J. Phys. Chem. C 2008, 112, 19405–19411
19405
Generation of a Highly Effective Corrosion Barrier on LiH Surfaces Jonathan Phillips,* Kennard V. Wilson, Dan Kelly, and John Tanski Los Alamos National Laboratory, Los Alamos New Mexico, 87545 ReceiVed: April 17, 2008; ReVised Manuscript ReceiVed: September 24, 2008
The kinetics of LiH powder/water reactions indicate a corrosion resistant layer can readily be generated on LiH by exposure to a mixture of CO2 and H2O. Specifically, LiH powder corrodes very rapidly even in low ppm concentration of water, but samples pretreated in CO2/H2O mixtures exhibit a lengthy ‘delay’ before the onset of a very slow (relative to untreated material in the same corrosion conditions) corrosion reaction. The longer the pretreatment, the longer the delay prior to corrosion onset and the ‘slower’ the ultimate corrosion rate. X-ray photoelectron spectroscopy indicates that the CO2/H2O pretreatment creates a thin (ca. 1000 nm) carbonate layer. Surprisingly, a review of the kinetics of LiH corrosion for both treated and untreated material shows a very low activation energy for corrosion. This suggests that diffusion through a ‘barrier layer’ is rate controlling even on ‘pristine’ material. This implies earlier measures of true ‘neat’ reaction kinetics of hydride powders may not be entirely accurate. However, from a materials engineering perspective these findings suggest not a problem but a solution to the dominant mode (H2O) of LiH corrosion. The data also suggest a possible method of CO2 sequestration with net energy release. Introduction Earlier reports from our team on the kinetics of LiH reaction with water established that a custom microbalance which allows materials to be exposed to flow streams containing precise concentrations of water (relative humidity, RH), over a range of temperatures, is an excellent tool for determination of the kinetics of material reaction with water.1 In particular, in that study we provided, surprisingly for the first time given the amount of effort that has been expended studying the corrosion rate of LiH,2-10 a rate expression for the corrosion process as a function of water concentration and temperature. Specifically, we showed1 that commercial samples of LiH, exposed to precise concentrations of water, gained weight at a constant rate. Other data were consistent with the postulate that the corrosion layer has a ‘trilayer’ form: LiOH/Li2O/LiH.2,11-14 The microbalance data were used to develop simple rate expressions for corrosion in water as a function of temperature and water concentration (first order) that matched the data with high fidelity. The expressions developed met expectations for a solid/gas reacting system, except the very low activation energy observed (
