CORROSION inhibitors are vital for protecting metallic substrates, either as standalone treatments present in surrounding electrolytes, or as leaching components in active protective coatings. While organic molecules offer tremendous versatility due to their nearly infinite structural tunability, their electrochemical performance still falls short of traditional chromate-based systems, especially under dynamic environments present in service conditions. This dissertation aims to analyse the potential of organic molecules as corrosion inhibitors for aerospace alloys by applying a systematic and multidisciplinary approach to evaluate, understand, and ultimately improve the electrochemical performance, stability, and long-termefficacy.... ... Read more

Electrochemical carbon dioxide reduction represents a promising pathway toward a circular carbon economy and the achievement of net zero emissions. Realising this potential requires catalysts that balance activity, selectivity, stability, earth-abundance, and economic feasibility. Molybdenum disulfide is an attractive candidate for this purpose due to its layered structure, tunable properties, and scalable synthesis, yet its performance in the electrochemical reduction of carbon dioxide remains limited by fundamental factors that affect conductivity, active site formation, and product selectivity. This thesis investigates strategies to enhance the catalytic behaviour of molybdenum disulfide by engineering its structural and electronic environment through alkali ion intercalation, vacancy induction, and the use of co-catalysts.

The thesis begins with an introduction to the electrochemical reduction of carbon dioxide and the challenges associated with achieving efficient and selective conversion. A detailed literature review follows, covering the properties of molybdenum disulfide, its reported behaviour in carbon dioxide reduction, and known structural limitations. Background on alkali ion intercalation, associated phase transitions, and force field considerations for molecular dynamics simulations is also provided to support the modelling work presented later.

The first research component examines the tunability of the electronic properties of molybdenum disulfide through controlled intercalation of alkali metal ions. Molecular dynamics simulations reveal the atomic scale mechanism of intercalation, demonstrating that the hydration shell of incoming ions forms an energy barrier that must be reorganised for successful insertion. Complementary experimental characterisation confirms that intercalation introduces additional defects and increases electronic conductivity. Potassium produces a more pronounced effect than sodium, consistent with its weaker hydration and greater structural impact. However, increased conductivity does not improve performance in carbon dioxide reduction. Instead, it correlates with a decline in catalytic efficiency, indicating that electronic enhancement alone is not sufficient to promote the desired reaction pathways.

Subsequent chapters, not detailed here, expand this investigation toward vacancy engineering and co-catalyst selection to influence product distribution and promote formation of higher carbon products. Combined molecular simulations and experimental studies provide insight into how local structure and interfacial environment govern the selectivity of the reaction.

Overall, this thesis demonstrates that the catalytic behaviour of molybdenum disulfide can be systematically tuned through structural modification and environmental control. The findings highlight key mechanistic factors that influence conductivity, defect formation, and selectivity, offering guidance for the rational design of improved catalysts for electrochemical carbon dioxide reduction. ... Read more

This work advances the understanding of local corrosion and inhibition of aerospace aluminum alloys by applying in situ reflected light microscopy, integrated with electrochemical measurements, to systematically analyze the behavior of many intermetallic particles and other local corrosion sites. This approach generates site-resolved datasets with high spatial and temporal resolution, capturing the diverse and dynamic nature of local corrosion phenomena. To fully harness this information, new data processing frameworks were implemented, enabling extraction of time-dependent parameters and robust classification of local corrosion and inhibition behaviors. These developments uncover new mechanistic insights into how corrosion and inhibition processes initiate, evolve, and interact across varying microstructures under different environments. Crucially, this approach moves beyond isolated observations of individual sites, enabling a broader and more representative understanding of local behavior across the materials. The resulting perspective provides insights that support the development of more targeted and efficient active corrosion protection strategies. ... Read more

In the aerospace industry, toxic and carcinogenic chromate-based inhibitors are still widely used in coatings to protect structural components of aircraft throughout their entire lifespan. Alternatives currently lack proven long-term performance, partly because accelerated ageing tests may provide a qualitative ranking of alternatives, but fail to accurately predict service lifetime in their real-world application. However, the reasons for these discrepancies between laboratory-based test results and in-service performance remain insufficiently understood.

This dissertation aims to deepen the understanding of the factors influencing coating degradation and their underlying mechanisms, both in practical applications and test environments. Such knowledge is essential for developing improved test methods capable of reliably comparing the performance of chromate-containing coatings with alternative systems. These advancements could significantly accelerate the development process of new coatings driving innovation in the paint and coating industry.

