The microstructural chemistry of the commercial aluminium alloy 7150, containing Al, Zn, Mg, Cu and some trace impurities, was investigated in detail. This alloy is a precipitation hardening alloy, deriving most of its strength from the fine distribution of solute rich precipitates formed during thermal processing. At peak strength this alloy suffers from the common problem of stress corrosion cracking, leading to unpredictable premature failure in the presence of a corrosive environment. Failure is mainly intergranular, thus the structure and chemistry of the grain boundary regions is of interest. A large number of previous investigations have failed to correlate any individual parameter with the stress corrosion cracking behaviour. As the analytical techniques have improved over the last three decades, more complex investigations of the microstructure and the microchemistry have been attempted, in order to more fully characterise the development of this alloy during thermal processing. This thesis presents the results of two of the highest resolution techniques available for microchemical analysis. Scanning transmission electron microscopy X-ray analysis, using a VG-HB501 dedicated scanning transmission electron microscope, enables chemical analysis with a 2nm electron probe, while atom probe analysis, using a VG-FIM100 atom probe with an additional position sensitive detector, enables single atom chemical identification with sub-nanometre spatial resolution. However, both of these techniques have their own experimental limitations which restrict the accuracy of the results obtainable. A detailed description of the many factors limiting both techniques is presented. Combining these techniques has enabled chemical analysis of all the microstructural features present in this alloy on the nanometre scale. A description of the chemical changes occurring during age hardening of this alloy is given in summary.

No description available.

The purpose of this thesis study is to assess the feasibility of using a fiber sorbent module system to remove t-butyl mercaptan (TBM), a common odorant, from pipeline grade natural gas. Odorants such as mercaptans are added to natural gas for safety reasons, but their combustion products are corrosive and decrease the lifetime of the turbines in which they are combusted. Therefore, it is desirable to remove the odorants to extend this lifetime. A TBM removal system attached to a 840 MW natural gas-fueled combined cycle power plant unit such as the one at Plant McDonough-Atkinson (Smyrna, GA) must process gas at a flow rate of approximately 180,000 standard cubic feet per minute. A single 85 MW GE 7EAQ gas turbine has a flow rate of approximately 15,000 standard cubic feet per minute, and will serve as the basis for a system design and process analysis study. The concentration of odorants in natural gas is typically 10 ppm or less. For the purposes of this study, the upper limit of 10 ppm TBM will be used. Zeolite 13X was selected as the model adsorbent for this study due to its high sorption capacity for mercaptans and its ease of incorporation into both fibers and pellets. Design calculations were performed to optimize and determine the feasibility of fiber modules for TBM removal, as well as assess their advantages over conventional pellet packed beds. An understanding of how critical parameters such as heat and mass transfer resistances, pressure drop, and capital and operating costs are affected by design specifications such as sorbent and bed dimensions, allows an optimal design for the needs of the model turbine to be found. Based on these design equations, a fiber sorbent module configuration that selectively and continuously removes TBM from natural gas is developed

No description available.

Corrosion of the reinforcing steel is a primary deterioration mechanism for reinforced concrete bridges. Heavy use of de-icing salts is believed to be a major contributor in Ontario to severe girder soffit spalling in certain cases. This thesis develops an assessment methodology to evaluate spalled bridges based on ultimate limit states. Specifically, a deterministic program is developed for assessment. It is subsequently compared to laboratory test results and used as a basis for a probabilistic reliability study. A modified area concept is proposed in this thesis to consider the effects of exposing reinforcement at various locations along the girder length. A multipoint analysis program, BEST (Bridge Evaluation Strength Tool), is developed that employs this concept, along with graphical spalling surveys and structural drawings, to evaluate reinforced concrete bridge girders. The program is adapted for a full bridge analysis and to consider the other effects of corrosion, such as bar section loss and bond deterioration. A case study bridge is evaluated to show that the BEST program offers a viable tool for the rapid assessment of spalled bridge girders and to facilitate the prioritization of rehabilitation projects. This evaluation indicates that the spatial distribution of the spalling along a girder, relative to bar splices and laps, has the most significant influence on structural capacity. Single girders show strength deficiencies in flexure and shear due to spalling. In general, the consideration of system effects improves the predicted bridge condition, while considering section loss and bond deterioration has the opposite effect. Laboratory work is used to validate the proposed model and identify a number of areas for future research. The laboratory test results also suggest that the current repair methods are effective in restoring bond and strength. In order to further explore potential uses for the BEST program, modifications are made so that it can be used to perform reliability analyses using Monte-Carlo simulation techniques. A simplified approach for estimating the reliability index as a function of the deterministic resistance ratio is proposed based on the reliability analysis results.

