Oxide Growth Mechanism on Zirconium Alloys

Abstract

In order to get a better understanding of the mechanisms governing corrosion of Zr-based alloys, several examinations have been performed on a variety of samples with uniform and nodular corrosion and different oxide layer thicknesses. The results point to a barrier layer concept. The oxide layer becomes porous at a critical thickness. Open porosity increases from 0.01% at 10 μm to 3% at 100 μm. Between the outer porous oxide and the metal, a dense interlayer exists. This is only ≤30 nm in nodular oxide but has been found to be several hundred nm in uniform post-transition oxide. The barrier layer is obviously influenced by the crystallization of the oxide at the interface. This crystallization leads either to a columnar monoclinic, an equiaxed tetragonal, or to a fine equiaxed monoclinic oxide. The latter, which probably forms only under the mineralizing effect of hydrogen, was found in nodular oxide. It easily cracks at the grain boundaries. Well developed columnar oxide is seen in uniform oxide, when corrosion resistance is high. Recrystallization seems to be responsible for the pore or microcrack formation at the transition. The intermetallic precipitates influence the corrosion behavior significantly. They probably oxidize slowly. This oxidation starts with the zirconium and is accompanied by iron diffusion into the surrounding oxide.

The measurement of stress and phase fraction distributions in pre and post-transition Zircaloy oxides using nano-beam synchrotron X-ray diffraction

“…These range from the accumulation of stresses giving rise to cracks in the oxide [12], the formation of defects such as microcracks and interlinking nano-pores [18,19], changes in the conductivity of the oxide layer [4], and transformation from the tetragonal to the monoclinic phase. This phase transformation has been associated with increasingly compressive stresses in the monoclinic phase at the point of transition [5], with cracking in the oxide [2], and with the linking-up of nano-porosity [15] thereby aiding the transport of the environment to the metal-oxide interface [1,15,20,21]. Pêcheur et al [22] associated the accelerated corrosion rate to the gradual transformation of the tetragonal phase to the monoclinic phase in the protective 'barrier layer'.…”

Oxidation kinetics of ZrNbO in steam: Differences between the pre- and post-transition stages

“…Considering the SEM micrographs of our samples (Fig.6), the oxide layer appears to be homogeneous (excepted a few cracks). Nevertheless, it has been found by impedance spectroscopy [10,27] that the oxide layer grown on ZrNbO could be divided into two sub-layers. For the kinetic modelling, two possibilities can be imagined, involving one or two oxide layers: -either one oxide layer with micropores and diffusion of adsorbed species via this porosity, -or two oxide layers: the microporous layer and a very thin and dense layer near the metal/oxide interface (for example, the native oxide layer), in which the diffusion of oxygen vacancies is very rapid.…”

Pressure effects on high temperature steam oxidation of zircaloy-4

“…The oxygen deficient zirconia near the metal interface seems to play an important role in maneuvering high compressive stress in oxide generated during oxidation [13]. Hart and Chaklader found that oxygen-deficient zirconia has superplasticity at a high temperature and that tetragonalmonoclinic inversion is not destructive in oxygen-deficient pure zirconia [14].…”

“…Therefore, the stability of the tetragonal phase is highly very dependent on the factors in the surrounding environment such as compressive stress, grain size and surface energy [13]. Oxygen vacancy is observed to be important in maintaining the stability of tetragonal zirconia.…”