The study consist of two separate research tracks: (i) forensic research into the degradation mechanisms of aircraft components after long-term in-service use and (ii) experimental research into degradation mechanisms in test environments. Each track focuses on two aspects: (i) corrosion and inhibitor action on aircraft metal alloys and (ii) coating degradation.

The forensic analysis examined four aircraft components that had been in-service for over 35 years, using visual inspection, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results showed that large areas of the coated components remained well-protected throughout the entire service life. However, three specific forms of degradation were identified: (i) erosion at the tip; (ii) corrosion around rivets and (iii) corrosion near fasteners at the leading edge. These findings demonstrate that even chromate-based coatings may not sustain the provision of long-term corrosion protection in complex multi-material areas.

Further forensic analysis focused on the protective mechanisms and degradation factors of the original coatings using electrochemical impedance spectroscopy (EIS), SEM and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Results confirmed that chromate-containing coatings are exceptionally effective; after more than 35 years of service, they outperformed some newly applied systems. This superior performance was attributed to chromate adsorption on corrosion products like aluminium hydroxide, which increased the pore resistance of the coating. Simultaneously, it was found that the polymers in the original coatings had degraded due to thermal oxidation. Temperature increase due to exposure to sunlight caused oxidation in the polymer, which accelerated moisture uptake. This, in turn, led to faster inhibitor leaching, compromising the coating barrier properties.

The experimental study compared two chromate-based coatings with two alternatives under various exposure condition, including a cyclic salt spray test (CSST), outdoor exposure and flight tests. Results revealed that the corrosion and inhibition mechanisms observed in the CSST did not align with those observed in flight tests. These differences were attributed to variations in time of wetness (TOW) during the relative humidity (RH) cycles, temperature fluctuations, differences in the type of deposited substances (such as salt) accumulating at the test specimens and excessive electrolyte exposure in CSST. Furthermore, galvanic coupling at fasteners was difficult to prevent leading to accelerated corrosion. Chromate-based systems provided partial active corrosion inhibition around fasteners, while alternative systems failed. However, the alternative systems offered improved corrosion resistance between aluminium-coated surfaces coupled with carbon fibre-reinforced polymer (CFRP). This improvement is due to the novel polymer formulation in the alternative systems, increasing their barrier properties as compared to the legacy polymers used in chromate-based systems.

Further experimental analysis on coating degradation under different exposure conditions, using EIS, SEM and ATR-FTIR, revealed that hydrolysis and thermal oxidation were the primary causes of polymer degradation during flight tests, with inhibitor leaching playing a comparatively minor role in the coating degradation. In contrast, inhibitor leaching was the dominant degradation factor in CSST and outdoor tests, significantly accelerated by UV radiation and excessive electrolyte exposure.
The study also highlighted the important role of the anodized oxide layer in coating systems. In chromate-based coating systems, chromate adsorption onto aluminium hydroxide within the pores of the anodized oxide layer, increase corrosion resistance, whereas in alternative systems, only the polymer inside the pores provides additional protection.

This dissertation provides valuable insights into factors for improving artificial ageing tests. Integrating thermal oxidation, increasing TOW during RH cycles and reducing electrolyte exposure into test protocols can enhance the predictive value of these tests. Additionally, incorporating complex material combinations with fasteners into updated sample configurations is considered crucial for realistic testing. These improvements can lead to more effective evaluations of alternative coating systems, accelerating the development and implementation of sustainable alternative coating systems. ... Read more

The electrochemical reduction of carbon dioxide (CO2RR) offers a sustainable route to convert CO2 into value-added chemicals and fuels, contributing to both carbon management and renewable energy storage. However, the competing intermediates of the reaction often results in poor selectivity and energy inefficiency. This thesis explores metal–organic frameworks (MOFs) as a tunable platform for studying and improving selectivity in CO2RR through atomically precise control of catalytic environments. By systematically tailoring the local coordination, geometry, and composition of Cu-based MOFs, this work establishes direct relationships between structure, intermediate stabilization, and product distribution, thereby advancing the rational design of selective CO2RR electrocatalysts. Overally, this thesis demonstrates that the local coordination environment and electronic structure of Cu-based MOFs dictate CO2RR selectivity. By integrating synthesis, operando spectroscopy, and DFT modeling, this work establishes coordination engineering as a powerful strategy for rationally designing selective and efficient MOF-based electrocatalysts for CO2 conversion. ... Read more

