Reactions of plutonium dioxide with water and hydrogen–oxygen mixtures: Mechanisms for corrosion of uranium and plutonium

Journal of Alloys and Compounds 314 (2001) 78–91

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Reactions of plutonium dioxide with water and hydrogen–oxygen mixtures: Mechanisms for corrosion of uranium and plutonium a, b b John M. Haschke *, Thomas H. Allen , Luis A. Morales b

a Kaiser-Hill Company, L.L.C., P.O. Box 464, Golden, CO 80402, USA Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA

Received 23 May 2000; received in revised form 21 August 2000; accepted 25 August 2000

Abstract Investigation shows that plutonium dioxide interacts chemically with water and catalytically with oxygen–hydrogen mixtures to form water. Water adsorbs strongly on the oxide below 1208C and desorbs as the temperature is increased to 2008C. Hydroxide formed by dissociative adsorption of water promotes formation of the higher oxide (PuO 21x ) plus H 2 , and in the presence of O 2 , drives a water-catalyzed cycle that accelerates formation of PuO 21x by the PuO 2 1O 2 reaction. Results are consistent with kinetic control of plutonium corrosion by the adherent oxide layer on the metal and imply that moisture-enhanced oxidation is driven by the water-catalyzed cycle. Evaluation of experimental results and literature data for U and Pu lead to a comprehensive corrosion mechanism applicable to both metals in dry air, water vapor, and moist air. In all cases, corrosion proceeds by diffusion of oxide ions through the oxide product, not by transport of hydrogen or hydroxide. Rates vary as changes in the concentration of adsorbed water determine the rate of O 22 formation and the gradient in oxygen concentration across the oxide diffusion barrier. Results account for a sharp decrease in the corrosion rate of Pu by water and moist air as the temperature approaches 2008C.  2001 Elsevier Science B.V. All rights reserved. Keywords: Uranium; Plutonium; Corrosion mechanism; Higher oxide; Catalysis

1. Introduction Oxidation and other corrosion reactions of uranium and plutonium are of concern because these actinides are used extensively in military and commercial applications and are important constituents of waste from nuclear processing facilities [1]. In addition to causing thermal excursions and transforming massive metal into a dispersible material form, corrosion may result in failure of containment vessels by expanding the solid phase or forming noncondensable gases [1,2]. Uranium and plutonium oxidize slowly in dry air or oxygen, but their corrosion rates are markedly enhanced by moisture. As shown in recent reports describing the oxidation kinetics of the metals over broad temperature and humidity ranges, the corrosion rates of U [3] and Pu [4] in water vapor are more than 10 2 to10 4 faster than in dry air. Effects of water concentrations as low as 1 ppm are seen during corrosion of unalloyed Pu in *Corresponding author. 11003 Willow Bend Drive, Waco, TX 76712, USA. Tel.: 11-254-399-0740; fax: 11-254-399-8876. E-mail address: [email protected] (J.M. Haschke).

air [5]. Use of moisture-control methods during storage and processing operations is necessary, but the nature of the enhancement process has remained speculative. Disappearance of moisture enhancement for both actinides at elevated temperatures leads to moisture-independent kinetic regimes where isobaric oxidation rates in water vapor and air are equal. The transition to moistureindependent behavior for U occurs gradually over the 250–5008C range [3]. For Pu, the change appears rather abruptly between 110 and 2008C [4,5], a temperature consistent with an early report that moisture has no kinetic effect above 2158C [6]. This change significantly improves the margin of safety for potentially pyrophoric salt residue forms containing plutonium metal by preventing runaway reaction of Pu with H 2 O at temperatures below 2008C [1]. Confidence in the ability to manage corrosion and autothermic reaction depends on understanding both the chemistry and mechanism of the moisture-enhanced oxidation. Surprisingly, similar corrosion kinetics are observed for uranium and plutonium in dry air and water vapor [3,4]. At room temperature, their oxidation rates in dry air are

0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01222-6

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essentially identical at 0.8 and 0.5 ng metal cm 22 min 21 , respectively. The activation energy of 67 kJ mol 21 for dry oxidation of U corresponds closely with the 79 kJ mol 21 value for Pu. Whereas exposure of U to saturated water vapor at 258C increases the corrosion rate by a factor of 400, that change induces a 250-fold increase in the rate for Pu. At temperatures below the transition points to moisture-independent oxidation, the corrosion rate of each metal is bounded by the slow rate in dry air or oxygen and the rapid rate in water vapor. Convergence of these Arrhenius curves at the transition point closes a temperature–humidity envelope that bounds the corrosion kinetics of each metal in humid air at low temperatures. Although differences are observed in the shapes of the envelopes and oxidation rates within the envelopes, the corrosion reactions of the two metals in air are remarkably similar and apparently proceed by the same mechanism. Combination of result for U and Pu provides a significantly expanded set of chemical and kinetic observations for interpreting moisture-enhanced corrosion. We employ data from this study and literature sources in describing the reactions of uranium and plutonium with dry air (equivalent to dry O 2 ), water vapor, and moist air (equivalent to moist O 2 ). Observations are applied in deriving a comprehensive mechanism that accounts for enhanced corrosion in moist conditions, as well as for its absence at elevated temperatures. Development of the mechanism is based on observations suggesting that corrosion kinetics are determined by properties of the oxide layer on the metal and are facilitated by results of experiments that examine interactions of plutonium dioxide with water vapor and oxygen–hydrogen mixtures.

2. Experimental methods Chemical and kinetic behavior of the gas–oxide systems were investigated over the 25–2508C range using microbalance (MB) and pressure–volume–temperature (PVT) methods [7,8]. During these tests, PuO 2 was exposed to water vapor or to an O 2 1H 2 mixture. Rates were obtained by measuring mass or pressure changes as a function of time (t). Dioxide specimens used in MB measurements were from the source used in an earlier investigation [9]. The oxide (4.8 m 2 g – 1 specific surface area) contained approximately 100 ppm Am as the major metallic impurity and its stoichiometry was obtained from the measured lattice ˚ and a correlation of a o with parameter of a o 55.4037(7) A O:Pu ratio [7] derived from data reported by Gardner et al. [10]. The PuO 2 used in PVT studies was prepared by transforming weapons-grade gallium alloy (10.2 g) into powder by repeated hydriding–dehydriding cycles and oxidizing with high-purity O 2 at 5008C in a volumecalibrated stainless steel system. The initial product was ground in an inert atmosphere, reheated in excess oxygen

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until the total O 2 uptake corresponded to the PuO 2.00 composition, and vacuum annealed at 7008C before being cooled to room temperature in 1.0 bar O 2 . A specific surface area of 463 m 2 g 21 for the product was estimated using a correlation of area with firing temperature [9]. Microbalance measurements of the interaction between PuO 2 and H 2 O were made with a Cahn RH microbalance located in an inert-atmosphere glovebox [7]. A weighed oxide specimen (approximately 0.1 g) was placed in a Pt sample cup and outgassed to constant mass in vacuum at 4008C before being exposed to water vapor maintained at 15 Torr with a constant-temperature water reservoir. Sample temperature was measured by a thermocouple placed near the cup and was increased stepwise from 25 to 50, 100, 150, and 2508C as the specimen mass was measured. XRD data for initial oxide and final products were obtained using a 114.56-mm-diameter Debye–Scherrer camera with Cu Ka radiation. Interaction of plutonium oxide with hydrogen and oxygen at 24.560.88C was investigated by exposing PuO 2 to a 2:1 molar mixture of D 2 and O 2 [8]. The oxidized stainless steel crucible and oxide were placed in a stainless steel PVT reactor (40.060.1 cm 3 free volume) fitted with metallic seals, pressure transducers, and internal thermocouples. The evacuated reactor was charged with 126.7 Torr of a gas mixture prepared by two-step pressurization of a small stainless steel reservoir with D 2 (99.9%) and O 2 (99.99%) at a 2:1 ratio. Pressure and temperature were measured continuously and gas samples (0.037 cm 3 ) were periodically obtained for mass spectrometric (MS) analysis with a quadrapole instrument. Spectra were corrected for fragmentation and ionization cross section. Deuterium was used in these experiments to investigate possible exchange of hydrogen between gaseous and condensed phases, but that concern is not pertinent to this report and a distinction between the D and H is not maintained. Kinetic results were derived from mass-t and P-t data and from results of periodic MS analyses. Ideal gas behavior was assumed and pressures were corrected for temperature variation and sampling. Rates (R) are reported in units of moles of gas reacted or formed per gram of oxide or square meter of oxide surface per unit time.

