Post-transition oxide

In Figure 5.9(a), a cross section SEM image of a specimen with 180 days exposure at 360^◦C is shown. It had attained an average final oxide thickness of 5.70 µm measured by SEM.

As shown in Figure5.9(a), a cross-sectional SEM examination of this specimen found that for the most part of the specimen, it is covered by a thick adherent corrosion film. In contrast to the pre-transition regime film, the oxide has a more uniform thickness, the undulation at the metal/oxide interface is less pronounced. Interfacial cracks observed in the pre-transition regime specimens are further away from the metal/oxide interface. Density of the cracks had increased after the transition, and also there are a lot very fine cracks appeared after the kinetic transition. New cracks were also observed at the metal/oxide interface.

Oxide surface cracks were also observed in the cross-sectional image in Figure5.9(b), small cracks that were normal to the surface, penetrating 500∼800 µm into the oxide.

(a) 2000x

(b) 4000x

Figure 5.9: SEM cross sectional view of ZIRLO^TM tube sample oxidised for 180 days at 360^◦C in primary water condition. The average thickness is 5.70 µm.

Images taken in BSE mode.

Noticeably, the oxide beneath these surface cracks is generally thicker compared with nearby regions that are free of surface cracks.

The oxide surface on ZIRLO^TM tube was examined in both SE and BSE mode (in Figure5.10). Grain boundary topography was observed as well as the surface cracks indicated in Figure 5.10(b). The roughness of oxide surface on the post-transition specimen (Figure 5.10(a)) was very much pronounced than the pre-transition oxide.

Surface cracks were observed which confirmed the cross section observation of surface cracks in Figure 5.9(b).

5.3.2 Zircaloy-4

5.3.2.1 Pre-transition oxide

SEM examination of the 25 days Zircaloy-4 specimen has shown interfacial crack formation as early as 25 days, with oxide thickness about 1∼1.5µm. Local corrosion has paused at cracked regions, indicated by the relatively thin oxide. Other regions that are free of cracks have uniform oxide thickness and very small undulations.

The oxide surface is relatively smooth and flat, no grain boundary topography was observed in Figure 5.11(b). Surface cracks on Zircaloy-4 specimens were observed as early as 93 days exposure (see Figure 5.12), crack length varies from couple of µm to over 20 µm with a width about 100 nm. The majority of these surface cracks are associated with surface features developed during the oxidation, such as surface topography of the original metal grain boundary, which was not observed on the early stage oxide. Surface examination on the 130 days Zircaloy-4 specimen (Figure 5.13) indicate that local swelling of the oxide surface is observed. Surface crack is usually associated with this type of surface swelling. Combining with the cross sectional observations of the oxide layer, usually a thicker oxide layer would be expected under these surface cracks, also the outer surface of the oxide would have

(a) SE mode

(b) BSE mode

Figure 5.10: SEM surface examination of ZIRLO^TM tube sample oxidised for 180 days at 360^◦C in primary water condition. (a) SE mode (b) BSE mode with

surface crack indicated in the dashed square.

(a) Cross section image

(b) Surface image

Figure 5.11: SEM examination of Zircaloy-4 sheet sample oxidised for 25 days at 360^◦C in primary water condition. (a) BSE mode cross sectional image (b) SE

mode surface image. The average thickness is 1.21 µm.

(a) SE mode

(b) BSE mode

Figure 5.12: SEM images of surface oxide on Zircaloy-4 sheet specimen oxidised for 93 days at 360^◦C in primary water condition. (a) SE mode image (b) BSE

mode image. The average thickness is 1.85 µm.

(a) SE mode

(b) BSE mode

Figure 5.13: SEM examination of Zircaloy-4 sheet sample oxidised for 130 days at 360^◦C in primary water condition. (a) SE mode image (b) BSE mode image.

The average thickness is 2.11 µm.

convex profile, probably due to the high local oxidation rate caused by forming an easy path way through the surface cracks.

The oxide layer on Zircaloy-4 specimens in the pre-transition regime shared some similarities with the ZIRLO^TM specimens, interfacial cracks were observed on all the pre-transition specimens, and their density of occurrence increases with increasing exposure time.

5.3.2.2 Post-transition oxide

The post-transition oxide of Zircaloy-4 is thinner than that of the ZIRLO^TM spec-imen at the same oxidation time, an average oxide thickness of 3.44 µm was de-termined by SEM. Cross sectional image of the 180 days oxide layer is shown in Figure5.14(a). Large number of pre-existing interfacial cracks were accumulated at a band region which is about 2 µm away from the outer surface, some finer cracks were also observed in this band region interconnecting the larger cracks. Hence, a large network of cracks would be expected.

Oxide surface roughness is more pronounced in the post-transition regime specimen, shown in Figure 5.14. As exposure time increases, the oxide outer surface become rougher, surface topographies arise from the metal substrate grain boundaries were observed in Figure 5.14. Some areas of surface oxide appeared to have “terracing”

features covering the whole areas of parent metal grains. Cracks are often associated with these features.

(a) Cross section image

(b) Surface image

Figure 5.14: SEM micrograph of the oxide formed on 180 days Zircaloy-4 sheet specimen that was oxidised in 360^◦C primary water condition. (a) BSE mode cross section image (b) SE mode surface image. The average thickness is 3.44 µm.