K. Sieradzki

Stress-corrosion cracking

Abstract We review stress-corrosion cracking (SCC) with an emphasis on recent developments in this field regarding the transgranular form of cracking. Evidence is presented indicating that transgranular SCC occurs via a series of discontinuous microcleavage events. Each event is typically 1 μm in extent and is triggered by a thin film of several hundred nm in thickness which has formed around the crack tip owing to an anodic process. This mechanism of SCC is called film-induced cleavage and we discuss its operation for many metals including stainless steels, Cu-Al alloys, α-brasses, and pure copper. Analytical calculations are presented which support the concept of film-induced cleavage. The compositional dependence of SCC in Cu-Al and Cu-Zn alloys is shown to correlate exactly with the compositional dependence of de-alloying the less noble metal element from these alloy systems. The selective dissolution process is discussed within the general framework of percolation theory.

Electrochemical Stability of Nanometer-Scale Pt Particles in Acidic Environments

Understanding and controlling the electrochemical stability or corrosion behavior of nanometer-scale solids is vitally important in a variety of applications such as nanoscale electronics, sensing, and catalysis. For many applications, the increased surface to volume ratio achieved by particle size reduction leads to lower materials cost and higher efficiency, but there are questions as to whether the intrinsic stability of materials also decreases with particle size. An important example of this relates to the stability of Pt catalysts in, for example, proton exchange fuel cells. In this Article, we use electrochemical scanning tunneling microscopy to, for the first time, directly examine the stability of individual Pt nanoparticles as a function of applied potential. We combine this experimental study with ab initio computations to determine the stability, passivation, and dissolution behavior of Pt as a function of particle size and potential. Both approaches clearly show that smaller Pt particles dissolve well below the bulk dissolution potential and through a different mechanism. Pt dissolution from a nanoparticle occurs by direct electro-oxidation of Pt to soluble Pt(2+) cations, unlike bulk Pt, which dissolves from the oxide. These results have important implications for understanding the stability of Pt and Pt alloy catalysts in fuel cell architectures, and for the stability of nanoparticles in general.

The Dealloying Critical Potential

This paper reexamines the concept of the dealloying critical potential by considering the critical potential to be a kinetically controlled morphological transition dependent not only on extensive system parameters such as alloy and electrolyte composition, but also on the rate of application of an intrinsic parameter such as the potential ramp rate. In the limit of a quasi-static potential sweep rate, an expression for the critical potential is derived which considers both compositional and morphological fluctuations on the alloy surface at the incipient point of either stability (passivation) or instability (bulk dealloying). In addition, we present detailed critical potential results for the entire range of Ag-Au alloy compositions in x M AgClO 4 + 1 M HClO 4 and x M AgNO 3 + 1 M HNO 3 (x = 10 -4 , 10 -3 , 10 -2 10 -1 , and 1). These results are shown to be consistent with the expression for the critical potential. Finally, we present and discuss ancillary experiments examining the effect of potential sweep rate on the determination of the critical potential.

Pattern formation during dealloying

Dealloying is a process during which an element is selectively dissolved from an alloy. If the starting material is single phase and within certain compositional limits dealloying results in the formation of bicontinuous structures on nanometer length scales. We introduce the topic of dealloying and discuss its phenomenology. Various models of dealloying and porous metal formation are presented. � 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Dealloying and corrosion of Al alloy 2024-t3

We present results of a comprehensive electrochemical study aimed at determining the factors that control the corrosion evolution of the important structural alloy Al 2024-T3. An important aspect of corrosion in this alloy is copper redistribution. The redistributed copper serves to enhance the kinetics of the corrosion process and presents some major difficulties with respect to development of new families of conversion coatings. Two major sources of mobile copper are discussed, the Al 2 CuMg S-phase and copper in solid solution. In each case, a dealloying mechanism is responsible for liberating copper that becomes available to redistribution by solid state and/or liquid phase transport mechanisms. This work is concerned with identifying the relative importance of these sources of copper and the active transport mechanisms. In order to decouple the sources of copper, we designed synthetic 2024 alloys that emulate the oxygen reduction kinetics of the real alloy. Our results indicate that. in principal, both sources of copper are operative during redistribution and, for the particular alloy that we examined, both contribute about equally to the amount of redistributed copper.

Computer simulations of corrosion: Selective dissolution of binary alloys

Abstract In this paper we present a new framework, based upon percolation theory, within which various aspects of corrosion in alloy systems may be understood. We have developed a new model of sele...

Curvature Effects in Alloy Dissolution

An elemental metal can dissolve at kink sites at low over potentials producing no new interfacial area. When an ideal solid solution alloy undergoes selective dissolution, this situation is not possible owing to atomic-scale disorder. Dissolution of the less noble constituent can proceed only by injection of regions of negative curvature into the solid surface, which increases the interfacial area. The authors present a thermodynamic analysis which accounts for these capillary effects in alloy dissolution. The phenomenon of the critical potential for macroscopic selective dissolution is analyzed in terms of a kinetic roughening transition. This transition results from a competition between curvature-dependent dissolution and surface diffusion. An expression for the critical potential as a function of alloy composition is developed. The dealloying threshold corresponds to a critical composition on the line of critical potentials defining the roughening transition.

Formation of nanoporous noble metal thin films by electrochemical dealloying of PtxSi1−x

We demonstrate the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying. Amorphous PtxSi1−x films (∼100–250nm thick) are formed by electron beam codeposition and dealloyed in aqueous HF solutions at an electrochemical potential sufficient to selectively remove Si while allowing self-assembly of Pt into a nanoporous structure. The Pt nanoporous layers have a pore size of 5–20nm, ligament thickness ∼5nm, a surface area enhancements >20 times, and an ultrafine grain polycrystalline microstructure.

Simple model for interface stresses with application to misfit dislocation generation in epitaxial thin films

A simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid–solid interfaces. An analysis of the thermodynamics of thin film epitaxy is presented that incorporates the effects of free surface and interface stresses, and an expression for the critical thickness for thin film epitaxy is obtained. Based on this analysis, the concept of effective pressures exerted by the thin film free surface and film–substrate interface is introduced. If it is assumed that misfit dislocations are generated at the film–substrate interface as a result of glide of threading dislocations, the thermodynamics and kinetics of stress relaxation can be discussed in terms of a balance of Peach–Koehler forces acting on the threading dislocations owing to the surface and interface pressures as well as to the coherency stress. An example is given that shows that, if the film has a relatively large surfac...

In Situ Scanning Tunneling Microscopy of Corrosion of Silver-Gold Alloys

An in situ scanning tunneling microscope (STM) was used to observe the morphological changes accompanying the selective dissolution of Ag from low-Ag content Ag-Au alloys in dilute perchloric acid. This study was undertaken to explore the role of surface diffusion in alloy corrosion processes. These results are interpreted within the framework of the kink-ledge-terrace model of a crystal surface and a recent model of alloy corrosion based on a variant of percolation theory. The corrosion process leads to roughening of the surface by dissolution of Ag atoms from terrace sites. Annealing or smoothening of the surface occurs by vacancy migration through clusters and the subsequent annihilation of clusters at terrace ledges.