3. Results and discussion

3.1. Interaction of PuO2 with H2 O Mass-time data obtained during exposure of the oxide to water vapor show surprisingly complex behavior as the temperature was incrementally increased from 25 to 2508C. Exposure of oxide to water at 258C for a 2-h period resulted in a mass increase of 0.40 mg m 22 . The mass remained stable for 5 h at 258C and did not change when the temperature was increased to 508C and held for 7 h. An additional weight gain of 0.45 mg m 22 was observed over

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a period of 15 h after the specimen temperature was raised to 1008C. A mass loss of 0.83 mg m 22 occurred at a progressively decreasing rate over a period of 20 h after the oxide was heated to 1508C. The mass remained constant for approximately 1 h after the temperature was raised to 2508C and then increased at a constant rate of 200650 nmol H 2 O m 22 h 21 until measurements were terminated after 65 h. A mass gain of 0.6 mg m 22 was observed after the sample was cooled to 258C. XRD analysis of the product after incremental heating to 2508C indicate a fcc lattice like that of PuO 2 . The ˚ was 0.004 A ˚ smaller than that measured a o of 5.3998(8) A of the starting oxide. The observed extent of H 2 O adsorption agrees with literature data in Fig. 1 [11]. Adsorption isotherms at 30, 50, and 858C show that the surface concentration of water, [H 2 O], is a linear function of temperature at constant humidity with values in the 0.4–0.5 mg m 22 range at 25–508C and 15 Torr H 2 O pressure (63% relative humidity). Failure to detect a mass loss after the temperature was increased from 25 to 508C is consistent with sluggish and irreversible desorption in dynamic vacuum [11]. The nature of the interaction between H 2 O and PuO 2 at 25–508C is derived from data showing that water adsorbs via a sequential multi-step process with a progressive change from strong chemisorption of hydroxide to weak

Fig. 1. Temperature dependence of the concentration of adsorbed water on PuO 2 at 35–858C and selected humidities relative to saturation (23.7 Torr) at 258C. Data are from graphical sources in Ref. [11].

physisorption of molecular water [12]. The first and second steps (0.1 mg m 22 each) involve dissociative reaction of H 2 O with lattice oxide ions to form two OH 2 ions; additional water adsorbs as successive H 2 O layers (0.2 mg m 22 each). Therefore, the initial mass increase at 25–508C corresponds to formation of a PuO(OH) 2 surface and coverage by one layer of molecular H 2 O. Behavior observed upon heating the water–oxide system to 1008C and that predicted by the [H 2 O]–temperature correlation in Fig. 1 is consistent with irreversibility of water adsorption. Adsorption of an additional 0.45 mg m 22 was observed at 1008C instead of desorption to a residual [H 2 O] of 0.20 mg m 22 as suggested by extrapolation of the data. Isothermal cycling experiments at 30–858C show that adsorption is irreversible [11]. A fraction of the adsorbate remained on the surface after isothermal desorption, but the initial adsorption curve was reproduced upon reexposure to water vapor. The amounts of irreversibly bound water, [H 2 O] i , increased for several cycles before reaching stable maxima of 0.69, 0.54, and 0.24 mg m 22 at 30, 50 and 858C, respectively [11]. These data define a linear [H 2 O] i –temperature curve corresponding to that for 80–85% humidity in Fig. 1 and imply that [H 2 O] i is also 0.2 mg m 22 at 1008C. Therefore, a total [H 2 O] of 0.4 mg m 22 (0.2 mg m 22 reversible and 0.2 mg m 22 irreversible) is predicted for that temperature using data in Fig. 1, but the measured [H 2 O] i at 1008C exceeds 0.55 mg m 22 and is incomplete after eight cycles [13]. The [H 2 O] 22 22 anticipated at 1008C exceeds 0.75 mg m (0.20 mg m 22 reversible and 0.55 mg m irreversible) in agreement with findings of this study. Results suggests that the equilibrium state for adsorption of water is not attained below 1008C or that the surface area of the oxide increased during heating at that temperature. A possible increase of 50% in oxide area is indicated by an additional mass gain upon cooling to 258C, but a three-fold increase is required to account for the observed mass. Formation of a Pu(OH) 4 surface also fails to account for the behavior and is unlikely because the tetrahydroxide is unstable and spontaneously disproportionates to PuO 2 [14]. Formation of PuO 21x by water is possible, but is too slow to account for the observed mass increase [15]. A likely explanation is that penetration of the oxide by OH 2 from dissociative chemisorbed water increases the PuO(OH) 2 thickness by a factor of three. Regardless of its origin, adsorption of additional water on heating to 1008C at constant H 2 O pressure is most consistent with failure to attain equilibrium at lower temperatures. Desorption (97%) upon extended heating at 1508C shows that adsorbed water is unstable at elevated temperatures, even in the presence of 15 Torr water vapor. Complete desorption at 2008C is implied because additional mass loss was not evident at 2508C. Although the adsorbate configuration attained at 1008C is apparently more stable than attained at low tempera-

J.M. Haschke et al. / Journal of Alloys and Compounds 314 (2001) 78 – 91

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tures, that state is also metastable because adsorbed water reacts with PuO 2 to form PuO 21x and H 2 .

Table 1 Experimental data and results for the reaction of a H 2 1O 2 mixture exposed to PuO 2 at room temperature a

PuO 2 (s) 1 xH 2 O(ads) → PuO 21x (s) 1 xH 2 (g)

Elapsed time (t) (days)

Total pressure (P) (Torr)

Reaction rate (R) (nmol O g 21 PuO 2 h 21 )

Qb

0 0.125 0.250 0.375 0.500 0.75 1.0 1.5 2.0 2.5 3.0 4.0 5.0 10 15 20 25 35 45 50 60 70 130 145

126.7 125.9 125.3 124.8 124.4 123.8 123.3 122.5 121.8 121.3 120.9 120.2 119.6 117.0 114.5 112.3 110.3 107.0 104.3 102.3 99.6 97.6 90.0 88.7

– 33 25 21 16 13 10 9.4 7.5 5.4 4.1 3.5 3.3 2.7 2.6 2.3 2.1 1.7 1.6 1.4 c 0.75 c 0.71 c 0.42 c 0.67 c

0 0.004 0.007 0.010 0.012 0.015 0.017 0.021 0.024 0.026 0.028 0.031 0.034 0.045 0.055 0.064 0.072 0.084 0.093 0.101 0.110 0.115 0.124 0.125

(1)

At 2508C, the mass change for this reaction is not masked by adsorption or desorption of water and the measured rate (200650 nmol H 2 O m 22 h 21 ) agrees with the rate of H 2 production (197 nmol H 2 m 22 h 21 ) defined by Arrhenius data for Eq. (1) [15]. Formation of higher oxide is confirmed by the measured change in oxide lattice parameter.

