Mössbauer Investigations of the Chemical States of Tin and Iron Atoms in Zirconium Alloy Oxide Film

Abstract

The chemical states of tin and iron atoms were investigated by Mössbauer spectroscopy on oxide films (of various thicknesses) formed on a Zr-1.3% Sn-1% Fe-0.5% Cr-0.12% O alloy (enriched with 119Sn and 57Fe) in an autoclave at 633 K with 10 ppm Li and 650 ppm B. Specimens were analyzed by Conversion Electron Mössbauer Spectroscopy (T = 298 K) and γ-ray transmission (T = 298 and 77 K) modes to investigate the surface and the bulk of the oxide films, respectively. The main results on the oxide were as follows: • For tin atoms: Sn2+ and Sn4+were clearly identified. In addition, metallic β-Sn was greatly suspected. • For iron atoms: Zr(Fe,Cr)2 precipitates, α-Fe, Fe3O4, Fe2+, and Fe3+, were clearly detected. Moreover, it was shown that during the oxide film growth the chemical states of tin and iron atoms change, in particular after the kinetic transition.

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

“…In the oxide layer of zirconium alloys, some alloying elements (Fe, Cr, Ni and Nb) precipitated in secondphase particles are known to be oxidized slower than the surrounding matrix [8][9][10][11][12]. This phenomenon has been reported as the delayed oxidation and would be a key factor in both the corrosion and hydrogen absorption mechanism as mentioned in the previous section.…”

The influence of reactor irradiation on the oxidation state of tin in Zr-0.76Fe-1.6Sn

“…%) were prepared by electric arc melting, and were sectioned into test coupons 1×10×15±0.5 mm in size. Detailed methods of alloy and specimen preparation are described in [1,2]. Oxide films were grown during both autoclave corrosion and similar conditions with simultaneous neutron irradiation.…”

“…In particular, the oxidation states of minor alloying elements, such as tin and iron, can strongly determine the corrosion behavior of Zircaloys with and without irradiation. Such data can be obtained using nuclear-physical methods such as Mössbauer spectroscopy [1,2], which can determine the state of tin atoms in zirconium alloys and oxide films.…”

Hydrogen Pickup Mechanism in Zirconium Alloys

“…Because the niobium solubility limit in the metal is at least one order of magnitude higher than that of iron or chromium [37], the niobium effect would be more significant than that of iron and chromium in space-charge compensation. (Tin is not considered because its oxidation state is not well defined [38][39][40] and could be mainly 4þ, in which case it would not affect space charge. )…”