CORROSION OF METALLIC MATERIALS IN SODIUM POLYSULFIDE

A method is described to determine corrosion rates of thin metal films in a sodium polysulfide melt at temperatures around 350-degrees-C by four-point dc resistance measurements, The steady state corrosion rate of aluminum at the free corrosion potential in Na2S4 at 350-degrees-C was 150 mum/year or less and the enthalpy of activation almost-equal-to 300 kJ/mol. The corrosion rate increased with the square of the cathodic current density. Anodic polarization resulted in an increase of the thickness of the passivating aluminum sulfide layer but had little influence on the corrosion rate. On the steel X10CrAl24 (AISI 446), growth of a sulfide film followed the parabolic law until a steady state corrosion rate was established at about 90 mum/year for 350-degrees-C. The corrosion rate of the steel X10CrNiMoTi 18 10 (AISI 316 Ti) not containing aluminum was 1.5 mm/year. Stellite 6 and glassy carbon corroded at rates below the 2 mum/year sensitivity of the method. A bypass cell is proposed to measure the effect of corrosion products deposited at beta''-alumina on the cell resistance of the sodium-sulfur battery. Specific resistances of sulfides which may appear as corrosion products correlated well to the observed increases of the resistance with different electrodes in the bypass cell.