Characterization of thermal connections using electrochemical and surface sensitive methods
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School of Chemical Engineering |
Master's thesis
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en
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167
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Abstract
This Thesis investigated the corrosion behavior of thermally connected aluminum joints in an industrial component, focusing on how alloy choice, joining parameters, surface machining, pretreatment, and cathodic dip coating systems influence performance. The experimental program combined surface roughness characterization, Modified Environmental Cycle Tests according to norm 1, and electrochemical methods to establish correlations between surface state, coating integrity, and corrosion kinetics. Three norm 1 variants (1, 2, and 3) were applied for durations of six and twelve weeks to simulate real-world cyclic exposure under varying severity. Electrochemical investigations employed two complementary setups: a large-area three-electrode configuration for coated samples and a microelectrode system for localized measurements on pretreated, uncoated surfaces. In these tests, open circuit potential (OCP), linear polarization resistance Rp, cyclic potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS), and galvanostatic load measurements were performed to quantify coating barrier properties, interfacial kinetics, and localized corrosion susceptibility. The results clearly show that the heat-affected zone (HAZ) is the most vulnerable region across all coatings, test severities, and durations. Corrosion consistently initiated and propagated in the HAZ, while the joining zone and flat sheet generally remained better protected. This trend intensified with longer exposure and under the aggressive test variant 3. Alloy composition also played a decisive role: M2 exhibited superior corrosion resistance compared to M1, particularly in the HAZ. This difference is attributed mainly to the higher magnesium content in M2, which promotes passivity and microstructural stability. However, the morphology of the two alloy types likely also influences their corrosion susceptibility. Surface preparation emerged as another critical factor. Method I alone produced low roughness and minimal profile length increase, resulting in excellent corrosion resistance even after twelve weeks of exposure. Optimized I delivered the smoothest finish and the lowest corrosion classes recorded. In contrast, methods II and III significantly increased roughness and relative profile length LR, amplifying corrosion susceptibility under cyclic exposure. The combined sequence of surface machining methods I, II, and III produced the highest profile length increase and the largest attacked surface fractions at long duration. Strong correlations between surface metrics and corrosion behavior were revealed. Corrosion classes increased proportionally with Rz, while the profile length ratio LR was more closely related to the relative attacked area. Once LR exceeded the profile length ratio threshold LR,c corrosion severity rose sharply, whereas LR values below a lower threshold consistently corresponded to low corrosion classes. These findings confirm that geometric enlargement of the surface and deformation-induced heterogeneity both amplify corrosion risk. Joining parameters and associated surface modifications also influenced coating performance. The joining parameter B lead to surface changes and introduced adhesion defects in the HAZ, leading to disbondment of the cathodic dip coating (CDC), particularly under Z coatings. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed needle-like Al6(Fe, Mn) intermetallics and Mg-rich eutectic phases in these regions, which likely impaired oxide layer formation and coating adhesion. Removing modified surface regions by machining method I mitigated this failure mode. Coating systems exhibited distinct performance hierarchies. The B CDC provided the most robust and long-term protection across all surface states, maintaining low corrosion classes even on surfaces machined with methods II and III while also showing minimal progression at scribes. The Z CDC performed well on smooth surfaces or regions machined with method I but was sensitive to the modified HAZ and heavy machining, with pronounced degradation under aggressive conditions. The T CDC offered excellent protection on planar surfaces but showed reduced stability at joining zones and heavily machined regions, particularly under severe cyclic exposure. Electrochemical investigations added additional insight into these observations, revealing how pretreatment and surface morphology influence interfacial stability and barrier integrity. Polarization resistance measurements confirmed that pretreated surfaces generally exhibit one to two orders of magnitude higher Rp compared to untreated references, demonstrating the effectiveness of conversion layer formation. The microcell measurements further resolved local differences between joining zone, HAZ, and surface, showing that Rp is consistently lowest at the joining zone and highest at the outer surface. However, hysteresis and breakdown potential analyses identified the HAZ as electrochemically most unstable, with wider hysteresis loops and more negative breakdown potentials, confirming its tendency for localized breakdown and slow repassivation. Furthermore, surfaces machined with methods II and III exhibit wider hysteresis compared to surfaces machined with method I or unmachined regions, mirroring their high corrosion classes observed in norm 1. The T pretreatment produced narrower hysteresis loops and higher charge transfer resistances on surfaces machined with method I, while Z pretreatments exhibited more positive breakdown potentials but delayed repassivation. EIS measurements on pretreated and coated samples revealed that most intact coatings could be modeled with a single Randles circuit, while degraded or roughened surfaces required additional time constants, indicating the formation of porous layers or defects filled with electrolyte. Under galvanostatic load, samples with higher roughness reached permanent voltage drops significantly faster and passed larger total charges, confirming faster barrier failure and electron transfer once electrolyte penetration occurred. These measurements confirmed that heterogeneity induced by machining accelerates pit initiation and compromises passive film stability, aligning with the trends observed in environmental cycling. From these findings, several practical recommendations emerge. Alloy selection should favor M2 over M1 for thermally joined components. Surface modifications created by joining parameter B should be avoided or their effects removed with method I before pretreatment and coating. Surface machining should prioritize method I only, preferably under optimized parameters, while methods II and III should be minimized. Quantitative surface limits should be defined, aiming for low Rz and LR values avoiding LR,c. For thermally joined assemblies, the B CDC offers the most reliable long-term protection, while T coatings may be suitable for planar geometries if edge coverage is improved. Pretreatment location had no measurable effect on roughness. The best-performing configuration identified in this study combines M2 alloy, optimized method I without II or III, and the B CDC, with careful removal of the edge caused by joining and adversely modified surfaces. This setup achieved the lowest corrosion classes across all test durations and severities. Overall, the integration of environmental and electrochemical data enables a comprehensive understanding of degradation mechanisms in thermally joined aluminum assemblies. The electrochemical techniques provided time resolved insight into interface kinetics and passive film evolution, bridging the gap between laboratory exposure and real-world performance. However, to fully validate the observed trends, the tests and measurements should be repeated to exclude potential influences from fluctuations in material paint shop quality, CDC bath chemistry, or variability in machining. This would increase statistical reliability and ensure that the observed differences are intrinsic to the material and process parameters rather than external variability. The work contributes to understanding degradation in joined 6xxx assemblies under cyclic exposure and clarifies the interplay between composition, microstructure, and coating performance. It introduces a dual-metric approach, Rz and LR, to define the ideal surface state prior to pretreatment and coating. These insights translate laboratory electrochemistry and environmental testing into possible manufacturing guidelines. Limitations include incomplete long-term data for T coatings, and variability in microcell measurements and roughness statistics. Future research should decouple geometric and metallurgical effects, refine pretreatment chemistry for challenges specific to the HAZ, and investigate the origin of adhesion failures associated with surface modifications caused by thermal joining in greater detail. In conclusion, corrosion durability in thermally joined aluminum components is determined primarily by HAZ integrity and surface preparation. Selecting M2, applying optimized method I, avoiding II and III, while using a robust CDC such as B can significantly enhance long-term corrosion resistance. These optimizations provide clear, implementable measures for improving the reliability of structural components.Description
Supervisor
Gasik, MichaelThesis advisor
Hofmann, Jan PhilippSchmidt, Christina