Corrosion reactions in molten sulphates

The redox potential in sulphate melts is controlled by the partial pressures of oxygen and SO₃ in equilibrium with the melt. The rate for the reduction of oxygen is limited by the solubility. Higher rates are observed for the reduction of SO₃ which is identified as the principal oxidizing species. Equilibrium potentials for reversible metal-metal ion couples for the principal components of superalloys are negative relative to the redox potential for the melt. The anodic oxidation of the metal therefore proceeds irreversibly in conjunction with the cathodic reduction of SO₃ An oxide layer is formed on the surface of the metal when the metal ion concentration at the surface, combined with the oxide ion concentration in the melt, which is related to the partial pressure of SO₃, exceeds the solubility limit for the oxide. The corrosion behaviour will depend on the mass transport processes through this oxide layer. Temperature gradients through the molten sulphate, and oxide ion concentration gradients established as a consequence of the corrosion reaction, may influence the morphology of the corrosion product. The gradient between the oxygen chemical potential at the outer surface of the molten sulphate, defined by the oxygen partial pressure in the gas and the oxygen chemical potential at the metal/metal oxide interface, defined by the dissociation equilibrium for the metal oxide, combined with the transport of SO₃ through the molten sulphate, increases the sulphur chemical potential at the metal surface and leads to the formation of the metal sulphide. Stress in the protective oxide layer caused by the growth of the sulphide phase at the interface between the metal and the oxide will eventually fracture the oxide and cause accelerated corrosion.     

Short-Crack Fracture Toughness of Silicon Carbide

The fracture toughness of four different silicon carbides was measured using single-edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11–32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial ( c ₀) to the instability ( c ^*) lengths. The results showed a rising crack-growth-resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack-growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.     

Human axillary extracts: Analysis of compounds from samples which influence menstrual timing

Previous studies have shown that menstrual cycle length and fertility are affected by a regular pattern of either: (1) intimate contact with men; or (2) contact with extracts from male axillary secretions; and (3) the axillary extracts of women. Experiments utilizing axillary extracts employed a controlled, double-blind research design using either extract or placebo. The study reported here employed quantitative gas chromatography-mass spectrometry (GC-MS) techniques to determine the concentration of several steroids in the male and female axillary secretion extracts. The steroids examined were: androsterone sulfate, dehydroepiandrosterone sulfate, and the volatile steroid androstenol. The results suggest the following: (1) The concentration of androstenol produced by secretion donors varied during the collection period. Males appear to produce more androstenol at certain times; women's secretions show a menstrual variation in androstenol; the highest concentrations of this compound appear to be produced in the midfollicular phase, prior to ovulation. (2) The concentration of dehydroepiandrosterone sulfate is significantly greater in men than in women. (3) In addition to the steroids, a series of aliphatic acids which range from C₂ to C₁₈ in chain length were found. The more volatile members of this series may contribute to the odor of the secretions.     

Surface activity of high-temperature perfluoropolyalkylether oil additives

Three additive materials for perfluoropolyalkylether-based high-temperature engine oils were studied, a bis-substituted dibenzothiazole, a diphenyl ether and a triphenyl phosphine. The focus of this work was to understand how the additives interacted with various surfaces to provide either anticorrosion or antiwear activity. In the case of the bis-substituted dibenzothiazole, the additive was found to form a thick, protective anticorrosion layer on the surface. The diphenyl ether served as another equally effective anticorrosion additive but failed to demonstrate surface activity. The third additive compound, triphenyl phosphine, was found to break down at the surface and form an inorganic polyphosphate which provided a level of both anticorrosion and antiwear function. This revised version was published online in August 2006 with corrections to the Cover Date.     

Label-free comparative analysis of proteomics mixtures using chromatographic alignment of high-resolution muLC-MS data

Label-free relative quantitative proteomics is a powerful tool for the survey of protein level changes between two biological samples. We have developed and applied an algorithm using chromatographic alignment of microLC-MS runs to improve the detection of differences between complex protein mixtures. We demonstrate the performance of our software by finding differences in E. coli protein abundance upon induction of the lac operon genes using isopropyl beta-D-thiogalactopyranoside. The use of our alignment gave a 4-fold decrease in mean relative retention time error and a 6-fold increase in the number of statistically significant differences between samples. Using a conservative threshold, we have identified 5290 total microLC-MS regions that have a different abundance between these samples. Of the detected difference regions, only 23% were mapped to MS/MS peptide identifications. We detected 74 proteins that had a greater relative abundance in the induced sample and 21 with a greater abundance in the uninduced sample. We have developed an effective tool for the label-free detection of differences between samples and demonstrate an increased sensitivity following chromatographic alignment.