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 O-2 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 O-2 and was maximal around 60-70% sat. because transport of O-2 was rapid in the soil pores partially filled with electrolyte. With further drying of the soil, the corrosion rate decreased while [O-2] remained constant. A concomitant increase of the electrolyte resistance R-s 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). (C) 2016 Elsevier Ltd. All rights reserved.