<b>Multiscale Characterization of Corrosion and Freeze-Thaw Induced Damage Mechanisms in Aluminum Alloys</b>
Creators: name: Kumar, Ankit
Publisher: name: Purdue University Graduate School
Year: 2025
Subjects: classificationCode: 401607
subject: Metals and alloy materials

description: Aluminum alloys are widely used in aerospace and marine structures due to their excellent strength-to-weight ratio. However, their susceptibility to corrosion in chloride-rich environments and their vulnerability to harsh temperature fluctuations present significant challenges to long-term structural integrity. Aircraft components are exposed to cyclic thermal and environmental stresses, such as temperature variations at high altitudes and saline conditions in coastal or polar regions, which lead to freeze-thaw cycles that accelerate material degradation and increase maintenance demands. While corrosion behavior at ambient conditions is well understood, the impact of corrosive freeze-thaw environments remains insufficiently explored.This research systematically investigates the degradation mechanisms and microstructural evolution of AA7075-T651 under cyclic freeze-thaw exposure in a chloride-containing environment. Advanced, multi-modal characterization techniques are employed to monitor damage progression and gain mechanistic insights. Using time-resolved X-ray computed tomography (XCT), a non-destructive and high-resolution imaging technique, corrosion damage evolution was observed over 2000 freeze-thaw cycles in a 3.5 wt.% NaCl solution. These observations were further correlated with electron microscopy to provide a detailed view of the evolving damage morphology and crystallographic effects. The results demonstrate that freeze-thaw cycling significantly accelerates degradation, evolving from localized pitting around intermetallic phases to crack initiation, propagation, and eventual material spallation, an effect significantly more severe than continuous immersion at room temperature. Further, <i>in situ</i> corrosive freeze-thaw experiments revealed a dual degradation mechanism: ice expansion during freezing induces mechanical stresses that drive crack growth, while thawing promotes intergranular corrosion, compromising structural integrity. To further investigate surface-level damage, a novel XCT-based approach was developed to track the real-time evolution of 3D surface roughness, enabling detailed roughness characterization of the corrosion layer interfaces. Additionally, the mechanical behavior of corrosion products layers was compared in both hydrated and dehydrated states. This was achieved using <i>in situ</i> nanoindentation, microstructural imaging, and compositional analysis across varying immersion durations. The study reveals that hydration state significantly influences mechanical properties, with distinct variations in Young’s modulus and hardness, as well as compositional heterogeneities that impact the overall degradation process.Overall, this work provides critical insights into the behavior of AA7075-T651 under complex environmental loading. The methodologies developed here offer a new standard for real-time corrosion monitoring and can be extended to other structural materials. These findings have important implications for predicting long-term durability and informing the design of protective coatings and maintenance strategies in aerospace and marine applications.