Temperature and Time-Dependent Ion Diffusion Across Stainless Steel-Zirconium Film Interface Studied by Scanning Surface
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Temperature and Time-Dependent Ion Diffusion Across Stainless Steel-Zirconium Film Interface Studied by Scanning Surface Potential. M.E. Hawley, K.J. Hollis, and P.O. Dickerson Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ABSTRACT Zirconium is of interest in the development of high-density diffusion barriers in nuclear reactor applications because of its low thermal neutron absorption cross section and good thermal and mechanical properties. It is used in these applications to protect the low enriched fuel from interacting directly with the cladding material to prevent swelling and cracking of the fuel. In this work, we investigated the use of plasma spraying as a method to produce high quality, dense Zr barrier coatings over large surface areas. A thin sheet of 21Cr-6Ni-9Mn stainless steel (SS) was used as the substrate material. Both sides of the SS sheet were coated, one side at a time. A transfer arc (TA) current between a torch and the substrate was used to vary the substrate temperature to explore the effect of temperature and time on the film grain size, interface quality, and film porosity. The films were characterized using light optical microscopy (LOM), scanning probe microscopy (SPM) and Kelvin probe force microscopy (KPFM) and EDS-SEM Although the film quality did improved with temperature, at the elevated substrate temperatures used in this study, metal atoms from the substrate diffused into and, at the highest temperature and longest time, through the Zr coatings. INTRODUCTION Because of its low thermal neutron absorption cross-section and good thermal and mechanical properties, zirconium is of interest in the development of high-density diffusion barriers for high performance reactor fuels. The barrier layer serves to protect the low enriched fuel from interacting directly with a fuel cladding material such as 6061 aluminum or stainless steel (SS) to help prevent swelling and cracking of the fuel. Another potential use is for coatings of reactor vessels and piping due to it high resistance to corrosive environments such as in the presence of alkali coolants. The goal of this work was to investigate the use of plasma spraying as a method to produce high quality, dense Zr coatings over large areas [1,2]. In this deposition process, a square wave, reverse polarity transfer arc (TA) is applied between a torch and the substrate surface to continuously clean the surface and heat the substrate before, during, and after deposition of the film. Very limited data is presently available on the properties of plasma sprayed Zr deposited on cladding materials [3,4]. In the present study we explored the effect of temperature and time on the microstructure and properties of zirconium films grown by thermal plasma spraying grown on thin 21Cr-6Ni-9Mn stainless steel foil substrates. By examining the samples in cross-section and applying a combination of characterization methods, one can correlate variations in chemical or phase properties with film grain size,
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