Cathodic Protection of Carbon Steel in the Tidal Zone: Characterization of the Mineral Layer Formed on the Steel Surface

The efficiency of cathodic protection in the tidal zone, in particular in its highest part, remains questionable. To address this problem, experimental vertical structures were designed and set in a commercial seaport. Each structure was composed of 10 cm × 10 cm carbon steel coupons and 50 cm × 10 cm carbon steel strips to obtain a 5.1 m long continuous structure extending all along the tidal zone. Cathodic protection of the structures was carried out with an Al-Zn-In galvanic anode permanently immersed in seawater. The mineral layers formed on the coupons after 32 and 52 months were analyzed by XRD and µ-Raman spectroscopy. The evolution of the mineral layer from the low water zone to the splash zone was due to (i) the decreasing efficiency of the cathodic protection and (ii) the changes in corrosion processes, from those typical of a permanent immersion to those typical of atmospheric corrosion. In particular, brucite Mg(OH)2 was found up to the high tide zone and, in agreement with the estimated degradation of the coupons, its formation was an indicator of the altitude at which the cathodic protection remained efficient.