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The corrosion protection of aluminium alloys is of high importance, particularly in the aerospace industry. The most widely used technologies utilise Chromium(VI) compounds for conversion coatings and primer additives in paint systems to provide corrosion protection to these alloys. These compounds are highly toxic, carcinogenic and detrimental to the environment, therefore the identification of alternative systems that are safe and environmentally benign, that meet or exceed the current levels of corrosion protection is vital. This research program examines the corrosion inhibition effectiveness of selected rare earth diphenyl phosphates (RE(dpp)3). These compounds incorporate known inhibitor species, namely rare earth metals, organics and phosphates into a single complex, with the aim of achieving synergistic inhibition in corrosive environments. A screening study utilising immersion and weight loss experiments identified Cerium diphenyl phosphate (Ce(dpp)3) and Mischmetal diphenyl phosphate (Mm(dpp)3) as the most effective inhibitors of corrosion for AA2024-T3. The inhibiting efficiency, mechanism of inhibition and surface interaction of these complexes on aluminium alloy AA2024-T3 was characterised using a range of electrochemical and surface techniques. A similar study was carried out using AA7075-T6 to assess the adaptability of the RE(dpp)3 compounds to protect different alloy compositions. The complexes were effective in significantly reducing the corrosion rate of the alloys, with both the cathodic and anodic corrosion processes being suppressed. This mixed inhibition was not attained with the constituent rare earth and diphenyl phosphate ions individually, indicating the need for the complex to remain intact in solution to achieve the high level of corrosion protection observed. The initiation and propagation of surface pits was effectively suppressed by the RE(dpp)3 complexes. The combination of electrochemical and surface characterisation techniques has for the first time allowed insights into the mechanism of action of these compounds on aluminium alloys, and indicated deposition was initiated at electrochemically active intermetallic particles. The mixed rare earth phases present in Mm(dpp)3 produced a synergistic effect, providing a greater degree of corrosion protection compared with Ce(dpp)3, particularly on AA2024-T3. In the final phase of this research project the RE(dpp)3 inhibitor compounds were incorporated into an epoxy coating system, and demonstrated that the initiation of filiform corrosion on AA2024-T3 could be reduced by up to a factor of 3 by their addition. The growth rate of filaments was also impeded.

The goal of this thesis is to probe the functionality of 2-mercaptobenzimidazole (MBI) as corrosion inhibitor of carbon-steel in both strong and weak aqueous acidic solutions (HCl and H2CO3). To achieve this target electrochemical techniques have been employed, in combination with substrate analysis. Concerning aqueous HCl media, results demonstrate that MBI is an effective corrosion inhibitor, functioning essentially equally well in 1 M, 0.1 M, and 0.01 M HCl concentrations. X-ray photoelectron spectra suggest that MBI is typically bound to the surface in two tautomeric forms (thione and thiol). Furthermore, these data indicate that substrate termination varies as a function of both HCl and MBI concentration, with the interface consisting of MBI bound to film-free carbon-steel on highly inhibited substrates. In further work, the impact of dissolved oxygen, solution temperature, and immersion time on MBI performance in HCl solutions has been assessed. The latter two parameters have considerable influence on MBI inhibition efficiency. More specifically, it was found that MBI decreases dramatically its inhibition efficiency between 60°C and 70°C in 1 M HCl, and also apparently work less well as substrate immersion time increases. As regards MBI performance in deaerated CO2-saturated NaCl (0.62 M) solution, results demonstrate that MBI effectively inhibits corrosion within the parameter space explored i.e. solution temperatures of 30°C and 55°C and total applied pressures (p(H2O) + p(CO2)) of 1 bar and 20 bar. The performance of MBI does not vary greatly for different combinations of these temperatures and pressures. Post immersion substrate characterisation with XRD and SEM indicate that no significant surface scaling occurs under these conditions.

