Effects of NaCl, pH, and Potential on the Static Creep Behavior of AA1100
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INTRODUCTION
THE effect of corrosive environments on the creep behavior of materials, i.e., corrosion creep, has been studied for years as a measurable demonstration of interactions between mechanical properties and environmental factors. This phenomenon was first discovered in the 1940s, when Cd single crystals stressed in sulfate and chloride solutions were found to display a softening effect.[1] Similar effects have also been observed in pure Cu,[2] Zn,[3] Mg,[4] stainless steel,[5] and many other different metals and alloy systems.[6] For Al and its alloys, which have been widely used in modern industry due to their light weight, high strength, and good corrosion resistance, it is also found by Kramer[3] that a continuous metal removal by electropolishing reduces the work hardening rate and accelerates the creep rate in excess of what would be expected from area changes alone, showing a clear environmental effect on their mechanical response to tensile stress. According to these observations, researchers have proposed various mechanisms for corrosion creep during further investigations.[7] While in some cases the accelerated creep rate is claimed to be attributed only to an increasing stress due to material loss,[8] it is more QUANHE WAN, Graduate Student in Materials Science, and DAVID J. QUESNEL, Professor of Mechanical Engineering and Materials Science, Mechanical Engineering Department, are with the University of Rochester, Rochester, NY 14642. Contact e-mail: [email protected] Manuscript submitted October 25, 2011. Article published online October 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
generally believed that dislocation-surface interactions, which are affected by corrosive environments, are the dominant factor that influences the creep behavior.[2,3,7] It has been suggested that such interactions reduce the surface energy and therefore accelerate the plastic flow of stressed metal.[2] Besides, the vacancies and divacancies induced by surface dissolution also interact with dislocations, promoting recovery via climb, and thereby modifying the plastic deformation characteristics.[7] In recent studies, the slow strain rate deformation expected near a crack tip under constant load has emerged as an important process occurring during stress corrosion cracking (SCC) inside metals.[5,9] Timedependent creep is the accepted cause of localized plastic deformation, which ruptures the stable oxide film protecting the inside of the crack.[9] Moreover, creep deformation may also assist the transport of hydrogen, which embrittles the crack tip metal.[10,11] Due to the difficulties people have encountered in the direct characterization of crack tip morphology and chemistry,[12] in this work, an approach based on examining the contributions of environmental variables to the corrosion creep processes is used to better understand the mechanisms of SCC that depend on slow strain rate processes taking place in the crack tip. By separating the complex geometry of the crack tip from the deformation processes, it shoul
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