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The stress corrosion behaviour of precipitation hardened Al-9Mg, Al-22Zn and Al-3Mg-6Zn alloys has been studied in aqueous environments and ethanol. The stress corrosion susceptibility defined as the reciprocal of failure time has been investigated as a function of alloy-environment system, isothermal aging treatment, microstructure, applied tensile stress, and temperature using smooth and notched specimens. Constant load tests, load-relaxation tests and tensile tests in different environments have been used to evaluate the stress corrosion characteristics of aluminum alloys. A limited study of Mg-9Al has also been carried out in aqueous environments. The process of stress corrosion generally consisted of three parts: 1) A slow initiation stage 2) a rapid propagation stage 3) mechanical fracture due to tensile overload. With a few exceptions, the initiation time was greater than the propagation time. The crack initiation and propagation rates were stress and thermally activated and could be expressed by a general equation of the form Rate = [formula omitted] where α is the applied tensile stress, Q is the apparent activation energy of the rate controlling process and A(0) and n are constants for a given alloy-environment system. The apparent activation energy of the rate controlling process was different in the two environments. It also changed between initiation and propagation stages. The aluminum alloys when ranked in order of increasing susceptibility were: 1) Al-3Mg-6Zn, 2) Al-9Mg, 3) Al-22Zn. The alloys which were given heat treatments correlating to the presence of coherent or partially coherent phases, were found to be most prone to stress corrosion cracking. The environments placed in an order of increasing aggressiveness were dessicant-dried air, double distilled water, ethanol, ambient air, deionized water and NaCl/K₂CrO₄solution. The ductility of susceptible aluminum alloys was found to be significantly decreased by NaCl/K₂CrO₄and deionized water at low strain rates and enhanced by dessicant-dried air. Fractography showed the cracking to be intergranular in aluminum alloys and transgranular in the Mg-Al alloy. The stress corrosion surface was characterised by a rough or corroded appearance while the mechanically fractured surface exhibited slip steps and dimples caused by void formation. The hydrogen mechanism of cracking was examined in light of hydrogen charging experiments and other evidence and was found to be unsatisfactory. Models involving either dissolution or deformation alone were also inadequate in explaining the present results. Therefore a new model was postulated which involves the generation of a continuous path of chemical heterogeneity by shearing and link up of coherent precipitates followed by their dissolution. The rate controlling step in the deformation process is believed to change during the transition from initiation to propagation. The postulated model is consistent with the present results but its further development must await better knowledge of the kinetics of dissolution of precipitates and that of deformation processes at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Single crystals and bicrystals have been used to study pitting corrosion and intergranular corrosion of Al and Al-Cu alloys in 0.5M NaCl solution. The critical pitting potential and pit density were examined as a function of a number of factors. These included crystallographic orientation; the bulk solution chemistry including CI- concentration, NO₃- addition and pH; the effect of Cu alloying; and the effects of homogenizing and aging on the alloy crystals. The susceptibility for pitting corrosion was found to depend on crystallographic orientation in pure Al with {111} showing maximum pitting and {011} and {001} exhibiting progressively less pitting. This crystallographic effect was not observed in the Al-3 wt% Cu alloy. The addition of Cu to pure Al was found to raise the Epit and produce a higher pit density on the surface. The increase of CI⁻ concentration was found to enhance pitting corrosion, producing a higher pit density and lowering the Epit. Addition of NO₃- to the solution decreases pitting corrosion, reduces the pit density and substantially shifts the Epit to a more noble potential. A model of pitting corrosion is proposed, based on a local kinetic balance between the repassivation process and the dissolution process at the bare metal surface at the base of a preexisting oxide flaw on the crystal surface. The model successfully accounts for the observed effects