Electrochemical formation and transformation of corrosion products on carbon steel under cathodic protection in seawater

Carbon steel coupons were permanently exposed to seawater in the low water zone of two French harbors for 5 years before to be subjected to cathodic protection (potential about −0.95 V vs. Ag/AgCl/seawater) for 1 year. The application of cathodic polarization to the previously unprotected coupons induced the transformation of GR(SO42-) into GR(CO32-). Other coupons were permanently protected by cathodic protection during the 6 years immersion. A thin film of green rust and mackinawite was observed on the steel surface. This film was covered by a thicker layer of calcareous deposits.

► Cathodic polarization of corroded carbon steel induces changes in the rust layers.
► Cathodic polarization transforms sulfated green rust into carbonated green rust.
► Residual dissolution of steel under cathodic polarization produces a thin rust layer.
► Green rust and FeS are formed on steel under cathodic protection in natural seawater.