*5.1. Corrosion Depth Profile of the Test Tube*

Figure 7a shows an OM image of cross-section of the specimen of the central straight pipe. The corrosion depth around the pipe circumference is relatively uniform. The maximum wall thickness of the corroded specimen is about 2.1 mm, as marked, which represents that the wall thickness of the as-received tube is no less than 2.1 mm. The fabrication tolerance is ±0.2 mm, so that the wall thickness of the as-received tube ranges from 2.1 to 2.2 mm, and it deviates 0.1–0.2 mm from its design size shown in Figure 5. This also implies that the thickness of the two 30-degree orifices deviates 0.087–0.173 mm from the design size. The corrosion depth of the specimen of the central straight pipe reaches a maximum of approximately 0.2–0.3 mm, and is 0.06–0.16 mm on average. The average corrosion rate ranges from 0.02 to 0.053 μm/h. Figure 7b shows the SEM image of cross-section of the specimen of the central straight pipe. No oxide film was observed on the specimen surface. Furthermore, there is LBE penetration into the grain boundary, which illustrates a typical corrosion scenario of 316L SS in LBE as stated previously in the corrosion model (Section 2.2). That is, Ni and Cr dissolve into LBE in the superficial area of the steel, and vacancies in the steel matrix are created as a result of depletion of Ni and Cr and of lower Fe content as well, and subsequently, Pb and Bi penetrate into the steel to fill the vacancies. During the ongoing penetration, selective leaching of Ni and Cr continues [38].

Figure 7c shows the OM images of cross-sections of the two orifices at various circumferential angles. The two orifices are the sloping parts seen in Figure 5. Clearly, the two orifice parts have been severely corroded, and the corrosion profiles are not uniform along the orifice surface. The quantitative corrosion depths of the two orifices along the orifice surface at various circumferential angles are plotted in Figure 8a,b. The corrosion depth was measured from the OM images by comparing the morphologies of the corroded orifice surfaces to those of the original surfaces. Each corrosion depth profile consists of forty datapoints measured equidistantly along the original orifice surface. The deviation of the measurement is about ±0.005 mm. The measured average corrosion depth was added by a fabrication deviation of 0.087 mm, as pointed out previously. The corrosion depth profiles are similar for all circumferential angles at each orifice. The least corrosion is seen at a circumferential angle of 180 degrees. It is, at present, unclear why this is so, although it may be attributable to the deposition of corrosion products. The maximum corrosion depth is nearly 1.0 mm for both orifices. The average corrosion depth of the inlet orifice and the outlet orifice is shown in Figure 8c,d,

respectively. Fabrication deviations of 0.087 mm and 0.174 mm are included in the calculation results of the average corrosion depth. The corresponding error bars are derived from the circumferential data in Figure 8a,b. The results indicate that the average depth of corrosion is larger in the outlet orifice than in the inlet orifice.

(**c**) OM images of the two orifices

**Figure 7.** OM and SEM images. (**a**) OM image of cross-sections of the specimen of the central straight pipe; (**b**) SEM image of cross-section of the specimen of the central straight pipe; (**c**) OM images of the two orifices at various circumferential angles.

**Figure 8.** (**a**,**b**) Corrosion depth profiles of the inlet and outlet orifices at various circumferential angles as a function of the distance along the orifice surface. Fabrication deviations of 0.087 mm are added in the measured corrosion depth; (**c**,**d**) average corrosion depth of inlet orifice and outlet orifice. Fabrication deviations of 0.087 mm and 0.174 mm are added in the measured average corrosion depth, respectively.
