Effect of Alloying Elements in Zircaloy on Photo-Electrochemical Characteristics of Zirconium Oxide Films

Abstract

Semiconducting characteristics of oxide film on pure Zr, Zr-Sn binary alloy, and Zr-Sn-X (X: Fe, Ni, or Cr) ternary alloy were evaluated by a photo-electrochemical method to study the effects of alloying elements on the oxidation mechanism of Zr alloy in a boiling water reactor (BWR) environment. Oxide films of the alloys showed the characteristics of an n-type semiconductor. Maximum photocurrent (Imax) was generated by an illumination of monochromatic light with a photon energy of 5 to 6 eV (i.e., the band gap energy of the Zr oxide semiconductor was 5 to 6 eV). Higher corrosion-resistant alloys showed a lower value of Imax. Maximum photocurrent decreased by an addition of Fe, Ni, or Cr, notably Fe or Ni, to Zr-Sn alloy. By repetition of cold-working and α-annealing subsequent to β-quenching, Imax increased. Such changes of Imax caused by changes in alloy chemical composition and material processing, were explained by the valence theory of oxide semiconductors (i.e., the decrease of Imax was considered to result from the increase of holes in the oxides due to the substitution of divalent cations (Ni2+) and trivalent cations (Fe3+, Cr3+ ) at Zr4+ cation sites). From these results, the effects of alloying elements on the oxygen vacancy structure of the surface oxide were believed to play a major role in controlling the corrosion resistance of zirconium alloys.

Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding

“…Instead, the oxidizing species have to diffuse through the oxide layer. Observation of the zirconium oxide system at normal operating temperatures has confirmed that zirconium oxide is an n-type semiconductor (39,40) and that the oxide growth occurs by oxygen anion migration through the oxide film, with the formation of new oxide taking place at the metal/oxide interface (41, 42).…”

Photoelectrochemical study of hydrogen in Zircalloy-2 oxide films

“…Linear relationships can be seen in each sample. The slope corresponds to the electrical conductivity of the oxide [4]. The slope of the hydrogen containing Zry-2 oxide film is higher than that of the hydrogen-free Zry-2 oxide films, whose thickness of the oxide films is almost the same.…”

Influence of protecting gel film on oxidation of zirconium alloys

“…10.013 concluded that electron conduction dominates the electrical conductivity of Zircaloy oxide films. It is well known [4][5][6] that ZrO 2 is predominantly an electronic high-resistivity semiconductor with a low amount of ionic conduction. The band gap is approximately 5 eV, the work function 4.0 eV and the relative permittivity 22.…”