# Effect of Laser Surface Treatment on the Corrosion Resistance of Zircaloy-4 at High Temperature
## 3 Results
### Oxide Film Morphologies  
Figure 7 shows the bright-field images of the cross-sectional oxide films of the LST and NLST Zircaloy-4 specimens after corroding in steam at 600 \({}^{\circ}\)C for 0 min. Both thicknesses of these two oxide films are \(\sim\)600 nm. In Figure 5, it can be seen that this corrosion is in the pre-transition stage. Figure 7a shows the morphology of the cross-sectional morphology of the oxide film formed on the surface of the LST Zricaloy-4 specimen. It is shown that some microcracks distribute in the oxide film, and the matrix grains are sub-grains. It means that the Zr sub-grains in the LST layer were not completely consumed during the corroding process. This oxide film resulted from the LST layer. However, more of microcracks appear in the oxide film of the NLST Zricaloy-4 specimen, as shown in Figure 7b. And the matrix grain is fully recrystallized. It is indicted that the LST layer can affect the oxide film morphology. In other words, LST can restrain microcrack formation in the oxide film. The reason for this phenomenon will be illustrated in the next part of this paper. Figure 8 shows the fracture surface morphologies of oxide films formed on the surface of the LST and NLST Zircaloy-4 specimens corroded in steam at 1100 \({}^{\circ}\)C for 0 min. In  
Figure 6: Weight gain vs. exposure time for the LST and NLST Zircaloy-4 specimens corroded in steam at 1100 \({}^{\circ}\)C for 2 h (the left arrow indicate the left Y-axis, the right arrow indicate the right Y-axis). Figure 7: Bright-field images of the cross-sectional oxide films of the LST (**a**) and NLST (**b**) Zricaloy-4 specimens after corroding in steam at 600 \({}^{\circ}\)C for 0 min. Figure 5, it can be seen that the corrosion is in the post-transition stage. Figure 8a,c show the images of the fracture surface morphologies of oxide film on the LST Zircaloy-4 specimen. Figure 8b,d show the images of the fracture surface morphologies of oxide film on the NLST Zircaloy-4 specimen. Comparing Figure 8a with Figure 8b, it can be seen that the thicknesses of the oxide films formed on the surfaces of the LST and the NLST Zircaloy-4 specimens are almost the same at \(\sim\)28 \(\upmu\)m. Figure 8c shows a magnified image of the local oxide in Figure 8a. It clearly shows that there are nearly no defects distributed in the oxide film. Figure 8d shows a magnified image of the local oxide. It can be seen that there are many defects distributed in the oxide such as microcracks and micropores. Comparing Figure 8c with Figure 8d, it can be seen that there is an obvious difference between the oxide film formed on the surface of the LST Zircaloy-4 specimen and that of the NLST Zircaloy-4 specimen. Figure 9 presents the normal XRD patterns of the oxide film formed on the surfaces of the LST and NLST Zircaloy-4 specimens corroded in steam at 1100 \({}^{\circ}\)C for 0 min. It is worth noting that the t-(101) Bragg peak intensity of the oxide film on the surface of the LST Ziccaloy-4 specimen is stronger than that of the NLST Zircaloy-4 specimen. In addition, the total integrated intensities \(I(hkl)\) of the m-(\(-\)111), m-(111) and t-(101) were used to determine the t-ZrO\({}_{2}\) volume fraction \(f_{T}\) with respect to the total volume of zirconia using the Garvie-Nicholson formula [38]. \[f_{T}=\frac{I_{T}(101)}{I_{T}(101)+I_{M}(111)+I_{M}(-111)}\]  
Here, the peak area and the peak height are treated as diffraction peak intensities to calculate \(f_{T}\). And then, the average value of the \(f_{T}\) calculated using peak area and the peak height as diffraction peak intensities is considered to be the real \(f_{T}\). After calculating, the \(f_{T}\) of the LST Zircaloy-4 specimen is \(\sim\)12.8%, and the \(f_{T}\) of the NLST Zircaloy-4 specimen is \(\sim\)6.8%. It is clearly indicated that the volume fraction of t-ZrO\({}_{2}\) in the oxide film formed on the surface of the LST Zircaloy-4 specimen is higher than that of the NLST Zircaloy-4 specimen after corroding in steam at 1100 \({}^{\circ}\)C for 0 min.