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

Abstract

The behavior of steel coupons buried in soil under cathodic protection (CP) was studied during wet/dry cycles using electrochemical impedance spectroscopy and voltammetry. The coupons were left at open circuit potential for 7 days before applying CP for 51 days at potentials of −0.8 and −1.0 V versus Cu/CuSO4 on coupons 1 and 2, respectively. Wet/dry cycling was achieved by first saturating the soil with an electrolyte inside a sealed electrochemical cell and by allowing the soil to dry by opening the top lid of the cell for various periods in the experiment. Surface analysis was performed after the experiments by X‐ray diffraction and Raman spectroscopy. The soil electrolyte resistance Rs depended mainly on the variations of soil moisture for coupon 1 but was strongly affected by the effects induced by CP for coupon 2. Residual corrosion rates of 17–18 and 7–10 µm/year for coupons 1 and 2, respectively, were estimated via voltammetry. The kinetic parameters vary with the polarization level so that the data obtained with a coupon polarized at a given potential cannot be used to predict the residual corrosion rate of a coupon polarized at another potential.

Study of Overprotective-Polarization of Steel Subjected to Cathodic Protection in Unsaturated Soil

“…The reference electrode was the Celco 5 (COREXCO, Décines Charpieu, France) copper-copper sulfate (Cu/CuSO 4 ) electrode (+0.316 V/SHE at 25 °C) specifically designed for soil experiments and commonly used in the field. It was also used in previous studies [ 10 , 11 , 23 , 24 , 25 , 26 , 27 ]. All electrochemical potentials were expressed with reference to this electrode, i.e., V vs. Cu/CuSO 4 .…”

“…The rest of the surface, which is in contact with air, does not participate in the electrochemical processes. More recently, the combined effects of cathodic polarization and variations of soil humidity were studied and it was shown that these effects depended on the applied potential [ 27 ]. It was also observed that the residual corrosion process led to a porous magnetite layer that influenced the cathodic reaction [ 27 ].…”

“…In some cases, the decrease of the potential indeed led to a decrease of the residual corrosion rate [ 20 , 27 ]. In other cases, an opposite effect was observed, suggesting that an excessively cathodic potential could be detrimental, more likely by hindering the formation of a protective layer of (residual) corrosion products [ 24 , 25 ].…”

“…They were left at open circuit potential (OCP) for the first seven days before CP was applied for the remaining 83 days of the experiment. EIS measurements were performed to study the evolution of the steel/electrolyte interface, while voltammetry was used to determine the residual corrosion rate using the methodology developed in previous works [ 25 , 26 , 27 ]. The characterization of the mineral layer covering the coupons at the end of the 90-day experiment was achieved by µ-Raman spectroscopy.…”

Assessment of cathodic protection effect on long‐distance gas transportation pipelines based on buried steel specimens

“…Bahekar and Gadve, [ 24 ] Diler et al, [ 25 ] and Brownlie et al [ 26 ] considered the influence of the protection current and its density, and they studied the corrosion of different cathodic protection currents on carbon steel to evaluate the influence of the cathodic protection system of steel structures. Ghobadi at al., [ 27 ] Attarchi et al, [ 28 ] and Mahlobo et al [ 29 ] used the finite element method to simulate and obtain the potential of steel structure cathodic protection system, and they studied the corrosion behavior of steel structure under cathodic protection, thereby quantitatively evaluating the cathodic protection effect. Ghanbari and Lillard, [ 30 ] Eichler and Isecke, [ 31 ] and Andrea et al [ 32 ] considered the influence of AC and DC stray current corrosion and analyzed the cathodic protection effect of various parameters on steel structures.…”