Coupled Tensile Stress Increases and Topography Evolution during Aluminum Corrosion
Authors: Capraz Omer , Ide Shinsuke , Shrotriya Pranav , Hebert Kurt 
Published: February 2020

Embrittlement of high-strength Al alloys resulting from water exposures thought to be caused by absorption of hydrogen produced by corrosion.1 Studies of Al dissolution processes indicate that corrosion itself produces large concentrations of subsurface hydrogen and defects, which may be relevant to degradation mechanisms. Hebert and coworkers detected extensive subsurface hydride formation during alkaline corrosion of Al, suggesting high hydrogen concentrations near the corroding surface.2 Studies of alkaline aluminum dissolution by electron microscopy and positron annihilation spectroscopy reveal large numbers of subsurface nanoscale voids which suggest condensation of near-surface vacancies produced during dissolution.3

Measurements of stress changes accompanying corrosion can detect defect formation, along with other microstructural changes relevant to degradation. Here we report in situ stress measurements during corrosion of 99.998% purity aluminum in alkaline solutions. The experiments utilized a new technique, phase-shifting curvature interferometry, the high curvature resolution of which enables measurement of corrosion-induced stress on bulk aluminum sheet and foils, and extension of the period of stress measurement to tens of minutes in order to capture the full evolution of stress transients during dissolution.4 Topographic changes were characterized quantitatively by Fourier transform analysis of scanning electron microscope (SEM) images, to identify length scales of the ridge-scallop surface pattern produced by dissolution.
Stress measurements during dissolution in NaOH solutions revealed large reproducible tensile force increases (in-plane stress integrated through thickness) that scale directly with the sample yield stress. This is apparently the first indication of near-surface plastic deformation directly induced by corrosion. The force increases to a steady state reached after around 10 min. Close agreement of force transients for as-received and electropolished sheet samples demonstrate that stress is not influenced by near-surface microstructure. Steady-state force levels are found to increase with NaOH concentration.
The force transients can be interpreted in terms of growth of a surface-adjacent plastic layer, within which the metal is at the yield stress. The thickness of this plastic layer can be approximated as the ratio of the measured force per sample width to the yield stress. Tensile force within the plastic layer is proposed to originate from the lattice contraction due to metal vacancies injected by dissolution.5 According to this mechanism, plastic layer growth should be governed by vacancy diffusion. Using a one-dimensional model for vacancy diffusion, closely comparable independent estimates of the vacancy diffusivity values are obtained from the measured steady-state force and time. These estimates are on the same magnitude as the vacancy diffusivity extrapolated from high-temperature measurements, 4 x 10-16 m2/s. Thus, the force measurements seem to be quantitatively consistent with the introduction of tensile stress by continuous vacancy formation at the dissolving surface. Vacancy injection can be rationalized by the high surface overpotential present during alkaline Al dissolution.
SEM images reveal the expected surface pattern of concave scallops surrounded by convex ridges. The characteristic scallop width increases to a steady-state during dissolution, following nearly the same trajectory as that of the measured tensile force. Indeed, a universal scaling factor of 120 MPa was found to relate force to scallop diameter, implying that the plastic layer thickness and mean scallop radius remain nearly the same during dissolution. Since nearly all the hydride particles and voids observed to form during dissolution are located at ridges,2,3 we propose that diffusing hydrogen and vacancies aggregate on ridges to produce immobile hydride particles and voids, that are then removed by metal dissolution. The scaling of force to scallop diameter implies that length scale for diffusion toward the sample bulk, governing the injection rate of hydrogen and vacancies, must remain comparable to the length scale for lateral diffusion to ridges, which controls the removal rate. The present results suggest strong enhancement of diffusion by vacancies near corroding surfaces, which may be relevant to crack propagation processes.
ACKNOWLEDGMENT
This work was supported by the National Science Foundation through NSF-CMMI-100748.
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