Quantifying Oxide Layer Growth at Low Pressures and Temperatures for Aluminum Alloy 6061

Aluminum alloy 6061 is commonly used as fuel cladding in nuclear research and test reactors. Over time, this cladding will oxidize and reduce heat transfer from the fuel to the reactor pool, which is a safety concern. Previous oxide growth predictions for these reactors are based on correlations developed from data taken at higher temperatures and pressures and may be overly conservative. This work assesses the validity of three existing oxide growth rate correlations at lower temperatures and pressures by measuring the oxide thickness formed over six months of exposure to water. In addition, we compare the corrosion rate of the as-received samples to the prefilmed samples that have a thin boehmite layer in order to assess the benefit of this surface modification. Samples were analyzed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction to characterize the oxide thickness, chemistry, and crystal structure, respectively. Open-circuit potential monitoring, electrochemical impedance spectroscopy, linear polarization resistance, and cyclic voltammetry were used to study corrosion resistance. Results show that the boehmite on the prefilmed samples is a stable, protective phase under the conditions tested. The non-prefilmed samples produced a partially crystalline unprotective oxide, with a linear rate of oxide growth. The prefilming process is deemed necessary for fuel cladding applications, and two of the three correlations considered herein conservatively predicted the oxide thickness of the prefilmed samples.