Precipitation Of Small Amounts Research Articles

Factors Involved in Stress Corrosion Cracking of Tubes from a Nuclear Power Plant Feedwater Heater

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The samples cut from tubes have been analyzed by different methods: visual examination, optical (metallographic) microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Following analyses, on the surface of tubes was highlighted the presence of many pits in which cracks have started. Inside the pits, impurities of the type S, Cl, K, Ca were detected too. The branched cracks most likely have occurred as a result of precipitation of small amounts of chlorides deposited and concentrated on the surface of the tubes over a long period of operation. The stresses that favored this type of corrosion cracking were both residual stresses and stresses occurring at the torsion and bending of the tubes, while the high water temperature from feed heater was also a favorable factor for corrosion. The final conclusion was that the tubes failed by a chloride induced stress corrosion cracking mechanism.

Physical and chemical constraints on platinum-group element behavior during crystallization of a basaltic komatiite liquid; example of the Proterozoic Delta Sill, New Quebec, Canada

The Delta sill, located in the southern part of the 2.03 to 1.92 Ga Cape Smith fold belt, northern Quebec, is composed of a peridotite, pyroxenite, gabbronorite, and anorthosite suite derived from a basaltic komatiite parental magma (MgO = 13.58-15.66%). Despite its rather limited thickness (215-275 m), it contains three platinum-group elements (PGE) horizons, two slightly enriched horizons, one in the peridotite and one in the lower gabbronorite, and a strongly PGE-enriched horizon (forming a reef) in the upper gabbronorite. The high PGE content in the reef implies a system with an R factor (volume of immiscible sulfide liquid/ volume of silicated magma) of 100 to 1.000. However, the small volume of magma involved suggests that several other processes controlled the PGE behavior. Some experimental works have shown that the CaO and FeO contents in parental magmas influence the sulfide capacity and consequently the sulfur fugacity. The last parameter, as well as the oxygen fugacity, have been previously recognized as having a fundamental influence on the PGE enrichment processes. Because of the relatively high CaO contents (7.21-11.26%) of parental magma and the existence of PGE enrichment, the Delta sill provides an opportunity to test the effects of varying sulfur and oxygen fugacities on the processes of PGE enrichment. Thus, the PGE concentration profiles observed in the two sections transecting the sill are in good agreement with the experimentally determined behavior of these elements in silicate melts as a function of oxygen and sulfur fugacities. First, the relatively high Ca content of the initial magma induced a high sulfide capacity in the liquid. Second, decreasing sulfide capacity (and correlatively, an increase in sulfur fugacity) produced by the precipitation of, calcic phases--notably clinopyroxene and plagioclase--account for the PGE anomalies as well as the late-stage crystallization of sulfides in the reef. In the lowermost PGE anomaly, Ir and Ru enrichment within the peridotitic layer can be clearly related to early ferrochromite crystallization due to increasing oxygen fugacity, whereas subsequent Pt enrichment can be related to the formation of an isoferroplatinum phase (Pt 3 Fe) associated with ferrochromite. In the second PGE anomaly, located in the lower gabbronorite, synchronous Ir and Ru, Rh, Pt, and Pd, and S enrichments are due to the precipitation of small amounts of sulfides under low oxygen fugacity. Both the Ir and Ru and the Rh, Pt, and Pd, left over from the first chromite-forming stage, are complexed by sulfur when the sulfide capacity increases, consequent to CaO removal from the melt by plagioclase fractionation. Finally, the third anomaly in the PGE reef, appears to record the crystallization of a differentiated Fe- and Ti-rich residual liquid containing fluid phases and sulfur. It reflects the migration of an S-bearing liquid following the important lowering of sulfide capacity in the liquid due to the increase of SiO 2 and TiO 2 in the liquid and to the precipitation of Ca-bearing minerals.

Magnetization study of UFe 2

Bulk magnetization measurements have been performed between 4 and 400 K on oriented samples cut from a single crystal of UFe 2 (cubic C15-type Laves phase) used in an earlier neutron-diffraction study. The easy axis of magnetization was determined to be 〈111〉 in all cases, but the samples differed from each other in bulk magnetic properties (particularly Curie temperatures, which varied from 147 to 162 K). These variations apparently reflect small differences in stoichiometry. The upper temperature limit of the measurements was determined by the increasing presence of an “impurity” magnetization presumably arising from the precipitation of small amounts (≈0.01%) of free iron. This result implies that samples of UFe 2 quenched from elevated temperatures are not stable, under equilibrium conditions, near room temperature. Subsequent measurements on polycrystalline samples deliberately prepared with excess uranium (UFe 2− x ) showed that the C15-type structure persists to at least UFe 1.7 and that the magnetic properties vary regularly with changes in stoichiometry. The bulk magnetic moment (1.09 μ B/mol) determined from a 〈111〉 oriented single crystal is significantly larger than the total moment obtained in the neutron-diffraction study on the same sample.

The predecomposition condition of supercooled austenite

1. It was shown experimentally that decomposition of supercooled austenite begins almost immediately on reaching the temperature below which the austenite becomes metastable. The precipitation of small amounts of decomposition products is due to mechanical stabilization of supercooled austenite, which is evident in the fairly rapid slow down of decomposition. The development of thermal stabilization is indicated by the relationship between the isothermal holding temperature and the amount of decomposition products required for the development of mechanical stabilization; in accordance with the data given in [14], the degree of thermal stabilization increases with the temperature. 2. It is probable that concentration fluctuations develop during isothermal holding, which absorb some amount of strain energy during deformation of the material that is sufficient for rebuilding of the lattice and creation of an interface, as the result of which particles of ferromagnetic phase are precipitated in slip systems. A substantial portion of this phase is unstable after removal of the stress field and is dissolved during further isothermal holding. 3. The development of mechanical stabilization due to precipitation of decomposition products is also evident in the fact that decomposition of deformed austenite lags behind the decomposition of undeformed austenite.