This study investigated the corrosion of an aluminum alloy with and without coatings under simulated corrosive environments. Coatings were selected and applied from commercial materials and techniques, consisting of microceramic, epoxy primer, and topcoat. The experiments for coated specimens were carried out under various corrosive conditions, specifically, hydrodynamic flow, immersion in salt water and DI water, varying temperatures, and simulated sun light. The hydrodynamic conditions were simulated using a rotating cylinder electrode (RCE) with rotational speeds of 200, 400, 600, 800, and 1000 rpm. The immersion in saltwater and sunlight illumination tests were applied for 8 hours, and the simulated exterior temperature variation were cycled for 7 days. Polarization techniques were used to study the corrosion mechanism and calculate the corrosion rate of coated specimens under simulated salt water. Microstructure of coated specimens were identified by using atomic force microscopy (AFM) and optical microscope (OM) analysis. Results indicated that corrosions in the simulated salt water were generally more serious than those in the distilled water. This was especially true for bare aluminum alloys, that its corrosion rates in the salt water exhibited two orders of magnitude higher than those in the DI water. The combination of an environmentally friendly electrodeposited ceramic coating with a primer and topcoat, which results in a chromium-free coating, exhibited a higher polarization resistance and a lower corrosion rate than the traditional chromate conversion coating combination. In addition, for all coated Al alloys, the corrosion rate increased with increasing rotation speed. For immersion portion, immersion in salt water accelerated the pitting corrosion process and increased the corrosion rate of the aluminum alloy five times higher as compared to the samples without immersion. For the varying temperature portion, the rates of corrosion nearly doubled for bare and ceramic coated Al alloy, under varying temperature conditions for 7 days. Besides, addition of ionic liquid inhibitors, such as BMIMBR and BEIMCl, exhibited great improvement of corrosion resistances of aluminum alloy in the salt water. The corrosion rates of aluminum in the presence of inhibitors were almost one order of magnitude lower than that in the absence of inhibitors.

The research in this thesis aims to develop new mechanistic knowledge for coating processes at 2024-Al alloy surfaces, ultimately to aid the design of new protective coatings. Coatings formed by phosphating, chromating, and permanganating were characterized especially by scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy, and scanning electron microscopy . The objective was to learn about growth (nm level) as a function of time for different coating baths, as well as a function of lateral position across the different surface microstructural regions, specifically on the μm-sized Al-Cu-Mg and Al-Cu-Fe-Mn particles which are embedded in the alloy matrix . The research characterizes coating thickness, composition, and morphology. The thesis emphasizes learning about the effect of different additives in zinc phosphating baths . It was found that the Ni²⁺ additive has two main roles : first, the rate of increase in local solution pH is limited by the slower kinetics of reactions involving Ni²⁺ compared to Zn²⁺, leading to thinner zinc phosphate (ZPO) coatings when Ni²⁺ is present. Second, most Ni²⁺ deposition occurs during the later stages of the coating process in the form of nickel phosphate and a Ni-Al oxide in the coating pores on the alloy surface, increasing the corrosion resistance. Aluminum fluoride precipitates first during the initial stages of the coating process, followed by aluminum phosphate, zinc oxide, and finally ZPO. When Ni²⁺ is present in the coating solution at 2000 ppm, ZnO predominates in the coating above the A-Cu-Fe-Mn particle while ZPO dominates on the rest of the surface. The Mn²⁺ additive gives a more even coating distribution (compared with Ni²⁺) across the whole surface. The Mn²⁺ -containing ZPO coating is similar to the chromate coating in terms of evenness, while there is more coating deposition at the second-phase particles for permanganate coatings. The oxides on the Al-Cu-Fe-Mn and matrix regions are similar before coating, thereby confirming that a variety of observed differences in ZPO coating characteristics at these regions arise from the different electrochemical characteristics of the underlying metals. Upon exposure to a corrosive solution, the ZPO coating provides more protection to the second-phase particles compared to the matrix.

The research in this thesis aims to develop new mechanistic knowledge for coating processes at 2024-Al alloy surfaces, ultimately to aid the design of new protective coatings. Coatings formed by phosphating, chromating, and permanganating were characterized especially by scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy, and scanning electron microscopy . The objective was to learn about growth (nm level) as a function of time for different coating baths, as well as a function of lateral position across the different surface microstructural regions, specifically on the μm-sized Al-Cu-Mg and Al-Cu-Fe-Mn particles which are embedded in the alloy matrix . The research characterizes coating thickness, composition, and morphology. The thesis emphasizes learning about the effect of different additives in zinc phosphating baths . It was found that the Ni²⁺ additive has two main roles : first, the rate of increase in local solution pH is limited by the slower kinetics of reactions involving Ni²⁺ compared to Zn²⁺, leading to thinner zinc phosphate (ZPO) coatings when Ni²⁺ is present. Second, most Ni²⁺ deposition occurs during the later stages of the coating process in the form of nickel phosphate and a Ni-Al oxide in the coating pores on the alloy surface, increasing the corrosion resistance. Aluminum fluoride precipitates first during the initial stages of the coating process, followed by aluminum phosphate, zinc oxide, and finally ZPO. When Ni²⁺ is present in the coating solution at 2000 ppm, ZnO predominates in the coating above the A-Cu-Fe-Mn particle while ZPO dominates on the rest of the surface. The Mn²⁺ additive gives a more even coating distribution (compared with Ni²⁺) across the whole surface. The Mn²⁺ -containing ZPO coating is similar to the chromate coating in terms of evenness, while there is more coating deposition at the second-phase particles for permanganate coatings. The oxides on the Al-Cu-Fe-Mn and matrix regions are similar before coating, thereby confirming that a variety of observed differences in ZPO coating characteristics at these regions arise from the different electrochemical characteristics of the underlying metals. Upon exposure to a corrosive solution, the ZPO coating provides more protection to the second-phase particles compared to the matrix. / Science, Faculty of / Chemistry, Department of / Graduate

