Certain elements, including noble metals, are identified to influence corrosion behavior of many metals in high-temperature water/steam and O2. We have previously reported effects of porous Pt coatings on the thermal oxidation of Zr, Fe, Ni, Cr, and GaAs in O2. Effects of Pt on oxidation of Zircaloy-2 in H2O have also been observed at temperatures near 400°C. An enhanced oxidation rate is observed in all of the studied systems upon a sufficiently high surface Pt-particle density. Even more interesting, low Pt-particle density in most cases leads to a decreased oxidation rate. In the case of Zr and Zircaloy-2 the beneficial effect of Pt is stronger when hydrogen is present in the metal substrate and in the oxide layer. From 18O/SIMS experiments it is concluded that, for a sufficiently high Pt-particle density on the surface, enhanced transport of dissociated oxygen, On− (n=0;2), towards the oxide/metal interface occurs in all of the studied systems. An oxygen spillover from Pt particles to the adjacent surface is a known phenomenon in catalysis and partly explains the results observed also in the current study of oxidation of Zr-based materials. Such a spillover involves a high surface diffusivity of dissociated oxygen, On−, which results in an increased gradient of On− across the oxide scale. In turn, the high oxygen gradient enhances the transport of oxygen towards the substrate/oxide-interface. This shows that not only diffusivity but also an effective activity of dissociated oxygen at the external oxide surface influences the oxidation rate. Naturally, the effects of Pt are maximized when small (nm-sized) and evenly distributed Pt particles are present on the surface. In our presentation a summary of obtained results of Pt additions on different materials will be given with suggested interpretations. Especially, the observed beneficial combination of Pt and hydrogen in the oxidation of Zr-based materials is discussed with a possible mechanistic explanation. Parallels are also drawn to the potential effect of Noble Metal Chemical Application (NMCA), used in a large fraction of the U.S. BWRs, on the corrosion resistance of Zr-based claddings.
Author Information
Anghel, Clara
Royal Institute of Technology DKV 51, Stockholm, SwedenStudsvik, Nyköping, Sweden
Hultquist, Gunnar
Royal Institute of Technology DKV 51, Stockholm, Sweden
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