H igh-temperature (HT) oxidation plays a significant role in various stages of the steelmaking process, including hot rolling. When exposed to high temperatures and oxygen partial pressure, the steel composition near the surface can be altered as alloying elements deplete. Additionally, the characteristics of the oxide scale, such as thickness and phase composition, vary depending on the oxidation conditions. Due to the experimental challenges of studying such rapid processes under extreme conditions, predictive models are necessary to estimate the substrate surface and oxide scale composition as well as the general oxidation rate of the alloy.... ... Read more

The exposure of certain carbon steels to sour environments can result in severe hydrogen induced cracking (HIC) damage in the oil and gas industry. Current mitigation techniques in this field have low reliability or are not able to provide long-term protection against such damage. Recent advancements in thermal spray technology have resulted in a promising and cost-effective solution. Improvements in particle velocity and deposition efficiency have enabled coatings to achieve higher density and uniformity. High-velocity air fuel (HVAF) thermal sprayed NiCrMoW coatings are particularly interesting due to their outstanding corrosion resistance and mechanical properties. To ensure that equipment is sufficiently protected against the harsh environment of this industry, a thick coating is desired. However, as coating thickness increases, the performance of thermal sprayed coatings is frequently affected by residual stresses and unfavourable microstructural features.

To identify this effect, three NiCrMoW coatings with thicknesses of 250, 375, and 500 \textmu m were applied with HVAF thermal spray technology on S235JR carbon steel. Samples were analyzed in order to evaluate differences in terms of microstructure, mechanical behaviour, HIC resistance, and corrosion resistance. An AK07 HVAF instrument in a controlled setting at the IOT research centre of the University of Aachen was used to ensure consistency among the coatings during the spraying process. Experiments to evaluate HIC resistance and corrosion resistance involved prolonged immersion in a sour environment, cathodic charging, open circuit potential measurements, and potentiodynamic polarization tests. Microstructural variation was examined with the use of SEM-EDS and optical microscopy. Additionally, subsurface microhardness measurements of the coating and underlying substrate were used to evaluate hardness and give an indication of the presence of residual stresses.

Findings indicate that the coatings exhibit excellent corrosion resistance. A small but noticeable decrease in resistance was however observed with increasing coating thickness. This decline can be attributed to two factors: an increase in the degree of oxidation and accumulation of residual stresses within the thicker coatings. Additionally, it is noteworthy that while the degree of oxidation and residual stresses increased with coating thickness, the porosity fraction decreased. Microstructural features in the coatings varied as a result of differences in thermal input, cooling passes and the influence of shot peening effects. Resistance to HIC of carbon steel in a sour environment was significantly improved by the application of the coatings in comparison with uncoated samples. This can be attributed to the excellent corrosion resistance, uniformity and absence of through-coating porosity in the coatings, the thickness did not have an influence. Furthermore, it was found that the galvanic interaction between the NiCrMoW coating and the S235JR carbon steel significantly accelerates the corrosion of the underlying substrate. Thicker coatings might be able to provide a greater physical defect-free barrier which can resist breaking, damage and erosion to prevent this galvanic effect. ... Read more

Over the last 15 years or so, research has revealed the great self-healing prospects possessed by asphaltic mixtures. Researchers have proposed novel methods to harness this capability, aiming to prolong the service life of asphalt pavement, particularly in porous asphalt. To date, the most promising of the healing methods is the combined capsule-induction system. This thesis aims to ascertain whether such a system would show positive results in stone mastic asphalt (SMA). Following that, an optimisation of the composition of self-healing SMA was proposed by assessing the mechanical and healing properties via laboratory testing. Finally, an evaluation of sustainability from an environmental perspective was done using Life Cycle Analysis (LCA) methodology.

Results of the healing assessment revealed that each combined healing system was able to recover between 58-63\% of its original fracture strength after 8 healing cycles, while the reference mix (without healing) was only able to regain 10\% fracture strength before failure after 2 cycles.

Inclusion of the combined healing system slightly reduced the strength, stiffness and water sensitivity of the SMA mixture compared to the reference. However, improved rutting resistance was observed in each self-healing case. Within the self-healing mixtures, increasing capsule content reduced asphalt density, stiffness and strength and resulted in an increase in asphalt void content.

The LCA results show that the self-healing system had environmental benefits in some facets such as a 14\% reduction in fossil fuel resource depletion and a 21\% reduction in land use. However, the present total known environmental costs of other impacts are approximately 15\% lower in the reference system based on a cradle to gate, and use phase analysis. Almost half of this total cost was attributed to maintenance activities. It was concluded that a 32\% increase in maintenance efficiency would ensure environmental viability of a self-healing mixture over a reference mixture within the constraints of the analysis conducted. ... Read more

Aluminium alloy AA2024-T3 is a lightweight and damage tolerant material, and is therefore often used in aerospace applications. However, this alloy is difficult to weld using conventional fusion welding techniques due to defects caused by the meld pool.

