of dissolution of AI(OH)~ in the acid (19). However, our experimental and theoretical attempts to explain similarly the process of dissolution of Zr in HF with ZrO~ as an intermediate product failed.

Rates of oxidation of uranium have been measured volumetrically at constant pressure in the temperature range 125°–250°C and at oxygen pressures from 20 to 800 mm. The oxidation is best described by two linear rate laws, a marked increase in rate occurring after an average of 50 µg O2/cm2 has been consumed. The first stage rate in µg/cm2‐min is given by the expression v1=1.48×104 P1/n e−E/RT where E=10,700 normalcal , P is oxygen pressure in millimeters of mercury, and 1/n varies with temperature. The reaction is believed to be controlled by process occurring at the gas‐oxide surface. The increase in rate leading to the second stage is not treated quantitatively, but is attributed to a net increase in surface area.

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The kinetics of oxidation of reactor grade plutonium and ultrapure plutonium were studied in oxygen and air at temperatures from 140° through 550 °C. Paralinear rate laws apply over the entire range with a change of mechanism between 300° and 400°C. In the low‐temperature range (140°– 300°C) the process is postulated to be controlled by a compact layer of Pu2O3 . Simultaneous growth of this layer and linear oxidation to PuO2 results in paralinear kinetics whose rate constants are knormalparabolic=1.6×1015 false[expfalse(−27,400/RTfalse)false]false(µnormalg/cm2)2/min knormalllnear=1.6×108 false[expfalse(−16,800/RTfalse)false]false(µnormalg/cm2false)/min Above 400 °C the oxide produced is more protective and is postulated to include a layer of PuO1.6 consistent with phase diagram studies. Paralinear kinetics are followed with simultaneous rate constants knormalparabolic=1.8×1012 false[expfalse(−22,500/RTfalse)false]false(µnormalg/cm2)2/min knormallinear=8.8×109 false[expfalse(−26,400/RTfalse)false]false(µnormalg/cm2false)/min The presence of moisture has no effect above 215 °C, but causes a marked increase in the oxidation at lower temperatures. As little as 180 ppm moisture caused the linear rate at 190 °C to accelerate an order of magnitude.The presence of 3.4 a/o aluminum in plutonium markedly reduced the oxidation rates at temperatures below 400 °C but only slightly reduced oxidation rates at higher temperatures.

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