3.2. Interaction of PuO2 with an H2 1 O2 mixture P-t data in Fig. 2 show that exposure of the hydrogen– oxygen mixture to the dioxide results in a relatively rapid initial pressure decrease. As shown by data in Table 1, the rate of pressure drop progressively slowed over time and ultimately became constant at a value that was maintained until failure of a seal on the PVT system resulted in termination of measurements after approximately 2 years. Gas-phase composition data in Table 2 show the presence of H 2 , O 2 , and trace amounts of CO and CO 2 , but no H 2 O. The H 2 :O 2 ratio was 2.27 (not 2.00) in the initial mixture and increased to 2.75 after 72 days. Increases are also observed in the fractions of CO, and CO 2 over time. The quantities of H 2 and O 2 in the PVT reactor at zero time and after 72 days are also given in Table 2 and the time

a

The initial D 2 :O 2 ratio of the mixture was 2.27:1. Q is the fraction of the oxide surface covered by a monolayer of hydroxide. c Whereas rates at low pressures are derived from the incremental P and t changes relative to the preceding data point, this rate is based on P-t data measured several days before and after the indicated median time. b

dependence of the reaction rate is shown by the ln R–time curve in Fig. 3. Mixtures of hydrogen and oxygen are thermodynamically unstable relative to water and their reaction is implied by concurrent disappearance. Results suggest formation of water by catalyzed H 2 and O 2 combination on the oxide surface and simultaneous formation of PuO 21x by watercatalyzed reaction of PuO 2 and O 2 . The initial pressure decrease in Fig. 2 results primarily from catalytic in-

Table 2 Mass spectrometric results for the gas phase after different time periods during exposure of the H 2 1O 2 reaction mixture to PuO 2 at room temperature Gaseous constituent Elapsed time (days) b

H2 O2 CO CO 2 a

Fig. 2. Time dependence of the pressure during exposure of PuO 2 to a 2:1 H 2 :O 2 mixture at 24.58C and an initial pressure of 126.7 Torr.

mmoles a

Mole percentage 0

24

72

0

72

69.43 30.55 0.014 0.010

72.13 27.62 0.14 0.10

72.41 26.38 0.35 0.87

190.2 83.7 0.04 0.03

152.6 55.4 0.74 1.8

Moles of gas are derived from mole percentages and PVT data. The percentages reported for H 2 are sums of values measured for H 2 and D 2 . b

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corrected for the amount of oxygen consumed by the PuO 2 1O 2 reaction. The pressure dependence of the H 2 –O 2 combination reaction is consistent with this interpretation and is defined by a general rate expression. R 5 k exp(2Ea /R*T )(PO 2 )u (PH 2 )v

(3)

R is a function of a proportionality constant (k), temperature, oxygen pressure (PO 2 ), and hydrogen pressure (PH 2 ). Temperature dependence is described by the Arrhenius term, which includes the activation energy (Ea ) and gas constant (R*). The exponents u and v define the dependencies of R on the pressures of oxygen and hydrogen, respectively, but cannot be determined from the experimental data. However, the pressure effect can be deduced for gas-phase composition at the 2:1 H 2 :O 2 composition because the molar ratio is not changed by preferential depletion of one reactant as reaction proceeds. Reaction is confined to a constant-composition section of P-t space and the isothermal rate expression reduces to a function of the total pressure. Fig. 3. Time dependence of ln R for reaction during exposure of PuO 2 to a 2:1 H 2 :O 2 mixture at 24.58C.

volvement of oxide in promoting association of dissociatively adsorbed hydrogen and oxygen. H 2 (ads) 1 1 / 2O 2 (ads) → 2H(ads) 1 O(ads) → H 2 O(ads) (2) The reservoir used for preparing the H 2 –O 2 mixture served as a blank for the contribution from surfaces of the PVT reactor. Periodic MS analysis showed that the rate of water formation on stainless steel was negligibly small at 258C. The absence of gas-phase H 2 O implies that product water adsorbed dissociatively as hydroxide on the oxide surface [12]. Oxide-catalyzed formation of water is associated with the time-dependent first and second segments of the ln R–time curve in Fig. 3. Catalysis of the reaction by active surface sites on the oxide and poisoning of that activity by adsorption of product water are consistent with the firstorder behavior observed for these processes. Linear ln R–time relationships result as OH 2 progressively blocks active surface sites and suppresses the reaction rate. Appearance of two segments implies that two sets of sites with different catalytic activity existed on the initial oxide. As shown in Table 1 and by the upper abscissa in Fig. 3, fractional coverage of the oxide surface by OH 2 (Q ) is 0.03 after 4 days, indicating that 3% of the surface was highly activity. Reaction continues at a decreasing rate along the second segment as sites in the less reactive set are progressively poisoned. The values of Q are derived using the specific surface area of the oxide and PVT data

R 5 k9(P)n

(4)

Values of n are defined by the slopes of ln R–ln P curves. An ln R–ln P analysis of data in Table 1 shows two linear segments corresponding to the first-order kinetic regions with a sharp slope change occurring at 4.0 days. Derived values of n for the first and second linear segments are 46.7 and 5.9, respectively. A 47th-order pressure dependence has no rational interpretation and incorrectly implies that extremely large rate changes are induced by small pressure changes. Even a sixth-order dependence cannot be explained. Radiolytic promotion of gas-phase combination is excluded because the 5% decrease in gas density after 4 days should be accompanied by a corresponding decrease in the rate, not by a ten-fold reduction. A consistent interpretation of these results is that a false pressure dependence results from poisoning of oxide activity. The observed n values depend on experimental parameters and are not characteristic of the oxide-catalyzed combination reaction. The time-independent (constant-rate) reaction observed after about 70 days (1680 h) in Fig. 3 is attributed to formation of PuO 21x by the water-catalyzed PuO 2 1O 2 reaction [15]. As implied by extension of the ln R–t line to shorter times, this reaction initiates after H 2 O appears and occurs simultaneously with H 2 –O 2 combination. Water produced by the H 2 1O 2 mixture reacts with PuO 2 to form higher oxide and hydrogen [Eq. (1)]. If O 2 is present, atomic hydrogen formed by the PuO 2 1H 2 O reaction does not associate to form H 2 , but reacts with dissociatively adsorbed oxygen to reform H 2 O. The net cycle is defined by summing Eqs. (5)–(7). PuO 2 (s) 1 xH 2 O(ads) → PuO 21x (s) 1 2xH(ads)

(5)

J.M. Haschke et al. / Journal of Alloys and Compounds 314 (2001) 78 – 91

x / 2O 2 (g)↔xO(ads)

(6)

2xH(ads) 1 xO(ads) → xH 2 O(ads) ]]]]]]]]]

(7)

PuO 2 (s) 1 x / 2O 2 (g) → PuO 21x (s)

(8)

Oxidation of PuO 2 by dry O 2 is not observed, but occurs at the measurable rate of the PuO 2 1H 2 O reaction when moisture is present. Occurrence of the water-catalyzed PuO 2 1O 2 reaction prior to the constant-rate stage is demonstrated by kinetic data. As shown by MS data in Table 2, the amount of H 2 reacted during the initial 72 days produced 37.6 mmol H 2 O, but the amount of O 2 consumed during that period (28.3 mmol) exceeded the stoichiometric value by 9.5 mmol and caused the observed increase in the H 2 :O 2 ratio. The average oxidation rate calculated from the excess O 2 consumption and specific surface area is 0.24 nmol O m 22 h 21 . The rate beyond 70 days is constant because the rate of H 2 –O 2 combination becomes insignificant. The longterm pressure decrease results from reaction of O 2 via Eq. (8) and yields an average gas-consumption rate of 0.55 nmol O 2 g 21 h 21 or 0.28 nmol O m 22 h 21 . The MS and PVT results are in excellent agreement with the rate of 0.2260.14 nmol O m 22 h 21 reported for the PuO 2 1H 2 O reaction at 258C and lie well within the uncertainty limits shown by the error bars for that measurement in Fig. 3 [15]. Occurrence of the water-catalyzed oxidation at the rate of the PuO 2 1H 2 O reaction in this study implies that the rates of oxidation by water and the water-catalyzed cycle are surprisingly insensitive to humidity at room temperature. Rates at the fractional Q values in Table 1 equal those for liquid water and near-saturation vapor [15] and imply that the oxide surface is saturated with respect to oxidation at Q less than 0.1. Results suggest that the PuO 2 1H 2 O reaction is moisture independent from below 1% relative humidity (315 ppm H 2 O) to saturation (23.7 Torr H 2 O) at 258C.