of the Cu alloy addition, and the solution composition variations on pitting corrosion. In the alloy bicrystals, it was observed that pitting corrosion in the grain boundary region was dependent on the composition and thermal history of the crystal. In most of the homogenized Al-Cu bicrystals, the presence of the grain boundary did not influence the pitting corrosion. In a 0.1 wt% Cu alloy with a tilt boundary of 28° about the <001> direction preferential pitting along the grain boundary was observed. The preferential pitting is attributed to nonequilibrium depletion of Cu at the high angle tilt boundary. Preferential attack is also observed at grain boundaries in as-grown and in aged bicrystals. This is attributed to Cu segregation in the crystals and the lower value of Epit associated with the Cu depleted regions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

by Ka-Man Mak. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references. / Text in English; abstract also in Chinese. / by Ka-Man Mak. / Acknowledgement --- p.i / Abstract --- p.ii / List of tables --- p.v / List of figures --- p.vi / Table of contents --- p.xi / Chapter Chapter One --- Introduction --- p.1-1 / Chapter 1.1 --- History of intermetallics --- p.1-1 / Chapter 1.2 --- Properties of intermetallic compounds --- p.1-5 / Chapter 1.2.1 --- Magnetic properties --- p.1-5 / Chapter 1.2.2 --- Chemical properties --- p.1-6 / Chapter 1.2.3 --- Semiconducting properties --- p.1-7 / Chapter 1.2.4 --- Superconducting properties --- p.1-7 / Chapter 1.2.5 --- Hydrogen storage --- p.1-8 / References --- p.1-9 / Chapter Chapter Two --- Background --- p.2-1 / Chapter 2.1 --- Some Behaviours of Intermetallics / Chapter 2.1.1 --- Intergranular and cleavage fracture --- p.2-1 / Chapter 2.1.2 --- Corrosion --- p.2-3 / Chapter 2.1.3 --- Oxidation in high-temperature intermetallics --- p.2-5 / Chapter 2.1.4 --- Hot corrosion --- p.2-8 / Chapter 2.2 --- Nickel aluminides --- p.2-9 / Chapter 2.2.1 --- Ni3Al --- p.2-10 / Chapter 2.2.2 --- NiAl --- p.2-12 / References --- p.2-14 / Chapter Chapter Three --- Oxidation --- p.3-1 / Chapter 3.1 --- Introduction --- p.3-1 / Chapter 3.2 --- Specimens preparation --- p.3-1 / Chapter 3.3 --- Experiment process --- p.3-5 / Chapter 3.3.1 --- Instrumentation --- p.3-5 / Chapter 3.3.2 --- Choosing of experimental temperature --- p.3-9 / Chapter 3.3.3 --- Methodology --- p.3-9 / Chapter 3.4 --- Results and Discussions --- p.3-10 / Chapter 3.4.1 --- Dependence of time --- p.3-10 / Chapter 3.4.2 --- Dependence of temperature --- p.3-14 / Chapter 3.4.3 --- Dependence of composition --- p.3-15 / Chapter 3.4.4 --- Activation energy of oxidation --- p.3-15 / Chapter 3.4.5 --- Oxidation morphology and mechanism --- p.3-16 / Chapter 3.5 --- Conclusions --- p.3-20 / References --- p.3-21 / Chapter Chapter Four --- Hot corrosion --- p.4-1 / Chapter 4.1 --- Introduction --- p.4-1 / Chapter 4.2 --- Specimens preparation --- p.4-1 / Chapter 4.3 --- Experiment process --- p.4-3 / Chapter 4.3.1 --- Instrumentation --- p.4-3 / Chapter 4.3.2 --- Choosing of experimental environment and temperature --- p.4-5 / Chapter 4.3.3 --- Methodology --- p.4-6 / Chapter 4.3.3.1 --- Experiment --- p.4-6 / Chapter 4.3.3.2 --- Experimental setup --- p.4-8 / Chapter 4.4 --- Results and discussions --- p.4-9 / Chapter 4.4.1 --- Dependence of time --- p.4-9 / Chapter 4.4.2 --- Dependence of temperature --- p.4-10 / Chapter 4.4.3 --- Comparison between hot corrosion with oxidation --- p.4-11 / Chapter 4.4.4 --- Dependence of composition --- p.4-12 / Chapter 4.4.4.1 --- Comparison between S1 and S2 --- p.4-12 / Chapter 4.4.4.2 --- Comparison between S3- S7 --- p.4-12 / Chapter 4.4.5 --- Results from XRPDS --- p.4-13 / Chapter 4.4.6 --- Study of microstructure --- p.4-13 / Chapter 4.4.6.1 --- Dependence on time --- p.4-14 / Chapter 4.4.6.2 --- Dependence on temperature --- p.4-14 / Chapter 4.4.6.3 --- Dependence on composition --- p.4-14 / Chapter 4.5 --- Corrosion mechanism --- p.4-15 / Chapter 4.5.1 --- Chemical reactions --- p.4-15 / Chapter 4.5.2 --- Corrosion process --- p.4-16 / Chapter 4.5.2.1 --- Temperature effect --- p.4-16 / Chapter 4.5.2.2 --- Composition dependence --- p.4-17 / Chapter 4.6 --- Conclusions --- p.4-18 / References --- p.4-19 / Chapter Chapter Five --- Conclusionsand suggestions for further studies --- p.5-1 / Chapter 5.1 --- Oxidation --- p.5-1 / Chapter 5.2 --- Hot corrosion --- p.5-2 / Chapter 5.3 --- Further development --- p.5-3

