Corrosion behavior of X80 pipeline steel in black soil with different relative water contents

• Stable impedance measurements were achieved using comb-tooth electrodes in the black soil environment, reducing the errors caused by environmental impedance. • The water content of black soil has a crucial impact on the corrosion resistance of X80 pipeline steel. • The influence of the differences in water absorption properties of minerals in black soil on the corrosion of X80 steel pipeline steel was studied. • The influence of water content in black soil on the cathodic corrosion process of X80 pipeline steel was studied. This study explores influences of relative water content in black soil and the differences in water absorption among major minerals (quartz sand, dolomite, montmorillonite) on corrosion behavior of X80 pipeline steel during a 60-day burial period through mass loss measurements, potentiodynamic polarization curves (PC), electrochemical impedance spectroscopy (EIS) tests, and characterization of corrosion morphology and products. Results indicate that average corrosion rate of X80 steel exhibits a "increasing followed by decreasing" trend with soil relative water content. Local corrosion occurs at 32% and 46% water content, while uniform corrosion is observed at 62% water content. Specifically, at low water content (Sr≈32%), nanoscale interlayers of montmorillonite remain unsaturated and continuously absorb capillary water from the micrometer scale pores of quartz sand and dolomite particles, leading to increasing soil resistivity and a steady reduction in the instantaneous corrosion rate of the steel as burial time extends. At moderate water content (Sr≈46%), the interlayers of montmorillonite are water-saturated, and the pores contain an appropriate amount of capillary water and moist air. Soil resistivity remains stable, and the steel surface is wetted by a thin liquid film and undergoes rusting, with the instantaneous corrosion rate first increasing then decreasing over time (associated with the barrier effect of the rust layer and the participation of Fe(III) oxides in the cathodic process). At high water content (Sr≈62%), continuous water contact is formed between steel and soil particles, and soil resistivity stays stable. Saturated adsorbed water, along with substantial capillary water and free water in the micrometer-scale pores of soil particles, forms a uniform thin liquid film on the steel surface, triggering uniform corrosion of X80 steel, with the instantaneous corrosion rate first decreasing, then increasing over time.