Friction stir welding (FSW) was developed by The Welding Institute (TWI) in 1991 in order to overcome welding difficulties associated with the conventional fusion welding of difficult to fusion weld materials such as AA2024-T3. Since then, efforts have been made to improve the process parameters. One such improvement was the development of a stationary shoulder tool, which reduces heat input by 30% and enables stronger welds with smaller heat affected zones (HAZ). However, since AA2024-T3 is susceptible to pitting corrosion, intergranular corrosion (IGC) and stress corrosion cracking (SCC), the welds need to be protected in order for any product to function in the long term. AA1050 is an often applied clad layer, which acts as a sacrificial anode with respect to the cathodic substrate. The microstructure and corresponding corrosion mechanisms are known for unwelded and FSW’d AA2024-T3, however, little research exists on the corrosion behaviour of stationary shoulder friction stir welded (SSFSW’d) bare and Alclad AA2024-T3 butt welds. Therefore, the main focus of this project is to investigate the mechanical and corrosion properties of SSFSW’d bare and Alclad AA2024-T3. This was done by performing optical microscopy, microhardness tests, open circuit potential measurements, linear polarisation resistance tests and potentiodynamic polarisation tests on the cross-sections of three weld configurations and thicknesses: 1.6 mm bare, 1.6 mm Alclad and 3.2 mm Alclad AA2024-T3 sheets. Simultaneously, tensile tests and immersion tests were also performed using dedicated tensile test and immersion specimens.

Analysis shows that the 1.6 mm bare weld is the strongest weld but also the most susceptible to pitting corrosion, compared to the Alclad welds. Cladding provides sufficient corrosion protection, even to an exposed section of the weld and when mixed into the weld. However, cladding lowers the tensile strength overall, and cladding mixed into the weld reduces the ductility of the weld compared to the bare weld. Furthermore, possible material flow issues at the root of the Alclad welds may cause voids, which lowers ductility and enables pitting at these locations. Nevertheless, even though specimens were immersed in 3.5% NaCl solution for 24 hours and small to severe pitting was visible on the specimens, the ultimate tensile strength was not affected compared to uncorroded weld specimens. However, a reduction of the maximum elongation of the bare specimen was observed after immersion. Similar to conventional FSW, the HAZ/TMAZ was the most susceptible to corrosion, due to the most active corrosion potential at this zone. Based on literature, this was deemed to be due the formation of S-phase precipitates along the grain boundaries in this zone. In all, stationary shoulder friction stir welded bare AA2024-T3 provide the best welds regarding mechanical properties, and should be protected against corrosion after welding instead of using preclad sheets, to avoid issues with the macrostructure and corresponding mechanical properties. ... Read more

Accelerated tests have been commonly used by the marine coating industry to evaluate the performance of a coating in a short duration of time. Replicating natural exposure in an artifi- cially simulated manner is a challenging task due to the various factors that induce corrosion in the natural environment. Continuous salt spray testing is a popular accelerated test, but it does not replicate the actual exposure scenario as intensifying the factors inducing corrosion does not produce the same result as natural exposure. However, it was found that alternating between wetting and drying of the coating correlates well with natural exposure. ISO 12944- 9, a standardized accelerated test for corrosion protection in the marine environment, is one such test which alternates between different exposure conditions. This is performed in order to replicate the exposure conditions that a marine structure experiences. This ultimately aids in understanding the performance and durability of the coating within a shorter period. Besides, accelerated tests can be used to correlate with the natural exposure, in order to circumvent the issue of testing coatings in the natural environment for years to understand the coatings performance and durability. This thesis aims to correlate the performance of a commercial marine vinylester coating re- inforced with glass platelets named Ecospeed, when exposed in a natural service life envi- ronment versus that when tested in an accelerated environment. For accelerated ageing, the standard ISO 12944-9 was used, which is the standard exposure conditions for coating sys- tems undergoing extreme conditions in the marine environment. For the natural exposure, two in-service vessels coated with Ecospeed were selected to test its performance. Both vessels were coated with Ecospeed for 10 and 15 years. The coating’s performance is evaluated using electrochemical impedance spectroscopy (EIS) in both the natural and accelerated environments. EIS measurements were performed at peri- odic intervals of 1000 hours until 3000 hours for the accelerated exposure. EIS measurements were obtained on the 10 and 15 years old ship at multiple locations on the hull. This was done to check the uniformity of corrosion protection and variation of coating performance across the ship. Pull-off adhesion tests were also conducted after 2000 hours of accelerated exposure and the strength was reduced to more than half its initial strength prior to exposure in ISO 12944-9. The results of the accelerated test indicated gradual reduction in the impedance of the coating over 3000 hours of accelerated test exposure. The magnitude of impedance at the lowest frequency of 10 ́1 Hz was 4.23 x 1010 Ω cm2 after 3000 hours of accelerated exposure and for the 15 year field exposure the magnitude of impedance was 4.27 x 109 Ω cm2, which is well above the minimum industry standard of 107Ω cm2. Considering the impedance values for both the accelerated and natural exposure, it can be said that the impedance of Ecospeed is atleast 2 orders of magnitude higher than the minimum industry standard requirement even after 15 years of natural exposure. The accelerated results of 1000 hours had good correlation with the impedance measurements performed on the 10 year old ship. The magnitude of impedance of the 15 year old ship was lower than the magnitude of impedance obtained from 2000 and 3000 hours of accelerated exposure. This means further exposure in the accelerated environment is needed to correlate with a 15 year old ship. ... Read more

