Oxide Growth Mechanism on Zirconium Alloys

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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.