil. Under the specific soil layer thickness, the steel suffers from the highest corrosion rate in aerobic soil, which is followed by the 5% CO2/N2-containing soil. The steel corrosion is negligible (0.006 mm/y only) when the soil is purged with N2. In addition to uniform corrosion, localized corrosion occurs on the steel under soil due to its heterogeneous nature at a much greater rate than the uniform corrosion. The maximum localized corrosion rates in 5% CO2/N2-containing 5 mm thick soil layer and in the 3 mm thick soil that is open to air are 1.03 mm/y and 0.72 mm/y, respectively. As a comparison, the uniform corrosion rates are 0.05 mm/y and 0.04 mm/y under the two conditions.