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As an alternative to the present toxic chromate-based coating system now in use, the Mg-rich primer technology has been designed to protect Al alloys (in particular Al 2024 T3) and developed in analogy to Zn-rich primers for steel substrate. As an expansion of this concept, metal-rich primer systems based on Mg alloy particles as pigments were studied. Five different Mg alloy pigments, AM60, AZ91B, LNR91, AM503 and AZG, were characterized by using the same epoxy-polyamide polymer as binder, a same dispersion additive and the same solvent. Different Mg alloy-rich primers were formulated by varying the Mg alloy particles and their pigment volume concentrations (PVC). The electrochemical performance of each Mg alloy-rich primer after the cyclic exposure in Prohesion chamber was investigated by electrochemical impedance Spectroscopy (EIS). The results indicated that all the Mg alloy-rich primers could provide cathodic protection for AA 2024 T3 substrates. However, the Mg alloys as pigments in metal-rich primers seemed to exhibit the different anti-corrosion protection performances, such as the barrier properties, due to the different properties of these pigments. In these investigations, multiple samples of each system were studied and statistical methods were used in analyzing the EIS data. From these results, the recommendation for improved EIS data analysis was made. CPVC studies were carried out on the Mg alloy-rich primers by using three Mg alloy pigments, AM60, AZ91B and LNR91. A modified model for predicting CPVC is proposed, and the results showed much better agreement between the CPVC values obtained from the experimental and mathematical methods. Using the data from the AM60 alloy pigment system, an estimate of experimental coarseness was done on a coating system, the first time such an estimate has been performed. By combining various surface analysis techniques, such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and confocal Raman microscopy, the oxidation products formed after exposure were identified. It was found that variation of Al content in Mg alloy could significantly affect the pH of the microenvironment in the primer films and result in the formation of various oxidation products. / Air Force Office of Scientific Research (Grant No. 49620-02-1-0398)

As an alternative to the present toxic chromate-based coating system now in use, the Mg-rich primer technology has been designed to protect A1 alloys (in particular A1 2024 T3) and developed in analogy to Zn-rich primers for steel substrate. As an expansion of this concept, metal-rich primer systems based on Mg alloy particles as pigments were studied. Five different Mg alloy pigments. AM60, A719B, LNR91, AM503 and AZG, were characterized by using the same epoxy-polyamide polymer as binder, a same dispersion additive and the same solvent. Different Mg alloy-rich primers were formulated by varying the Mg alloy particles and their pigment volume concentrations (PVC). The electrochemical performance of each Mg alloy-rich primer alter the cyclic exposure in Prohesion chamber was investigated by electrochemical impedance Spectroscopy (EIS). The results indicated that all the Mg alloy-rich primers could provide cathodic protection for AA 2024 T3 substrates. However, the Mg alloys as pigments in metal-rich primers seemed to exhibit the different anti-corrosion protection performances, such as the barrier properties, due to the different properties of these pigments. In these investigations, multiple samples of each system were studied and statistical methods were used in analyzing the EIS data. From these results. the recommendation for improved EIS data analysis was made. CPVC studies were carried out on the Mg alloy-rich primers by using three Mg alloy pigments, AM60, A2918 and LNR91. A modified model for predicting CPVC is proposed, and the results showed much better agreement between the CPVC values obtained from the experimental and mathematical methods. Using the data from the AM60 alloy pigment system, an estimate of experimental coarseness was done on a coating system, the first time such an estimate has been performed. By combining various surface analysis techniques, such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and confocal Raman microscopy, the oxidation products formed alter exposure were identified. It was found that variation of A1 content in Mg alloy could significantly affect the pH of the microenvironment in the primer films and result in the formation of various oxidation products. / Air Force Office of Scientific Research (AFOSR) (Grant No. 49620-02-1-0398)

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