Aniline oligomers have become a very interesting topic for research because of their potential application not only in organic electronics but also in smart coatings for corrosion treatment of iron and steel. A majority of the studies in the literature are focussed on the bulk or direct interaction between the organic molecules with metal substrates, without considering the native oxide film. In order to develop smart coatings (has redox activity and self-healing ability) for iron and steel, one must first understand how these oligomers interact with the native iron oxide film. In this thesis, we develop new knowledge from our fundamental understanding of the interactions of redox-active aniline oligomers with the iron oxide surface. Phenyl capped aniline dimer with two oxidation states [fully reduced (DPPD) and fully oxidized (B2Q1)] and phenyl-capped aniline tetramer (PCAT) with three oxidation states [fully reduced (B5), half-oxidized (B4Q1), fully oxidized (B3Q2)] were chosen for investigation. The former is the smallest redox active aniline oligomer but with one fewer oxidation states than polyaniline whereas the latter mimics the redox system as well as corrosion inhibition properties of polyaniline. Moreover, the phenyl-caps help both of these molecules to resist polymerization on the surface. Raman spectroscopy, mid-IR spectroscopy, atomic force microscopy (AFM), temperature programmed desorption (TPD) and electrochemical impedance spectroscopy (EIS) were used to study interactions. We demonstrate that charge transfer and interconversion to different oxidation states take place during interactions between each of these molecules with iron (III) oxides surfaces. During interaction with the surface, all three tetramer molecules and DPPD prefer standing on their edge orientation, whereas B2Q1 molecules tend to orient in lying down direction on the same surface. Having amino groups in the chain helps reduced and half oxidized molecules to strongly hydrogen bond with the surface and make them static on the surface. On the other hand, a lack of amino groups makes oxidized molecules mobile and loosely bound to the surface. Interactions and change of oxidation states impact the corrosion inhibition properties of PCAT. Strong ability of sticking to the surface and not fully oxidizing (B3Q2) during interactions makes B5 molecules superior corrosion inhibitors than B4Q1 and B3Q2 molecules. Transformation into B3Q2 form at the beginning of interaction allows B4Q1 to moderately inhibit corrosion but as it transforms back to its original form with time it becomes the 2nd best corrosion protector of iron oxide surface after B5. The study of all oxidation states and their surface interactions with iron oxide surface will open up pathways towards of designing smart coatings using aniline oligomers and other redox-active molecules. / Thesis / Doctor of Philosophy (PhD) / Aniline oligomers have become a very interesting topic for research because of their potential application not only in organic electronics but also in smart coatings for corrosion treatment of iron and steel. A majority of the studies in the literature are focussed on the bulk or direct interaction between the organic molecules with metal substrates, without considering the native oxide film. In order to develop smart coatings (has redox activity and self-healing ability) for iron and steel, one must first understand how these oligomers interact with the native iron oxide film. In this thesis, we develop new knowledge from our fundamental understanding of the interactions of redox-active aniline oligomers with the iron oxide surface. Phenyl capped aniline dimer with two oxidation states [fully reduced (DPPD) and fully oxidized (B2Q1)] and phenyl-capped aniline tetramer (PCAT) with three oxidation states [fully reduced (B5), half-oxidized (B4Q1), fully oxidized (B3Q2)] were chosen for investigation. The former is the smallest redox active aniline oligomer but with one fewer oxidation states than polyaniline whereas the latter mimics the redox system as well as corrosion inhibition properties of polyaniline. Moreover, the phenyl-caps help both of these molecules to resist polymerization on the surface. Raman spectroscopy, mid-IR spectroscopy, atomic force microscopy (AFM), temperature programmed desorption (TPD) and electrochemical impedance spectroscopy (EIS) were used to study interactions. We demonstrate that charge transfer and interconversion to different oxidation states take place during interactions between each of these molecules with iron (III) oxides surfaces. During interaction with the surface, all three tetramer molecules and DPPD prefer standing on their edge orientation, whereas B2Q1 molecules tend to orient in lying down direction on the same surface. Having amino groups in the chain helps reduced and half oxidized molecules to strongly hydrogen bond with the surface and make them static on the surface. On the other hand, a lack of amino groups makes oxidized molecules mobile and loosely bound to the surface. Interactions and change of oxidation states impact the corrosion inhibition properties of PCAT. Strong ability of sticking to the surface and not fully oxidizing (B3Q2) during interactions makes B5 molecules superior corrosion inhibitors than B4Q1 and B3Q2 molecules. Transformation into B3Q2 form at the beginning of interaction allows B4Q1 to moderately inhibit corrosion but as it transforms back to its original form with time it becomes the 2nd best corrosion protector of iron oxide surface after B5. The study of all oxidation states and their surface interactions with iron oxide surface will open up pathways towards of designing smart coatings using aniline oligomers and other redox-active molecules.

<p> Poly aniline (PANI) is capable of inhibiting corrosion on iron by inducing the formation of a passive oxide film. The mechanism by which PANI does this is unknown to the scientific community. We have used photoemission spectroscopy of thin films of a model PANI oligomer to investigate the PCAT -iron interaction. </p> <p> The oligomer chosen was a phenyl-capped aniline tetramer (PC AT). Thin films of PCAT were prepared by in-vacuum physical vapor deposition to obtain extremely thin films of thickness ranging from -5A to over 1 Onm. </p> <p> Films were investigated with a photoemission electron microscope (PEEM) using synchrotron radiation to obtain spatially resolved valence band photoemission spectra. Analysis of PEEM results suggest that PCAT is capable of migrating several microns along the substrate surface, and causes a decrease in substrate work function wherever present. </p> <p> High-resolution core level and valence band photoemission spectroscopy using a laboratory-based photon source was used to characterize the substrate and PCAT properties near the PCAT-substrate interface. Characterization of an in-situ thin film deposition reveals that the iron substrate exhibits band bending in it oxide as well as a decrease in work function by 0.7eV upon adsorption of PCAT. </p> / Thesis / Master of Science (MSc)

