Aluminum alloys are widely used in aerospace applications due to its high strength-to-weight ratio but is prone to corrosion, especially in chloride-containing environments and freeze-thaw conditions. Aircraft structures experience cyclic thermal and environmental stresses, such as temperature fluctuations during high-altitude flights and exposure to saline conditions in coastal and polar regions, accelerating degradation and increasing maintenance needs. While corrosion mechanisms at constant temperatures are well-studied, the effects of freeze-thaw exposure remain underexplored. This study aims to systematically investigate the microstructural evolution and degradation mechanisms of AA7075-T651 under freeze-thaw cycling, employing advanced characterization techniques to quantify material degradation and establish mechanistic insights.

 

Using three-dimensional (3D) X-ray computed tomography (XCT), a non-destructive, high-resolution imaging technique ideal for tracking the spatial evolution of corrosion damage over time, corrosion damage was monitored over 2000 freeze-thaw cycles in a 3.5 wt.% NaCl solution and correlated with electron microscopy. The results reveal that freeze-thaw cycling accelerates material degradation, transitioning from pitting to crack initiation and eventually spallation, compared to continuous immersion. In situ freeze-thaw experiments identified a dual degradation mechanism: volumetric ice expansion during freezing induces cyclic stresses that drive crack propagation, while thawing promotes intergranular corrosion, leading to structural integrity loss.

 

To further elucidate surface corrosion mechanisms, a novel XCT-based methodology was developed to track in situ real-time 3D surface roughness evolution. Additionally, comparing the mechanical behavior of hydrated and dehydrated corrosion products helps understand how environmental conditions affect the material properties and degradation processes. This study is the first to systematically investigate the in situ micromechanical behavior of corrosion products formed on AA7075-T651. Using in situ nanoindentation, microstructural characterization, and compositional analysis, the mechanical properties of corrosion layers were examined in both hydrated and dehydrated states over varying immersion times. The findings reveal significant differences in mechanical properties between hydrated and dried corrosion products, along with heterogeneities in corrosion product composition and evolution, which critically impact material performance.

 

These findings provide foundational insights into predicting material durability under cyclic environmental stresses and inform the development of optimized protective coatings and predictive models for aerospace and marine applications. The in situ methodologies introduced here represent a new standard in corrosion science, enabling precise real-time tracking of degradation processes and corrosion product evolution. Beyond AA7075-T651, these approaches are broadly applicable to other structural materials exposed to extreme environmental fluctuations, enhancing reliability assessments and material design strategies.