Properties of a Multilayer Coating for Applications in High Level Waste Packaging

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Title: Properties of a Multilayer Coating for Applications in High Level Waste Packaging
Author: Scheffing, Candice Camille
Advisors: Jagannadham Kasichainula, Committee Member
Man-Sung Yim, Committee Co-Chair
Mohamed Bourham, Committee Co-Chair
Kevin Gross, Committee Member
Abstract: Materials features that are being considered for the Yucca Mountain high level waste repository include corrosion, hydrogen, and radiation effects as well as structural strength. The current plan for protection of the environment from high level waste placed inside Yucca Mountain includes a defense-in-depth design with multiple engineering barriers. The outer engineered barrier is a large drip shield made of titanium grade-7. Titanium was chosen for its high corrosion resistance and structural strength. This titanium drip shield is an elaborate design that will be expensive and may be susceptible to hydrogen embrittlement or hydrogen-induced cracking. An alternative, multi-layer coating design is proposed that will provide corrosion resistance and act as a barrier to hydrogen diffusion. The coating proposed is composed of the hydrogen barrier titanium nitride (TiN), corrosion resistant zirconium oxide (ZrO₂), and wear resistant diamond-like carbide (DLC). TiN and ZrO₂ coatings were deposited on stainless steel substrates using magnetron sputtering and laser ablation. Analysis of the corrosion resistance of TiN and the multilayer coating, TiN + ZrO₂, has been performed at Lawrence Livermore National Laboratory (LLNL) using simulated waters, representative of the Yucca Mountain environment. The hydrogen barrier properties of TiN have also been analyzed using low temperature resistance measurements and secondary ion mass spectrometry (SIMS) analysis. Using cyclic polarization testing, TiN was found to be resistant to SCW and BSW, Yucca Mountain simulated waters, with a passive region of 760 ± 342 mV in SCW and 408 ± 67 mV in BSW. The added ZrO₂ layer increased the passive region to 822 ± 108 mV in SCW, and increased the passive region in BSW to 1002 ± 260 mV. The ZrO₂ did not significantly increase the passive region in SCW but dramatically increased the passive region in BSW, considered to be the worst-case scenario for Yucca Mountain. Further testing in SCW using multiple coatings of TiN increased the passive region to 1050 ¬± 31.1 mV. Long term corrosion tests were also performed on TiN coated 316L weight-loss samples. After exposure for 6 months in corrosion tanks, the maximum corrosion rate observed was 0.530 μm y⁻¹. This level was observed in the 90 °C aqueous SCW environment, and the coating had been completely stripped away while immersed in the tank. Hydrogen diffusion testing was done on TiN coated stainless steel samples by exposing coated and non-coated samples to hydrogen at an elevated temperature for 3 hours. SIMS analysis indicated that for the TiN-coated samples there was no increase in hydrogen for the exposed sample; rather the hydrogen content of the substrate was lower than the non-exposed sample. The non-coated samples tested in SIMS did not show a difference in hydrogen content between the hydrogen exposed and non-exposed samples. More tests need to be done to confirm claims that TiN is a good diffusion barrier to hydrogen. The multilayer TiN + ZrO₂ coating provides good resistance against corrosion as shown by the large cyclic polarization passive regions. It does not, however, provide comparable corrosion resistance to titanium, indicating it is not as protective as titanium. The hydrogen barrier property of TiN is an additional property that pure titanium does not have and may be useful for Yucca Mountain. Additional corrosion tests can be done on the ZrO₂ coatings to find a more accurate corrosion passive region for the various simulated Yucca Mountain water. The feasibility of this multi-layer coating will also be an important next step to determine in what ways the coating can be a better choice than titanium as an alternative to the drip shield design at Yucca Mountain. *This research was performed under appointment of the Office of Civilian Radioactive Waste Management Fellowship Program administered by Oak Ridge Institute for Science and Education under a contract between the U.S. Department of Energy and the Oak Ridge Associated Universities.
Date: 2005-04-27
Degree: M
Discipline: Nuclear Engineering

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