Laser surface melting (LSM) of aluminium alloys with high power continuous wave (CW) CO2 and Nd:YAG lasers has been shown to produce dendritic/cellular microstructures with refined second-phase particles distributed along the dendritic boundaries. Although refinement of the microstructure and extension of the solute solubility in the matrix can be achieved, the refined second-phase particles still act as preferential sites to initiate localized corrosion. In contrast to a CW laser, an excimer laser with a UV wavelength and pulses width in the range of nanoseconds, resulting in extremely high cooling rate up to 1011 K/s, is expected to generate a further refining of the near-surface microstructure and hence, improved corrosion performance. In this project, a Lumonics IPEX 848 KrF excimer laser, with a wavelength of 248 nm and pulse width of 13 ns, has been used for surface melting of an AA2024-T351 alloy. The aim is to investigate the microstructure and the resultant corrosion behaviour of the laser treated surface, and its contribution to alloy performance. The laser fluence was fixed at 7 J/cm2 and the number of pulses per unit area was varied as 10, 25 and 50 pulses respectively. Microstructural characterisation and compositional analysis have been performed by SEM/EDX, TEM/EDX and XRD to disclose solidification phenomena and phase transformations. The results show that the melted layers, with a melt depth from 3 to 7 µm, have been achieved, that is far more chemically uniform than the bulk alloy. In particular, the relatively fine precipitates and dispersoids in the matrix have been dissolved, while large constituent intermetallic particles at the melted layer/matrix interface have been partially melted. In addition, solute-rich bands, containing particularly copper, were formed within the melted layers, especially at the melted layer/matrix interface. SKPFM also reveals that the laser-melted layers exhibit a uniform surface potential distribution. The corrosion performance of AA2024-T351 alloy before and after LSM has been evaluated by anodic polarisation in deaerated and aerated 0.1 M NaCl solution, and immersion tests in 0.1 NaCl solutions. Exfoliation corrosion immersion test ASTM G34- 01 (EXCO test) was also carried out to evaluate the intergranular corrosion (IGC)/exfoliation resistance of the alloy. The results show that the untreated alloy exhibits severe pitting corrosion and IGC. After LSM, significant improvement of corrosion resistance has been achieved. However, delamination of the laser melted layer from the matrix was evident after an EXCO test for 6 hours. The absence of significant corrosion product may suggest a stress-related mechanism. In order to investigate the effect of LSM on anodising of AA2024-T351 alloy and its influence on the corrosion resistance, excimer LSM has been applied as a pre-treatment method prior to anodising in 0.46 M H2SO4 solution. The results show that LSM significantly improved the corrosion performance following anodising compared with the alloy anodised without LSM and LSM alone.

The mechanical properties of pure Aluminium, Al-0.2%Cu and Al-0.4%Cu at large strains were studied by channel-die compression at three different temperatures: 77K, 200K and 300K. The results were interpreted in terms of work hardening rate versus stress (0/r) diagrams. The evolution of the structure was studied on a range of scales from macroscopic to microscopic, by optical study of slip lines, X-ray diffraction for texture measurements, Electron Back-Scattering Kikuchi Patterns for local texture measurements, and by Transmission Electron Microscopy for microstructural information. Intense shear banding was observed at large strains in all alloys at all temperatures. The texture evolution was shown to be consistent with this change in deformation mode. At low temperatures, stage HI of deformation was shown to be represented by a straight line in the 6lr diagram. Increasing the temperature lead to a dramatic decrease in work hardening rate and to an increasing concavity of the 0lr plots. The addition of solutes to pure Aluminium was shown to result in an increase of the work hardening rate, which could be represented by a simple translation of the 0/r plots on the stress axis. At large strains, all three materials experienced a stage (stage IV) of constant work hardening at low rate. The stage IV work hardening rate decreased with increasing temperature, and was not influenced by solute content. The stage Ill-Stage IV transition was very sharp at 77K and smoother at higher testing temperatures. Phenomenological models were developed for the prediction of the influence of temperature and solute content on work hardening. Moderate strains were modelled taking into account the evolution of the dislocation density into two different populations during the deformation. The influence of solutes on work hardening was modelled by considering how segregation of solute atoms at the dislocation cores influences dynamic recovery. Stage IV work hardening was considered to arise from the accumulation of dislocation debris resulting from the dynamic recovery events. / Thesis / Master of Engineering (ME)

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