This thesis addresses the potential for use of cobalt(III) complexes for functionalisation of lightly oxidised iron surfaces. In Chapter 2 the preparation of cobalt(III) complexes of a series of ligands based on 1,1,1-tris(aminomethyl)ethane is described. The synthesis was approached in two ways. Firstly, preparation of functionalised triol molecules as precursors to functionalised triamine ligands was investigated. This approach utilised the Tollens condensation of aldehydes with formaldehyde. In a second approach, the functionalisation of tetrakis(aminomethyl)methane in which one amine arm has been differentiated was used. The tetraamine was reacted with benzaldehyde and reduced with borohydride ion to give a secondary amine molecule that was then functionalised using alkyl or aryl sulfonyl chloride molecules. Chapter 3 describes the measurement of the binding of some cobalt(III) complexes to the surface of high surface area goethite. It was observed that complexes that have three exchangeable ligands bind more strongly than those with two exchangeable ligands. This is rationalised as being due to there being more bonds to the surface formed by complexes with three exchangeable ligands. It was also observed that complexes with three exchangeable ligands give greater surface coverage than those with two. This is likely due to the larger cross sectional area of the complexes with two exchangeable ligands in comparison to that of those with three, which blocks potential adjacent sites. Preliminary experiments on the use of the contact angle, SEM, EDS and QCM to characterise complex binding are explored in Chapter 4 . The results from the EDS and QCM experiments show that these may be valuable tools for measuring this binding and the subsequent surface properties, but have not yielded detailed results at this point. In Chapter 5 the use of cobalt(III) complexes as inhibitors of corrosion of iron in hydrochloric acid is investigated. All the complexes tested, even those that showed no binding to goethite surfaces, inhibit the corrosion to some degree. The level of inhibition is dependent on the complex, with [Co(tren)Cl2]Cl showing maximum inhibition of 81% and [Co(tame)Cl3] showing maximum inhibition of 53%. For some of the complexes, their concentration in solution over the course of the experiment was monitored by UV-vis. It was found that the complex disappears in a zero order reaction, the rate of which is dependent on the complex. However, the exact nature of this reaction is unknown. Furthermore, it was observed that inhibition of corrosion continues after the complex is no longer observed in solution. There is a difficulty in rationalising the inhibition being dependent on the complex identity, but not its continued presence in solution. Consequently, the mechanism of corrosion inhibition that explains all of these observations is still not known.

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A eficiência de antocianinas como inibidores de corrosão da liga de alumínio 2024-T3 foi avaliada através de medidas eletroquímicas como a espectroscopia de impedância eletroquímica e polarizações anódica e catódica. Soluções de NaCl 0,025 mol.L-1 contendo diferentes concentrações (800, 1000, 2000, 3000, 5000 e 7200 ppm) de antocianinas extraídas da uva foram preparadas para analisar o efeito da presença destas substâncias quando em contato com a liga de alumínio 2024-T3. Medidas de espectroscopia de impedância eletroquímica apresentaram uma diminuição sobre a dispersão dos pontos nas regiões de baixas frequências com o acréscimo do tempo de contato das antocianinas com a superfície da liga de alumínio 2024-T3 de 1 hora para 3 dias de imersão. Além disso, o comportamento indutivo nestas regiões decresceu após 3 dias de imersão com uma elevação considerável da resistência à polarização e da eficiência de inibição em 1000 ppm de antocianinas. Sob polarizações anódica e catódica, também foi verificado um melhoramento generalizado das propriedades anticorrosivas como potencial de corrosão, corrente de corrosão e eficiência de proteção contra a corrosão, para a concentração de 1000 ppm de antocianinas. Análises de microscopia eletrônica de varredura, microscopia de força atômica e espectroscopia por dispersão de energia, confirmam a adsorção de antocianinas na superfície da liga, assim como medidas de absorbância após 3 dias de imersão. / The efficiency of anthocyanins as corrosion inhibitor of the 2024-T3 aluminum alloy was evaluated by electrochemical measurements such as electrochemical impedance spectroscopy and anodic and cathodic polarizations. NaCl solutions 0.025 mol.L-1 containing different concentrations (800, 1000, 2000, 3000, 5000 and 7200 ppm) of anthocyanins extracted from grape were prepared to examine the effect of the presence of these substances when in contact with the 2024-T3 aluminum alloy. Electrochemical impedance spectroscopy measurements showed a decrease on the dispersion of points in the lower frequency regions with an increase of contact time of anthocyanins with the surface of the 2024-T3 aluminum alloy from 1 hour to 3 days of immersion. In addition, the inductive behavior in these regions decreased after 3 days of immersion with a considerable increase of the polarization resistance and inhibition efficiency of 1000 ppm of anthocyanins. Under cathodic and anodic polarizations, it was noted a general improvement in the anticorrosive properties such as corrosion potential, corrosion current and efficiency of protection against corrosion for the concentration of 1000 ppm of anthocyanins. Analyses of scanning electron microscopy, atomic force microscopy and energy dispersive spectroscopy confirm anthocyanins adsorption on to the surface alloy, as well as absorbance measurements in a spectrophotometer after 3 days of immersion.