General corrosion of alloy 22: Experimental determination of model parameters from electrochemical impedance spectroscop
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I. INTRODUCTION
THE proposed design of the high-level nuclear waste (HLNW) repository at Yucca Mountain, Nevada relies primarily on engineered barriers to prevent release of radioactive and toxic materials into the environment.[1] At the time of this writing, Alloy 22 (UNS N06022, 56Ni-22Cr-13.5Mo-3W-3Fe)[2] had been chosen as the corrosion resistant material (CRM) for the outer shell of the waste package (WP) container.[3,4] The present United States Department of Energy (DOE) WP design[5,6] has a 20-mm-thick Alloy 22 outer layer surrounding a 50-mm-thick type 316 nuclear grade stainless steel inner container. The 20-mm-thick Alloy 22 outer layer is intended to survive at least 10,000 years, eventually breaching to the type 316 stainless steel inner container by uniform corrosion. It is important to note that the WP design is based on the 10,000 year regulatory lifetime originally proposed for the repository; recent deliberations on the regulatory time suggest that this time may need to be increased by a factor of as much as 100. The current design is based on uniform corrosion rates; it considers the chance of stabilization of localized corrosion (pitting and crevice corrosion) for time periods L. GLEN McMILLION, Research Scientist, is with the Department of Metallurgical and Materials Science, University of Nevada, Reno, NV 89557. D.A. JONES, formerly Professor, Department of Metallurgical and Materials Science, University of Nevada, is deceased. ADAN SUN, Graduate Research Assistant, and DIGBY D. MacDONALD, Distinguished Professor and Materials Science and Engineering Director, are with the Center for Electrochemical Science and Technology, Pennsylvania State University, University Park, PA 16802. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
in excess of 10,000 years to be negligible under what is believed will be the prevailing environmental conditions of the proposed repository.[7] Alloy 22 owes its corrosion-resistant properties to the formation of a thin protective film, a few nanometers thick, called the “barrier oxide layer” of the passive film. On nickelbase alloys such as Alloy 22 that contain chromium as the primary alloying element responsible for corrosion resistance, the barrier layer is typically between 0.5- and 4-nm thick, depending upon the alloy and the environment.[8–12] This film greatly reduces the rate of uniform corrosion under a wide range of environmental chemical, electrochemical, and physical conditions. Alloy 22 includes molybdenum as an alloying element to extend corrosion resistance to reducing environments and to decrease the susceptibility to pitting corrosion.[13–18] However, under some conditions, localized corrosion still occurs as a result of destabilization of the passi
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