Pretreatment has become a necessary process to enhance the surface chemistry and morphology of aluminium alloys before implementing subsequent corrosion protection measures, especially organic coating application. Traditionally, hexavalent-chromium-based pretreatment processes were frequently used in many industries, but hexavalent-chromium-based chemistries are now recognized as a potentially carcinogenic hazard and environmentally harmful. The development of alternative, non-toxic and eco-friendly corrosion inhibitor and pretreatment technologies has become of pivotal importance to industries as a basic license to operate and to reach a sustainable society as a whole.
In this work, sodium galactarate (NaGal) was investigated as a potential green corrosion inhibiting chemical for hexavalent-chromium-free pretreatment and to serve as a coating adhesion promoter on aluminium alloy AW3003. The pretreatment layer formation was examined at five different pHs; 3, 4, 7, 10, and 11. The surface analysis was performed by Fourier transform infrared – reflection absorption spectroscopy (FTIR-RAS), contact angle measurement and white light interferometry (WLI). Samples with pretreatment layers formed in acidic and alkaline environment showed an increase in polar energy and surface roughness, which are strongly related to the coating adhesion properties, in line with the failure stresses obtained from the pull-off adhesion tests. The corrosion resistance was evaluated through the electrochemical behavior that was measured by linear polarization resistance (LPR), potentiodynamic polarization (PP), electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements. Immersion testing was performed to evaluate the pitting corrosion behavior. The pretreated sample exhibited less pitting than the reference sample, especially in an alkaline environment where aluminium alloys severely corrode due to limited protectiveness of the oxide layer. The pretreatment prevented the corrosion products from accumulating on the sample surface, limiting stain and smut layer redeposition, subsequently reducing the pitting corrosion.
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Currently, as the utilization of offshore wind energy continues to increase, floating wind turbines are expected to be widely used. As the fixed system of the turbine, the safety of the mooring chain has gradually attracted the attention of researchers. Mooring chains immersed in seawater are mainly subjected to various mechanical loads and corrosion. Therefore, premature failure and frequent replacement are the main problems of mooring chain systems. In order to avoid huge losses of safety caused by structural failure, it is urgent to establish mooring integrity management, accurately identify hazards, and evaluate the service life of the mooring system. Considering the requirements of sufficient mechanical properties and economic benefits, high-strength low-alloy steel has gradually replaced low-carbon steel as the main material for mooring chains. However, there are few studies on the detailed corrosion process of mooring chain steel.
This research aims to explore the corrosion process of mooring chain steel and the influence of marine environmental factors on the corrosion process. Traditional electrochemical techniques, morphology observation and new in-situ non-destructive technique acoustic emission are used to investigate the corrosion process. The experiment includes the exploration of the corrosion process of steel under natural and accelerated conditions. Experiments on the influence of flow velocity and temperature are also included. The corrosion process of mooring chain steel is successfully explored during the monitoring process. Acoustic emission signals related to corrosion are separated. Their sources are reasonably identified. The effects of water flow velocity and temperature of the corrosion process are summarized.
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Trivalent chromium (Cr(III)) is one of the most promising non-toxic replacements for hexavalent chromium (Cr(VI)) coatings in the steel packaging industry. The application of a chromium layer to packaging steel is essential for providing a protective layer on the steel packaging’s external surface area, which prevents corrosion during its use. However, the deposition process of Cr-metal from Cr(III) solution cannot produce a deposit thicker than 10 μm with sufficient corrosion and wear resistance without the application of a complexing agent. This study provides both an experimental research as an comparative computational model. Experiments are set up to analyse the effect of the formate ion HCOO(-) concentration as a complexing agent in Cr_2(SO_4)_3 electrolytes, which results are compared to the output of the used computational model. The results provide insights into the initial composition of species in plating electrolytes, which is essential for the determination of the deposition mechanism from Cr(III) electrolytes to improve the electroplating process. Ligand exchange and bonding with HCOO(-) in chromium complexes is studied with Ultraviolet-visible light (UV-VIS) spectroscopy and Attenuated Total Reflectance Fourier InfraRed spectroscopy (ATR-FTIR) at various concentrations of HCOO(-). Additionally, computational modelling is performed with Density functional theory (DFT) to predict the spontaneous character of various ligand substitutions, and simulations of spectroscopic spectra are performed as a reference to mimic the experimentally observed data. It was found that ligand exchange with HCOO(-) in Cr_2(SO_4)_3 solutions occurs spontaneously within two days at elevated temperatures at the analysed concentrations. The experimental data shows, there is a transition point in the complex formation between the ratio of [Cr(3+)]:[ HCOO(-)] is respectively [1]:[1.61] and [1]:[3.32]. This indicates that the complex formation decreases past the ratio of [Cr(3+)]:[ HCOO(-) ] = c.a.[1]:[3] and that the concentration at which the most chromium-formate complexes can be observed is within the range of [Cr(3+)]:[ HCOO(-) ] = [1]:[1.61] to [Cr(3+)]:[ HCOO(-) ]= [1]:[3.32]. The formation of chromium-formate complexes is beneficial for the deposition process, as the addition of formate increases the amount of chromium deposited on the steel surface. Furthermore, it is found that the presence of SO_4(-2), originating from the chromium salt, is beneficial for the ligand exchange of HCOO(-). Also, the bonding of formate to chromium is found to be as monodentate binding. ... Read more

