Cathodic protection of carbon steel in the tidal zone: evolution of the mineral layer from immersion zone to splash zone

This study deals with cathodic protection of carbon steel in the tidal zone. Actually, the metal can be polarized even when it is not immersed in seawater because a liquid film remains on the surface during the emersion period. The efficiency of cathodic protection then depends on the properties of this thin liquid film. The aim of the present work was to study the mineral layer formed on carbon steel coupons under cathodic protection in the tidal zone. A specific experimental system was designed, with 10X10 cm steel coupons simulating a continuous vertical structure of 5.1 m height, i.e. extending all along the tidal zone, from the low water zone to the splash zone. An Al-Zn-In galvanic anode permanently immersed in seawater ensured the protection of the coupons. After 32 months of experiment, the mineral layer covering the coupons was scraped, rapidly carried to the laboratory for analysis 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 of corrosion processes (from those typical of a permanent immersion to those typical of atmospheric corrosion).