The
NanoSIMS has been used to study the oxidation mechanisms when commercial low tin ZIRLO™1 and Zircaloy 4 materials are exposed to corroding environments. Zr alloys were oxidized for various times (34 days, 80 days and 160 days) in an autoclave to simulate Pressurized Water nuclear Reactor conditions (pure water (+ Li, B) at 360°C and 18 Mpa). Isotopic tracers
18O and
2H were then added for an additional duration of 20 days to reveal active oxidation sites by NanoSIMS imaging of cross-sections near the surface.
Results:
Porosity mediated transitions between corrosion regimes occur at critical oxide thicknesses. Hydrides were present in all samples, even those with short oxidation times. Clear evidence has been shown for different characteristic distributions of 18O before and after the kinetic transitions, and this behaviour has been correlated with the development of porosity in the oxide which allows the corroding medium to penetrate locally to the metal/oxide interface.
Above images: Low tin ZIRLO - Hydride distribution using deuterium tracer. Colour merge images showing the relative locations of the 18O (red), 16O (blue) and 2H (green) signals. Hydrides (in green) are found below the metal/oxide interface but little deuterium was detected in the oxide layer.
(A) 34 + 20 days : a protective oxide (in blue) is globally present, blocking the penetration of 18O-water (in red),
(B) 80 + 20 days : the too thick, cracked, original oxide (in blue) is no more protective and has let 18O-water (in red) reach and oxidize the metal,
(C) 160 + 20 days : this new compact, protective layer (lower one in blue) forbids the oxidation by the 18O-water.
Three strong advantages of the NanoSIMS are illustrated by this study:
- The NanoSIMS enables navigating over and imaging larger sample areas compared to TEM or Atom Probe, thus giving a more representative overview of the sample.
- The NanoSIMS makes it possible to map light elements such as boron and deuterium (2H) even at low concentrations.
- The NanoSIMS permits to use stable isotopes (D, 13C, 18O, 15N,…) to follow chemical reactions in solids.
From: An investigation of the oxidation behaviour of zirconium alloys using isotopic tracers and high resolution SIMS. Sean S. Yardley, Katie L. Moore, Na Ni Jang Fei Wei, Stuart Lyon, Michael Preuss, Sergio Lozano-Perez, Chris R.M. Grovenor. Journal of Nuclear Materials 443 (2013) 436–443.