Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.     

Corrosion of Fe-10Al-Cr alloys by coal char

Corrosion of iron-base alloys at 982°C (1800°F) by coal char is observed and the phase morphologies are discussed. No sulfidation was observed at 50 hr exposure. After 100 hr internal aluminum-rich sulfides were observed along with thick outer scales of iron oxide. The species causing the hightemperature-induced corrosion are probably sulfides and sulfates, present in most coal chars. Possible mechanisms for the corrosion are also discussed.Department of Materials Science and Mineral Engineering.     

Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization

Partially crystalline Si₃N₄, with nanosized crystals and a specific surface area greater than 200 m²/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH₃ to 1000°C. This polymer does not contain N initially. Crystallization to high-purity α-Si₃N₄ proceeds with additional heating above 1400°C under N₂. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N₂, are consistent with an amorphous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes.     

Cement mortar impregnated with poly(methyl methacrylate-co-styrene)

Polymer-impregnated mortars were prepared by copolymerization of a monomer mixture of methyl methacrylate and styrene in the ratios of 13:87 and 40:60 using Co-60 gamma radiation. The copolymerization characteristics viz. the rate of polymerization, the extent of monomer loss, polymer loading, etc., were studied. The nature and molecular weight of the extractable polymer from the composite were determined. The flexural strength of the copolymer-impregnated composites was found to be better than that of the composites impregnated with component homopolymers.     

Has water-saving irrigation recovered groundwater in the Hebei Province plains of China?

This study performed a quantitative evaluation of the impact of water-saving irrigation on the groundwater regime in the Hebei Province plains area. In this work, the change in groundwater regime and the contributions of precipitation and water-saving irrigation development were investigated. The results indicate that the groundwater overdraft has been mitigated to some extent, mainly due to changes in precipitation and the implementation of water-saving irrigation, with contributions of 64.3% and 35.7%, respectively, when considering only these two factors. Water-saving irrigation is accepted as an important means for reducing groundwater depletion, but should be used in conjunction with other measures.     

Microwave joining and repair of composite materials

A novel technique that uses microwave power for joining and repair of thermoplastic and thermoset composites and ceramics is discussed. Enhanced microwave heating resulting from the use of conducting polymers and chiral microinclusions shows considerable promise for joining and repair of composites. The method is attractive because it produces clean and reliable interfacial joints, it is fast, it does not entail alteration of the bulk materials, and it does not result in volumetric heating. System details, including magnetron, waveguides, circulators, coupling iris, and the applicator for delivering microwave power, are described. Material requirements for the efficient absorption of microwave power are discussed. Microwave heating can be increased by doping the components to be joined or by the use of microwave adhesive films. Recent developments in electromagnetic chirality for microwave absorption are introduced and the design of composite materials for enhanced absorptivity is discussed. Results are presented for the welding and repair of thermoplastic, thermoset, and ceramic components.     

Structure of polyurethane elastomers. X-ray diffraction and conformational analysis of MDI-propandiol and MDI-ethylene glycol hard segments

X-ray diffraction and conformational analysis have been used to investigate the structure of polyurethane hard segments prepared from diphenylmethane 4,4′-diisocyanate (MDI) with propandiol (PDO) and ethylene glycol (EDO) as chain extenders. The results are compared with those obtained previously for MDI-butandiol (BDO) hard segments. In the latter system, the poly(MDI-BDO) chains are fully extended with an all-trans conformation for the O(CH₂)₄O unit, and a monomer repeat of 18.95 Å. The unit cell is triclinic with a tilted base plane such that adjacent chains are staggered along the fibre axis. In contrast both poly(MDI-PDO) and poly(MDI-EDO) adopt unstaggered structures, i.e. the chains are in register and the unit cell base planes are perpendicular to the chain axis. The monomer repeats of 16.2 Å for poly(MDI-PDO) and 15.0 Å for poly(MDI-EDO) are shorter than the predicted repeats for fully-extended chains, indicating that these polymers have contracted conformations containing some gauche CH₂ groups. Conformational analysis shows that the 16.2 Å repeat for poly(MDI-PDO) can be achieved with the O(CH₂)₃O unit in the trans-gauche⁺-gauche⁺-trans or gauche⁺-trans-trans-gauche⁺ conformations. Similarly the 15.0 Å repeat for poly(MDI-EDO) is predicted for the gauche⁺-trans-gauche^? conformation for the O(CH₂)₂O unit. These conformations are of higher energy than the all-trans fully-extended chains. This may explain the higher crystalline perfection of the poly(MDI-BDO) hard segments, for which crystallization in the all-trans form will probably provide a greater driving force for phase separation.     

Conformational analysis of poly(MDI-butandiol) hard segment in polyurethane elastomers

As part of our study of the structure of the hard segments of polyurethane elastomers, we have used conformational analysis to predict the conformation of the polyurethane chain formed from diphenylmethane 4,4′-diisocyanate (MDI), with butandiol as the chain extender. X-ray diffraction indicates the chain conformation is highly extended in the solid state, with a monomer repeat of 18.95 Å. The chain conformation can be predicted from the interaction between the large contiguous groups. Semiempirical and CNDO/2 molecular orbital calculations have been used to investigate the phenyl-phenyl, phenyl-urethane, and urethane-butandiol interactions. Minimum energy for the diphenyl-methane section occurs for a CCH₂C bridge angle of θ = 110°. However, the more likely conformation in the polymer is probably that at the subsidiary minimum at θ = 118°, in which the phenyl rings are mutually perpendicular, which is comparable to the conformation seen for model compounds. For the phenyl-urethane interactions the energy minimum is at $\text{ч = ±90°}$, with the combination $\text{(ч}{}_1\text{, ч}{}_2\text{) = (?90°, +90°)}$ leading to the necessary extended conformation. Calculations for the butandiol segment favour an all-trans conformation which is coplanar with the urethane groups. The predicted conformation of the polymer chain has a fibre repeat which matches the observed value, and this cannot be attained if the butandiol unit contains any gauche bonds. Model compounds show large variations for the phenyl-urethane conformation from the predicted minimum at $\text{ч = ±90°}$, indicating the importance of intermolecular stacking forces in determining the polymer conformation. However, provided the adjacent butandiol and urethane groups are coplanar, the fibre repeat is relatively insensitive to variations of ч.     

Self-emulsifiable soybean oil phosphate ester polyols for low-VOC corrosion resistant coatings

A series of soybean oil phosphate ester polyols (SOPEP) was prepared by reaction of fully epoxidized soybean oil with phosphoric acid and simultanoeous hydrolysis in the presence of a polar solvent. The polyols were characterized by determination of acid value, oxirane number, hydroxyl value, molecular weight (GPC), and FTIR spectra. These polyols with varying amounts of acid phosphate groups could be self-emulsified to form aqueous dispersions after neutralization with organic base. These aqueous dispersion showed varying degrees of stability and their appearance ranged from opaque dispersions to translucent to clear solutions. Waterborne coating compositions were prepared using these aqueous dispersions as principal components and their thermally cured film properties were studied. it was found that by careful selection and formulation, SOPEPs can be successfully used for low-VOC waterborne coating formulations. SOPEPs with 3.5% phosphate ester content showed visibly superior corrosion resistance properties.