All reinforced concrete (RC) design theories are based on the assumption that concrete exhibits a perfect bond with the steel reinforcement. The bond between steel and concrete is essential to the transfer of the load applied from the concrete to the steel reinforcement. When steel bars are corroded, the concrete cracks, and the strength of the bond between the steel bars and the concrete is decreased. Structures such as bridges and marine structures are prone to corrosion. These structures are usually also subjected to repeated loading. Repeated loading can initiate cracks in the concrete surrounding the steel bars that propagate as the number of load cycles increases leading to the destruction of the concrete-steel interface and slip of the steel bars inside the concrete. The combined effect of corrosion and repeated loading reduces the service life of RC structures. This study investigated the effect of anchorage length and confinement from supports, stirrups and carbon fibre reinforced polymer (CFRP) on the bond behaviour of corroded and uncorroded reinforced concrete beams subjected to monotonic and repeated loading. Fifty-seven large-scale reinforced concrete beams (152*254*2000 mm) were tested for the purpose of this study. The variables were stirrup spacing (75 mm and 150 mm), anchorage length (200 mm, 350 mm and 650 mm), corrosion level (mild corrosion and high corrosion level), repair condition (wrapped or unwrapped with FRP sheets in the anchorage zone) and the fatigue load range. From this study, it was found that the resistance to bond stresses (forces) between the steel and concrete were provided mainly by the concrete keys. The bond stresses increased with the number of the concrete keys engaged. The factors that affected the number of concrete keys engaged were: confinement from the supports, confinement from the stirrups, confinement due to wrapping with FRP sheets and change in anchorage length. Decreasing the stirrup spacing from 150 mm to 75 mm increased the number of concrete keys engaged thus increasing the bond capacity and changed the mode of failure under monotonic loading from splitting to pullout. The beams with the first stirrup spacing (150 mm c/c) when tested under repeated loading failed by bond fatigue while the beams with the second stirrup spacing (75 mm c/c) failed by flexure at the end of a debonded region that started from the support. The failure mechanism is discussed for each case. The change in anchorage length from 200 mm to 350 mm increased the static and fatigue bond capacity of the beams by 60% and 12.5% respectively. The debonding for this group of beams (200 mm and 350 mm anchorage length) subjected to monotonic loading started from the pocket and propagated towards the support while the debonding for the 350 mm anchorage length beams subjected to repeated loading started at the location of a crack that widened while fatiguing the beam and propagated towards the support. The change in anchorage length from 350 mm to 650 mm did not affect the monotonic bond capacity of the beams since in this case, debonding was initiated from the supports and the change in anchorage length had little effect. The confinement with FRP sheets caused the concrete keys at both the top and bottom of the bar to be crushed and increased the bond stress of the wrapped beams. The bond strength of the beams repaired with CFRP sheet was governed by the strength of the FRP sheets for all anchorage lengths and corrosion levels. The CFRP repair of the 200 mm anchorage length set of beams increased the capacity of the uncorroded beams by 80% and the capacity of the corroded beams by about 25% under static and repeated loading compared to the control (uncorroded and unrepaired) beam. The CFRP repair of the 350 mm anchorage length set of beams changed the mode of failure from bond to flexure. The fatigue life for the beams varied linearly on a logarithmic scale with the load range applied with a shallow slope. Corroding the 200 mm anchorage length set of beams to a mild corrosion level decreased their fatigue strength by 34% compared to the control beams. Corroding the 350 mm anchorage length set of beams to a mild corrosion level did not affect the fatigue strength for the single beam that failed in bond. Finally a probabilistic approach was used to allow the design engineers to estimate the design fatigue life for similar beams with 95% probability for a given normalized stress ratio.