For decades, developments towards a chromate-free protective system have been a crucial quest in the aerospace industry. In this search, innovation starts with a comprehensive understanding of the aircraft complex multi-material structures and the corrosion mechanisms involved. These complex structures, with aluminium the most dominant substrate, are used to optimize the strength to weight ratio, fatigue properties and operational performance in aircraft design. On the other hand, the use of multi-materials can cause an accelerated corrosion attack, called galvanic corrosion. This type of corrosion can be advantageously applied to galvanically protect a material but can also occur undesirably with serious accelerated degradation as result.

This thesis aims to increase our understanding of corrosion on AA7XXXClad alloys as stand-alone material, as well as in configurations relevant for galvanic corrosion with other metals. The understanding of these phenomena should support the definitions of new hypotheses on how these alloys can be better protected using chromate-free coating technologies. Two commonly used aluminium alloys in the aerospace industry have been investigated in this study because a selective galvanic stimulated dissolution of the cladding layer material was found after an accelerated corrosion test. This selective dissolution makes it difficult for the corrosion inhibitors to reach the exact site of corrosion propagation in the cladding layer, reducing the inhibition efficiency and allowing corrosion to propagate.

Since Zinc is the main alloying element in the substrate and cladding material, the role of Zn with respect to corrosion initiation and propagation was investigated first. Subsequently, the behaviour of clad alloys under galvanic corrosion conditions, and finally how inhibition under these conditions can be reliably assessed. This was performed by a combination of multiple electrochemical techniques and microscopic analysis. The results demonstrate that Zn plays a significant role in the dissolution of the cladding layer and may be held responsible for the selective dissolution observed. In addition, an experimental procedure was developed to measure the coupled galvanic parameters and to simulate the galvanic corrosion degradation in industrial desired timeframes. Although simulation of the phenomenon has shown to be promising with in-situ experiments, it was not fully observed. Furthermore, to test the performance of corrosion inhibitors, a procedure is developed with and without the use of coatings. In general, it can be concluded that limiting the cathodic reactions is of paramount importance to reduce the galvanic corrosion current. ... Read more

