What is flash rust in zinc-rich coatings

The cause of flash rusting during the application of water-based anti-corrosion coatings seems to remain unresolved. There is a saying that goes: \"The flash rust problem in water-based coatings refers to the appearance of rust spots on newly applied water-based coatings during the drying process; this phenomenon occurs mainly at the corroded interfaces or on the surfaces of steel that have recently been activated.\" The formation of flash rust may be related to the dissolution and precipitation of iron compounds. The severity of flash rust depends on the coating system; coatings such as polyvinyl acetate latex are prone to flash rust when exposed to low pH values, low temperatures, and high humidity, as these conditions increase the contact time between water and the substrate. ” In fact, since flash rust is an electrochemical phenomenon, all active metals that are more reactive than hydrogen will encounter this problem when using water-based coatings. In our coating tests on substrates such as steel, cast iron, aluminum and aluminum alloys, and galvanized sheets, we found that materials prone to flash rusting are generally: cast iron, aluminum, carbon steel, galvanized sheets (sanded), tinplate (sanded), iron sheets or wire, and aluminum alloys. Based on various experiments and feedback from peers, cast iron is not only the substrate most prone to flash rust, but it is also one of the most difficult substrates to address this issue. Electrochemical phenomena are the root cause of flash rust, yet the conditions under which flash rust occurs during the application of water-based coatings are quite complex. Apart from the differences in substrates discussed above, factors such as the pH value of the paint, the temperature and humidity of the application environment, wind speed, and the thickness of the paint film all have an impact on the phenomenon of flash rust.

There is a theory that flash rusting is an essential step in the spraying of inorganic zinc-rich primers; appropriate flash rusting helps with the adhesion of the primer. This is also why high humidity is required. Excessive flash rusting, however, is detrimental