Thesis (M.S.)--University of Hawaii at Manoa, 2006. / Includes bibliographical references (leaves 113-114). / xi, 132 leaves, bound ill. 29 cm

In this study the unique Magotteaux Mill® system was used to control the grinding chemical conditions, which may be adjusted by varying grinding media, purging gas and pH, during grinding. An electrochemical apparatus was used to investigate oxidation-reduction behaviour of grinding media and sulphide mineral electrodes, as well as their galvanic interaction in-situ of the Magotteaux Mill®. Galvanic interaction between the grinding media (mild steel, 15% chromium, 21% chromium and 30% chromium media) and the sulphide minerals (bornite, arsenopyrite and pyrite) was initially quantified in-situ of the mill by electrochemical techniques under different grinding atmospheres (nitrogen, air and oxygen). An innovative mathematical theoretical model was developed to describe the effect of galvanic interaction on oxidation rates of the grinding media during grinding, which was verified by the experimental data. Galvanic interaction enhanced the oxidation of the grinding media and produced more oxidized iron species in the mill discharge. It was observed that oxidized iron species (EDTA extractable iron) was linear with galvanic current between the grinding media and the sulphide minerals, in agreement with the prediction of the theoretical model. The effect of grinding conditions on pulp chemistry, surface properties and floatability was investigated by the measurement of dissolved oxygen (DO), pH, pulp potential (Eh), ethylene diamine-tetra acetic acid (EDTA) extraction, X-ray photoelectron spectroscopy (XPS) and floatation recovery. / Thesis (PhDAppliedScience)--University of South Australia, 2005.

Metal dusting is a kind of catastrophic corrosion phenomenon that can be observed in several of petrochemical processes. It occurs on iron-, nickel- and cobalt-base metals in carbonaceous atmospheres at high temperature when gaseous carbon activity is higher than one. The process is particularly rapid for ferritic alloys The aim of this project was to compare the dusting kinetics of pure iron and a 2.25Cr-1Mo alloy steel under CO-H2-H2O atmosphere at 650??C. Polished (3??m) samples of iron and the steel were exposed to flowing CO-H2-H2O gas atmospheres at 650??C, when the gases were supersaturated with respect to graphite. The partial pressure of CO was varied between 0.25 and 0.9 atm, and the carbon activity was varied from 2.35 to 16, in order to obtain a series of experimental conditions. In most experiments, pO2 was less than 7.37E-24 atm, and no iron oxide could form. However, Cr2O3 would always have been stable. When exposed to these gases, both iron and steel developed a surface scale of Fe3C which was buried beneath a deposit of carbon, containing iron-rich nanoparticles (the dust). Examination by Scanning Electron Microscopy allowed the observation of fine and coarse carbon nanotubes, and also spiral filaments. However, the morphology of the graphitic carbon was not sensitive to pCO and aC. Moreover, the carbon deposit was gas permeable, allowing continuing gas access to the underlying metal. At a fixed=4.5, the carburizing rate clearly increased with CO content from 0.25 to 0.68 atm. However, increasing the CO content to higher value led to decreased rates, indicating that carburizing rate reaches a maximum value at pCO=0.68 atm. When pCO was fixed at 0.25 atm and 0.68 atm, and carbon activity was varied. The induction period was extended by the formation of protective oxide layers at low values of carbon activity (aC= 2.35 and 2.55) where pO2 exceed the iron oxide formation value. For other reaction conditions, the carbon uptake rate for iron and steel did not increase with aC. The present work showed that the carbon deposition rates were not proportional to pCO or pCOpH2. Instead, the rate was affected by the partial pressure of all three reaction gases, and the carbon uptake rate for both materials could be expressed at r=k1pCOpH2+k2pCO2+k3pH22 and the rate constant k3 has a negative value, corresponding to coke gasification. From XRD analyses, it was found that cementite was the only iron-containing phase in the dusting product. The cementite particles acted as catalysts for carbon deposition from the gas. The same deposition process at the surface of the cementite layer led to its disintegration, thereby producing the particles. This disintegration process was faster on the steel than on pure iron. Consequently, the rates of both metal wastage and coke accumulation were faster for the steel. It is concluded that chromium and molybdenum do not stabilize the carbide but accelerate its disintegration process. It is suggested that Cr2O3 fine particles in the cementite layers provide more nucleation sites in the cementite layer on steel, explaining its more rapid dusting kinetics. However, appropriate methods of proving this assumption, such as TEM and FIB, are required.

