Influence of soil moisture on the residual corrosion rates of buried carbon steel structures under cathodic protection

Abstract

Carbon steel coupons buried in a sand soil with various moisture conditions were left at open circuit potential during 14-17 days before to be subjected to cathodic protection for 78–90 days. Voltammetry was used to follow the corrosion rate with time. The residual corrosion rate under cathodic protection was estimated via a mathematical adjustment of the polarization curves with a theoretical kinetic law, which allows the determination of the anodic part of the current. The ohmic drop was corrected after measurement of soil resistance with electrochemical impedance spectroscopy. At open circuit potential, a link between soil resistance and corrosion rate revealed that these two parameters were mainly influenced by the active area of the coupon. Thus, the various measured corrosion rates (from 40 to 400μm yr−1) mainly reflect variations of the active area. Under cathodic protection, the active area increases and the residual corrosion rates tend to a unique value for all coupons. Average corrosion rates deduced from voltammetry were consistent with weight loss measurements. Analysis of the products formed on the coupons illustrated the influence of aeration and water content. The predominant compound was α-FeOOH in the 25%/saturation soil, carbonate green rust in the saturated soil.