• Energy Research

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.

Traditional moulds for jewellery casting are made Of SiO2 refractory particles agglomerated by a bonding phase. Typical precious alloys are moulded around 1100 degreesC, a temperature that might lead to partial thermal decomposition of the bonding phase, typically CaSO4. The degradation process usually causes release of gas, e.g. SOx, which is responsible for high porosity and roughness in the casting. These gas imperfections are responsible for about 10% of the overall casting failures. The study of novel bonding phases, developed to eliminate gas defects and improve mechanical strength, is reported in this paper. In traditional moulds, bonding is generated during the investment stage, the new phases are obtained during ceramic burnout. New moulds are made from a mixture of quartz and CaO powders, and in the investment stage, quartz particles are embedded in a network of Ca(OH)(2). In the next firing, Ca(OH)(2) first dehydrates and then reacts with the surface Of SiO2 particles to form CaxSiOx+2 phases. These interfacial compounds provide a refractory scaffold for SiO2. The process has been studied by TG-DTA. Different firing temperatures have been tested and the silicate moulds have been studied by XRD. Mechanical and casting performances have been evaluated by compression tests and microstructural analysis. Cast comparison with respect to the traditional refractory is also illustrated. (C) 2004 Elsevier B.V. All rights reserved.