Material compatibility during removal of antimony by oxidative process using hydrogen peroxide

Dissolution and removal of antimony activity from the reactor surfaces was found to be difficult during decontamination campaigns using reductive processes. Alternatively, oxidative process using hydrogen peroxide wherein antimony gets converted to more soluble Sb⁵⁺ species that can be easily removed over ion exchange resin was thought of. In this paper, compatibility of various alloys viz., carbon steel, monel/incoloy and zircoloy in the presence of hydrogen peroxide is presented. Besides, the effect of hydrogen peroxide on the protective oxide coatings that may be formed over the system surfaces over the period of operation was also studied. Corrosion of carbon steel as a function of concentration of H₂O₂, temperature and pH was carried out using electrochemical techniques. The concentration of H₂O₂ varied from 0 to 100 ppm and pH of the solution was varied using lithium hydroxide. At pH 8.5 and 10.2, corrosion of carbon steel increased with concentration of H₂O₂. At pH 9.5, carbon steel showed a lower corrosion rate of 2.0×10^-3 μm/h with small variation with concentration of H₂O₂. In order to evaluate the effect of H₂O₂ on the oxides formed over surfaces, insitu Raman spectroscopy using flow-cell was carried out. Raman spectra in the absence of H₂O₂ showed major vibrational mode at 667 cm^-1, which is characteristic of magnetite phase. There was no change in the spectra in the presence of 10 ppm of H₂O₂ upto 80degC. On increasing the concentration of H₂O₂, a shoulder appeared at 710 cm^-1 along with the major magnetite phase. Effect of H₂O₂ on the passivated carbon steel surface was monitored by EIS with 50 ppm H₂O₂ at 60 and 80degC and pH 10.2 for 24h. The EIS data taken at OCP with one hour interval was fitting to Randle's circuit, the charge transfer resistance increased with time and reached a maximum at 65 kΩ/cm² and there after decreased, and indicating a formation of a very thin film initially that dissolves subsequently. The gravimetric analysis of the specimen showed no significant weight loss and spectroscopic mapping of the specimen before and after exposure showed no change in the oxide phase. From these observations it is seen that the phase conversion is very small that is limited to only few nanometer layers. (author)