Causes of Flash Rusting During Water-Based Coating Application

The causes of flash rusting during the application of water-based anti-corrosion coatings for steel structures have long remained unclear. Previously, experts suggested that flash rust in water-based coatings occurs during the drying process of newly applied coatings on construction projects, primarily appearing at corrosion interfaces or on recently activated steel surfaces. The formation of flash rust may be related to the dissolution and precipitation of iron compounds. Its severity depends on the coating system; when the coating has a low pH, and under conditions of low temperature and high humidity, the contact time between water and the substrate is prolonged, making flash rust more likely to occur.

The film formation process of water-based coatings involves multiple stages. No metal surface is absolutely pure or perfectly smooth, resulting in numerous small areas with differing potentials. Current flows between these areas with varying potentials, triggering metal oxidation and forming rust spots—known as flash rust. These various explanations reveal that flash rust formation is an electrochemical process. Since water-based coatings are conductive, they create pathways for charge migration between areas with potential differences. Consequently, corrosion occurs rapidly during the short film-forming period of water-based coatings, resulting in flash rust. Clearly, the longer the wet film persists, the more severe the flash rust becomes. This explains why flash rust is more pronounced under low-temperature, high-humidity conditions.