Carbon dioxide (CO2) corrosion is a major problem in the oil and gas production industry. The survival of mild steel equipment is to a large extent conditional on the formation and stamina of protective iron carbonate (FeCO3) films. Damage to protective films allegedly leads to accelerated corrosion attacks and increases the risk of failures. In single-phase flows, film removal phenomena are broadly ascribed to two intrinsic mechanisms: mechanical removal by hydrodynamic forces and/or chemical removal by dissolution. The fact that both mechanisms usually act simultaneously in practice puts their combined action in the forefront regarding its significance and relevance for the industry. Yet, virtually no information is available on the exact conjoint mechanism of protective FeCO3 film removal in single-phase environments. The obscurity is largely due to the uncertainty regarding the roles of hydrodynamic forces and mass transfer, where both are closely related to turbulence intensity levels. The aim of this dissertation was to clarify the roles of the two basic FeCO3 film removal mechanisms during the conjoint removal in undisturbed, single-phase flow in terms of their relative contribution and possible synergistic interaction. The proposed aim was accomplished by applying an innovative analytical approach, in which inherently coupled processes of film formation and removal were decoupled. Also, the two intrinsic removal mechanisms were studied separately in the initial stages, before they were combined to provide a complete picture of the conjoint mechanism. An integrated approach to studying film formation/removal mechanisms involved advanced electrochemical techniques for following film growth/removal, complemented by detailedScanning Electron Microscopy/Energy Dispersive Spectroscopy/X-Ray Mapping characterisations of protective/residual films. A single-phase, highly turbulent flow field was attained by employing a rotating cylinder configuration. A standard corrosion experimental setup was extended to accommodate more complex film studies. A comprehensive flow characterisation around the rotating cylinder was carried out by means of flow visualisation and mass transfer measurements under turbulent flow conditions. While the former facilitated proper design of film formation experiments, the latter led to an empirical mass transfer correlation that enabled quantification of film dissolution rates. Furthermore, although some information on film growth kinetics is available, customised experimentation was necessary to identify the key parameters needed to obtain films with desired characteristics. Sound procedures for FeCO3 film growth were established, which led to the reproducible formation of realistic, protective films after a few days. The results of the pure mechanical removal of protective FeCO3 films have shown that its kinetics are rather slow even at high velocities and have caused a delayed, partial macroscopic type of damage. Yet, the findings demonstrate that the currently widely accepted view, that film removal by hydrodynamic forces in the absence of film dissolution in undisturbed, single-phase flows does not occur, is wrong. The strong correlation found between velocity and pure chemical film removal kinetics implicitly followed via corrosion rates suggests that the dissolution of protective FeCO3 films is under mass transfer control. Pure dissolution has faster removal kinetics and is far more detrimental to film integrity even at relatively high pH (just below saturation) than pure mechanical removal at the same Reynolds number. It has been found that the controlled pure dissolution mechanism led to only partial and selective film removal, where the more dissolution-resistant crystalline top film layer and the dissolution-prone inner layer were differently affected both in terms of the type of damage and its severity. A strong synergistic effect between mechanical and chemical film removal mechanisms has been identified during their simultaneous action. The quantified synergistic share in fully established conjoint film removal (during the steady, linear corrosion rate increase) expressed via corrosion rate gradients increased from 19.4% to 29.7% for the corresponding increase in the rotational speed from 7,000 rpm to 10,000 rpm. The synergism comprised two modes of mutual interactions: enhanced mechanical removal due to dissolution (M/D) and enhanced dissolution due to mechanical removal (D/M). In contrast to the independent action of integral removal mechanisms, where dissolution appears to be more destructive, the interaction between the two was primarily dominated by drastically accelerated mechanical film removal kinetics, that is, M/D rather than D/M mode, the latter of which was inferior. A fundamentally improved understanding of film removal mechanisms in single-phase flows has been reached as a result of the present project, thereby creating a solid foundation for future modelling and a more effective prevention and control of flow accelerated corrosion, not only in CO2 corrosive environments, but also in a wide range of industrial settings.