Examinations of the Corrosion Mechanism of Zirconium Alloys

Abstract

Several mechanism-related aspects of the corrosion of zirconium alloys have been investigated using different examination techniques. The microstructure of different types of oxide layers was analyzed by transmission electron microscopy (TEM). Uniform oxide mainly consists of m-ZrO2 and a smaller fraction of t-ZrO2 with columnar grains and some amount of equiaxed crystallites. Nodular oxides show a high open porosity and the grain shape tends to the equiaxed type. A fine network of pores along grain boundaries was found in oxides grown in water containing lithium. An enrichment of lithium within such oxides could be found by glow discharge optical spectroscopy (GD-OES) depth profiling. In all oxides, a compact, void-free oxide layer was observed at the metal/oxide interface. Compressive stresses within the oxide layer measured by an X-ray diffraction technique were significantly higher compared to previously published values. Electrical potential measurements on oxide scales showed the influence of the intermetallic precipitates on the potential drop across the oxide. In long-time corrosion tests of Zircaloy with varying temperatures, memory effects caused by the cyclic formation of barrier layers could be observed. It was concluded that the corrosion mechanism of zirconium-based alloys is a barrier-layer controlled process. The protective properties of this barrier layer determine the overall corrosion resistance of zirconium alloys.

The influence of alloying elements on the corrosion of Zr alloys

“…Various experimental studies have demonstrated an increased fraction of the non-equilbrium tetragonal phase close to the metal-oxide interface [3,4,5], where it is stabilised by a combination of i) small grain size, ii) compressive stress (from the Pilling-Bedworth ratio of 1.56 on transformation from α-Zr to ZrO 2 ) and iii) the inclusion of alloying elements as dopants incorporated into the growing oxide layer [6,7,8]. As corrosion progresses, the metal/oxide interface (the point at which the electrochemical reaction takes place [8,9]) moves away from the newly formed oxide.…”

Ab initio study of hydrogen diffusion in zirconium oxide

“…There has been considerable research about the oxide layer including its microstructure [1][2][3], crystallographic structure [4,5], stress distribution [6][7][8], and hydrogen diffusivity [9][10][11][12]. Transmission electron microscopy observations revealed that oxide layers near metal/oxide interfaces include tetragonal zirconium oxide [4,5], which is metastable at the cladding temperature and pressure.…”

“…Transmission electron microscopy observations revealed that oxide layers near metal/oxide interfaces include tetragonal zirconium oxide [4,5], which is metastable at the cladding temperature and pressure. This implies that the oxide layer is subjected to a high compressive stress.…”

Corrosion behavior of Zr−Nb alloys in 360°C water and 500°C supercritical water

“…However, the tetragonal structure can be stabilized in the corrosion process of the Zr alloys. The mechanism of the stabilization of the tetragonal phase in the oxide can be explained in several ways such as the high compressive stress in the oxide, the small grain size or the presence of the point defect [11,[17][18][19][20][21][22]. Godlewski et al obtained the distribution of the tetragonal phase fraction in the oxide by a Raman spectros- copy and clarified that the tetragonal fraction is usually higher in the interface region than in the surface region in the oxide [23].…”