Revealing the growth kinetics of atmospheric corrosion pitting in aluminum via in situ microtomography

Abstract

Understanding the mechanistic relationship between the environment, microstructure, and local kinetics of atmospheric corrosion damage remains a central challenge. To address this challenge, this study used laboratory-based X-ray tomography to directly observe attack in-operando over an extended period, enabling insights into the evolving growth kinetics and morphology of individual pits over months of exposure. Damage progression associated with nine pits in a 99.9% pure aluminum wire exposed to chloride salts in humid air was characterized. Most pits grew at a nominally linear rate up until pit death, which occurred within 12–24 h of nucleation. Exceptions to this were observed, with three pits exhibiting bimodal growth kinetics and growing for 40 or more hours. This was explained by secondary droplets that formed near the pits, increasing the cathode area. A corrosion-driven drying mechanism likely contributed to pit death in both cases. Pits first grew into the material followed by lateral expansion.

The evolution of pit morphology and growth kinetics in aluminum during atmospheric corrosion

“…intermetallic particles [35][36][37] . In the absence of intermetallic particles, pits are known to preferentially attack grain boundaries, dislocation boundaries, and grains oriented with the {111} plane normal to the surface 19,38 . Regarding the latter, it is known that, in Al, the pitting potential of the {111} plane is relatively low compared to other orientations 19,38 .…”

“…Post-mortem sectioning using focused ion beam (FIB) and scanning electron microscopy (SEM) can thus augment in-situ XCT data, e.g. to assess relationships between corrosion damage and the microstructure at submicron length scales 19,20 .…”

“…Electrochemical polarization has been employed to provide direct relationships between electrochemical measurements and corrosion damage morphology 4,7,21 . Because corrosion under immersed and atmospheric conditions are often incomparable 22 , in-situ XCT has also been used to measure pit growth rates and the evolution of environmentally assisted cracking in humid, atmospheric, and chloride-containing environments 19,[22][23][24][25][26] .…”

“…The relationship between the morphology of this pit and the microstructure was not assessed, though the authors suggested that intergranular corrosion may have occurred. In commercial-purity aluminum, using a 7 h temporal resolution, it was determined that pits generally grew at constant rates up to repassivation except when droplet spreading occurred 19 . After droplet spreading, the rate of pit growth increased substantially, suggesting a strong tie between the electrolyte at the surface and the local kinetic response of the pit.…”

Beamline K11 DIAD: a new instrument for dual imaging and diffraction at Diamond Light Source

“…To fully understand the properties of many materials, there is a scientific need to understand the 3D microstructure, the phase composition and the stress distribution of the material. Examples include corrosion, where growth kinetics are determined by the shape of a corrosion pit as well as the presence of (also intermediate) corrosion products (Noell et al, 2020); the mechanical stability of bone, where fracture depends on the local bone microstructure, the presence of micro-cracks and the local stress distribution around these cracks (Ma et al, 2016); and the discharging of batteries, where fracture due to repeated stress and strain during the battery charging/discharging cycles leads to premature failure (Llewellyn et al, 2020). ISSN 1600-5775 Therefore, new tools are needed to provide simultaneous information about the 3D microstructure, the phases present and/or the local stress distribution.…”