Tensile Stress Induced by Aluminum Corrosion

Stress corrosion cracking (SCC) is a critical problem affecting the safety and viability of both existing energy conversion systems and ones under consideration for future development. In SCC, chemical interactions of a metal with the environment during corrosion accelerate degradation of materials under tensile applied stress, by reducing the critical stress intensity for crack propagation. Many competing mechanisms for the effect of corrosion in SCC have been put forth, including formation of brittle oxide or hydride phases, stress concentration at corrosion pits, and absorption of hydrogen. An additional mechanism is based on observed generation of tensile stress during corrosion of SCC-susceptible alloys (1,2). Corrosion-induced tensile stress would combine with externally applied stress to assist crack initiation and growth. Tensile stress may result, for example, from the lattice contraction due to vacancies produced by corrosion. This effect has been examined in the alkaline dissolution of Al, where lattice contraction is observed accompanied by extensive H absorption (3). The contraction was attributed to vacancies stabilized by association with hydrogen. In the same system, corrosion produces large concentrations of subsurface nanoscale voids, also revealing the presence of near-surface tensile stress (4)

Modeling Stress Distributions in Anodic Alumina Films Prior to the Onset of Pore Formation

Porous anodic oxide (PAO) films are produced when reactive metals such as Al and Ti are electrochemically oxidized in baths that dissolve the oxide. Research in PAObased devices has been stimulated by the self-organized hexagonally ordered pore arrays found for some anodizing conditions. The initiation and ordering of pores follows a morphological instability of the initially planar barrier oxide, upon reaching a critical oxide thickness

Morphological Instability Leading to the Formation of Self-Ordered Porous Anodic Oxide Films

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, Zr, Nb, Hf, and W in baths that dissolve the oxide. Procedures to grow films with highly ordered arrangements of nanoscale pores have led to the extensive use of PAO films as templates for nanostructured devices. The porous film geometry may be controlled precisely via the film formation voltage and bath composition (1). Recently, tracer studies and modeling showed that transport in the amorphous oxide involves both electrical migration and plastic flow (2,3). The oxide seems to behave as an incompressible material during steady-state growth of the porous film. Linear stability analysis incorporating the assumption of incompressibility predicted important features of PAO (4). These include the constant ratio of interpore distance to anodizing voltage on Al for any electrolyte composition; narrow ranges of oxidation efficiency (the fraction of oxidized metal atoms that remain in the oxide) producing ordered PAO films on Al and Ti; and the inability to produced ordered films composed of divalent metal oxides. However, the analysis did not predict the observed onset of instability at a critical oxide thickness, and the observed dependence of the interpore distance on the electrolyte composition

In Situ Stress Measurement During Aluminum Anodizing Using Phase-Shifting Curvature Interferometry

Stress measurements yield insight into technologically relevant deformation and failure mechanisms in electrodeposition, battery reactions, corrosion and anodic oxidation. Aluminum anodizing experiments were performed to demonstrate the effectiveness of phase-shifting curvature interferometry as a new technique for high-resolution in situ stress measurement. This method uses interferometry to monitor surface curvature changes, from which stress evolution is inferred. Phase-shifting of the reflected beams enhanced measurement sensitivity, and the separation of the optical path from the electrochemical cell in the present system provided increased stability. Curvature changes as small as 10−3 km−1 were detected, at least comparable to the resolution of state-of-the-art multiple beam deflectometry. It was demonstrated that small curvature change rates of 10−3 km−1s−1 could be reliably measured, indicating that the technique can be applied to bulk samples. The dependence of the stress change during anodizing on current density (tensile at low current density, but increasingly compressive at higher current densities) was quantitatively consistent with earlier multiple-beam deflectometry measurements. The close similarity between the results of these different high-resolution measurements helps to resolve conflicting reports of anodizing-induced stress changes found in the literature

The role of stress in self-ordered porous anodic oxide formation and corrosion of aluminum

