Effects of cathodic protection breakdowns and resumptions on the steel corrosion products layer in the tidal zone

Carbon steel coupons were set in the tidal zone of two seaports (Le Havre and Nantes-Saint Nazaire) for 7 years with a cathodic protection applied permanently, stopped after 6 years or applied only the last year. The products covering the coupons were thoroughly characterized by X-ray diffraction and ae-Raman spectroscopy, and the corrosion rates were estimated by residual thickness measurements. Permanently protected steel coupons are characterized by corrosion rates ten times smaller than those measured for unprotected coupons. The products of the residual corrosion process, similar to those observed at the open circuit potential, form a thin layer on the steel surface under the calcareous deposit. Stopping cathodic protection after 6 years induces a re-acceleration of the degradation, as revealed by the accumulation of the usual corrosion products. This confirms that cathodic protection has mainly a moderating effect on the kinetics of a process basically unchanged. Finally, the delayed application of the protection can induce the transformation of previously formed corrosion products. For instance, sulfate green rust is integrally transformed into carbonate green rust. Other compounds, such as magnetite, are not affected. The importance of the modifications induced by cathodic protection consequently depends on the composition of the pre-existing rust layer.