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A method to determine the defect structures in hyper-stoichiometric UO(2+x) using a combination of XRD and Raman spectroscopy has been developed. A sequence of phase transitions, from cubic to tetragonal symmetry, occurs with increasing degree of non-stoichiometry. This sequence proceeds from a cubic phase through an intermediate t''-type tetragonal (axial ratio c/a = 1) phase to a final t-type tetragonal (c/a not = 1) phase. Four distinct structural defect regions can be identified in the stoichiometry range, UO(2) to U(3)O(7): (i) a random point defect structure (x (in UO(2+x)) < or = 0.05); (ii) a non-stoichiometry region (0.05 < or = x < or = 0.15) over which point defects are gradually eliminated and replaced by the Willis 2:2:2 cluster; (iii) a mixture of Willis and cuboctahedral clusters (0.15 < or = x < or = 0.23); (iv) the cuboctahedral cluster (x > or = 0.23). The geometry and steric arrangement of these defects is primarily determined by the concentration of the excess-oxygen interstitials.

The initial stages of corrosion of iron by unstirred saturated aqueous H2S solutions at 21~ and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H2SO4 or NaOH), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal FeSl-x) is formed by both solidstate and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid-state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; the quantity of precipitated material peaks near pH --4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pit = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21~ was monitored by x-ray diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.The initial corrosion of iron or carbon steel by hydrogen sulfide saturated water at low temperatures is complex (1-5). There is considerable confusion regarding product identity in the earlier literature (5, 6) but it is now established that the initial corrosion involves the formation of up to three iron monosulfide phases (1-4): mackinawite (tetragonal FeSI-x), cubic ferrous sulfide, and troilite (stoichiometric hexagonal FeS). Solubility measurements (7, 8) and interconversion studies (1, 2, 9) indicate the order of stability troilite > mackinawite > cubic ferrous sulfide. Troilite is a stable phase in the Fe-S system (10), but it is metastable with respect to pyrite in the solutions used in the present study (11). Mackinawite appears to be metastable under all conditions, but it occurs widely both naturally (9, 12) and as a precipitate under a wide range of laboratory conditions (8, 13) as well as in corrosion products. Little is known about cubic ferrous sulfide, other than its occurrence as a corro= sion product, and its ease of conversion to mackinawite (1, 2).Although there is a substantial body of literature describing the occurrence of these phases, no systematic examination of the chemical conditions of their formation on corroding surfaces has hitherto been reported. The purpose of the present study was to make such an examination, and to elucidate the mechanisms whereby the three solid corrosion products are formed. We have therefore examined the sequence of corrosion of iron in unstirred saturated aqueous H2S at 21~ as a function of pH, reaction time, and applied current.This work formed part of a study of corrosion and deposition p...

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