The phenomenon of plastic flow induced by electrochemical reactions near room temperature is significant in porous anodic oxide (PAO) films, charging of lithium batteries and stress-corrosion cracking (SCC). As this phenomenon is poorly understood, fundamental insight into flow from our work may provide useful information for these problems. In-situ monitoring of the stress state allows direct correlation between stress and the current or potential, thus providing fundamental insight into technologically important deformation and failure mechanisms induced by electrochemical reactions. A phase-shifting curvature interferometry was designed to investigate the stress generation mechanisms on different systems. Resolution of our curvature interferometry was found to be ten times more powerful than that obtained by state-of-art multiple deflectometry technique and the curvature interferometry helps to resolve the conflicting reports in the literature. During this work, formation of surface patterns during both aqueous corrosion of aluminum and formation of PAO films were investigated. Interestingly, for both cases, stress induced plastic flow controls the formation of surface patterns. Pore formation mechanisms during anodizing of the porous aluminum oxide films was investigated . PAO films are formed by the electrochemical oxidation of metals such as aluminum and titanium in a solution where oxide is moderately soluble. They have been used extensively to design numerous devices for optical, catalytic, and biological and energy related applications, due to their vertically aligned-geometry, high-specific surface area and tunable geometry by adjusting process variables. These structures have developed empirically, in the absence of understanding the process mechanism. Previous experimental studies of anodizing-induced stress have extensively focused on the measurement of average stress, however the measurement of stress evolution during anodizing does not provide sufficient information to understand the potential stress mechanisms. We developed a new method, which enables us to discriminate the potential stress mechanisms during anodizing and characterize the evolution of the stress profile during film growth. Using stress measurement and characterization techniques, we demonstrated the evolution of the stress profile during the film formation and discussed the role of stress on the PAO film formation. Compressive stress builds up linearly during the anodizing, while barrier oxide film gets thicker until the onset of the pore initiation. Both barrier layer thickness and the integrated oxide stress decreased rapidly to the steady-state period when pore initiation began. The morphology change and stress transients points out the transition from elastic to plastic oxide behavior, similar to those observed in other situations such as lithium intercalation into silicon. The stress profile is consistent with the stress gradient needed to drive plastic flow observed experimentally. We also addressed the dependence of overall stress generation on applied current density. Apparently, stress caused by expansion or contraction of oxide and metal interface depends on the volume change due to overall reactions. In the last chapter, the stress generation during alkaline Al corrosion will be discussed. The enhancement of mechanical degradation by corrosion is the basis for the damage process such as stress-corrosion cracking. Understanding the synergistic effect of stress on stress-corrosion cracking mechanism is necessary to design new materials to improve the safety and viability of existing energy conversion systems. the high-resolution in-situ stress measurements during Al corrosion in alkaline solution was presented, supported by characterization techniques and Fast Fourier Transform analysis. Unprecedented curvature resolution of curvature interferometry permits the monitoring of stress during extended periods of corrosion of thick metal samples. Evolution of concaved-shaped surface patterns is in a great harmony with recorded tensile stress. Furthermore, absolute value of tensile stress onset of the plasticity depends on the dissolution rate of metal and yield stress of metal. The measurements reveal corrosion-induced tensile stress generation, leading to surface plasticity. This finding is evidence that corrosion can directly bring about plasticity, and may be relevant to mechanism of corrosion-induced degradation

Stress evolution of anodic alumina films prior to the pore formation

Porous anodic oxide (PAO) films are grown by electrochemical oxidation of metals in a solution that dissolve the oxide. During the film formation, voltage increases linearly until the peak, then declines to steady-state value. PAO finds extensive usage as templates and substrates in many applications, such as solar cells and optical devices. However, the mechanism underlying the observed initiation and evolution of self-organized PAO structure is not understood. Recent studies on pore formation points out the role of plastic flow on pore initiation process [1–3]. In this study, we characterized stress profiles and its evolution during the film formation prior to the pore initiation. Phase-shifting curvature interferometry was used to monitor sample curvature change during film growth and subsequent dissolution. Oxide films were grown at constant current densities in 0.4 M H3PO4 until different thicknesses and subsequently current was turned off to dissolve grown oxide film. The stress profile of oxide film was revealed by in-situ monitoring the curvature change during dissolution of oxide film period after anodizing [4] . In addition, morphological evolution of oxide film during film growth was characterized using SEM. Oxide films growth until different thicknesses values up to onset of pore initiation instability. The measured stress change during film growth was in excellent agreement with prior measurements [5]. Measured stress profiles showed that for thin films \u3c20 nm, compressive stress was evenly dispersed through the thickness. However, for thicker films, the stress is concentrated within 20-nm thick layer near the solution interface. Transition in the stress profile coincides with oxide film thickness associated with initial roughening instability at the solution interface [6]. SEM images also showed that the first instability initiated at an oxide thickness of 20 nm with stable surface roughness pattern with a length scale of 20 nm. After the initial instability, the stress level near the solution interface became increasingly compressive as oxide film thickens. This behavior continued until the moment of self-ordered pore initiation when the both oxide thickness and the integrated oxide stress rapidly decreased to steady-state values. Morphological change during anodizing coincides with the stress transient, which could be attributed to the relaxation of elastic stress due to onset of plastic flow. Thus, plastic yielding in the oxide may induce a second instability mechanism involving pore initiation, leading to final self-ordered pore pattern. ACKNOWLEDGMENTS Support was provided by the National Science Foundation (CMMI-100748). REFERENCES [1] Garcia-Vergara, S.J., et al. Electrochim. Acta. 2006, 52, 681. [2] Houser, J.E., Hebert, K.R. Nature Mater. 2009, 8, 415. [3] Oh, J., Thompson, C.V. Electrochim. Acta. 2011, 56, 4044. [4] Çapraz, Ö.Ö., Shotriya, P., Hebert, K.R. J. Electrochem. Soc. 2014, 161, D256. [5] Çapraz, Ö.Ö., Hebert, K.R., Shotriya, P. J. Electrochem. Soc. 2013, 160, D501. [6] Hebert, K.R., et al. Nature Mater. 2012, 11, 162