• Energy Research

Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawaterwas investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential -1.2V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg4Al2(OH)14·2H2O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6Al2(OH)18·4H2O islands.

Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawaterwas investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential -1.2V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg4Al2(OH)14·2H2O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6Al2(OH)18·4H2O islands.

Cathodic protection of metals in seawater is known to be influenced by chemical-physical parameters affecting cathodic processes (oxygen discharge, hydrogen evolution and calcareous deposit precipitation). In shallowseawater, these parameters are influenced by sunlight photoperiod and photosynthetic activity. The results presented here represent the first step in studies dedicated to cathodic protection in shallow photic seawater. This paper reports on carbon steel protected at -850mV vs. Ag/AgCl (oxygen limiting current regime) in the presence of sunlight radiation but in the absence of biological and photosynthetic activity, the role of which deserves future research. Comparison of results obtained by exposing electrochemical cells to daylight cycles in both biologically inactivated natural seawater and in NaCl 3.5 wt.% solutions showed that sunlight affects current densities and that calcareous deposit interfere with lightcurrents effects. Sunlight radiation and induced heating of the solution have been separated, highlighting results not otherwise obvious: (1) observed current waves concomitant with sunlight radiation depend fundamentally on solar radiation, (2) solar radiation can determine current enhancements from early to late phases of aragonite crystal growth, (3) a three-day-old CaCO3 layer reduces but does not eliminate the amplitude of the current waves. Theoretical calculations for oxygen limiting currents and additional field tests showed that sunlight, rather than bulk solution heating, is the main cause of daily current enhancements. This was confirmed by polarizations performed at -850 and -1000mV vs. Ag/AgCl (constant bulk temperature), during which the electrode was irradiated with artificial lighting. This test also confirmed O2 discharge to be the cathodic process involved. A mechanism of radiation conversion to heat in the oxygen diffusion layer region is proposed.

Cathodic protection of metals in seawater is known to be influenced by chemical-physical parameters affecting cathodic processes (oxygen discharge, hydrogen evolution and calcareous deposit precipitation). In shallowseawater, these parameters are influenced by sunlight photoperiod and photosynthetic activity. The results presented here represent the first step in studies dedicated to cathodic protection in shallow photic seawater. This paper reports on carbon steel protected at -850mV vs. Ag/AgCl (oxygen limiting current regime) in the presence of sunlight radiation but in the absence of biological and photosynthetic activity, the role of which deserves future research. Comparison of results obtained by exposing electrochemical cells to daylight cycles in both biologically inactivated natural seawater and in NaCl 3.5 wt.% solutions showed that sunlight affects current densities and that calcareous deposit interfere with lightcurrents effects. Sunlight radiation and induced heating of the solution have been separated, highlighting results not otherwise obvious: (1) observed current waves concomitant with sunlight radiation depend fundamentally on solar radiation, (2) solar radiation can determine current enhancements from early to late phases of aragonite crystal growth, (3) a three-day-old CaCO3 layer reduces but does not eliminate the amplitude of the current waves. Theoretical calculations for oxygen limiting currents and additional field tests showed that sunlight, rather than bulk solution heating, is the main cause of daily current enhancements. This was confirmed by polarizations performed at -850 and -1000mV vs. Ag/AgCl (constant bulk temperature), during which the electrode was irradiated with artificial lighting. This test also confirmed O2 discharge to be the cathodic process involved. A mechanism of radiation conversion to heat in the oxygen diffusion layer region is proposed.