3.3. Implications for plutonium corrosion The corrosion kinetics of plutonium in air and moist atmospheres is undoubtedly controlled by properties of the coherent layer of oxide product and its temperature dependent interaction with water. Water adsorbs on oxide as hydroxide and as molecular H 2 O at temperatures up to 1008C, but desorbs upon heating above 1508C. Formation of water by association of the elements on the oxide surface at 258C and at 50–3008C [16] shows that H 2 and O 2 dissociatively adsorb on the oxide over a broad temperature range. Adsorbed water reacts with PuO 2 to form PuO 21x and drives water-catalyzed formation of the higher oxide by O 2 [15]. Insensitivity of PuO 21x formation to [H 2 O] at room temperature implies that the watercatalyzed PuO 2 1O 2 reaction occurs over a wide humidity

83

range. Reformation of H 2 O via the cycle results in continuing oxidation by O 2 in closed systems containing only traces of moisture.

4. Physicochemical properties relevant to corrosion

4.1. Properties of metal–oxygen–hydrogen compounds Observations suggest that the corrosion rates of U and Pu are determined by the product layers formed on the metal surfaces and credible mechanisms for moistureenhanced corrosion must be consistent with product properties. Identification of the product layer is essential and consideration of thermodynamic, structural, and transport properties of binary compounds with oxygen and hydrogen is necessary. Ternary oxide hydrides are known for plutonium [17], but their involvement in corrosion is unlikely. Properties of binary uranium and plutonium oxides are extensively reviewed by Colmenares [18,19]. In air at temperatures between 25 and 5008C, the dioxide phase extends from stoichiometric UO 2 to superstoichiometric UO 21x (x,0.20). At higher O / U ratios, another phase of variable stiochiometry (UO 2.256x ) forms near the U 4 O 9 composition. Structures of these higher-composition fcc oxides are derived from that of fluorite(CaF 2 )-type UO 2 ˚ by incorporating excess oxygen in octahed(a o 55.470 A) ral interstices of the structure. The bcc U 4 O 9 phase (a o |21 ˚ is formed by long-range ordering of interstitial oxygen A) and is oxidized to orthorhombic U 3 O 8 in air. UO 21x is an intrinsic p-type semiconductor. The diffusion coefficient (D) for oxygen self-diffusion is 10 9 greater than that for uranium self-diffusion. Ea for oxygen diffusion decreases with increasing x from 250 kJ mol 21 for x50.001 to 90 kJ mol 21 for x50.02 to 0.15. In the equally complex Pu–O system [7,15,19], a lowcomposition oxide (a-Pu 2 O 3 ) formed by auto-reduction of the dioxide layer on the metal under non-oxidizing conditions is a non-stoichiometric phase (Pu 2 O 36x ) with a bcc ˚ derived by ordered removal of structure (a o 511.02 A) one-fourth of the oxygen from PuO 2 . Behavior of the ˚ is remarkably similar fluorite-type dioxide (a o 55.396 A) to that of UO 2 ; PuO 22x (x#0.02) exists under reducing conditions at room temperature and PuO 21x (maximum x.0.25) forms in the presence of moisture [15]. PuO 2 is a p-type semiconductor and similar behavior is expected for PuO 21x by analogy to UO 21x . Ea values for oxygen self-diffusion in PuO 22x range from 95 to 190 kJ mol 21 [20] and are remarkably similar to those for UO 21x . Uranium and plutonium form stable binary hydrides with rather dissimilar properties [21,22]. Metallic b-UH 3 ˚ loses hydrogen to form (anti-W3 O structure, a o 56.644 A) UH 32x upon heating and vaporizes incongruently to form hydrogen-saturated metal. Equilibrium H 2 pressures in the U–UH 32x two-phase region are 3 nbar at 258C and 5 mbar

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˚ at 2508C. The fluorite-type PuH 2 phase (a o 55.359 A) forms PuH 22x and hydrogen-saturated metal when heated and readily accommodates hydrogen in octahedral interstices of the fluorite structure to form an extended PuH 21x (x#1) solid solution upon exposure to hydrogen. Equilibrium H 2 pressures in the Pu–PuH 22x region are 10 221 bar at 258C and 3 mbar at 2508C.

4.2. Chemistry of corrosion reactions Corrosion of uranium and plutonium in dry air, water vapor, and moist air proceeds with the formation of binary oxides [3,5,18]. Reaction of U with dry air produces superstoichiometric oxide with x,0.25. U(s) 1 (2 1 x) / 2O 2 (g) → UO 21x (s)

(9)

The oxide formed by oxidation of Pu in dry air is PuO 2 [18]. Binary oxide is also formed by the reaction of U with water vapor. U(s) 1 (2 1 x)H 2 O(g) → UO 21x (s) 1 (2 1 x)H 2 (g)

(10)

Concurrent formation of UH 3 at temperatures less than 1008C was suggested in early work [23,24], but was not verified experimentally. Recent studies confirm that the product of U1H 2 O is UO 2 [25] or UO 21x [26]. The product layer formed on Pu by reaction of H 2 O at 2508C is composed of successive PuO 21x , PuO 2 , and a-Pu 2 O 3 layers with 15–20% of the oxide–metal interface occupied by hydride inclusions [27]. Estimation of the product ratio from the extents of oxide and hydride formation indicates that the PuH 2 fraction is approximately 0.15, a value substantially less than the 0.67 fraction expected for stoichiometric reaction of water. Hydride is not observed in products formed at low temperatures [5] and corrosion of Pu by water vapor is accurately described by Eq. (10). Behavior in moist air is complex for both metals. U 3 O 8 forms during corrosion of uranium at 200–5008C [28]. During oxidation of U in moist oxygen at 1008C (Fig. 4), O 2 is consumed at an accelerated rate, the H 2 O concentration remains constant, and neither UH 3 nor significant H 2 is formed [23,29]. H 2 is produced after O 2 is depleted. Oxidation of Pu in moist air occurs at the accelerated rate of the Pu1H 2 O reaction and consumes O 2 without forming H 2 or hydride [5].

4.3. General kinetic observations Corrosion of uranium and plutonium in dry and humid atmospheres proceeds via a sequence of steps common to corrosion reactions controlled by protective product layers. Molecular reactants adsorb on the product surface and dissociate to form species that migrate through the product layer. Reaction depends on transport of electrons from the

Fig. 4. Time dependence of constituents in the gas phase during reaction of uranium metal with a 3:2 O 2 :H 2 O mixture at 1008C. Data are from a graphical source in Ref. [28].

metal to the gas–solid interface, dissociation of adsorbed reactant, and diffusion of mobile species through the product layer. In the final step, the mobile species react to form product, electrons, and heat at the metal interface. The reaction rate may depend on a single step or on the combined effect of several steps. Rate control by diffusion of reactant through the layer of product oxide is suggested by kinetic data for U and Pu [17,18]. Freshly machined or burnished metal surfaces readily tarnish upon exposure to dry, air, water vapor, or moist air. The isothermal corrosion rate, which progressively decreases and ultimately becomes constant is described as ‘paralinear’ because the extent of reaction follows a parabolic time dependence during the initial stage and a linear time dependence during the constant-rate stage. Parabolic behavior is characteristic of a diffusion controlled process in which the reaction rate slows as the thickness of the oxide layer increases. Linear behavior results from maintenance of steady-state diffusion barrier by continuing spallation of oxide and continuing oxidation [3]. At low temperatures, the linear stage is entered after the product layer is several micrometers thick [30] and is the primary reaction for corrosion of metal. The rate of a diffusion-controlled reaction is determined by the flux (J) of the diffusing species as defined by a one-dimensional diffusion model.

