# 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  
SUPERDYMA shape steel is a Zn-Al-Mg-Si coated steel from Nippon Steel and Sumitomo Metal in Japan, with a coating composition consisting of zinc as the main substrate in combination with aluminum (about 11%), magnesium (about 3%), and a trace amount of silicon (about 0.2%). The corrosion rate of SUPERDYMA is only one-thirtieth as fast as that of the general hot-dip galvanized steel, because the Zn-Al-Mg-Si coated layer produces a compact protection layer during the corrosion process and then contributes to the effective suppression of corrosion. A protection layer is also generated in the corrosion process of hot-dip galvanized steel, but has an interior effect on the suppression of corrosion as a result of the lower density of the protection layer, leading to water and oxygen penetration. In contrast, Zn-Al coated steel contains 55% aluminium and thus has a stronger corrosion resistance [19]. Similar to aluminum, the ability of Zn-Al coated steel to resist acidic corrosion is superior to that of Zn-Al-Mg-Si coated steel, while its ability to resist alkali corrosion is inferior to that of general hot-dip galvanized steel [20; 21]. Although the type 304 stainless steel (SS) or other higher number steels have a better acid and alkali resistance, their ability to resist chloride is weaker. Overall, Zn-Al-Mg-Si coated steel has an excellent corrosion resistance and alkali resistance, but normal acidic resistance. Furthermore, the thicker the coating layer, the better the corrosion resistance. Shape steel K27 (thickness of coating layer is 0.068 mm) was selected as the substrate on which the screws were fastened in this study. The five types of screw, from left to right, were the 304 SS screw, the 410 SS screw, the XP zinc-tin alloy coated steel screw, the galvanized steel screw capped with a type 316 SS nut, and the chromate-passivated galvanized steel screw. Each type had an equal number of screws (n = 4), including two of with an outward tail (upper row) and two with \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Method Item** & **Neutral Salt Spray (NSS)** & **Acetic Acid Salt Spray (ASS)** &
\begin{tabular}{c} **Copper-Accelerated Acetic** \\ **Salt Spray (CASS)** \\ \end{tabular} \\ \hline Temperature & (35 \(\pm\) 2) \({}^{\circ}\)C & (35 \(\pm\) 2) \({}^{\circ}\)C & (50 \(\pm\) 2) \({}^{\circ}\)C \\ \hline pH (collected solution) & (6.5–7.2) & (3.1–3.3) & (3.1–3.3) \\ \hline Concentration of sodium chloride & & (50 \(\pm\) 5) g/L \\ (collected solution) & & (50 \(\pm\) 5) mL/h \\ \hline Average collection rate for a & & (1.5 \(\pm\) 0.5) mL/h \\ horizontal collecting area of 80 cm\({}^{2}\) & & (1.5 \(\pm\) 0.5) mL/h \\ \hline Periods of test & 2, 6, 24, 48, 96, 168, 240, 480, 720, and 1008 h \\ \hline \hline \end{tabular} The \(\pm\) tolerances given are the allowable operational fluctuations, which are defined as the positive and negative deviations from the setting of the sensor at the operational control set point during equilibrium conditions. This does not mean that the set value may vary by plus/minus the amount indicated from the given value.  
\end{table} 
Table 1: Comparison table of salt spray test conditions. an outward head (lower row), and a total of 20 screws without any rust had metallic luster before testing, as observed in Figure 4a. A white substance started to emerge on the surface of the K27 shape steel after the five-day (120 h) test, as observed in Figure 4b. This white substance was unable to be removed through washing with water. In other words, it was not a salt deposit, but the protection layer generated from the Zn-Al-Mg-Si coating dissolution during the corrosion process. Zinc hydroxide Zn(OH)\({}_{2}\) and simonkolleite Zn\({}_{5}\)(OH)\({}_{8}\)Cl\({}_{2}\)-H\({}_{2}\)O were shown to be the materials of composition for the white substance [22; 23]. The white protection layer was particularly obvious in the interface of the fastened screws.