Due to the demand of reducing the weight of aircrafts to reduce the carbon consumptions without compromising the mechanical properties, aluminium alloy AA2024-T3 has been widely applied in the aircrafts’ manufacture. However, the alloying elements which ensure the mechanical properties also could increase the susceptibility of localized corrosion. The chromate conversion coatings (CCCs) have been used as the efficient anti-corrosion protection for aluminium alloys. Due to their toxicity, many countries and regions in the world have been banned CCCs from many industries. Numerous alternatives were studied for decades to replace the CCCs. Among all of them, the sol-gel technology is a promising method to inhibit the corrosion of aluminium alloy AA2024-T3. Furthermore, the addition of corrosion inhibitor into the sol-gel coating additionally increases active corrosion protection of aluminium alloys. This thesis focuses on the sol-gel coatings corrosion protection of aluminium alloy AA2024-T3 with the incorporation of lithium and cerium salt as corrosion inhibitors. The corrosion protection for the intact coatings was analyzed by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS). An active corrosion protection for the scribed coatings was investigated by observation of immersion test with digital optical microscope and the electrochemical impedance spectroscopy (EIS). The main findings of the present work are the following: (1) the sol-gel coating decreases the corrosion. (2) the addition of corrosion inhibitors, LiNO3 and Ce(NO3)3, into the solgel coating decreases the corrosion current. (3) the combination of LiNO3 and Ce (NO3)3 provides the best corrosion protection for intact coatings applied on AA2024-T3. ... Read more

Due to their simple structure and the possibility for in situ doping, light emitting electrochemical cells (LECs) are shaping up to be a cheap and easily produced alternative to LEDs. To date, little research has been done to combine the luminescent properties of colloidal quantum dots (QDs) with the LEC concept. In this thesis, steps are taken towards developing a QD-based LEC. The LECs were produced based on the semiconducting polymer MEH-PPV, ZnO and CdSe/CdS/ZnS quantum dots. Furthermore, drift-diffusion simulations were performed to discover general trends in LECs. MEH-PPV devices performed as expected. ZnO QDs were found to be unsuitable for LECs due to their high intrinsic conductivity and low luminescence efficiency, while CdSe/CdS/ZnS QDs were identified as promising candidates. Results from the drift-diffusion simulations suggested that unlike in LEDs, the type of electrode material has little effect on device operation. Charge carrier mobility was identified as the limiting parameter for luminescence intensity. It is thus suggested that improving carrier mobility in QD films is the most important step towards QD-based LECs. ... Read more

Rapid industrialization and use of carbon based fuels has caused a drastic increase in the atmospheric CO₂ levels in the last few decades. The rising anthropogenic CO₂ levels pose a significant threat to the environment as evidenced by the increase in the mean global temperature levels, and the rising ocean levels. To mitigate the challenges associated with rising CO₂ levels, there is an urgent need to move towards carbon neutral sources of energy and to curb carbon emission from large scale point emitters such as industries. Additionally, emitted CO₂ could be converted into energy dense organic fuels using carbon-neutral forms of energy. This not only helps in reducing the carbon emissions but also balances the intermittent nature of renewable energy supply. CO₂ could also be converted into platform chemicals such as ethylene/CO, which can be further up-converted or directly used in industry. Ethylene is particularly interesting due to its high
energy density and wide industrial usage as a precursor in the polymer industry. Electroreduction of CO₂ provides one such approach to electrochemically convert CO₂ produced at large scale emitters to useful organic compounds. Different metallic catalysts are known to catalyse the electrochemical reduction towards different products, which follows from Sabatier’s principle. In this study copper is used as the model catalyst due to its unique ability to electrochemically convert CO₂ to multi-carbon products, such as ethylene. From a cell design perspective conventional electrochemical reduction of CO₂ in aq. media suffers from low production rates due to the low solubility of CO₂ in aq. electrolytes which makes it not feasible from an industrial standpoint. To overcome the low production rates, this study was carried out on novel gas diffusion electrodes. Another factor limiting the implementation of CO₂ electrolysers on an industrial scale, is the scalability of the catalyst synthesis. To improve this, electrodeposition of copper catalysts was employed. Electrodeposition is a well-established industrial technique and integrable within the existing infrastructure. Electrodeposition facilitates in-situ growth of the catalyst on gas diffusion layers, thereby providing a facile alternative to the conventional multi-step process for catalyst synthesis. Different morphologies of copper were synthesized by varying the electrodeposition process parameters. Copper nanowires were also synthesized by using templated electro-deposition techniques. The catalysts were characterised before and after the CO₂ reduction experiments by Scanning ElectronMicroscopy, and X- Ray Diffraction. CO₂ reduction experiments using the synthesised copper catalysts were carried out over a range of potentials. A peak Faradaic Efficiency (FE%) of 15% was measured at -1.5 V vs RHE (uncompensated) for ethylene, 19% FE at -1.1 V vs RHE for formic acid, and 13% FE at -1.5 V vs RHE for methane. It was also seen that the catalyst suffered from stability issues which were overcome by using pulsed electrolysis. Using pulsed electrolysis the lifetime of the catalyst was increased from 30 minutes to 15 hours. ... Read more