J.M. Haschke et al. / Journal of Alloys and Compounds 314 (2001) 78 – 91

J 5 cD(dC / dt )

(11)

D is the temperature-dependent diffusion coefficient of the migrating species in the product, dC / dt is the gradient in reactant concentration C across the layer of thickness t, and c is a proportionality constant. At constant product thickness, the isothermal rate depends only on the difference in reactant concentration across the diffusion barrier.

5. Assessment of proposed mechanisms Published mechanisms for moisture-enhanced corrosion of U and Pu in air consistently involve formation of an oxide diffusion layer and transport of reactant across that layer. Two primary methods for rate enhancement by moisture are proposed: (1) disruption of the protective oxide layer by formation of hydride or hydrogen at the product–metal interface and (2) rapid transport of a chemical species other than oxygen. Both approaches involve diffusion of a hydrogen-containing species through the oxide layer. Their consistency with material properties and experimental observation merits examination. In an early report by Wilkinson [28], accelerated corrosion by moist air is attributed to mechanical disruption of the oxide layer. Hydrogen ions (H 1 ) formed by autoionization of adsorbed H 2 O diffuse through the oxide layer, disrupt the protective oxide layer by forming UH 3 or H 2 , and ‘cause the rate of oxidation to increase and become linear.’ The fate of OH 2 remaining on the oxide surface is not specified. A similar mechanism proposed by Kondo et al. [24] attributes moisture-enhancement to fracture of the protective oxide layer by stresses associated with hydride formation at the oxide–metal interface. Penetration of the oxide layer by H 2 O produces UO 21x , UH 32x , and H, a species that returns to the gas–solid interface and reacts with O 2 to form H 2 O. Fracture of the oxide layer reactivates formation of oxide and hydride in a stage ‘observed as a linear rate law.’ An enhancement mechanism proposed by McD. Baker et al. [29] involves diffusion of hydroxide through the oxide layer. Transformation of isotopically labeled oxygen (O *2 ) into H 2 O* is attributed to formation of OH 2 by autoionization of water at the gas–oxide interface, transport of OH 2 , and formation of UO 2 and H atoms at the metal interface. Atomic hydrogen returns to the gas–solid interface and forms H 2 O* by reacting with neutral HO* produced via surface reaction of H 1 with O *2 and electrons. Colmenares and coworkers [18,31] suggest that acceleration of the U1H 2 O results from rapid diffusion of OH 2 formed by dissociative chemisorption of H 2 O on UO 2 . Reaction of OH 2 at the oxide–metal interface produces UO 2 and H 1 , a species that returns to the gas–oxide interface, reacts with electrons, and associates to form H 2 . Linear kinetics are interpreted as implying that ‘oxidation is independent of oxide thickness.’ Additional

85

mechanisms involving diffusion of OH 2 [32] and both OH 2 and O 22 [33] are proposed for oxidation in moist air. Published mechanisms account for enhanced corrosion of Pu in water vapor, but are not proposed for moistureenhanced corrosion in air. Thompson [34] suggests that the reaction forms PuO 2 and PuH 2 , a product that reacts with water to form oxide at an accelerated rate and is regenerated by reaction of the H 2 product with Pu. A mechanism proposed by Waber [35] involves autoionization of water, diffusion of OH 2 to the oxide–metal interface, and participation of Pu(OH) 3 and Pu(OH) 4 intermediates prior to formation of PuO 2 plus H 2 O. H 2 is produced by association of H 1 with electrons at the surface. A common feature of proposed mechanisms for moisture-enhanced corrosion in moist air is diffusion of hydrogen (H or H 1 ) or a hydrogen containing species (H 2 O or OH 2) to the oxide–metal interface. Hydride formation is rapid if hydrogen-containing species reach the oxide–metal interface and the H 2 pressures required for its stabilization are very low (Section 4.1). Experiments show that hydride forms at the oxide–metal interface during oxidation of Pu coated with the monoxide monohydride (PuOH) and catalyzes corrosion by factors of 10 10 to 10 13 [4,36]. The hydride is quantitatively retained as a PuH 21x core inside the product after all metal is reacted. Transport of OH 2 to the oxide–metal interface must produce an oxide–hydride mixture containing 40% UH 3 or 50% PuH 2 . The absence of stoichiometric amounts of hydride products implies that mechanisms involving diffusion of hydrogen-containing species are invalid. A logical extension of this conclusion is that oxygen is the only diffusing species during corrosion in both dry and humid atmospheres. Development of a consistent mechanism for enhanced corrosion of uranium and plutonium at humid conditions is needed. That mechanism must account for accelerated oxidation at a constant isothermal rate via a process that produces neither hydride nor H 2 , agrees with results of isotopic studies, and is absent at elevated temperatures.

6. A comprehensive corrosion mechanism

6.1. The mechanistic concept A comprehensive mechanism for corrosion of U and Pu in dry air, water vapor, and humid air is derived using findings of the present study and results described in the literature. Observation of paralinear behavior at all conditions indicates that corrosion rates are controlled by diffusion of reactant through a coherent oxide layer of constant average thickness. As shown by Eq. (11), the isothermal flux of reactant diffusing across a constantthickness layer is determined by the gradient in concentration of the diffusing species. Results of the present study suggest that dissociative adsorption of water as OH 2

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promotes higher oxide formation, a process that enhances the corrosion rate by increasing the oxide ion (O 22) concentration at the gas–oxide interface. An accelerated rate of higher-oxide formation by water is attributed to facile transformation of chemisorbed OH 2 into lattice O 22 . The concentration of OH 2 on the oxide surface is high and O 22 formation proceeds by a singleelectron transfer to OH 2 . In contrast, oxide ion formation by oxygen requires the transfer of two electrons. OH 2 resides on an anionic site prior to reaction and the O 22 product is readily accommodated into the oxide lattice. Hydrogen atoms produced at the oxide surface associate as H 2 . Moisture enhancement is not observed at elevated temperatures because of thermal instability of OH 2 and desorption of water. If both water and oxygen are present, adsorbed H 2 O reacts to form higher oxide at an accelerated rate. Hydrogen atoms produced during formation of O 22 at the gas– oxide interface react with dissociatively adsorbed oxygen to reform H 2 O, a product that either desorbs or reacts to form higher oxide. The H 2 O concentration remains constant, O 2 reacts at an accelerated rate, and neither hydride nor H 2 is formed.

