# Effect of Laser Surface Treatment on the Corrosion Resistance of Zircaloy-4 at High Temperature
## 4 Discussion  
Generally speaking, the corrosion processes of Zirconium alloys can be divided into pre-transition and post-transition stages. In the pre-transition stage, the oxide film is dense and has a protective effect, but in the post-transition stage, the oxide film lost protection effect and the corrosion rate obviously increased [39; 40; 41]. Therefore, the corrosion rate is determined by the properties of the oxide film. It is well known that when the thickness of the oxide films of the zirconium alloys exceeds a critical value 2-3 \(\upmu\)m during the corrosion process, it corresponds to an increase in the oxidation rate [42; 43; 44]. So, the properties of the oxide film formed in the earlier stage will affect the corrosion behavior of the later corrosion stage. In Figure 6, we can see that the LST and the NLST Zircaloy-4 specimens corroding in the steam at 600 \({}^{\circ}\)C are in the pre-transition stage. And in Figure 7, it is clearly shown that more of microcracks appear in the oxide film of the NLST Zircaloy-4 specimen than that of the LST Zircaloy-4 specimen. In Figure 8, we can also see that the oxide film formed on the surface of the LST Zircaloy-4 specimen is denser than that of the NLST Zircaloy-4 specimen. Moreover, there are nearly no defects distributed in the oxide film of the LST Zircaloy-4 specimen, But, microcracks and micropores are clearly shown in the oxide film of the NLST Zircaloy-4 specimen. In Figure 5, we can see that the Fe-Cr precipitates are dissolved and disappear in the LST layer. It is well known that the oxidation rates of the precipitates in zirconium alloys are generally slower than those of \(\alpha\)-Zr [45; 46; 47; 48]. Therefore, the precipitates are incorporated into the oxide film. And then, these precipitated can be full oxidized in the oxide film at last. The Pilling Bedworth ratio (P.B. ratio) of Fe is 1.77, and that of Cr is 2.02; they are both larger than that of Zr: 1.56 [49]. After the precipitated oxidization, local additional stress in the oxide film will be generated, and the t\(\rightarrow\)m transformation can be initiated [50]. And then, this process can result in microcracks formation. The illustration of the microcracks' formation, which resulted from precipitates in the oxide film, was shown in detail in a work published by [PERSON] et al. [51]. These cracks will cause the oxide film to lose its protective effect and then increase the corrosion rate. Therefore, the corrosion resistance of the NLST Zircaloy-4 specimen is worse than that of the LST Zircaloy-4 specimen, as shown in Figure 6.  
On the other hand, during the corrosion of zirconium alloys, t-m transformation could cause microcrack formation and oxide film damage due to the volumetric expansion of  
Figure 9: X-ray diffraction patterns of the oxide films formed on surfaces of the LST and NLST Zircaloy-4 specimens after corroding in steam at 1100 \({}^{\circ}\)C for 0 min. \(\sim\)5% [52; 53]. Since some elements have a valence of +2 or +3, they could be added to stabilize commercial t-ZrO\({}_{2}\) ceramics [54; 55; 56]. So, Fe and Cr could be the elements for stabilizing the t-ZrO\({}_{2}\) phase since they can introduce many intrinsic oxygen vacancies [57; 58]. In Figure 5, nearly no Fe-Cr precipitates appear in the LST layer; this means that the solid solution of Fe and Cr in \(\alpha\)-Zr is higher than that in the common matrix. And then, these solid solutions of Fe and Cr in \(\alpha\)-Zr could stabilize the t-ZrO\({}_{2}\) phase. Therefore, more t-ZrO\({}_{2}\) could be distributed in the oxide film formed on the surface of the LST Zircaloy-4 specimen than that of the NLST Zircaloy-4 specimen. This is consistent with our XRD result, as shown in Figure 9. The volume fraction of t-ZrO\({}_{2}\) is higher, so the number of microcrack that resulted from t-m transformation could be less.