03687 ON APPARENT BI-LINEAR CORROSION RATE BEHAVIOR OF TI GRADE 7 IN BASIC SATURATED WATER (BSW-12) BELOW AND ABOVE 80°C

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Product Number: 51300-03687-SG

ISBN: 03687 2003 CP

Author: Fred Hua, Gerald Gordon

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Ti Grade 7 (UNS R52400) is proposed as the corrosion resistant material for fabricating the waste package drip shield, for the potential nuclear waste repository at Yucca Mountain. Although Ti Grade 7 possesses excellent resistance to both crevice and general corrosion, it has been hypothesized that the corrosion mechanism of Ti Grade 7 in basic saturated water (BSW) could be different below and above a threshold temperature around 80°C. In this work, the bi-linear corrosion rate behavior of annealed and welded Ti Grade 7 in BSW-12 was studied based on weight loss, corrosion rate, activation energy, and macro- and micro-surface analysis in a temperature range from 70 to 90°C. When considering the bi-linear corrosion rate behavior around 80°C, it is hypothetical that the corrosion of Ti Grade 7 below ~ 80°C is an athermal process, while above this temperature the corrosion process is a thermally activated mechanism. The oxide film thickness, the Ti/O elemental atomic ratio in oxide layer, the corrosion rate and activation energy all seem to be consistent with the apparent post-test specimen color, suggesting a transition in corrosion mechanism around 80°C with the crystal structure of the oxide remaining unchanged. However, further investigations based on the changes in oxide film composition, crystal structure and thickness with exposure time and temperature will reduce remaining uncertainties associated with the bi-linear corrosion rate hypothesis. Keywords: Ti Grade 7,BSW, UNS R52400, Waste Container, BSW, Yucca Mountain Project, HLW, Oxide film thickness, Oxide Crystal structure, Passive dissolution, Activation Energy.