A Brief Analysis of the Causes of Flash Rust in Water-Based Coatings

The film formation process of water-based coatings involves a series of steps. No metal surface is absolutely pure and smooth, inevitably creating small areas with differing electrical potentials. Current flows between these areas with varying potentials, leading to metal oxidation and the formation of rust spots—commonly known as flash rust. 

The issue of flash rust in water-based coatings typically refers to rust spots appearing during application or drying, primarily occurring at corrosion interfaces or newly exposed steel surfaces. The causes of flash rust during application may be related to the dissolution and precipitation of iron compounds. The severity of flash rust depends on the coating system; for example, polyvinyl acetate emulsions are prone to flash rust under low pH, low temperature, and high humidity conditions, where prolonged contact time between water and the substrate increases susceptibility.

 Since water-based coatings are conductive themselves, charge migration pathways form between different areas, leading to corrosion occurring within the short timeframe of the water-based coating's film formation process and resulting in flash rust. Clearly, the longer the wet film remains, the more severe the flash rusting becomes, explaining why low-temperature, high-humidity conditions exacerbate this phenomenon.

While electrochemical phenomena are the fundamental cause of flash rusting, the conditions contributing to it during water-based coating application are highly complex. Beyond substrate variations mentioned above, factors such as coating pH, environmental temperature and humidity, wind speed, and film thickness all influence flash rusting occurrence.