Waves and vortices of rust on the surface of corroding steel

Two-dimensional wave patterns have been observed on corroding steel plates. A thin layer of oxygen-containing nitric acid covering the surface of the low-carbon steel induces the corrosion process. The spectrum of the observed patterns includes propagating pulses, rotating spirals, and serpentine structures. The propagation velocity is on the order of millimeters per second and increases with acidity. Waves can be observed with the naked eye as traveling brownish bands on the passivated grayish steel. Spiral wave tips deposit black lines onto the surface, documenting the tip trajectory and influencing the propagation of subsequent spirals. A simple chemical model for the mechanism of pattern formation is suggested.