Kinetics and characterisation of calcareous deposits under cathodic protection in natural sea water

Calcareous deposits due to cathodic protection in marine environment have been analyzed by chronoamperometry, ac and EHD impedances, and microscopy examination. In a first part devoted to synthetic sea-water, it has been shown that Mg(OH)(2) is formed first, then aragonite calcium carbonate crystals developed with an increasing size till approximately 50 mu m before coalescence. Impedances measurements showed that oxygen diffusion proceeds through a porous medium towards an interface partly blocked by aragonite. Both the blocked fraction of the interface and the size of the blocked sites can be measured, providing an on-line characterization of the elementary current potential curves for oxygen reduction, necessary for an automatic monitoring of cathodic protection. In a second part, first results obtained in natural sea-water indicated the importance of clay suspensions in the deposition kinetics.