This paper deals with calcareous deposit precipitation occurring on cathodically protected carbon steel in natural seawater. Daylight cycles related effects were here investigated. Being O2 both cathodic reactant and key molecule of aerobic environments, a potential involving oxygen limiting current without hydrogen evolution (–850 mV vs. Ag/AgCl) was employed. Comparison of natural seawater with NaCl 3.5% wt. solution data showed that sunlight radiation plays a primary role with respect to temperature in affecting cathodic currents even in presence of electro-accreted aragonite. Calcareous deposit morphology and composition was investigated through SEM, highlighting properties and peculiarities.
Cathodic protection is a widely employed technique which allows the oxidative power of environment affecting metals to be controlled. This can be achieved making the structure working as a cathode, where the reducing power necessary for the onset of cathodic processes can be supplied by impressed current or coupling to a less noble metal. This second way is preferably used in seawater where the metallic structure is protected with sacrificial anodes of aluminum, zinc or magnesium based alloys. In aerobic environments, current requirement for completely protect the structure is reached when the oxygen that comes in contact with the metallic surface is reduced (Foster et al. 1986). Hence, all the factors that control oxygen diffusion towards the surface also control the cathodic current behavior. As a result of the cathodic polarization is the alkalization at the metal/solution interface, where the increase of OH- shifts the carbon dioxide system equilibrium towards the formation of carbonate and allowing the precipitation of CaCO3 to take place (Barchiche et al. 2003). The commonly called calcareous deposit is not formed only by CaCO3; if the alkalinity is enhanced with further cathodic polarization, when pH value of 9.5 is reached Mg(OH)2 starts to precipitate (Hartt et al. 1984).