6.2. Corrosion in dry air 6.2.1. Chemistry and oxide properties in dry air As discussed in the early work by Wilkinson [28], the fundamental nature of the oxide layer must allow for transport of oxygen from the gas–oxide interface to the oxide–metal interface and movement of electrons in the opposite direction. Mass transport is limited to diffusion of charged species because the mobility of atoms and molecules is insignificant in oxide. Departure from ideal oxide stoichiometry by formation of oxygen interstitials or vacancies is necessary for establishing a concentration gradient across the layer. Semiconduction in the oxides is consistent with an ionic process involving hopping of electrons (or holes) between neighboring cations such as U(IV)–U(V) and Pu(III)–Pu(IV) in the lattice [19]. 6.2.2. Kinetic behavior in dry air Paralinear time dependencies are observed for oxidation of both uranium [28] and plutonium [6] in dry air. As discussed in Section 4.3, this behavior and observed activation energies are consistent with diffusion control during the linear stage of reaction. Ea values of 67 kJ mol 21 for oxidation of U [33] and 79 kJ mol 21 for oxidation of Pu [4] during the linear stage are in good agreement with the value of 95 kJ mol 21 measured for oxygen self-diffusion in both UO 21x and PuO 22x . Precise agreement is not expected because the oxygen transport rate is sensitive to other thermal effects such as the thickness of the steady-state oxide layer [30]. Dependence of oxidation rate on oxygen pressure is modest and proportional to P(O 2 )m with values of m

ranging from 0.1 to 0.4 for U [19] and from near zero to 0.5 for Pu [37] as temperature and pressure are varied. Colmenares shows that the oxidation rate is proportional to electrical conductivity and correlates dependence on O 2 pressure with results of electrical conductivity modeling [19]. Observed m-values correspond with pressure dependencies for doubly ionized clusters formed by association of O 22 on octahedral interstices in the fluorite with U(V) holes on cation sites in the oxide lattice. The defect clusters formed during oxidation of U in dry air have characteristic positron annihilation lifetimes near 400 ps that are readily distinguished from those of other clusters, as well as from the lifetimes of U and UH 3 [18,26]. Migration of oxygen via the octahedral interstices is suggested by secondary ion mass spectrometry (SIMS) profiles of oxide layers formed by sequential oxidation of U by isotopically distinguishable oxidants [33]. Depth profiles show that a thin layer of normal oxide formed on the surface of chemically etched metal prior to reaction with labeled oxidants remains at the outer surface, implying that oxide ions on tetrahedral sites of the UO 2 lattice are immobile and that O 22 is transported to the oxide– metal interface through octahedral interstices. Transport of electrons and oxygen in the oxides occurs via a concerted process consistent with the conclusion that intrinsic semiconduction in UO 2 is best explained by hopping of electrons (or holes) to neighboring cations, not by the band model [38]. As electrons move toward the gas–oxide interface, U(V) holes are inherently moved toward the metal–oxide interface. Oxygen is transported as O 22 ions follow the migrating holes.

6.2.3. A mechanism for oxidation in dry air Formulation of the oxidation mechanism for oxidation of uranium and plutonium in dry air is based on the following observations: (1) compositions in the oxides depart from stoichiometry and allow for establishment of concentration gradients across the product layers. (2) The time dependence of the extent of oxidation is paralinear, indicating that the process is controlled by diffusion of reactant through the oxide layer of constant average thickness. (3) The temperature dependence of the reaction agrees with that for oxygen self-diffusion in the oxide. (4) The pressure dependence of oxidation correlates with formation of interstitial clusters that carry O 22 toward the oxide–metal as electrons simultaneously move toward the gas–oxide interface. The initial mechanistic step is dissociative adsorption of O 2 at the gas–oxide interface. O 2 (g)↔O 2 (ads)↔2O(ads)

(12)

This reversible process is followed by interaction of atomic oxygen with electrons to form O 22 ions and U(V) or Pu(V,VI) holes, products that associate to form defect clusters.

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2O(ads) 1 4e 2 → 2O 22 (ads) → 2O 22 (lat)

(13)

The steady-state oxygen concentration at the gas–oxide interface is determined by the rates of O 22 formation via Eq. (13) and of diffusion to the oxide–metal interface. The O 22 formation rate depends on the surface concentration of O and is most likely controlled by the rate of transferring two electrons to oxygen. Entry of O 22 into the lattice is facilitated by the presence of concurrently formed positive holes. As electrons move toward the gas–solid interface, clusters containing O 22 ions and positive holes move toward the oxide–metal interface where dioxide forms and product electrons electrons neutralize the positive holes. U(Pu)(s) 1 2O 22 (lat) → UO 2 (PuO 2 )(s) 1 4e 2

(14)

Summation of Eqs. (12)–(14) describes the formation of dioxide according to Eq. (9).

6.3. Corrosion in water vapor 6.3.1. Chemistry and oxide properties in water vapor Reaction of U and Pu with water is rapid compared to corrosion in dry air and forms H 2 , as well as UO 21x or PuO 21x . As demonstrated by formation of PuO 21x and H 2 by the PuO 2 1H 2 O reaction, metal participates primarily as an oxygen sink that maintains a large gradient in oxygen concentration across the oxide layer. The oxide formed on plutonium during reaction of water vapor is composed of successive layers that vary in composition from PuO 21x at the gas–oxide interface, to PuO 2 , and ultimately to Pu 2 O 3 at the oxide–metal interface [27]. Rate control by diffusion through the product layer is implied because one side of the oxide layer approaches equilibrium with the gas phase and the other side approaches equilibrium with the metal, a situation that exists only if the equilibrium regions are separated by slow transport of O 22 through dioxide. Results of the present study and earlier work [11,12,15] show that plutonium dioxide interacts strongly with water via several processes. Molecular water adsorbs reversibly at low temperatures and is apparently eliminated for all humidity levels at 122698C, the average intercept of the [H 2 O]–temperature curves in Fig. 1. Water dissociatively adsorbed as OH 2 is removed by heating at 150–2008C in water vapor. Formation of PuO 21x is energetically favored at all temperatures in the 25–3508C range [15], but dissociative adsorption of water as OH 2 is kinetically favored below 2008C. Calculations by Colmenares suggest that dissociative adsorption of water on UO 2 is expected [18,26]. As discussed in Sections 4.2 and 5, formation of H 2 instead of hydride implies that O 22 is the only diffusing species during H 2 O reaction. Positron annihilation data show a defect lifetime of 425 ps for oxide formed by U1H 2 O, implying that the same defect cluster associated

87

with oxidation in dry oxygen is present in oxide formed by water [26].

6.3.2. Kinetic behavior in water vapor The paralinear time dependence observed for the reaction of water with Pu [39] is also indicated by interferometric data for U [40–42]. These results disagree with reports that kinetics of U1H 2 O are linear and independent of oxide thickness [18,26]. Spallation begins when the uranium oxide thickness is about 1 mm [18], a value consistent with data for plutonium [30] and corresponding to a total mass gain of 0.10–0.15 mg cm 22 during the parabolic step. With one exception, data points are for gains of 1 mg cm 22 or more [26] and are incapable of detecting a parabolic step. Preference is given to the more sensitive data. As indicated by an activation energy of 54 kJ mol 21 for the U1H 2 O reaction [3], the temperature dependence for U corrosion is similar to that for dry air. Appearance of a substantially higher Ea (141 kJ mol 21 ) for the Pu1H 2 O reaction [5] is attributed to increased adsorption of water on PuO 2 near 1008C. A significant temperature effect for both metals is the convergence of corrosion rates for water vapor and dry air at elevated temperatures. Convergence for Pu is preceded by a sharp decrease in rate at 110– 2008C [5], the temperature range in which OH 2 thermally desorbs in the presence of water vapor. Closure of the temperature–humidity envelope for uranium is apparently driven by a similar desorption process [3]. Behavior suggests that moisture-enhanced corrosion results from a high concentration of adsorbed water. Square-root dependence of the corrosion rate on water pressure for both U [3,18] and Pu [5] implies that the enhanced corrosion rate is determined by the OH 2 concentration. As noted by Stakebake [37], this implies that the rate is proportional to the hydroxide concentration, [OH 2 ], formed by reaction of water with lattice oxygen. H 2 O(g) 1 O 22 (lat) → 2OH 2 (ads, lat)

(15)

The relationship of [OH 2 ] to water pressure, P(H 2 O), is defined by the equilibrium constant, K, for this reaction. K 5 ([OH 2 ] 2 ) /([O 22 ] P(H 2 O))

(16) 22

Since the lattice oxide concentration, [O ], is constant, [OH 2 ] is proportional to P(H 2 O)1 / 2 . The observed P(H 2 O)1 / 2 dependence implies that the corrosion rate is proportional to [OH 2 ] and supports the conclusion that moisture enhancement is determined by the concentration of dissociatively adsorbed water.

