Abstract
Abstract : The corrosion behavior of carbon steel was studied in artificial soils saturated to 50%, 70% and in wet/dry cycles for exposure times of 37 up to 90 days. In each experiment, three carbon steel coupons were buried in soil in the presence of cathodic protection at various polarization levels. Various electrochemical techniques were employed to study the corrosion behavior. The voltammetry around open circuit potential (VAOCP) method was used to study the corrosion rate of the unprotected steel coupons while voltammetry was applied in order to deduce the residual corrosion rate of the coupons under cathodic protection (CP). The electrochemical impedance spectroscopy (EIS) results revealed that very high soil electrolyte resistance in soil saturated at 50% which led to very low corrosion rates and consequently difficult to study the CP efficiency. The VAOCP method reported that the highest corrosion rates, ~370 μm/yr, were obtained in soil saturated at 65-70% for the unprotected coupons. However, the dry/wet cycles in soil also gave higher corrosion rates but with no clear saturation level as the aeration/deaeration facilitated O2 transport inconsistently. The monitoring of CP on the protected coupons was performed via voltammetry and revealed that polarization levels -0.8 V (insufficient) and -1.2 V (excessive) vs. Cu/CuSO4 did not provide full protection of the coupons in soil. The residual corrosion rates were found to be 12 μm/yr at CP -0.8 V vs. Cu/CuSO4 and 30 μm/yr at CP -1.2 V vs. Cu/CuSO4 (>10 μm/yr recommended by the EN 12954: 2001 Standard for full protection). In the case of excessive polarization, the EIS revealed unstable mineral layer covering the steel surface more likely due to hydrogen evolution reaction. The best protection of steel was achieved at the polarization level of -1 V vs. Cu/CuSO4 with the residual corrosion rate estimated at 5-7 μm/yr. In such unsaturated soils, cathodic protection (CP) led to an increase of interfacial pH and to an increase of the active area of the steel linked to electro-capillarity effects. The increase of interfacial pH led to the formation of a pseudo-passive oxide layer on the steel surface. Raman spectroscopy analysis of the coupon under CP revealed that the mineral layer covering the steel surface was composed of a calcareous mineral layer, magnetite, green rust and lepidocrocite. CP efficiency, primarily linked to the polarization level, also depended on the properties of the calcareous deposit and those of the possible pseudo-passive oxide layer.
D.Phil. (Engineering Management)