The development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy.

摘要

This study has been directed towards developing environmentally friendly protective coatings for the depleted uranium (DU)-0.75 wt.% titanium alloy. The surface of the alloy was studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy. The acicular martensitic microstructure is revealed after chemical etching. Several impurities have been identified and their prevalence has been found to vary from sample to sample.;The electrochemistry of the alloy was studied using open circuit potential measurements and potentiodynamic polarization techniques. The effect of various oxyanions, MoO42-, PO43-, VO43-, MnO4-, SiO 44- and WO42-, on the electrochemical behavior of the DU alloy in nitric acid has been explored and MoO4 2- was chosen for further studies.;Surface activation has been examined using fluorine compounds or concentrated nitric acid. Residual fluoride from the activation process was found to interfere with coating formation. Surface activation by nitric acid yields a more robust coating. The corrosion protection of the coatings was evaluated by potentiodynamic polarization testing in quiescent 0.05 M NaCl. The coatings were studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy.;The acid-activated surface forms a coating that offers corrosion protection after a period of aging. X-ray photoelectron spectroscopy revealed that the protective coating is primarily a polymolybdate bound to a uranyl ion. Rutherford backscattering spectroscopy on the acid-activated coatings shows uranium dioxide migrating to the surface. The coatings on the fluoride-activated samples have been found to contain molybdenum trioxide and a minor component of an Mo(V) oxide.;Raman and Infrared spectroscopies have been performed to examine the chemistry during aging. Raman indicates that the coating has an octamolybdate structure and FTIR shows that uranyl hydroxide is present in the as-made coating but no longer apparent in the aged coatings, supporting the proposed mechanism describing the aging of the coating.;Electrochemical impedance spectroscopy has been performed on the coating and compared with the untreated DU alloy. The as-made coating yielded Nyquist plots that were similar to the untreated samples and contained capacitive inductive loops. The aged coating exhibits significantly different behavior and has been modeled with a four element equivalent circuit.