Heterogeneous Scale Growth During Steam Corrosion of Zircaloy-4 and 500°C

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Abstract
Various Zircaloy-4 materials have been exposed in steam at 500°C for times between 2 and 72 h. The observations indicate that two major factors govern scale growth: crystal orientation and precipitate density. Furthermore, the study of coarse grained materials shows that when two adjacent grains corrode at very different rates, the catastrophic oxidation of one of them has little tendency to propagate into its more highly resistant neighbor. This “barrier” effect to the lateral extension of local regions of thick scale is particularly efficient when the individual grains have markedly dissimilar characteristics. This is especially true for acicular and alpha-beta structures. In equiaxed structures, the corrosion rate depends on both structure and texture. Structures with very fine precipitates (ϕ > 0.1 μm) show excellent nodular corrosion resistance. On the contrary, when the precipitates are coarse (ϕ < 0.2 μm), the nodular corrosion resistance is very poor. In both of these cases, the texture has little influence. However, for intermediate precipitate dimensions (0.1 μm < ϕ < 0.2 μm), the effect of orientation becomes preponderant. If a grain whose orientation favors rapid corrosion is surrounded by more resistant neighbors, then the localized corrosion will not develop. On the contrary, if the adjacent grains are also propitiously oriented for enhanced oxidation, a critical nucleus size is attained and the resulting corrosion nodule can continue to grow. This mechanism assumes that the initiation site includes a minimum number of grains.