Graduation Year

2003

Document Type

Thesis

Degree

M.S.C.E.

Degree Granting Department

Civil Engineering

Major Professor

Sagues, Alberto A.

Keywords

passive behavior, corrosion, josephinite, nuclear waste

Abstract

Monson ABSTRACT The U.S. Department of Energy (DOE) has been engaged in a viability study for a potential underground geological repository in Yucca Mountain, Nevada. The repository is being designed for disposal of high level nuclear waste. A reference design for the repository has focused on the use of natural and manmade barriers to assure that radionucleide release will not be significant though an extended time period on the order of 10,000 years. The reference design utilizes manmade metallic components that are expected to last for this time period.

The specified metallic materials depend on a phenomenon known as metallic passivity to achieve their expected service lives. It is difficult to demonstrate this type of service life for these metallic materials as they have only been in commercial use for less than 100 years. There have been metal artifacts and metallic materials that have survived for long time periods, however, little is known about whether these artifacts have been exposed to conditions where they have been immune to corrosion, exhibiting passive behavior, or actively corroding at an extremely low rate. A demonstration of metallic passive behavior being maintained over many thousands of years would greatly increase confidence in the expectation that passive behavior could be maintained on the repository waste package

A demonstration of metallic passive behavior being maintained over many thousands of years would greatly increase confidence in the expectation that passive behavior could be maintained on the repository waste package materials. Long-lived metallic materials, such as iron, copper, nickel, and alloys based on these metals are materials that demonstrate passive behavior and have been identified in the literature as possible analogues, potentially useful to provide additional confidence in making projections of such long-term passive behavior.1, 4, 28, 45 This paper presents a study into some aspects of the corrosion behavior of Josephinite.

Josephinite is a naturally occurring assemblage of a metallic alloy of nickel and iron in conjunction with a host rock. The typical metallic composition is approximately 70% nickel and 30% iron. The material has been reported in association with geologic features with age into the millions of years. The study used corrosion measurement techniques to assess the behavior of the mineral immersed in aqueous solutions of various pH. Corrosion measurement techniques utilized included potentiodynamic polarization, open circuit corrosion potential, and electrochemical impedance spectroscopy.

Other techniques utilized in the study included visual and metallographic examinations with both optical and scanning electron microscopy. Test results from this study indicate that passive behavior characterizes Josephinite specimens immersed in naturally aerated buffered aqueous solutions in a range of pH from 6 to 9. This range has been reported for the geographic area where Josephinite materials are found in southwest Oregon. This suggests that passive behavior may be responsible for the material longevity as opposed to the material being immune or undergoing slow but active corrosion.

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