Electrochemical Monitoring of Steel/Soil Interfaces during Wet/Dry Cycles

Abstract

Carbon steel coupons were buried for 6 months in an artificial clay soil initially set at 75% saturation with a 0.01 M NaCl solution. Wet/dry cycles were applied to the system so as to induce strong modifications of the steel/soil interface. The steel coupons were set in a large soil box, as far as possible from each other, so that they behave differently as the drying/wetting of the soil was heterogeneous. The electrochemical analysis of the steel/soil interface was achieved by combining voltammetry, used to determine corrosion potential and corrosion rate, and electrochemical impedance spectroscopy, used to estimate the active area of the electrode via the determination of the soil electrolyte resistance. This analysis allowed monitoring the evolution of the steel/soil interface during the wetting and drying stages of the system and gave information on the specific behavior of each coupon.

Cathodic protection of buried steel structures: Processes occurring at the steel/soil interface during wet/dry cycles

“…One can see that the shape of the diagrams did not change, whereas R s decreased back to a low value of 1,240 Ω · cm 2 , very similar to that measured at Day 1, that is, 1,290 Ω · cm 2 . This illustrates clearly the link between soil moisture and resistivity and shows how R s can give information on soil moisture . Finally, the cell was opened again at Day 47.…”

“…In agreement, the OCP values, listed in Table , are low. They remained between −0.767 and −0.780 V versus Cu/CuSO 4 , which is typical of a soil close to saturation where the transport of O 2 is slow because it can only take place via diffusion in the liquid phase …”

“…Therefore, the liquid would tend to spread on the surface when CP is applied. In unsaturated soils, this phenomenon increases the active area of the electrode, that is the soil electrolyte/metal contact area (i.e., the “wet” area) and consequently decreases R s . It was even proposed, in unsaturated soils in the absence of CP, that R s was mainly linked to the soil electrolyte/metal contact area and, more precisely, inversely proportional to this area …”

“…The two prepared coupons were used as working electrodes (WE) and thus called WE CP1 and WE CP2 . The reference electrodes were the Celco 5 (COREXCO) copper–copper sulphate (Cu/CuSO 4 ) electrodes ( E = +0.316 V/SHE at 25°C) specifically designed for soil experiments, commonly used in the field and also used in previous studies . All electrochemical potentials in this study are expressed with reference to this electrode, that is, V versus Cu/CuSO 4 .…”

“…The present work follows this series of studies devoted, on the one hand to the corrosion processes of carbon steel in soils occurring during wet/dry cycling and, on the other hand, to the CP of steel structures buried in soils with constant moisture . The idea was to study the behavior of steel coupons buried in soil and subjected to CP during wet/dry cycles.…”