## 3 Results and Discussion
### 3.1. Wettability Test Results
For the sample etched for 20 h, the size of the cylinder was significantly smaller due to the long etching time. Observing the magnified images in Figure 3d3,d4 of the surface morphology, relatively light nano-layered structure can be found, which is not as obvious as in Figure 3c4. Through the analysis of the microstructure morphology of the sample surface, it can be found that the nano-layered structure of the cylindrical surface after 15 h of corrosion is the most prominent, and the surface is also the shortest time-consuming transition from the hydrophilic state to the super-hydrophobic state. The cylindrical surface etched for 20 h has less nano-layered structures than the one etched for 15 h, and it takes longer for the surface to change from a hydrophilic state to a super-hydrophobic state. However, the nano-layered structure on the cylindrical surface that was corroded for 5 h and 10 h was less, and the transition from hydrophilic to superhydrophobic could not be realized. Figure 4a shows the surface XPS spectra of samples corroded for 15 h in a room atmosphere for 1 day, and Figure 4b shows the surface XPS spectra of samples corroded for 15 h in a room atmosphere for 15 days. The surface elements of the samples left for 1 day and 15 days are the same; they are Ni, Fe, O, Cr, C, and Si, all of which are 316 L  
Figure 3: The micro-topography of the sample surface at different times of corrosion. (**a**) 5 h; (**b**) 10 h; (**c**) 15 h; (**d**) 20 h.  
stainless steel. The difference lies in the element content. The content of the C element increased from 57.57% for 1 day to 66.45% for 15 days, and the content of the O element dropped from 38.45% for 1 day to 27.4% for 15 days. The content of the Fe element did not change significantly, and it increased by 2.17%. The increase in the content of the C element is consistent with the experimental results of [PERSON]. [PERSON] [28], indicating that after 15 days indoors, the microstructure adsorbs the organic matter in the air and reduces the surface energy. This is the fundamental reason why the surface of the sample changes from hydrophilic to superhydrophobic. Figure 5 shows the change in the contact angle of the water droplets on the surface of the 3D-printed + modified samples after being placed outdoors for different times. It can be seen intuitively from the figure that after two months of outdoor placement, the contact angle of the sample surface has changed from the initial 153.4\({}^{\circ}\) to 150.8\({}^{\circ}\). It still has excellent superhydrophobic properties, indicating that a 3D-printed + modified superhydrophobic surface has good durability under natural outdoor conditions. Figure 6a-c are the pictures of the contact angles of water droplets on the surface of the 3D-printed + corrosion samples (corrosion 15 h) after being placed in an outdoor natural environment for 5 days, 10 days, and 15 days, respectively. It can be seen from  
Figure 4: The surface XPS spectra of samples corroded for 15 h in a room atmosphere for 1 day (**a**) and 15 days (**b**). Figure 5: Contact angle of 3D-printed + modified samples placed outdoors for different time. the picture that on the 5 th day, the contact angle of the water droplets on the surface was 151.3\({}^{\circ}\), and the sample still had excellent superhydrophobic properties. However, on the 10 th day, the contact angle of the water droplets on the surface dropped to 90.2\({}^{\circ}\), and the superhydrophobic properties disappeared. On the 15 th day, the wettability of the surface of the sample changed again, and the contact angle of the water droplets on the surface was only 29.8\({}^{\circ}\), which changed from a hydrophobic surface to a hydrophilic surface. It can be found that the durability of 3D-printed + corrosion superhydrophobic samples in outdoor natural environments is less than 10 days; the durability is not good.