Influence of soil moisture on the corrosion processes of carbon steel in artificial soil: Active area and differential aeration cells

Carbon steel coupons were buried for 4 months in an artificial silt loam soil initially set at 75% of saturation with a 0.01 M NaCl solution before to be subjected to wet/dry cycles. The corrosion process was monitored by electrochemical impedance spectroscopy and voltammetry around OCP. The evolutions of soil moisture and O2 concentration at the vicinity of the coupons were also recorded. For high water contents (≥60%sat.) the corrosion rate was linked to the transport of O2 and was maximal around 60-70%sat. because transport of O2 was rapid in the soil pores partially filled with electrolyte. With further drying of the soil, the corrosion rate decreased while [O2] remained constant. A concomitant increase of the electrolyte resistance Rs showed that both Rs and corrosion rate variations were due to the decrease of the active area of the electrode. Actually, for soil moistures below 60%sat., the corrected corrosion rate, expressed with respect to the active area, still increases while soil moisture decreases. Galvanic couplings between aerated and de-aerated zones led to localized corrosion processes with estimated rates up to 1.5 mm yr−1.