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

Various electrochemical methods were used to understand the behavior of steel buried in unsaturated artificial soil in the presence of cathodic protection (CP) applied at polarization levels corresponding to correct CP or overprotection. Carbon steel coupons were buried for 90 days, and the steel/electrolyte interface was studied at various exposure times. The coupons remained at open circuit potential (OCP) for the first seven days before CP was applied at potentials of -1.0 and -1.2 V vs. Cu/CuSO4 for the remaining 83 days. Voltammetry revealed that the corrosion rate decreased from similar to 330 mu m yr(-1) at OCP to similar to 7 mu m yr(-1) for an applied potential of -1.0 V vs. Cu/CuSO4. CP effectiveness increased with time due to the formation of a protective layer on the steel surface. Raman spectroscopy revealed that this layer mainly consisted of magnetite. EIS confirmed the progressive increase of the protective ability of the magnetite-rich layer. At -1.2 V vs. Cu/CuSO4, the residual corrosion rate of steel fluctuated between 8 and 15 mu m yr(-1). EIS indicated that the protective ability of the magnetite-rich layer deteriorated after day 63. As water reduction proved significant at this potential, it is proposed that the released H-2 bubbles damage the protective layer.

Automated summary

The research found that under correct cathodic protection, the corrosion rate of steel significantly decreased, with the formation of a magnetite layer enhancing the protective effect. However, under overprotection conditions, the efficacy of the protective layer decreased over time, possibly due to the significant water reduction reaction.