6.3.3. A mechanism for oxidation in water vapor The proposed mechanism for reaction of uranium and plutonium with water vapor is based on the following observations: (1) corrosion of the metals by water vapor proceeds with formation of oxide and H 2 . (2) The oxide

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products formed by reaction of H 2 O are chemically identical to those formed in dry air. (3) The time dependence of oxidation is paralinear. (4) Water chemisorbs as hydroxide on the oxide in the low-temperature range where rate enhancement is observed and desorbs at high temperatures where rate enhancement terminates. (5) A squareroot dependence on water pressure implies that the corrosion rate is proportional to [OH 2 ]. (6) Absence of product hydride precludes transport of OH 2 to the oxide–metal interface. Except for the chemistry of O 22 formation at the gas– oxide interface, oxidation of U and Pu by water proceeds via the same mechanism as in air. Hydroxide ions associate with electrons to form O 22 ions and H atoms. OH 2 (ads, lat) 1 e 2 → O 22 (lat) 1 H(ads)

(17)

Half of the product O 22 ions replaces those reacted during OH 2 formation (Eq. (15)). The remaining oxide ions occupy interstitial sites, migrate to the oxide–metal interface as clusters, and react as described by Eq. (14). Association of product H atoms on the surface leads to formation and desorption of H 2 . The rate of oxide ion formation is rapid compared to that for O 2 because [OH 2 ] is high and O 22 is formed by transfer of a single electron. As indicated by formation of PuO 21x at the gas–oxide interface during Pu oxidation by water vapor [27], production of O 22 at an accelerated rate enhances the oxidation rate by increasing the gradient in oxygen concentration across the constant-thickness oxide layer. The thermally induced decrease in [OH 2 ] and accompanying drop in the O 22 formation rate result in a decreasing corrosion rate and ultimate disappearance of moisture enhancement at elevated temperatures.

6.4. Corrosion in moist air 6.4.1. Chemistry and oxide properties in moist air Oxides formed by corrosion of Pu in moist air are indistinguishable from those formed in dry air or water vapor, but formation of U 3 O 8 is observed at 200–5008C [28]. Corrosion of U by a 1.5:1.0 molar mixture of O 2 and H 2 O (Fig. 4) proceeds with consumption of O 2 at an accelerated constant rate, while the concentration of H 2 O remains constant and only a trace of H 2 appears [29]. Reaction of H 2 O is accompanied by formation of H 2 after O 2 is depleted. A complementary study shows that the U1H 2 O reaction forms H 2 at a constant isothermal rate until a quantity of O 2 is injected into the system [24]. Formation of H 2 ceases at that point and resumes at the initial linear rate after O 2 is depleted. Corrosion of Pu by moist air proceeds with reaction of O 2 at the accelerated rate of the Pu1H 2 O reaction without forming H 2 [5]. Isotopic studies with 18 O (O*) show that corrosion of U by a H 2 O1O 2* mixture at 1008C produces H 2 O* and incorporates O* in the oxide product while preserving O *2

purity [29]. Although questions are raised about these results [43], depth-profile characterization of oxides formed by reaction of U with H 2 O, H 2 O*, H 2 O*1O 2 mixtures, and H 2 O1O 2* mixtures shows that oxygen from both H 2 O and O 2 are incorporated in the oxide [33]. Profiles of the layer formed by H 2 O* show that labeled oxide formed beneath a thin preexisting surface layer of unlabeled oxide contained a constant O* content of about 85%. Profiles of oxides formed by reaction of H 2 O*1O 2 mixtures show that O* enrichment was highest in the product beneath the outer unlabeled layer and decreased with increasing depth. Conversely, the content of unlabeled O in the oxide formed by H 2 O1O 2* mixtures was about 90% near the surface, but decreased relative to O* with increasing depth. In addition to implying that oxygen in the dioxide lattice does not exchange during reaction and that transport of oxygen occurs via an interstitial mechanism, these results show that the initial oxide product is highly enriched in the isotope from H 2 O and that the content of the isotope from O 2 progressively increases with increasing depth. Catalytic activity of oxide promotes formation of water from the elements and plays a key role in water-catalyzed oxidation of dioxide to PuO 21x by O 2 . Moisture-enhanced oxidation of uranium oxide in air is also observed [44], but the chemistry is not described. Corrosion chemistry of the metals parallels that of the oxides and suggests that watercatalyzed formation of higher oxides participates in moisture-enhanced reaction.

6.4.2. Kinetic behavior in moist air A paralinear time dependence for the reaction of U is shown by measurements during corrosion in moist oxygen [31]. Result are consistent with observations for U and Pu in dry air and water vapor and confirm that reaction in moist air is controlled by diffusion, even though temperature and pressure dependencies of corrosion kinetics diverge [3–5]. Behavior of Pu in moist air is similar to that in water vapor. The rate is independent of O 2 pressure and has a square-root dependence on H 2 O pressure [5]. Ea varies 21 systematically with H 2 O pressure from 79 kJ mol in dry 21 air to 141 kJ mol in moisture-saturated air. However, reaction in moist air proceeds with consumption of O 2 instead of H 2 O and without formation of H 2 . Data for oxidation of U in moist air show three regions of kinetic dependence on oxygen pressure [3,18]. At low O 2 pressures (,15 mTorr or 20 ppm), behavior is identical to that observed for Pu in moist air. The corrosion rate depends on H 2 O pressure and decrease sharply as O 2 pressure is increased from of 15 to 150 mTorr, but data are inadequate for quantifying pressure dependencies. In the third region above 150 mTorr O 2 , corrosion is independent of oxygen pressure and defined by three temperature regimes. The rate is equal or less than that for dry air at

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temperatures below 408C. At 40–1008C, the rate is enhanced by water in excess of an undefined threshold pressure (,2 Torr) and depends on temperature (Ea 5109 kJ mol 21 ). At 100–4008C, the rate remains insensitive to changes in O 2 pressure, but varies with H 2 O pressure and temperature. The pressure exponent of water varies systematically from zero at 1008C to first-order at 2508C in accordance with the Langmuir adsorption model [3,33]. A progressive decrease in rate with H 2 O pressure at 250– 4008C is consistent with increasing instability of adsorbed water at high temperatures and with closure of the temperature–humidity envelope near 4508C [3].

6.4.3. A mechanism for oxidation in moist air The corrosion mechanism is defined by the behavior of Pu in moist air and by that of U in moist atmospheres with less than 15 mbar O 2 . A modified mechanism is necessary to account for kinetic behavior of U in moist air. The proposed mechanism for corrosion of Pu in moist air and U in moist O 2 at low pressures is based on the following observations: (1) formation of oxide is accompanied by reaction of O 2 at an accelerated rate, by maintenance of a constant H 2 O concentration, and by the absence of H 2 formation. (2) Oxide products are chemically identical to those formed by reaction of dry air and water vapor. (3) The time dependence of reaction is paralinear. (4) The temperature range for moisture enhancement of oxidation coincides with the stability range for adsorption of water. (5) The corrosion rate has a square-root dependence on water pressure. (6) Isotopic data suggest that oxidation proceeds by reaction of H 2 O and show that O 2 is transformed into both H 2 O and oxide during corrosion. As with corrosion of U and Pu in dry air and water vapor, corrosion rates in moist air are controlled by diffusion of O 22 through an oxide layer of steady-state thickness. The only difference between the mechanisms in 22 moist air and water vapor is in the chemistry of O 2 formation. In both cases, OH apparently reacts with an 22 electron to form O and H. In the presence of O 2 , atomic hydrogen combines with dissociatively adsorbed oxygen to reform water as observed during the reaction of PuO 2 with moist air (Eqs. (5)–(7)). Product water either desorbs as gaseous H 2 O or reacts to form oxide by the catalytic cycle [15]. Formation of H 2 O* by H 2 O1O 2* mixtures is a consequence of the unidirectional reaction that produces higher oxide (O 22 on lattice sites) via the water-catalyzed cycle. H 2 O(ads) 1 O 22 (lat) → 2OH 2 (ads)

