Uncovering the Growth Kinetics of Atmospheric Corrosion Pits in Aluminum Using X-Ray Computed Tomography

Abstract

Understanding how the local kinetics of atmospheric corrosion damage are controlled by environment and microstructure remains a central challenge in the field. Conventional approaches such as mass-change studies provide little insight into the growth kinetics of individual pits. To overcome the challenges of observing pit evolution in-situ, the current study employed laboratory-based X-ray computed microtomography (XCT) to directly observe pitting in-situ over months of exposure. This approach enabled direct observation of how the growth kinetics and morphology of individual pits evolved. Atmospheric corrosion damage in Al wires of various purities, including 99.99% Al and 1100 Al, exposed to chloride salts in humid air was studied. Initial studies suggested that most pits grow at a linear rate until pit repassivation. Following repassivation, no further growth was observed. Subsequent studies at higher temporal resolutions indicated that, in some cases, growth rate is non-linear. A corrosion-driven drying mechanism appears to lead to pit death in both cases. Pit growth occurred at significantly faster rates in samples containing second-phase, Iron-rich particles. Droplet size also played an important role in pit growth kinetics. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.