Hexavalent chromium has been the industry standard for corrosion protection for many years. Its unsurpassed active corrosion inhibiting capabilities, its incredible versatility and its economic benefits made it a popular all-rounder. Nowadays the widely known toxic and carcinogenic nature have restricted its use within the European Union. More and more research in the field of corrosion science has been focussing on finding safer alternatives, since hexavalent chromium was officially added the US annual report on carcinogens in the 1980s. Before it was used in almost every step of corrosion protective schemes consisting of a pre-treatment, a primer and a topcoat. In this work a novel approach to two industrial anodising pre-treatments (sulfuric acid anodising and tartaric sulfuric acid anodising) was investigated in order to improve the corrosion performance of the corrosion sensitive aluminium alloy 2024-T3. Both are currently used as alternatives to the historically often applied chromic acid anodising procedure, which contains hexavalent chromium compounds. In this work the effect of the anodising electrolyte viscosity, the anodising interelectrode distance and the addition of ceric sulphate to the anodising bath were investigated. Different fractions of ethylene glycol were used to vary the electrolyte viscosity. All anodising procedures used a fixed anodising voltage, temperature, acid concentration and agitation speed. These parameters were not changed. All samples were cleaned before anodising. In order to assess the corrosion performance linear sweep voltammetry, electrochemical impedance spectroscopy and immersion tests were deployed. Furthermore scanning electron microscopy with energy dispersive X-ray spectroscopy was used to evaluate the chemical composition of the anodised substrates.
It was found that an increase of the electrolyte viscosity results in a decrease of the anodising current density, which was related to a decrease in the overall thickness of the oxide layer created by the anodising process. The addition of 25 vol% ethylene glycol did not show any significant changes in corrosion performance although some indications of a slight improvement were found. Slightly smaller pores and a tighter barrier layer were proposed to be a possible explanation. A fraction of 75 vol% on the other hand dramatically deteriorated the corrosion performance, due to much slower oxide growth kinetics resulting in a much thinner oxide. The addition of ceric sulphate did not lead to any significant improvements in the corrosion performance for any of the tested procedures with one exception. The tartaric acid based procedure without ethylene glycol addition did show a significant improvement. Negatively charged complexes of cerium and tartaric acid compounds, which are supposed to be drawn towards the substrate during anodising, were proposed to be a possible explanation. The interelectrode distance did not show any significant differences except for the tartaric acid based procedure with ceric sulphate but without ethylene glycol addition. A higher electric field strength as a results of the smaller interelectrode distance was held responsible. The electric field strength should directly affect the amount of cerium complexes attracted towards the substrate, increasing the chance of cerium ending up as residues in the pores of the anodic oxide layer. ... Read more

Copper and copper alloys have been used widely in different applications, such as electric power transmissions and heat exchanges. However, they are susceptible to corrosion in different environments leading to damages and catastrophic failures. Different corrosion-inhibition strategies are used to retard corrosion of copper and copper alloys. Among them, corrosion inhibitors are successfully used to inhibit their corrosion. This thesis focuses on the corrosion inhibition of brasses (the copper-zinc binary alloys) using two kinds of organic inhibitors, namely 2-mercapto-1-methyl-benzimidazole (1H-HB-2T) and 2-mercapto-benzimidazole (MBI). Corrosion inhibition of the substrates using the inhibitors were screened using linear polarization resistance (LPR) measurements. Subsequently, electrochemical impedance spectroscopy (EIS) was used to study the detailed corrosion inhibition processes and mechanisms. Finally, two modes (RAS and ATR) of Fourier transform infrared spectroscopy (FTIR) were adopted for in-situ and ex-situ tests to study the adsorption mechanisms of inhibitor molecules on the surfaces. The research results showed that: (1) the composition (phase type) of the brass controls the interaction of inhibitor on surfaces; (2) an increased Zn content of brass leads to a decreased corrosion resistance; (3) 2-mercapto-1-methyl-benzimidazole (1H-HB-2T) exhibits better corrosion efficiency than 2-mercapto-benzimidazole (MBI) indicating the importance of the methyl functional group to increase the inhibitor’s efficiency; (4) 1HHB-2T exhibits a spontaneous structure transformation from thiol to thione and (5) within a maximum 5-hour exposure, the adsorption of the inhibitors to the metal surface is complete. ... Read more