Environmental release mechanisms of tungsten and alloying elements from tungsten heavy alloys.

摘要

Tungsten heavy alloys and composites, containing 88% to 48% tungsten with the rest usually made up with Ni, Co, Fe and Cu in different combinations and ratios, have been used in a wide variety of applications. They are currently being used or proposed for use as replacements for lead and depleted uranium in small arms munitions and kinetic energy penetrators. The prevailing notion has been that tungsten is inert and therefore environmentally benign. However, new findings challenge the alleged inert nature of tungsten materials.; Tungsten heavy alloys typically posses a biphasic microstructure consisting of a tungsten phase bound by an alloy matrix. Little information is available on the potential for release of tungsten or alloying components from these heavy alloys and subsequent possible environmental impact and/or health effects. This work investigates the mechanisms involved in the environmental degradation of these alloys, the kinetics of release of tungsten and alloy components and their effects on the environment. Sample alloys subjected to corrosion tests revealed significant release of tungsten heavy alloy components due to galvanic potential differences between the tungsten phase and the alloying binder phase. Morphological and spectroscopic analyses were performed using SEM/EDS and grazing incident angle XRD. In four samples studied, the binder phase suffers degradation relative to the tungsten phase while the converse was observed in the fifth sample. Analyses of the extent of corrosion and metal release show that inclusion of Co in alloy formulation reduces the potential for W release while Fe increases it. Inclusion of copper was responsible for a reverse galvanic effect leading to extensive degradation and release of tungsten from the grains.; Dissolution kinetics experiments performed corroborate the results of the corrosion tests. Tungsten was observed to dissolve at different rates in the presence of other metals depending on standard electrode potential differences. The dissolution of tungsten, strongly dependent on the availability of hydroxyl ions OH-, is enhanced when these ions are produced during the dissolution of the other metals. Effects on pH, DO and ORP were observed and correlated with the observed trends in concentrations of dissolved tungsten and the alloying metals. Kinetic and equilibrium modeling limitations are discussed.; The speciation of tungsten in solution comprises of a rather complicated system dependent on pH, concentration, counter ions and aging time. Polyoxotungstate species have been suspected of possible adverse biological effects. Results of FTIR-ATR and Mass Spectroscopy studies of the effects of aging on the speciation of tungsten in solution suggest changes occur in molecular and bond structure over time. Results also indicate that even at environmental concentration levels (mg L-1) of tungsten and within relevant environmental pH ranges, polyoxotungstate species are formed.; With the anticipated increase in the utilization of tungsten and tungsten based materials, results obtained from this work are of significance in fostering a proactive approach to environmental stewardship of new and existing tungsten applications.