A Positron Annihilation Study of Corrosion of Aluminum and Aluminum Alloy by NaOH
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TRODUCTION
CORROSION of metals and alloys was studied for many decades because of its technological importance.[1] Most existing studies focus on aqueous corrosion, oxidation of metals, surface electrochemistry, and solution-phase transport processes. Defects in alloys play an important role in the process of corrosion. However, the effects of defect properties on corrosion in metals are not fully understood. Positron annihilation spectroscopy (PAS) was widely used as a probe for detecting defects on an atomic scale in solids, such as vacancies, vacancy clusters, dislocations, and nanometer-scale voids.[2] In aluminum and aluminum alloys, vacancies and other defects produced by heat treatments and quenching were characterized using positron techniques.[3,4] Positron techniques are nondestructive, and their sensitivity exceeds that of electron microscopy in many cases. The use of variableenergy positron annihilation spectroscopy (VEPAS) allows one to probe the defect profile in depth in solids, and is particularly suitable for analysis of surface and near-surface defects.[5] The foundation of VEPAS’ utility is its sensitivity to neutral or negatively charged openvolume point defects over a wide range of sizes (from missing single atoms to large clusters) and concentrations (from approximately one to a thousand defects in Y.C. WU, Professor, is with the School of Physics and Technology, Hubei Nuclear Solid Physics Key Laboratory, Wuhan University, Wuhan 430072, P.R. China. T. ZHAI, Associate Professor, is with the Chemical and Materials Engineering Department, University of Kentucky, Lexington, KY 40506-0046. Contact e-mail: tzhai@engr. uky.edu P.G. COLEMAN, Professor, is with the Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom. Manuscript submitted March 31, 2011. Article published online August 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
every ten million atoms) at depths from the surface to a few micrometers. A positron implanted into a sample to a mean depth related to their incident energy loses almost all its original energy in a negligible time, and diffuses for a further tenth of a billionth of a second before being annihilated by its antiparticle, the electron. The most common form of VEPAS is based on the measurement of the extent of Doppler broadening of the annihilation radiation (at mc2 = 511 keV) caused by the momentum of the electrons at the various annihilation sites; there is a characteristic electron momentum distribution, and hence Doppler broadening, associated with each site (e.g., bulk, surface, or defect), and the measured total broadening of the annihilation line is a linear combination of contributions from each possible site. In VEPAS, the Doppler broadening is represented by a singlenumber parameter, usually the sharpness S, being the central fraction of the annihilation line. In metal, for example, if a fraction of the positrons are trapped in missing-atom sites (called vacancies), then the average momentum of the electrons ‘‘seen’’ by the trapped positrons is
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