White Rust Formation and Discoloration of Hot‐Dip Galvanized Steel During Heat Treatment

ABSTRACT A severe corrosion failure occurred on galvanized steel automotive parts after heat treatment at 900°C, manifesting as substantial, loose, and porous white rust on the surface. Additionally, the color of the galvanized steel changed from light yellowish green to dark brown with prolonged heating. In this study, the mechanisms behind the corrosion failure and the color change were investigated using multiple analytical techniques (including SEM, EDS, and XPS) as well as simulation experiments. The results indicated that the white rust was zinc oxide (ZnO), and its formation was caused by the presence of sodium sulfate (Na 2 SO 4 ) in the rust‐preventive oil on the surface of the galvanized sheet. At 900°C, molten Na 2 SO 4 triggered high‐temperature hot corrosion, leading to the formation of extensive, loose, and easily detachable white rust on the surface. For shorter heat treatment durations, ZnO and FeO formed on the galvanized sheet surface, giving it a light yellowish‐green appearance. As the heating time increased, substrate‐sourced Fe and Mn elements diffused to the surface and oxidized into MnO 2 and Fe 2 O 3 , which caused the surface to turn dark brown.