Tensile stress and plastic deformation in aluminum induced by aqueous corrosion

Abstract

Measurement of near-surface stress generated by metallic corrosion can reveal defects and material property changes relevant to structural degradation by stress corrosion cracking. In this study, stress and topography evolution were characterized during alkaline corrosion of high-purity aluminum. In situ stress measurements revealed tensile increases of the force per width (stress integrated over depth) that scale directly with the sample yield stress. Spectral analysis of the uniform dimpled surface pattern produced by corrosion showed that transient changes of its characteristic wavelength track closely with the force per width. Together, the stress and topography measurements imply that corrosion creates a plastically deformed metal layer. Tensile stress is attributed to the lattice contraction associated with metal vacancies introduced during dissolution. In agreement with this hypothesis, a vacancy diffusion model successfully predicted the time dependence and magnitude of the force response, as well as the observed scaling relation between plastic layer thickness and pattern wavelength. The driving force for vacancy formation is thought to arise from high hydrogen and low aluminum chemical potentials near the corroding surface, the latter imposed by the high dissolution potential relative to the equilibrium potential of aluminum.