# A Novel Accelerated Corrosion Test for Supporting Devices in a Floating Photovoltaic System
## 3 Results and Discussion
### SUPERDYMA Shape Steel and Five Types of Screw
This can be interpreted by the fact that magnesium in Zn-Al-Mg-Si coating layer has lower standard reduction potential (anode). When in contact with other metals, such as iron, zinc, or aluminum (cathode), the magnesium corroded before the others, which led to the production of the white protective layer around the screws. The larger the reduction potential difference of the heterogeneous metals, the more easily the white protection layer generated quicky because of its strong causticity. What is more, the smaller area cathode, the larger the current density and the more severe the corrosion. As observed in Figure 4c, after the eight-day (192 h) test, the surface of the K27 shape steel was almost completely covered by the white protection layer, but a local region was already rusted. This result indicates that most of the surface of the K27 shape steel was resistant to corrosion, while only a partial area of the white protection layer was fully corroded and rusted further into the ground layer steel. If the time for rust generation needs to be delayed, a shape steel with thicker coating layer could be employed. Figure 3: Test system for the salt spray corrosion. Figures 5 and 6 show the results of the five types of screws after the five-day (120 h) test and eight-day (192 h) test, respectively. As observed from the heads of the screws, both the type 316 SS nut capped on the galvanized steel screw and the type 304 SS screw presented with no rust, yet the type 304 SS screw lost its metallic luster and the other remained. On the contrary, the type 410 SS screw not only lost its metallic luster, but red rust formation was also found. These results suggest that type 316 SS (containing chromium, nickel, and molybdenum) had the highest corrosion resistance, followed by type 304 SS (containing chromium and nickel) and type 410 SS (containing chromium). In addition, a small amount of red rust was also produced on the chromate-passivated  
Figure 4: The appearance of SUPERDYMA shape steel: (**a**) before the copper-accelerated acetic acid salt spray (CASS) test (0 h), (**b**) after the five-day CASS test (120 h), and (**c**) after the eight-day CASS test (192 h). galvanized steel screw, and some yellow substances were found on the XP zinc-tin alloy coated steel screw instead of rust. As observed from the tails of the screws, only little red rust as generated on the 304 SS steel screw in comparison with the 410 steel screw, which was consistent with the data that the 304 SS steel screw had a higher corrosion resistance ability than the 410 SS steel screw, as mentioned in the previous paragraph. The tail of the galvanized steel screw capped with a type 316 SS nut was a general galvanized steel screw, upon which more red rust was produced compared with the chromate-passivated galvanized steel screw. This could be attributed to the chromium-containing chemical conversion coating on the surface of the chromate-passivated galvanized steel screws, which had a better corrosion resistance than the general galvanized steel screws. Moreover, the XP zinc-tin alloy coated steel screw produced numerous yellow substances instead of red rust on the original silver white surface. We believe that those yellow substances are the protection layer resulting from the zinc-tin alloy coating layer dissolution, and not only suppress further corrosion, but also possess a strong corrosion resistance ability because of its fairly compact structure. The composition of the yellow substance was stannous hydroxide Sn(OH)2 and Sn4(OH)6 Cl2 [24; 25; 26; 27]. Last, a comparison between the heads and tails of the screws revealed that the corrosion degree of the tails of the screws was more severe than the heads of the screws. A possible explanation for this is that the coating layer or protection layer of the screws were prone to thinning and even to partial peeling when screws were used as tapping substrates, which reduced the corrosion resistance greatly.