Reaction kinetics of plutonium with oxygen, water and humid air: Moisture enhancement of the corrosion rate

Kinetic measurements for the reactions of unalloyed plutonium with water vapor and oxygen in the 35 to 400 degrees C range show that moisture enhancement of the corrosion rate is confined to a temperature regime below 200 degrees C. Zn combination with kinetic data from literature sources and measurements with air at 25 degrees C, the data define a rate-temperature envelope in which the corrosion reaction is accelerated by increasing water pressure. The lower boundary of the envelope is defined by an Arrhenius relationship based on rates measured in the moisture-independent regime above 200 degrees C and in the moisture-dependent: regime at lower temperatures. As defined by experimental data and the equilibrium vapor pressure of water, the upper envelope boundary indicates that moisture dependence is confined to the -25 to 200 degrees C range with a rate maximum near 110 degrees C. Activation energies within the envelope vary systematically with water pressure from 17.9 kcal mol(-1) in dry (less than 1 ppm H2O) air to 34.4 kcal mol(-1) in water-saturated air. The corrosion rate is independent of oxygen pressure and has a square-root dependence on water pressure. The chemistry of moisture-enhanced corrosion is defined by results showing that plutonium oxide catalyzes the combination of hydrogen and oxygen. Hydrogen formed by the inherently rapid reaction of water with plutonium catalytically recombines with oxygen at the gas-oxide interface of the reacting metal. Kinetic anomalies attributed to reaction of beta-phase plutonium in earlier studies are correlated with the presence of trace amounts of moisture in the experimental systems. Accelerated corrosion in moist argon is discussed and effects of alloying and humidity on the corrosion rate are compared.