(18)

2OH 2 (ads) 1 2e 2 → 2O 22 (lat) 1 2H(ads)

(19)

1 / 2O *2 (g) → O*(ads)

(20)

2H(ads) 1 O*(ads) → H 2 O*(ads) ]]]]]]]]]

(21)

89

H 2 O(ads) 1 1 / 2O 2* (g) 1 2e 2 → H 2 O*(ads) 1 O 22 (lat) (22) Purity of residual O *2 is preserved and each H 2 O that reacts is replaced by a molecule of H 2 O*. Desorption enriches the gas phase in H 2 O* and reaction increases the O* content of the oxide product. Therefore, oxide formed initially near the gas–oxide interface is rich in unlabeled oxygen. As continuing reaction progressively transforms O *2 into H 2 O*, the mole fraction of H 2 O* increases and the O* content of the oxide layer increases with increasing depth, as observed in SIMS profiles [33]. Conversely, H 2 O*1O 2 mixtures produce oxide rich in O* near the gas–oxide interface and with a progressively lower O* concentration with increasing depth. Simultaneous diffusion of O 22 and OH 2 is not required to account for the isotopic trends in depth profiles [33]. The high level of gas-phase H 2 O* enrichment reported by McD. Baker et al. [28] is not confirmed by a later study [43], but this apparent discrepancy is resolved by the proposed mechanism. Formation of high-purity H 2 O* by an H 2 O1O 2* mixture is a consequence of a high initial O *2 :H 2 O molar ratio and of extended reaction that replaced H 2 O with H 2 O*. High enrichment is not possible with mixtures having initial O *2 :H 2 O ratios less than 1:2. Moisture enhancement predicted by the mechanism is not observed for U corrosion at elevated oxygen pressures and low temperatures [3], conditions that favor formation of higher oxides. Composition of the oxide layer is controlled by the rates of competing oxidation and reduction reactions [17]. PuO 2 is the only surface phase indicated by XRD analysis of metal oxidized at room temperature, but moisture enhancement of oxidation implies that PuO 21x is present [27]. PuO 21x and PuO 2 are observed at 2508C. Data for the product formed at 3508C show that the PuO 2 thickness is sharply reduced by autoreduction to Pu 2 O 3 at the oxide–metal interface and a thin PuO 21x layer remains at the gas–solid interface. Thick PuO 21x appears only at intermediate temperatures where its formation rate is rapid relative to the rates of the competing reactions. Oxidation of UO 2.25 to U 3 O 8 is favorable in air [45] and oxidation is promoted by moisture [44]. Behavior of U 3 O 8 at the gas–oxide interface during corrosion of U in moist air at 200–5008C [28] parallels that for PuO 21x and suggests that a thin layer of the higher oxide is present at low temperatures. Suppression of moisture-enhanced corrosion in moist air at low temperatures is consistent with slow diffusion of oxygen through a U 3 O 8 layer. The orthorhombic a-U 3 O 8 structure [46] is formed by chains of vertex-shared, pentagonal–bipyramidal coordination polyhedra. A suppressed oxidation rate and a high Ea are consistent with restricted O 22 mobility in the narrow channels formed by edge-share polyhedral chains. The rate remains diffusionlimited above 1008C, but the concentration of adsorbed water is insufficient to sustain the maximum rate and

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water-dependent kinetics appear. This concept formalizes a proposal that the kinetic anomaly results from adsorption of O 2 on surface sites [18,31].

7. Conclusions Experimental results show that PuO 2 is a reactive and catalytically active material that alters the kinetic behavior of systems containing hydrogen, oxygen, and water. Dissociative adsorption of water and catalytic activity promote formation of PuO 21x at temperatures below 2008C. This conclusion is supported by coincidence of a sharp decrease in the corrosion rate of Pu with desorption of water from the oxide. These observations establish a basis for defining the corrosion chemistry and developing a comprehensive mechanism that accounts for the corrosion kinetics of U and Pu in dry air and water vapor, as well as for moisture enhancement of the corrosion rate in air. Kinetics of U and Pu corrosion are undoubtedly controlled by properties of the product oxide formed on the metal surface. The high chemical potential for reaction is the only metallic property of importance. Observation of a parabolic time dependence during isothermal and isobaric measurements at each condition might suggest that the corrosion rate is determined solely by the temperaturedependent diffusion of oxygen through the oxide layer. The thickness of the adherent oxide layer increases with temperature [30] and the gradient in oxygen concentration across the layer is a complex function of temperature and oxidant concentration. Kinetic behavior is determined by the combined effects of surface chemistry, thickness, and transport properties of the oxide. Unlike the metals, the oxides of U and Pu are similar and lead to remarkably similar corrosion kinetics. Chemistry of corrosion by water suggests that, unlike oxygen, hydrogen is not readily transported across the oxide layers on U and Pu. Although generation of H at twice the rate of O 22 production maintains a gradient in hydrogen concentration across the layer and hydride formation is thermodynamically favorable, detectable amounts of hydride do not form at low temperatures. Competition from oxygen for electrons and blockage of interstitial sites in the higher oxides may be important factors in limiting hydrogen transport. In closed systems containing metal and moist air, the pressure decreases initially as O 2 is consumed by the water-catalyzed oxidation, increases as H 2 is formed by reaction, and ultimate decreases as hydride is formed by reaction of H 2 . The comprehensive mechanism for corrosion finds application in assessing the hazards posed by potentially pyrophoric waste forms containing Pu. The corrosion rate in water vapor at 1108C equals that for oxidation of metal in dry air at 4008C [5] and heat generation by moistureenhanced corrosion of Pu holds potential for initiating runaway reaction. However, evaluation of the pyrophoric

potential posed by plutonium-containing salts leads to the conclusion that moisture cannot initiate an autothermic reaction because the corrosion rate decreases by more than 10 3 as temperature increases from 110 and 2008C [1]. Validity of the assessment rests on the veracity of that kinetic anomaly. The derived corrosion mechanism shows that moisture-enhanced corrosion is driven by dissociatively adsorbed water on the oxide and experimental results show that water desorbs at 120–2008C. These results agree with the observed rate decrease at 110–2008C and with entry into a moisture-independent kinetic regime at temperatures beyond 200–2158C [5,6]. Generation of hydrogen by the PuO 2 1H 2 O reaction in storage containers of plutonium oxide is also a concern [2]. Formation of explosive mixtures by reaction of water with PuO 2 in air-filled containers has been suggested, but is not possible because moisture-enhanced corrosion proceeds via the water-catalyzed cycle. Association of H atoms with dissociatively adsorbed oxygen reforms H 2 O and prevents accumulation of H 2 whenever O 2 is present. H 2 appears only after O 2 is depleted. The complex nature and inadequate understanding of actinide oxide chemistry are evident. A consistent picture of corrosion kinetics for U and Pu emerges from the results of this study, but further investigation is needed to determine the consequences of catalytic and transport properties of the oxides.

Acknowledgements Experimental work was performed at Los Alamos National Laboratory under auspices of Energy Contract W-7405-ENG-36. Data evaluation and report preparation were supported by Kaiser-Hill Company, LLC, Golden, CO.

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