Corrosion of carbon steel in artificial soil: Processes occurring during wet/ dry transitions studied with a multi-coupon electrode

A multi-coupon electrode, composed of 16 square (5 mm x 5 mm) carbon steel coupons, was buried at room temperature during 125 days in an artificial silt loam soil. The soil was initially set at 75% of the saturation with a 0.01 M NaCl + 0.01 M NaHCO3 solution in a specific electrochemical cell. The experimental cell was hermetically closed for 32 days before to be opened so that the effects induced by the progressive drying of the soil could be studied. The OCP and the soil electrolyte resistance R-s, determined by electrochemical impedance spectroscopy, were then monitored over time for each coupon. During the drying process, different behaviors were observed for the various coupons, demonstrating the heterogeneity of this process. XRD analysis of the corrosion product layer covering each coupon at the end of the experiment revealed significant variations of composition from one coupon to another. The composition of the corrosion product layers could be interpreted in link with the OCP and R-s measured values.